◊ 5-General Anesthesia ◊
It is a state of unconsciousness, insensitivity to pain and stimuli from the environment, and absence of motor reflexes to such stimuli due to transit paralytic action of the motor center produced by process of controlled reversible intoxication of the CNS without interfering with vital centers. General anesthesia can be classified according to physical characters of anesthetic agent into injectable anesthesia and volatile anesthesia.
General anesthetic can be classified into volatile or gaseous anesthetic that can be inhaled into the lung, transmitted to circulation and reaches the CNS to produce its action; or nonvolatile injectable anesthetic agent that can be administered orally, rectally, or injected intra-peritoneal, intra-venous or intra-muscular after which it is released to circulation and reaches CNS to produce its effect.
Regarding equine, there are numerous problems in anesthetizing this species. These problems are related to the size of the animal, and its temperament that render surgery under local analgesia in standing position more safe for the animal than general anesthesia although it may be more dangerous for the surgeon. Size of the animal determines both the ease of restrain and type of breathing circuit when inhalation anesthesia adopted. Accordingly, foals or small sized horse require minimal restrain and the inhalation anesthesia apparatus of human is suitable for that species, while larger sized horses require restraining by heavy sedation or intravenous anesthesia prior but the inhalation anesthesia apparatus of human causes too much resistance to breathing. Animals of bad temper require the use of agents that can be injected rapidly to avoid dislodging of the needle during injection. Weight of the animal can compromise both arterial blood supply and venous drainage of the lower muscles during general anesthesia. Especially when hypotensive anesthetic is too be used (like halothane). Accordingly if preventive measures are not considered, muscular ischemia and necrosis will ensue leading to lameness after surgery.
Ruminants are not good subjects for general anesthesia due to;
1-The danger of regurgitation and inhalation of ingesta is much greater in these species compared to other common domestic species.
2-Their docile temperament allows majority of surgical procedures to be carried out by local anesthesia (± sedation) without much difficulty, and many techniques are available (see local anesthesia lecture)
3-However, some procedures, with economic justifications, are better performed under general anesthesia, and with certain precautions, general anesthesia can be carried out safely without complications
4-Adult cattle carries greater risk of developing myopathies and neuropathies following prolonged recumbency, so good positioning and protective padding must be ensured
5-Following recumbency esophageal opening is submerged in ruminal contents, normal eructation can not occur, and gas accumulates. The degree of bloat depends on the amount of fermentation and on the length of time that gas is allowed to accumulate
6-Gross distension of the rumen becomes a hazard if anesthesia or recumbency is prolonged and regurgitation can follow from this
7-The weight of the abdominal viscera and their contents prevents the diaphragm from moving freely on inspiration and ventilation becomes shallow, rapid and inefficient for gas exchange within the lungs.
8-In unfortunate circumstances, the aspirated regurgitatants obstruct the airway, cause asphyxia, and bring the patient to death within 24 hours of developing the complication.
The danger of regurgitation can be minimized by;
1-Starvation prior to anesthesia
2-Water deprivation prior to anesthesia
3-In lateral recumbency, elevating the neck to avoid easy regurgitation and positioning the head sloped down to facilitate drainage of saliva (large amount produced) and other intraoral materials.
4-Passing down a stomach tube so as to allow drainage of ruminal materials (and also accumulated gas) during the recumbency
5-Cleansing solid materials in the mouth at the end of anesthesia, and leaving the ET tube with the cuff inflated until the animal is in sternal recumbency, is swallowing and is able to withdraw its tongue into the mouth
Use of general anesthesia: -
General anesthesia offers the ideal situation for;
2-Proper handling of tissue
Reflexes that disappeared during general anesthesia: -
5-Skin pinching reflex
6-Pharyngeal, laryngeal and cough reflexes
7-Ocular reflex (corneal)
Transportation of anesthetic agent to the brain and from brain: -
General anesthetics are two types mainly inhalation anesthetics and injectable ones. With respect to inhalation anesthetics, following inhalation of anesthetic agent it passes to alveoli of the lung, then it diffuse to the blood, and finally it distributed through out the body fluids and tissues. Brain tissue has high lipid contents and rapidly acquires anesthetic concentration equal to that of the blood. However the process of uptake of anesthetic depends up on many factors one of which is the blood pressure and the heart rate.
Heart rate has direct relationship with saturation of the brain with anesthetic, and the faster the heart rate as during excitement or struggling that may associate absence of tranquilization prior to anesthesia, may predispose the brain to rapid uptake of anesthetic agent. Finally the drug eliminated from the body in reverse manner and eliminated by expiration although small amounts may be lost via skin or wound.
Anesthetic drugs administered by injection are mixed with blood directly and transmitted to the brain, while oral administration of anesthetics predispose them to absorption to portal circulation and passes to the liver first then after it passes to circulation to reach the brain. Elimination of these drugs occurs via metabolization in the liver and excretion in urine.
I-Nonvolatile Anesthetic Agents (Injectable)
Intravenous anesthesia is particularly useful either for the induction of anesthesia that is maintained by inhalation technique or when short duration anesthesia is needed, and it should be kept in mind that their effect is not as fast reversal as inhalation type. It is mandatory that the person deal with that type of anesthesia have knowledge about intravenous administration of these drugs and familiar with type of anesthesia produced so that overdose can be avoided.
Clinical level of anesthesia is related to both intensity of surgical stimulation and degree of cerebral depression. Undisturbed animal may have depressed breathing and relaxed abdominal muscles giving the picture of satisfactory anesthesia (corresponding to 2nd or 3rd plane of 3rd stage anesthesia), but surgical stimulation of that animal may stimulate breathing and induce tonicity of muscles may be with reflex motion of limbs (corresponding to 2nd stage or 1st plane of 3rd stage anesthesia). On the other hand, increasing the dose of anesthesia may produce severe respiratory depression and loss of consciousness during absence of surgical stimulation
A-Chloral hydrate: -
Chloral hydrate is primarily a basal narcotic drug that can be injected strictly IV in concentrations of 10% (to avoid the probability of necrosis associating peri-vascular filtration). The drug may be used with barbiturate or magnesium sulfate, however it is unlikely that these combinations have advantages over the chloral hydrate alone. The dose should be increased up to 6-7 gm/ 50 kg, 10% concentration, and injected strictly intra-venous to induce deep narcosis that bordering the line of anesthesia. Generally this dose is not anesthetic and the anesthetic dose is very close to toxic dose that is why this drug is not used for anesthesia, and it should be noted that the toxic dose is 18.5 gm/ 50 kg.
1-Absence of excitement during induction or recovery
2-Wide safety in healthy animals
3-Absence of post anesthetic nausea or malaise
4-Rapid on set (4-5 mins)
1-Necrosis of peri-vascular tissue when it accidentally infiltrated
2-Prolonged recumbency (1.5-2 hrs) is a significant inconvenience
The drug can be used for induction of anesthesia in horse for minor operations like castration, but longer duration surgeries it is not convenient to use additional doses to maintain anesthesia to avoid the side effect on liver, so it is advised in such situations to make shift to inhalation anesthesia or intravenous barbiturate.
B- Magnesium sulfate: -
It is not used alone for induction of anesthesia because of its narrow safety margin, and the marked cardiovascular and respiratory depression. Its use alone is restricted to euthanasia and one liter of saturated solution is sufficient for euthanasia of adult horse.
C- Chloral hydrate + Magnesium sulfate (2-3 parts:1 part): -
A mixture of chloral hydrate 5 gm/ 50 kg and magnesium sulfate (2:1 or 3:1) can be used for induction of anesthesia in horse.
[5 gm/ 50 kg chloral hydrate + 1.5 - 2.5 gm/ 50 kg magnesium sulfate (3:1 or 2:1)]
1-Less irritant 2-Faster on set of anesthesia
3-Higher depth of anesthesia 4-Lower toxicity
D-Magnesium sulfate + Chloral hydrate + Acetylepromazine: -
It can be used for sedation and induction general anesthesia, generally 3 ml/50 kg of solution (containing 3% magnesium sulfate and 7% chloral hydrate) and 3 mg/ 50 kg acetylpromazine produce sedation in horse
[3 ml (7% chloral hydrate + 3% magnesium sulfate) + 3 mg acetylepromazine] / 50 kg
E-Magnesium sulfate+Chloral hydrate+Pentobarbitone Na (Equithesin): -
It is a satisfactory general anesthesia in equine although it causes prolonged recovery.
[28 gm chloral hydrate + 14 gm magnesium sulfate + 6.5 gm pentobarbiturate + 1000 water]
75 ml/ 50 kg
(20 gm chloral hydrate + 10 gm magnesium sulfate + 4.3 gm pentobarbitone sodium)
1-Smooth induction without excitement and smooth rapid recovery
2-Complete muscle relaxation and complete anesthesia
3-Wide safety margin
1-Short anesthetic period (15-30 mins) that increases to 40-60 mins by adding 15% of the dose
2-It should be used fresh
F-Ketamine H Cl (Ketalar®, Vetalar®, or Ketaset®): -
It is non-barbiturate general anesthetic of rapid onset and short duration. It produces anesthesia that differ from conventional anesthetic as the patient is described as being dissociated rather than being unconscious with the opened eyes and existence of some degree of swallowing reflex.
Clinical effect: -
2-Poor muscle relaxant with induction of toinc-clonic spasm of limbs even in absence of surgical stimulations
3-Salivation that may cause obstruction of respiratory passage despite of presence of laryngeal and pharyngeal reflexes
4-Mild respiratory depression manifested by increased rate without compensation of decreased tidal volume
5-It raises arterial blood pressure controversial to other injectable general anesthetics
6-Its use alone causes good analgesia, poor muscle relaxation, convulsion, and violent recovery period
7-It works well when combined with xylazine or diazepam, with longer duration of action and absence of convulsions
Ketamine alone causes stimulation of CNS rather than depression with tremors or convulsions and poor muscle relaxation. However incorporation of xylazine produces an excellent induction with rapid very quite recovery. This combination is much safer than xylazine thiopentone mixture. This method is associated with much gradual onset of recumbency controversial to the effect of thiopentone. Respiratory depression is minimal so that maintenance by inhalation anesthesia can be achieved. Generally it is not recommended to prolong anesthesia induced by this mixture by additional injection of ketamine to avoid its associating excitatory effect.
Xylazine 1.1 mg/ Kg IV + Ketamine 2.2 mg/ kg IV lateral recumbency after 1-2 mins
Xylazine H Cl 2.2 mg/ kg IM
Glyceryl quaiacolate 55 mg/kg IV 5% in dextrose 5%
Ketamine 1.7 mg/kg IV
Ketamine alone will not cause seizure in cattle but the quality of induction is poor. However it is associated with increased muscle rigidity and excessive salivation and it may cause increased heart rate, cardiac output, and blood pressure. Generally it is better to be used in combination with other sedatives (most commonly with benzodiazepines).
Calves from one week to one year can be anesthetized in the following manner
1-Ketamine 20 mg/ kg IM or IV
2-Ketamine 10 mg/ kg IM + Xylazine 0.2 mg/ kg IM
In calves 0.25 mg/kg of diazepam and 5 mg/kg ketamine can be combined and injected IV. Butorphanol 0.1-0.2 mg/kg IV can be included in this combination for better analgesia and muscle relaxation. This regimen provides approximately 15 minute of anesthesia
B-Mature cattle: -
[Xylazine 0.2 mg/ kg (IM) or 0.1 mg/ kg IV + Ketamine 2 mg/ kg IV]
Injection of xylazine prior to ketamine provides that ketamine produce excellent anesthesia, quite induction, good muscle relaxation, and smooth uncomplicated recovery. Endotracheal intubation can be performed directly after injection of xylazine and induction of sedation, to avoid asphyxia or drenching pneumonia due to either excessive production of saliva or inability to swallow. Prolongation of duration can be assured by drip infusion of ketamine 2 mg/ml and recovery is completed 45 mins following stopping of infusion.
[Xylazine 0.1-0.2 mg/ kg (IM) or 0.05-0.1 mg/ kg IV + Butorphanol 0.1-0.2 mg/kg IV + Ketamine 2 mg/ kg IV]
Another technique includes injection of Xylazine to adult cattle either IM at 0.1 – 0.2 mg/kg or IV at 0.05-0.1 mg/kg to produce deep sedation often recumbency. Butorphanol 0.1-0.2 mg/kg IV can be included in this combination for better analgesia and muscle relaxation. Then after Ketamine is then given IV in doses of 2 mg/kg to induce anesthesia. Often, Endotracheal intubation can be performed soon after the xylazine injection and before ketamine is given and whenever possible this should be done, as ketamine appears to produce copious salivation or an inability to swallow the normal saliva volume. Hypoxia due to hypoventilation during the use of this combination has also been reported. For this reason, supplemental oxygen is recommended.
[Diazepam 0.1 mg/kg + ketamine 2 mg/kg given IV]
This combination will produce less cardiovascular depression than xylazine-ketamine. This combination is used for induction in xylazine premedicated animals in the dose of diazepam 0.1 mg/kg and ketamine 2 mg/kg given IV inducing anesthesia in 60 seconds
Ketamine on its own has analgesic effect in small ruminants yet they aware of surrounding, and maintained ability to eructate, cough, and swallowing. It can be used alone or with other drugs to reduce excitation and increase muscle relaxation.
Animals should be pre-medicated with atropine sulfate 0.4 mg/ kg IM, and incorporation of xylazine and/or diazepam increases skeletal muscle relaxation and suppresses excitatory effect of ketamine.
In small ruminants 0.25 mg/kg of diazepam and 5 mg/kg ketamine can be combined and injected IV. Butorphanol 0.1-0.2 mg/kg IV can be included in this combination for better analgesia and muscle relaxation. This regimen provides approximately 15 minute of anesthesia.
a-Ketamine 20 mg/ kg (IM or slow IV)
b-Ketamine 11 mg/ kg + Xylazine 0.22 mg/ Kg IM
c-Ketamine 4 mg/ kg + Xylazine 0.05 mg/ kg IV + Diazepam 1-2 mg/ kg
Anesthetic dose is very close to convulsion dose so it is not recommended as sole anesthetic agent in that species. However pre-medication with diazepam and xylazine (even via epidural rout) prolongs duration of analgesia and reduces convulsions.
5 mg/ Kg IM (premedicated with Xylazine or promazine)
30 mins and recovery is 90 mins
As the drug can be used IM, it makes it very useful agent for induction of anesthesia in the cat that is difficult to be handled.
a-Ketamine 11-22 mg/ Kg IM
b-Ketamine 5 mg/kg IV + after 1 mg/kg Xylazine IM + 0.3 mg atropine
30 mins and recovery is 90 mins
Barbiturates (thiopental sodium, thiopentone sodium, or pentobarbital sodium) are sodium salt of barbituric acid and when dissolved in water they act as weak acids and the effect (depth of anesthesia) of these derivatives is directly proportional to the acidity of the solution and blood pH. The drug acts via depression of CNS by blocking the passage of impulses to cerebral cortex.
The drug is used mainly for induction of general anesthesia in dogs while in equine its use is limited to obtaining muscular relaxation with chloral hydrate and magnesium sulfate.
On injection, about 30% of the dose should be injected rapidly IV and the rest injected slowly till absence of reflex
Clinical effect: -
2-Circulatory depression centrally and peripherally with dropping of blood pressure
3-Lowering body temperature due to reduction of basal metabolic rate
4-Complete muscle relaxation
1-Pentobarbital sodium: -
1-White powder or crystalline granules
2-Soulable in water or alcohol to form colorless solution
Freshly prepared solution can be used orally, IV, or intra-peritoneal
6-18 hrs and cats my not arouse for as long as 24-72 hrs
1-The drug is able to cross the placenta leading to high mortality fetus accordingly it is not recommended for cesarean section.
2-Anesthetised animals exhibit the same signs of anesthesia in reverse order as crying, shivering, involuntary movement, and finally standing with staggering gait.
2-Thiopental sodium: -
The ultra-short acting thiobarbiturate, thiopental, provides approximately 10-15 minutes of anesthesia when used alone. Recovery is through redistribution of the agent from the brain into the other tissues. Maintenance of anesthesia through continuing use of thiopental is not recommended due to accumulative effect and resultant prolonged recovery. Maintenance of anesthesia for longer periods of time can be accomplished through the use of inhalation anesthesia. 6-10 mg/kg in unpremedicated animals provides 10-15 minute of recumbency. Thiopental (2g) can be combined with guaifenesin (50g) and can be administered at 100 mg/kg guaifenesin-4 mg/kg thiopental titrated to effect. Pentobarbital, a short acting barbiturate was a commonly used injectable anesthetic agent in ruminants but is largely replaced by contemporary induction agents.
1-Yellow crystalline powder unstable in aqueous solution or air, accordingly solution should be freshly prepared and kept at 5-6 C° to retard deterioration. Old solution becomes turbid with precipitated crystals.
2-It is preferred to be used in diluted solution (2.5%) for;
-The accidental toxicity is less likely to occur
-To avoid spasm of vein and thrombosis
-To avoid skin necrosis and sloughing if the peri-vascular tissue is infiltrated
3-The drug is strictly intravenous general anesthetic
3-Thiopentone sodium (Nesdonal® or Intraval®): -
It is ultra-short-acting barbiturate and injection of 1 g/ 90 kg (10 % conc) causes the horse to lie down 30 seconds post injection for 3-4 mins then after signs of recovery becomes apparent and complete recovery (usually violent in horse and quite in cattle) occurs after 35-45 mins. Accordingly this drug is useful for induction of anesthesia in standing cows or horses to avoid casting. Accordingly it is suitable for induction of anesthesia that maintained by halothane.
Although it can cause anesthesia for 10 mins in horse, it is unwise to prolong anesthesia for more than 20 mins by repeated injection. Many authors don't consider it suitable agent neither for induction nor for maintenance of anesthesia, however it is useful for use in small does for animals that awake during maintenance anesthesia by other agents. However the induced narcosis by this drug depends up on;
1-The rate of distribution of the drug in the non-fatty tissues
2-The rate of up take by fatty tissue
3-Amount of the injected drug
4-Rate or speed of injection
Both of the last two factors are related, and it means that rapid injection of small amount induces high plasma concentration, high brain level, and rapid induction of deep narcosis. However this is followed by rapid distribution of the high plasma concentration to the non-fatty tissue with rapid reduction of both plasma and brain concentrations leading to rapid decrease in the narcosis depth.
On the other hand, slow injection of large amount of the drug maintains high plasma level as the drug is distributed through body tissue, and this indicated the need for large amount of the drug to obtain deep narcosis, while recovery depends up on drug up take by fatty tissue as the non-fatty tissue concentration will be high at the end of injection.
Rate of detoxication is slow indicating that its repeated injection for maintenance of anesthesia predisposes to prolonged narcosis due to saturation of both fatty and non-fatty tissue by repeated injection.
1-It crosses blood-brain barrier with very great speed so its rapid injection produces short period of narcosis with rapid recovery.
2-Anesthesia appears about 1 minute after intravenous injection accordingly the CNS depression differs from that of other anesthetics as the excitement stage may be not seen,
3-The drug has little or even no analgesic effect so the reflex to painful stimuli may be not abolished unless very deep stage of anesthesia is achieved. However pre-medication with analgesic drugs enables operative procedures to be performed at lower plasma levels of thiopentone that would be insufficient if thiopentone used alone.
4-The drug should be freshly prepared and after preparation it can be used for up to 5 days when kept under room temperature.
1-On heart: -
Rapid IV injection causes transient fall in blood pressure, moreover the drug has direct depressant effect on myocardium, and causes cardiac arrhythmia, so it is not preferred to be used or to be rapidly inject it in patients with cardiac diseases.
2-On respiratory system: -
It causes period of apnoea after injection due to central depression by high plasma level, also sensitivity of the center to Co2 decrease as narcosis becomes deeper, leading to increased arterial level of Co2. The high level of blood Co2 predisposes to acidosis that increases the un-dissociated fraction of thiopentone (the only fat soluble part). However recorded apnoea never lasts more than 20 seconds and requires no artificial respiration.
3-Laryngeal and bronchial reflexes: -
Their sensitivity increases under light narcosis and this is due to inability of light narcosis to block the afferent side of the reflex pathway like other anesthetics do.
Large doses causes hepatic damage and the dose should be reduced with patients suffer from liver disease.
The drug should be used carefully in patients with history of renal diseases as ureamia prolongs duration of narcosis.
1 gm/ 90 kg (11 mg/ kg) for horse or cow IV, or 5.5 mg/ kg when the animal premedicated with xylazine or acepromazine
25 mg/kg 2.5% strictly IV for dog
This low concentration ensures that there will be no spasm or thrombosis of vein, and when the drug infiltrates the peri-vascular tissue there will be limited probability of necrosis
1-It can be used for standing horses without the need for restraining as an induction anesthesia
2-It can be used in small does for horses that awake during maintenance anesthesia by other agents
3-It is useful in horses subjected to induction by xylazine/ketamine and maintained by halothane, and still moving, as prevention of moving requires high dose of halothane that may cause hypotension and this can be avoided by IV injection of 0.5 gm thiopentone.
1-Sudden falling of the horses after injection predisposes heavy breeds for skeletal injuries
2-Recovery associated with violent movement that may cause injury to the horses controversial to cattle. However this can be avoided by premedicating the horse with 0.4 mg/ kg acepromazine or 0.5 mg/ kg xylazine.
3-Pervascular problems when accidental perivascular injection in high concentrations performed
4-Despite the safety of the drug in cows and horses, it is not preferred, even in small does, for calves under 2 weeks or foals of age less than 1 month as it causes very prolonged recovery
5-The associated respiratory depression interfere with normal inhalation of inhalation anesthetic agent used for maintenance anesthesia. This could be overcome by halving the dose of thiopentone with premedication by chloral hydrate, acepromazine or xylazine
6-It is not suitable for animals with cardiovascular diseases
4-Thialbarbitone sodium: -
Its potency is 50% that of thiopentone but it has lesser depressant effect on respiratory system at any level of narcosis
1-Pale yellow water soluble powder
2-Its solution is stable for one week in refrigerator
3-Used as thiopentone sodium but it has lesser respiratory depression and lesser fall in blood pressure.
4-It increases salivation and should be pre-medicated with atropine
25 mg/kg 10% freshly prepared solution
5-Pentobarbitone sodium: -
It acts as narcotic, sedative, and anesthetic by depressing CNS, however it has weak analgesic effect. It takes fairly longer duration to cross the blood-brain barrier and the maximum depth of narcosis is not reached until 3-5 mins post injection, thus a much slow rate of injection is indicated.
It exists as solution of 6.5% concentration and it can be used for induction of basal narcosis in dogs by slow IV injection of 20-25 mg/kg.
It can be used as sedative in cattle and horse, generally it can be used for sedation and narcosis in cattle by using 0.5-1 ml /50 kg (1-2 gm 20% IV) but when it is injected in cattle in a dose rate of 3 gm /500 kg 20% IV, recumbency and loss of consciousness ensues and this can be utilized for induction of basal narcosis prior to induction of general anesthesia.
In small bovine, 1 gm /50 over 4 mins infusion time induce light anesthesia for 30 mins, however the animal will not be able to regain its feet before 3 hours, generally this technique should be avoided in calves younger than 1 month to avoid the prolonged narcosis that may last 2 days and may cause death due to pulmonary edema.
The drug shouldn't be used for narcosis in equine as it is causes prolonged recovery rate that is associated with narcotic excitement. However in cattle and horse, small doses IV 15-20 ml 6.5% (i.e. 1-1.25 gm) prolongs narcosis induced by chloral hydrate. By then it is slowly injected as soon as chloral hydrate depression becomes inadequate, however this injection has the advantages that;
1-The recumbency period is reduced than when another injection of chloral hydrate is given
2-Recovery associated with no excitement in horse
3-More than one injection can be given (up to 4gm) during the course of long operations
1-It can cross placental-barrier to foetal circulation tat inhibits foetal respiration
2-It stimulates release of antidiuretic hormones with reduction of renal out put
3-It has no toxic action on myocardium
4-It depresses vasomotor center leading to peripheral vasodilatation and subsequent fall in blood pressure
5-It has no adverse effect on the liver unless high doses are used in already damaged liver
In the horse it causes prolonged recovery associated with narcotic excitement
Propofol can be used in small ruminants or in calves for the induction and maintenance of general anesthesia. It provides rapid induction and it is very rapidly eliminated from the plasma. 5-6 mg/kg IV produces 4-9 minutes of anesthesia Maintenance of anesthesia can be achieved using a constant rate of infusion. Expense is the primary limiting factor (along with impractically large volume for rapid administration) for use of this agent in large ruminants.
Advantages of injectable general anesthetics: -
1-The most direct rout in reaching the CNS with very short induction phase
2-Very economic when compared with inhalation general anesthesia
3-Most injectable general anesthetics are stable and not flammable
4-Most injectable anesthetics can be utilized intra-peritoneally
5-Can be utilized with minimal need for assistance
Disadvantages of injectable general anesthetics: -
1-Longer recovery period
2-Management and control of over dose is not as accurate as inhalation
II-Volatile or Gaseous Substances (Inhalation Anesthesia): -
Generally it is the most dangerous anesthetic agents when compared with other agents and deaths had been recorded during induction due to ventricular fibrillation. It can be used by any of the 4 methods of induction of anesthesia and reduction of deaths can be achieved either by the use of phenothiazine premedication to reduce the required amount for induction anesthesia, or by induction of anesthesia by thiopentone sodium. Also deep anesthesia should be avoided as it causes inadequate ventilation and hypotension.
1-Heavy and clear color liquid with boiling point of 61 ºC
2-Sweet smelling of pleasant odor so it is easy and not unpleasant to inhale
3-It is neither inflammable nor explosive
4-Non-irritant vapour but the liquid is irritant to skin and mucous membranes and causes burn if spilt on them
5-Decomposed by light or air so it should be stored in cool dark place in dark bottle and the residue in the anesthetic machine should be discarded
6-Decomposed by alkali but at the same time it can be used with soda lime as long as it is of good quality
1-Respiratory system: -
The respiratory center is influenced both directly and indirectly and in the early stages respiration is deep and accelerated by struggling. While during anesthesia the susceptibility of the center to Co2 stimulation is depressed and breathing becomes slow and shallow. In addition the center is directly affected by the general fall in blood pressure. It produces low cardiac out put and low blood pressure and it produces simultaneous respiratory and circulatory arrest controversial to halothane that causes respiratory failure earlier than circulatory one.
Struggling during induction of anesthesia produces deep accelerated respiration, however as anesthesia ensues, progressive respiratory depression occurs leading to slow shallow respiration and the respiratory center becomes less sensitive to carbon dioxide.
Regarding circulatory system, it affects the heart itself, medullary center, and the peripheral blood vessels.
During early stages of induction it causes slow heart rate due to vagal stimulation that may cause heart failure, but once anesthesia becomes well established, the slow heart rate becomes regular, but dilatation of the heart and reduction of it contraction force has been noticed.
During early stages of induction, and due to struggling, the increased heart rate associated with reflex stimulation of the center leading to increased blood pressure, but during anesthesia the center is depressed, the peripheral blood vessels dilate and blood pressure falls. In addition to this central effect the walls of the blood vessels themselves are affected and this is a contributory factor in the fall in blood pressure.
It has toxic effect on heart leading to fatty degeneration even with therapeutic doses. Although the entire heart can be affected dilatation of left atrium can be noticed as it is the 1st part of the heart exposed to inhaled vapour followed by ventricles that receive the agent by coronary arteries and there is evidence that adrenaline causes ventricular fibrillation in presence of chloroform. The heart in such condition is much more susceptible to arrest as a result of vagal stimulation, toxic effect of chloroform on the myocardium and adrenaline release and this is the main cause of heart failure during induction.
3-Body temperature: -
It falls progressively during anesthesia due to
a-Peripheral vasodilatation with increased heat loss
b-Reduction of heat production by the reduced tone immovable muscles
Exposure to chloroform anesthesia to any duration produces cloudy swelling, fatty degeneration, glycogen depletion and central necrosis of hepatic lobules that develops 6-10 hrs post inhalation and requires approximately 14 days for repair. Clinical signs include acute acidosis, severe vomiting (in pets), and ictrus.
The direct effect of the drug on both of heart and kidney produces anuria during anesthesia, which is followed by poly urea and albuminuria after recovery. Severe acidosis is associated by acetonuria
Recorded death 1-10 days post anesthesia occurs due to cardiac, hepatic, and renal lesions accordingly the chloroform is not recommended for patients with cardiac, respiratory, renal, or hepatic insufficiency.
B-Ether (Diethyl ether): -
Its use in cattle is save as well as in other animals, and it can't be used by open or semi-open methods and it is better to be used for maintenance of anesthesia (not for induction) in narcotized cow or horse (by thiopentone or chloral hydrate) by using closed or semi-closed methods due to its high cost and irritation of mucosa with increased bronchial and salivary secretion especially in calves.
However it has quiet rapid recovery in cow about 60 mins after termination of anesthesia, but in horse recovery is usually associated with excitement when thiopentone is used for induction so premedication with phenothiazine reduces that excitement during recovery.
2-Highly volatile (34 ºC boiling point)
3-It has pungent odor so its use for induction of anesthesia is difficult for anesthetist and unpleasant for the animal
4-In presence of air or oxygen it forms toxic aldehydes by oxidation
5-Lighter than chloroform but heavier than air 2.6 times
6-Inflammable and explosive
Accordingly inflammable vapour tends to accumulate near the floor
7-It is decomposed by air, light, or heat therefore the liquid should be stored in sealed dark containers in cool dark place
8-Its vapour irritant to all tissues and its liquid causes burns when spilt on skin or mucous membranes
9-Effect on tissues
a-Respiratory mucosa: -
The irritant effect of the vapour causes breath holding during induction of anesthesia, increased flow of respiratory and salivary secretions that interfere with respiration and lowers immune resistance with increased susceptibility of post operative pneumonia. However it is mild respiratory stimulant.
b-Renal mucosa: -
The irritant effect of the agent reduces urine out put during anesthesia and induces post anesthetic albuminurea
c-Hepatic and cardiac tissues: -
It has minimal effect on liver and cardiac muscle
10-It produces post-operative nausea and the animal refuse to eat several hours post surgery
11-It has wide safety margin and it is one of safest anesthetic agent
C-Ethyl chloride: -
1-It is a potent general anesthetic but doesn't produce good muscle relaxation in dog
2-It boiling point is 12 ºC, so it exist in gaseous form at room temperature
3-Under pressure and low temperature it is a colorless liquid with ethereal odor and it is supplied as glass tube with spring loaded metal spraying nozzles
4-Its vapour is heavier than air and explosive
5-Non-irritant and causes easy rapid induction
6-It has narrow safety margin
used for induction not for maintenance of anesthesia and its use is restricted to birds and for very short period in cats
7-Over doses may ensue during induction and it causes;
It causes depression of myocardium, and ventricular fibrillation that may causes death. It also causes fall in blood pressure due to direct depression of vasoconstrictor center, peripheral vasodilatation, and cardiac slowing.
b-Hepatic and renal system: -
It causes hepatitis, renal damage, and renal failure
D-Nitrous oxide: -
Nitrous oxide is the oldest anesthetic agent and its use as principle anesthetic in veterinary field is limited. As it is not potent anesthetic, produces no muscle relaxation, has tendency for induction of asphyxia in unskilled anesthetist, and requires very expensive apparatus for using it alone in horse, it can only be used with O2 for vaporization of ether or chloroform for reduction of amount of potent inhalant anesthetic agents (ether or chloroform) that has depressing effect on cardiovascular system.
Controversial to all inhalant anesthetics, nitrous oxide can be administered to patients in high concentrations as it has minimal irritating effect.
Moreover its analgesic effect in human is much more pronounced than animals, and the cost of its use is higher than other inhalant anesthetics.
The drug can be used for maintenance of anesthesia induced by barbiturates with operations that don't affect sensitive structures (because it is weak anesthetic) and when there is no need for muscle relaxation (as it is poor muscle relaxant).
It has smooth rapid recovery especially when minimal intravenous supplementary agents have been used, and when used alone for induction and maintenance, the animal gains its feet 10 minutes after termination of anesthesia.
1-It is colorless gas
2-Odorless or faint pleasant smell gas
occupational exposure to it may cause significant health hazards
3-It is neither inflammable nor explosive
4-Exist as liquid at room temperature under pressure
5-It is not irritant and not toxic (in absence of hypoxia) so it can be used for long duration anesthesia
1-It is neither inflammable nor explosive
2-It is non toxic (in absence of asphyxia)
3-It has rapid recovery with no post surgical complications
1-It is not potent, has weak narcotic and weak anesthetic action so it is preferred to be used for maintenance of anesthesia in unconscious animals previously medicated with thiopentone sodium
2-It is not suitable for large animals due to the high cost as a result of both the large gas flow rate required in large animals and the cost of the necessary apparatus
3-It is poor muscle relaxant
4-It is associated with high incidence of asphyxia
5-It enters gas containing cavities and causes over expansion (intestine)
E-Halothane (Fluthane®): -
It is potent, safe, mild irritating, of very low toxicity, has fast recovery, has minimal effect on liver and kidney, so it is considered as the most popular in veterinary anesthesia, but due to its high cost, it shouldn't be used by open or semi-open methods and it is preferred not to use it for induction.
Induction of anesthesia in 450 kg cow or horse requires 30-40 ml that can be maintained for 1 hour by further 25-30 ml and the next hour requires lesser amount that means that the required dose for maintenance is lesser than that required for induction. As a result of its high cost, induction can be achieved by IV thiopentone that can be maintained for 1 hour by the same 25-30 ml halothane, however the use of thiopentone sodium predisposes to delayed recovery.
When anesthesia induced in horses by xylazine/ketamine, and maintained on halothane, the animal gains its feet 30 mins after discontinuation of halothane anesthesia, but this duration duplicated if the same horse was subjected to induction by acepromazine/thiopentone.
1-It is not inflammable and non explosive
2-Boiling point is 50 ºC
3-It has characteristic but not unpleasant odor
4-Slowly decomposed by light but not affected by soda lime
1-It is mild irritating and causes minimal mucous membrane irritation so it has smooth rapid easy induction and maintenance
2-It has fast recovery free from excitement as the animal supports himself on brisket 6-7 mins after termination of anesthesia with incoordination after 10-15 mins, and finally it gains its feet after 15-30 mins
3-It is potent 2 times more than chloroform and 4 times more than ether, has wide safety margin (twice more safe than chloroform or ether) and of very low toxicity so it is a very satisfactory anesthetic in cattle
4-It has minimal effect on liver and kidney
5-It has low incidence of inappetence after anesthesia
6-It causes reasonable muscle relaxation
1-It is of high cost so it should be used by closed or semi-closed system
2-It is not wise to produce profound muscle relaxation by it due to low blood pressure that decreases by increasing the depth of anesthesia
3-It decreases blood pressure and slows pulse that return to normal after 30 mins
4-It causes direct depression of myocardium and central depression of vasomotor center leading to cutaneous vasodilatation
5-It produces low cardiac out put and low blood pressure but it causes respiratory failure long time before circulatory failure
6-It causes relaxation of smooth muscle fibers so it delays involution of uterus after cesarean and predisposes to post partum uterine hemorrhage
7-It has potent respiratory depression effect that causes difficulty in maintenance of anesthesia induced by intravenous agents especially on using large dose of a drug that has depressant effect on respiration like barbiturates, used for induction, accordingly xylazine/ketamine is preferred for induction if halothane is to be used for maintenance.
Methods Of Induction Of Inhalation Anesthesia..
1-Open method or rag and bottle anesthesia..
It can be used for to volatilize ether or chloroform via dropping them by dropper bottle on the surface of a piece of gauze or lint that is held over the face of the animal, this gauze or lint may be stretched over wire frame to form a mask with preservation of free flow of air between the mask and the face. This technique can also be applied by using ethyl chloride spray.
1-Lack of control of ventilation, as oxygen enrichment should be always given during general anesthesia
2-Excessive loss of volatile agents that is not only unpleasant for the surgery team but also explosions and fires may ensue if the used anesthetic agent is inflammable
3-Dilution of anesthetic agent to unknown extent
4-Difficulty of maintaining stable anesthetic state
5-Animal that become more lightly anesthetized tends to awake and animal that becomes more deeply anesthetized tends to have respiratory depression and to become more depressed so that air dilution decreases that increases anesthesia concentration and make the animal nearer to death.
2-Semi-open method (perhalation)..
All respired air is made to pass through the mask on which the vapourization of anesthetic agent is occurs. The space between the mask and the face is closed with double thick layer of Gamgee. The 1st layer of Gamgee has small hole in its center so that when it is in position the nose and the mouth are the only exposed parts of the face. Then after the nose and mouth are covered with the mask over which the 2nd layer of Gamgee, with large central hole, is applied. The sponge containing the anesthetic agent shouldn't come in direct contact with the nostril to avoid burns when using irritant agents like chloroform.
Same as open method
Hinz’s Box: -
It is a glass or transparent plastic box used for inhalation anesthesia (ether) in cat and dog.
1-Require no restraint and no assistant
2-Minimized anesthetic emergency
3-Animal passes into a state of anesthesia with minimal amount of struggling
It has two systems, with or withour carbon dioxide absorption. Vapours flow from anesthetic apparatus into reservoir bag from which the animal inhales through endo-tracheal tube, while part of the exhaled mixture passes out into the atmosphere through an expiratory valve.
With Magill system the rebreathing is prevented by maintaining the total gas flow rate from cylinders slightly in excess of the patient respiratory minute volume. The animal inhales from the bag and wide-bore tubing, and the exhaled mixture passes back up the tubing displacing its gas contents back to the bag till filling it.
The exhaled gas never reaches the bag due to the greater capacity of the tube and once the bag is distended, the build-up of pressure inside the system causes the expiratory valve to open so that the terminal part of expiration passes out of the valve into the atmosphere. During the pause after expiration and before the next inspiration, fresh gas from the anesthetic apparatus sweep the reminder or the 1st part of exhaled gas from the corrugated tube out through the expiratory valve.
With other systems it is more difficult to prevent rebreathing since the inflowing gases from the anesthetic apparatus will not wash expired gases out through the expiratory valve this difficulty can be over came by interposing soda lime canister between animal and the bag and this system is referred as semi-closed system with Co2 absorption.
A similar system can also be arranged with circular type of absorber unite by opening the expiratory valve near the face mask or endo-tracheal tube. Soda lime canister with semi closed system permits lowering in gas consumption but it requires considerable practicing as the expiratory valve setting and the gas flow rates require continuous adjustment throughout the anesthetic period.
For pets, none of these systems is suitable and they require T-piece system with an open end tube that acts as reservoir with no valve. The exhaled gas swept out of the open end of the reservoir tube, by fresh gas flow in from the anesthetic apparatus during expiratory pause.
Unless the capacity of the reservoir tube is at least equal to the tidal volume of the animal, the inspired gas will be diluted with air. The T-piece system included three types
1-T-piece with expiratory limb of lesser capacity than tidal volume
2-T-piece without expiratory limb
3-T-piece with expiratory limb of greater capacity than tidal volume
The last type is concluded to be the best type and it has open-ended bag attached to the distal end of expiratory limb, accordingly fresh gas flow of 2.5-3 times the minute volume of respiration is required to eliminate rebreathing.
4-Closed method with Co2 absorption..
In this method, vapours flow from anesthetic apparatus into reservoir bag from which the animal inhales through endo-tracheal tube. The exhaled anesthetic agent that is mixed with Co2, is directed to closed bag at which Co2 is removed by soda lime and O2 is added to satisfy metabolic requirements, then the same vapour is rebreathed.
The soda lime may contain dye that act as an indicator for the degree of saturation or absorbing capacity of soda lime. Soda lime consists of 90% calcium hydroxide, 5% sodium hydroxide, and 5% silicates and water to prevent powdering.
1-It is a simple method that consumes less amount of anesthetic agent (no wastage to atmosphere)
2-Lower probability of fires and explosions
1-Resistant to respiration because of the packed soda lime that renders this method unsuitable for cats, puppies, and small sized adult dogs
2-Conservation of heat and water vapour afforded by this method may cause heat stroke in dog and sheep.
There are two systems in use for closed-circuit Co2 absorption techniques of anesthesia;
1-To-and-fro system: -
A canister of soda lime is interposed between the animal and the rebreathing bag and fresh gas is fed to the system as close to the animal as possible to effect changes in the mixture rapidly.
1-Simple and efficient
2-Inexpensive and may be improvised
2-No wastage of anesthetic agent
1-Difficulty in maintaining the heavy awkward apparatus in a gas-tight condition
2-The inspired gas becomes undesirable hot due to chemical reaction between soda lime and Co2
3-High probability of bronchitis due to inhalation of irritating dust from the soda lime canister
4-It is not really efficient absorber of Co2 due to exhaustion of soda lime at canister end near to the patient
2-Circle system: -
This system for Co2 absorption incorporates inspiratory and expiratory tubes with unidirectional valves to ensure one way flow of gases, and rebreathing bag and soda lime canister are placed between these tubes. The valve and tubing offer considerable resistance to breathing and strain on the animal.
It is not suitable for cats and small dogs because of the resistance to breathing and inevitable degree of rebreathing at the T-piece connection to the patient, however the later could be over came by placing unidirectional valve close to the face piece.
1-Simple and efficient
2-It is more efficient absorbers of Co2 than to-and-fro unites due to their constant dead space so that all the soda lime is available to the respired gas
2-Valve and tubing offer appreciable resistance to breathing that may causes strain on animal
3-Exhusion of soda lime is sudden than to-and-fro so inspired Co2 may become excessive suddenly
4-It is not suitable for field work
Breathing System And Anesthetic Machine…
The primary purpose of that machine and breathing system are;
1-Delivary of O2 and exact amount of anesthetic agent
2-Removal of Co2 from exhaled gas by dilution, non-breathing valves, or Co2 absorber
3-Provision of positive pressure ventilation
Basically the anesthetic apparatus for inhalation anesthesia is consisted of;
1-Source of O2 and compressed medical gases as Nitrous oxide
2-Pressure gauge, regulator or reducing valve, and flowmeter for each gas
3-Vapourizing bottle for the volatile anesthetic liquid
1-Oxygen and or compressed medical gases cylinder: -
It is either exists as single cylinders that connected directly to the machine by hose; or as bank of large cylinders or bulk tanks from which gases are piped through copper alloy tubes that ends at the wall of surgery room.
2-Pressure gauge, reducing valve (regulator), and flowmeter (rotameter): -
Pressure gauge should be supplied to each compressed gas to indicate gas pressure in the cylinder and the full scale of the reading gauge should be 1/3 greater than the maximum cylinder pressure.
Reducing valve is necessary for;
a-Re-adjustment of the flow as the cylinder pressure decrease over time
b-Easy production of small variations of gas flow by supplying low gas pressure to the control valve spindle. The direct control of high pressure cylinder by simple type needle valve produces large changes in flow rate even by very small movement of control valve spindle.
c-Limitation of the connecting tubing pressure to a low level, that the possibility of bursting of the tube (when the flow is shut off by the flowmeter control) is reduced.
Flowmeter or rotameter is consists of a glass tube inside which a rotating bobbin freely move. Diameter of the glass tube increases from below and up ward to permit floating of the bobbin and flow of gas around it. The gas enters the flowmeter from its bottom and exits its top. The level of bobbin becomes high, both the gas flow and the space between the bobbin and the glass wall, increases. The glass is graduated to demonstrate the rate of flow.
3-Vapourizing bottle for the volatile anesthetic liquid: -
Vaporizer is used mainly for vaporization of volatile anesthetic like ether or chloroform.
a-Un-calibrated vaporizer: -
The Boyle vaporizer is designed as to allow for fairly fine control of the strength of anesthetic vapour. The method of varying the concentration of anesthetic vapour utilizes a permanent partition to prevent the direct passage of gasses from the flowmeter to the patient. When the control lever is in the off position, all the gases are diverted around the partition but away from the bottle. With the tap on the on position, the entire gasses pass thought the bottle containing the liquid anesthetic agent.
The control tap can be placed in any intermediate position and this determines how much of the total gas flow, passes through the bottle. Another mean of control of the rate of vaporization is the fenestrated J-shape tube that passes below or over the surface of the anesthetic agent, and the deeper the position of the tube the more bubbles formation and the greater the vaporization rate.
b-Calibrated vaporizer: -
It is designed to deliver known concentration of volatile anesthetics. It consists of vaporizing chamber and bypass. The fresh gas stream flowing into the vaporizer is divided into two portions. The larger one passes through the bypass and the smaller passes through the vaporizer chamber to be saturated with vapour that ensures;
1-No sudden burst of high vapour concentration when the vaporizer is first switched on
2-No affection of vapour out put by shaking
Vaporization occurs by removal of heat from the warm liquid with resultant fall in its temperature, so the fall in liquid temperature should be checked to preserve continuous vaporization and to prevent reduction of concentration of out put over the time. Preservation of higher temperature of the liquid can be achieved either by;
1-Compensation of the temperature by bimetallic strep valve that arranged to act as a control of the volume of the gas passing through vaporizing chamber. Reduction of liquid temperature stimulates further opening of the valve that allows more gas to pass through the chamber.
2-Using vaporizer made of copper that has high thermal conductivity to allow conduction of room temperature to the liquid that supplies the liquid with the necessary heat for vaporization.
Stages Of General Anesthesia..
1-Stage I (stage of voluntary excitement, analgesia or induction): -
This period extends from the beginning of induction of anesthesia to stage of loss of consciousness. At the beginning the animal is conscious and may struggle to avoid being anesthetized leading to release of adrenaline that causes;
a-Increase of the respiratory and pulse rates
b-Dilatation of the pupil
Urine and feaces may be voided and some animal species may show salivation during this stage but towards the end of that stage the animal;
a-Losses ability to stand
b-Acquires lateral recumbency
2-Stage II (Stage of involuntary excitement or Delerium stage): -
Generally it is know as the earliest period of loss of consciousness and characterized by
a-Exaggerated reflex to stimuli and violent movements of the limbs accordingly the animal should be restraint to avoid injury of workers in surgery room
b-Release of adrenaline is continued leading to increased heart rate and dilation of the pupil
c-Respiration is very irregular
d-Salivation may be profuse in cattle and cat
e-Cough and pharyngeal (swallowing and vomiting) reflexes exist but they disappeared near the end of that stage
3-Stgae III (Stage of surgical anesthesia): -
This stage is characterized by loss of consciousness and can be classified into three planes
A-Plane of light anesthesia: -
Generally this plane is suitable for simple surgical procedures like incision of abscess or diagnostic investigations and it is characterized by;
1-Eyeball moves from side to side (jerky movement) with absence of eye reflex
3-Pedal reflex exists in pets
B-Plane of medium anesthesia: -
This plane is suitable for all surgical procedures except for laparotomy and it is characterized by;
1-Eyeball fixed centrally in all animals and ventrally in dog
2-Sluggish muscle relaxation
C-Plane of deep anesthesia: -
This plane is suitable for all surgical procedures including laparotomy and it is characterized by;
1-Eyeball fixed centrally in the dog
2-Pedal reflex disappears in pets
3-Decreased respiration depth with longer pause between inspiration and expiration
4-Stage IV (Over dose or paralytic stage): -
1-Severe depression of CNS
2-Pupil dilation with fish-eye appearance due to cessation of lachrymal secretion
3-Increase pulse rate
4-Gasping respiration due to complete paralysis of inter-costal muscles without paralysis of the diaphragm
5-Relaxation of anal and bladder sphincters with voiding of urine and feces
6-The animal enters stage of death with coma and cyanosis
Complications Of Over Doses Of Anesthesia..
1-Syncopy, and pathologic affection of respiration and circulation
Treatment Of Overdoses Inhalation Anesthesia..
1-Stop inhalation of gas
2-Stimulate respiration by remove restraint, supplying the animal with oxygen and Co2, applying artificial respiration, forward pulling of the tongue with removal of saliva, and applying piece of cotton moisten with ammonia near the nose
3-Adminsteration of cardiac stimulant
4-Warming of the animal
Problems Encountered In General Anesthesia In Bovine..
General anesthesia in bovine is so dangerous as a result of presence of rumen that might predispose the animal for asphyxia or drenching pneumonia, so that nerve block and infiltration analgesia are preferred especially if the magnitude of the operation requires standing position of the animal as rumenotomy. However in some of the more sophisticated surgical procedures, general anesthesia becomes mandatory for the sake of asepsis, and complete restraint as in case of repair of penile hematoma, inguinal herniorrhaphy, or cercumcission and correction of preputeal adhesion. Generally, the problems encountered in bovine general anesthesia are;
1-Bloat and regurgitation: -
The excessively formed gases in the rumen can’t be regurgitated normally with the animal in lateral recumbency, due to the higher level of ruminal fluid than cardiac opening and this large amount of gases causes;
a-Interference with the already impaired respiration, caused by the recumbence position and pressure on diaphragm
b-Pressure on the cardiac opening that is augmented by the depth of anesthesia, so that the fluid flows from the rumen, collected in the pharyngeal region, and aspirated leading to drenching pneumonia or death
It results from;
a-Compression of the lower lung by lateral recumbency
b-Increased diaphragmatic breathing by deep anesthesia
c-Pressure on diaphragm by abdominal viscera
Importance of endotracheal tube: -
a-It reduces many of potential problems
b-It provides a potent air way
c-It prevents aspiration of saliva as well as regurgitation of food materials
3-Radial paralysis: -
This condition may last few minutes up to few weeks and its occurrence depends up on;
a-Length of time of recumbency
c-Thickness of subcutaneous fat
Generally this condition can be avoided by placing inflated tire under the point of shoulder at the time of casting
Use of atropine sulfate reduces amount of saliva and increases its viscosity