Veterinary anesthesia

Veterinary anesthesiaYasser Haidar Ahmad MD Anesthesia

DefinitionGreek term anaisthaesia meaning insensibility, used to describe the loss of sensation to the entire or any part of the body.Anesthesia is induced by drugs that depress the activity of nervous tissue locally, regionally, or within the central nervous system

Reasons for Administering Anesthesia

First and foremost, anesthetics alleviate pain and induce muscle relaxation, essential for safe surgery. Other important uses include restraint, safe transportation of wild and exotic animals, various diagnostic and therapeutic procedures, euthanasia, and the humane slaughter of food animals.

Effects of anesthetic drugsAnalgesia: freedom from or absence of pain2.Tranquilization: results in behavioral change wherein anxiety is relieved and the patient becomes relaxed but remains aware of its surroundings. In this state, it may appear to be indifferent to minor pain.3. Sedation is a state characterized by central depression accompanied by drowsiness. The patient is generally unaware of its surroundings but responsive to painful manipulation

4. Narcosis is a drug-induced state of deep sleep from which the patient cannot be easily aroused. Narcosis may or may not be accompanied by analgesia.5. Hypnosis is a condition of artificially induced sleep, or a trance resembling sleep, resulting from moderate depression of the CNS from which the patient is readily aroused.6. Local analgesia (anesthesia) is a loss of sensation in circumscribed body area. 7. Regional analgesia (anesthesia) is insensibility in a larger, though limited, body area (e.g., paralumbar nerve blockade).8. General anesthesia is drug-induced unconsciousness that is characterized by controlled but reversible depression of the CNS and analgesia. In this state, the patient is not arousable by noxious stimulation. Sensory, motor, and autonomic reflex functions are attenuated

9. Surgical anesthesia is the state/plane of general anesthesia that provides unconsciousness, muscular relaxation, and analgesia sufficient for painless surgery.10. Balanced anesthesia is induced by multiple drugs. Drugs are targeted to specifically attenuate individual components of the anesthetic state; that is, consciousness, analgesia, muscle relaxation, and alteration of autonomic reflexes.11. Dissociative anesthesia is induced by drugs (e.g., ketamine) that dissociate the thalamocortic and limbic systems. This form of anesthesia is characterized by a cataleptoid state in which the eyes remain open and swallowing reflexes remain

Types of AnesthesiaInhalation: anesthetic gases or vapors are inhaled in combination with oxygenInjectable: IV, IM, SCOral or rectal: liquid and suppositoriesLocal: anesthetic drug is applied topically, injected locally around the surgical site

pharmacologyAnesthesia is of necessity a reversible process.Knowledge of the factors underlying production of anesthesia and those that modify it is essential. The dose of anesthetic and the techniques for its administration are based on the average normal healthy animalVariation in response to standard dose result of factors related to CNS, metabolic activity existing disease or pathology and the uptake and distribution of the anesthetic.

Biological variationPharmacogenetic differencesPharmacokineticsFactors modifying pharmacokineticsCellular effects and teratogenicity

Biological variationElimination of anesthetics depends on the species and the metabolic processes within the animal.Small animals have a higher basal metabolic rate per unit of surface area than large animals. The smaller the animal the larger is the dose Animals with large quantities of fat have low metabolic rate and require less anesthetic.Animals in poorer condition requires less anesthetic In newborns, the basal metabolic rate is lowResponse to barbiturates varies in dogs of differing ages. Very young and adult are most sensitive whereas those of age range 3 to 12 months are least sensitiveConflicting evidence regarding sex difference

Pharmacogenetic differencesVariation in the dose response to drug because of genetic related factors.the heritable difference in the ability of rabbits to hydrolyze atropine and cocaine.genetic variations in response to pentobarbital in miceSome breeds of swine are susceptible to malignant hyperthermiaPlasma cholinesterase deficiency

PharmacokineticsGeneral anesthesia is produced by the action of an anesthetic on the brain and spinal cord. The agent must therefore achieve access to the central nervous tissueInhalation anesthetics are eliminated primarily by exhalation. Thus, providing respiration and circulation are maintained, inhalants are readily eliminated from the bodyInjectable agents depend on redistribution within the body, biotransformation in the liver and excretion via the kidneysLess control over the elimination process with injectable anesthetics. for this reason, some consider them to be more dangerous than inhalant anesthetics

Anesthetics are commonly administered by intravenous injectionIntravenous administration bypasses the absorption phase of the drug with the consequences that onset and intensity of action are less variableThe body have multiple compartments differentiated by blood supply and tissue-blood partition coefficient.After initial IV injection mixing and dilution occur and initial blood concentration of the drug is established Binding of drugs to plasma protein, in which form they cannot readily penetrate cellular membranes, and the removal of drugs by tissues that store, metabolize, and excrete them are both important factors that lower the effective concentration of drugs at their site of action.

The fraction of bound drug increases with decreasing drug concentration and vice versa.The drug is distributed to various tissue according to their perfusion, their capacity for drug and the partial pressure gradient between blood and tissueThe vessel rich group of tissues achieves equilibrium with the blood more quickly than do other group of tissuesAlthough fat and muscle have same blood flow, the higher solubility of anesthetic in fat than in muscle accounts for the greater time required to achieve equilibrium for fat than for muscle.

As the plasma concentration falls anesthetic reenters the blood from the vessel-rich tissue to be redistributed from the brain to muscle and fat and lightens anesthesia.The ultimate effect of any general anesthetic is contingent to its ability to cross the blood-brain barrierLimits the penetration of nonlipophilic, ionized, or protein-bound drugs.Within moment of tissue uptake and redistribution elimination of the drug begin. The liver is the primary site of biotransformation, whereas the kidney is primarily responsible for excretionThe liver is the primary site of biotransformation and converts most anestheticcs from lipophilic nonpolar to polar water-soluble derivatives capable of excretion by the kidneys

Factors modifying pharmacokineticsConcentration and rate of injection: the more dilute and slower injection the less is the effectModification of cardiac output, ventilation, ventilationperfusion ratios, and/or alveolar-capillary diffusion from any cause will influence both the uptake and elimination of inhalant drugs, most especially those of greater solubility. Variation in distribution of blood to the vessel rich and vessel poor tissue. In shock, the proportion of the cardiac output flowing to the brain is increased, and the potential for redistribution is reduced. dilution of the drug is also diminished, as is hepatic and renal blood flow .Induction is thus rapid, the dose required is reduced, and recovery is delayed.

In shock brain CO inc an redistribution dec. decr blood volume decr decre dilution so induction is rapid dose reduced recovery delayed. Ex surgical bleed. Dec biotransformation n renal excretion21

Fear, struggling or fever increase CO , decrease circulation time and prolong the time necessary for equilibration of inhalant anesthetic concentration between alveoli and pulmonary capillaries. Muscle and skin blood flow is increased, induction of inhalant anesthesia is delayed, and more anesthetic is required. animals showing a period of excitement during induction of inhalant anesthesia always require more anesthetic. Preanesthetic sedation is often advantageous.a large meal of meat may increase the metabolic rate of dogs as much as 90% above the basal level. It is usually 12 to 18 h after the last meal before the basal metabolic rate is attained in carnivorous animals.Nitrous oxide increases pressure and volume in compliant spaces.Drug interaction: antibiotics and muscle relaxant

Increase CO prolong z time for equilibrium. Muscle blood flow incrinduction is delayed. Animal showing period of excitement at induction nee more anesth..danger.. Nitrous oxide and pneumothorax and intestinal obstruction barbiturate weak acid narcotics and muscle relaxant r weak base..ex plasma acidosis .decrease binding make more drug available so increase sensitivity22

Most noninhaled drugs are weak acids (barbiturates) or weak bases (narcotics, narcotic antagonists, and muscle relaxants). Once the drug is injected, equilibrium between ionized and nonionized forms of drug depends on the pH of the blood or tissues and the dissociation constant (pKa ) of the drug. Plasma acidosis, for instance, increases intracellular barbiturate but decreases intracellular narcotic concentration. Drug availability at the site of action or of elimination is also modified by the degree of protein binding. Protein binding is diminished by uremia, hypoproteinemia, dehydration administration of various drugs can either stimulate or inhibit hepatic microsomal drug-metabolizing enzymes Hepatocellular disease causes reduced protein (primarily albumin) Production and may delay drug biotransformation.Renal disease Hyperthyroidism is accompanied by an elevated metabolic rate and may increase anesthetic requirement, whereas hypothyroidism is generally accompanied by a lowering of the metabolic rate and a reduced anesthetic requirement

teratogenicityFirst trimester of pregnancyExposure of rats to nitrous oxide on day 9 of gestation has been shown to cause fetal resorption and skeletal and soft tissue anomaliesInhalational anesthetics and chicks

Skeletal and soft tissue deformity24

Assessment of Anesthetic actionGeneral Anesthesia requires:UnconsciousnessInsensitivity to painMuscle relaxationAbsence of reflex responseThe degree to which these are required for specific procedures varies. Anesthetists must therefore select the most suitable drugs and be able to assess the degree to which the varying effects are induced. Different species has different response to similar anesthetic dosethe progressive changes produced by the administration of anesthetic drugs have been classified into four stages. Recognizing the signs characteristic of these stages enables anesthetists to determine whether the required CNS depression has been achieved or whether it is insufficient or too much.

Insufficient in other species anesthetist must be familiar with drug caracteristic in order to use them effectively and safely.. More propofol more CNS depresiion without analgesia effect25

Stages of General AnesthesiaStage 1 or stage of voluntary movementLast from initial administration to loss of consciousnessMost variableAnimal may struggle violently and hold their breathRapid heartbeat and pupillary dilatationSalivation is frequent in some species, as are urination and defecation. With the approach of stage II, animals become progressively ataxic, lose their ability to stand, and assume lateral recumbency.

Variability due to drugs anesthestist, the way animal is restrained rate of inductionWith approach to stage 2 animal become progressivly ataxic unable to stand26

Stage 2 or stage of delirium or involuntary movementsLoss of all voluntary control marks the change from stage I.Lasts from loss of consciousness to the onset of regular pattern of breathingPatients react to external stimuli by violent reflex struggling, breath holding, tachypnea, and hyperventilation. The larynx of cats and pigs is very sensitive at this stage, and stimulation may cause laryngeal spasms.Eyelash and palpebral reflexes are prominent. Nystagmus commonly occurs in horses. Pupils widely dilatedWhine, cry or bellowSalivation , vomiting or regurgitation in ruminantsIn view of the exaggerated reflex responses during this stage, stimulation of any kind should be avoided

Loss of voluntary control id the mark change from stage 1Epinephrine cause heartbeat increasesaLIVATION IN CATS. VOMITING IN DOGS CATS AND GOATSin view of exagerated reflexes stimulation of any kind should be avoided27

Stage 3 stage of surgical anesthesiaCharacterized by unconsciousness with progressive depression of the reflexes.Muscular relaxation develops, and ventilation becomes slow and regular.Vomiting and swallowing reflexes are lostIn human divided into planes 1 to 4Light medium and deep If CNS depression is allowed to increase further,patients will progress to stage IV

Stage 4In this stage, the CNS is extremely depressed and respiration ceaseBlood pressure is at the shock level, capillary refill of visible mucous membranes is markedly delayed, and the pupils are widely dilated.Anal and bladder sphincters relaxDeath quickly intervenes unless immediate resuscitative steps are taken.If the anesthetic is withdrawn and artificial respiration is initiated before myocardial collapse, these effects may be overcome and patients will go through the various stages in reverse. The stages just described are best seen when inhalant anesthetics are administered, probably because considerable time is required for an anesthetic concentration to accrue in the CNS. This allows the various signs to become apparent. With some intravenous anesthetics (e.g., dissociatives) or the concurrent use of preanesthetic sedatives, anesthetic-induced depression is difficult to assess, and signs of anesthetic depression are not uniformly apparent.

Death unless anesthetic withdrawn artificial respiration maybe overcome and reverse stages29

Signs of anesthesiaRespirationCirculationOcular signsPharyngeal and upper airway reflexes

respirationIncrease during stage 1Irregular in stage 2 with breath holdingOnce again regular in stage 3 surgical manipulation stimulates respiration, whereas premedication depresses itDuring stage III, the intercostal muscles andthe diaphragm weaken progressively.The depth of respiration declines progressively, thoracic movement decreases, respiration becomes largely or entirely abdominal

Especially if preanesthetic sedation is not used31

circulationIndirect methods of blood pressure monitoringveterinarians often do not measure pressure, but must depend on the pulse rate or bleeding at the surgical site, and on induction of momentary blanching by compressing an exposed mucous membrane (such as the conjunctiva, oral mucosa, or tongue) to give some indication of the circulatory status of patients. The mucous membranes may show pallor as a result of hemorrhage or shock and cyanosis caused by hypoxiaIn rodent feet and ears are observedPulse is strong in stage 1 and 2 Pain stimulation in light stage III may induce tachycardiaBlood pressure decrease and pulse weakens as the anesthetic depth increase

Conjuctiva oral mucosa or tongue may show palor or cyanos32

Ocular signs include eyeball position and movement, photomotor reflexes and pupillary size, lacrimation, and palpebral, corneal, and conjunctival reflexes. Variable in most species and should never replace observation of respiratory and circulatory signs.in horses nystagmus occurs with the onset of stage II and continues through light surgical anesthesiaThe palpebral reflex becomes sluggish in all species when surgical anesthesia is attainedLacrimation is a sign of light surgical anesthesia in horsescorneal reflexes is variable, but is usually lost shortly after the palpebral reflex. In horses, the corneal reflex persists into deeper anesthesia.

Nystagmus in horse in stage 2 and continue through light surgical anesthesiaPalpebral reflexes induced by touching the eyelids is lost33

Pharyngeal and upper-airway reflexes Suppression of these reflexes is of particular importance for endotracheal intubation and for induction of anesthesia in animals, such as ruminants, with full stomachs.Coughing and laryngospasm in response to intubation are lost in light surgical anesthesiaThe intensity of these responses varies with the animal species concerned: especially intense in the cat, and relatively mild in cattle and horses. swallowing and vomiting disappear with stage 3 onset like the laryngeal reflexes, the swallowing reflex persists into medium anesthesia in cats.

Anesthesia recoveryAs anesthetic drugs are eliminated from the CNS, the degree of anesthesia lightens and reverse progress through the stages of anesthesia occurs. Induction techniques are usually selected and performed to minimize the duration of stage II, in which excitement and motor activity may occurDuring recovery, however, stage II can be prolonged. Every effort should be made to avoid stimulation of the animal at this timeShort acting anesthetics and postoperative sedation can minimize recovery delirium.effective anesthesia is not only that which just obliterates a patient's response to painful stimuli without excessive depression of vital functions, but also that from which recovery is relatively rapid and uneventful

Stage 2 exictement35

Preanesthetic evaluationThe purpose of the preanesthetic patient evaluation is to determine a patient's physical status: Disease, pain and stressThe goal is to determine any deviations from the norm that will affect anesthetic uptake, action, elimination, and safety.CNS cardiopulmonary, hepatic and renalAid to selection of anesthetic techniquethe sicker a patient (the poorer the physical status) is, the greater is the likelihood of adverse events or death.

Physical status is determined by:History: previous and current health; presenting complaint, its severity, and its duration; concurrent symptoms of disease (e.g., diarrhea, vomitingexposure to drugs; prior anesthetic history; and recent feeding.. the possibility of a full or distended stomach or of hepatic or renal disease, and the prior or concurrent administration of drugs Inspection: attitude condition stressPalpation, auscultationLaboratory :radiographs Laboratory tests should be done on the basis of the physical exam and history

Abnormalities should be noted and corrected37

Classification of physical statusFollowing examination, the physical status of the patient should be classified as to its general state of health according to the American Society of Anesthesiologists (ASA) classification and the information recordedshould be done in the owner's presence, if possible, so that a prognosis can be given personally. This allows the client to ask questions and enables the veterinarian to communicate the risks of anesthesia and allay any fears regarding management of the patient.

Normal healthy: castration Mild systemic disease: fractureSevere systemic disease; fever, dehydration anemia and cachexiaSystemic disease that is a constant threat to life: uremia toxemiaNot expected to survive1 day without operation: extreme shock and dehydration

PainWarms animal of potential tissue damageNociception: noxious mechanical, chemical or thermal transduced into electrical signs by pain receptors, nociceptorsTransmitted to the spinal cord and projected to the brainChronic pain can be responsible for long-term structural changes within the CNS, leading to the development or modification of memory patterns that change animal behaviorvisual analogue scale (VAS): A trained observer rates pain along a 10-cm line from no pain to worst possible pain and, based on this rating, prescribes appropriate therapy.initiating therapy before pain is initiated (preemptive therapy).

Damage and no damage.. Inflamation cancer41

Stresssevere or chronic pain is responsible for temporary periods of dramatic increases in sympathoadrenal and neuroendocrine activity, which is commonly referred to as the stress responseDistress, an exaggerated form of stress, is present when the biological cost of stress negatively affects the biological functions critical to survivalbehavioral changes, including hyperresponsiveness, hyperarousal, vigilance, and agitationchanges in an animal's behavior may be the most noninvasive and promising method to monitor the severity of an animal's pain and associated stress

Patient preparationit is best to have patients off feed for 12 h previously.Birds, neonates and mammals may become hypoglycemic within a few hours of starvationInduction of anesthesia in animals having a full stomach should be avoided : Aspiration riskDistension of the rumen in sheep and larger ruminants has been shown to impair ventilation, with consequent hypoxemia and hypercapniamany older animals suffer from renal disease. water is usually offered up to the time that preanesthetic agents are administered.Systemic antibiotics 1 or 2 h before anesthetic inductionFluid for dehydrated animals Anemia and hypovolemia should be corrected by administration of whole blood or blood components and balanced electrolyte solutions.

Oxygen administration by nasal catheter or mask if the patient will accept it : Animals with upper-airway obstruction by masses or abscesses, pneumothorax, hemothorax, pyothorax, diaphragmatic hernia, and gastric or rumen distention are often in a marginal state of oxygenationa tracheotomy may be performed under local anesthesiainotropes, antiarrhythmic drugs, and diuresis should be made prior to anesthesia for patients with decompensated heart diseaseit is desirable to encourage defecation and/or urination by giving animals access to a run prior to inductionIn cases of hepatic or renal insufficiency the mode of anesthetic elimination should receive strong consideration, with inhalation anesthesia preferred.

During anesthesia, patients should, if possible, be restrained in a normal physiological position. Complications include hypoventilation, nerve and/or muscle damage, and impaired venous return. Horses are especially susceptible to myopathy and/or neuropathya periodic change in position may be beneficial during long surgical procedures. In all species, the head should be extended to provide a free airway and to prevent kinking of the endotracheal tubeIn ruminants, it is desirable to have the head tilted down to enable drainage of salivaOn induction, if active regurgitation begins with large volumes of ruminal contents flowing into the pharyngeal cavity, pressure should be applied immediately by externally grasping the esophagus dorsal to the trachea to prevent further flow

an endotracheal tube can be inserted into the esophagus and the cuff rapidly inflated, directing the flow through the tube away from the laryngeal opening while an endotracheal tube is properly placed to protect the airway from contamination.

Selection of an AnestheticThe ideal anesthetic is one that1. Does not depend on metabolism for its termination of action and elimination2. Enables rapid induction, quick alteration in anesthesia depth, and rapid recovery3. Does not depress cardiopulmonary function4. Does not irritate any tissue5. Is inexpensive, stable, noninflammable, and nonexplosive6. Requires no special equipment for administration

No anesthetic drug possesses all of these qualities. Therefore, selection of an anesthetic is a compromise based on appraisal of the situation. Factors to be considered include1. The patient's species, breed, and age2. The patient's physical status3. The time required for the surgical (or other) procedure, its type and severity, and the surgeon's skill4. Familiarity with the proposed anesthetic technique5. Equipment and personnel available

veterinarians will have greatest success with drugs they have used most frequently and with which they are most familiar. short procedures are done with short-acting agents, such as thiobarbiturates, propofol, alphaxalone-CD, and etomidate, or with combinations using dissociative, tranquilizing, and/or opioid drugs. Where longer anesthesia is required, inhalation or balanced anesthetic techniques can usually be safely used. Species differences may prevent the use of some drugs procaine is frequently lethal for parakeets and, in high doses, morphine can be excitatory in cats

Brachycephalic dogs, because of their pendulous soft palate and restricted respiratory passages, may have difficulty in breathing even when awake. This is particularly true in hot, humid weather. During anesthesia, the degree of airway obstruction is compounded, and these animals often have severe respiratory strider unless a patent airway is maintained via endotracheal intubation.It is wise to use short-lasting agents that are rapidly eliminated (e.g., propofol) so that a prolonged recovery period is avoided and patient control of the airway recovers rapidly. the anesthetic protocol should be one that provides convenience to the surgeon so that the procedure can be completed efficiently, insures patient comfort, allows for appropriate monitoring, and can be administered with confidence by the veterinarian

Record Keeping and MonitoringFor every mistake that is made for not knowing, a hundred are made for not looking.the hazard of death can never be eliminated completely; it can, however, be minimized, particularly if one is willing to investigate and to learn from mistakes items that should be recorded are1. Species, breed, age, gender, weight, and physical status of the animal2. Surgical procedure or other reason for anesthesia3. Preanesthetic agents given4. Anesthetic agents used and method of administration5. Person administering anesthesia (veterinarian, technician,student)6. Duration of anesthesia7. Supportive measures8. Difficulties encountered and methods of correction

Minimally, the pulse and respiratory rate should be monitored at 5-min intervals and charted at 10-min intervals The aspects of anesthetic management that deserve careful attention include patient circulation, oxygenation, and ventilation.

AftercareIn brachycephalic breeds or in animals in which respiratory function is compromised, an endotracheal tube should remain in place until upper-airway reflexes and jaw movements returnCare is necessary to assure freedom of the airway from blankets or paperPredisposition to postoperative respiratory failure may result from continuing drug-induced respiratory depression, postextubation spasm or glottic edema, other respiratory obstructions, diffusion hypoxia, and persistent hypoventilation and/or atelectasis during anesthesiaif necessary, supportive respiratory therapy should be initiated.Under no circumstances should an anesthetized animal be placed in the same cage with a conscious one. Cannibalism has been known to occurIt is unwise to send anesthetized animals home