Anesthesia is intended to be a controlled, benign and reversible process. Unfortunately, the anesthetic drugs produce their effects primarily by limited depression of vital processes.
Anesthesia is intended to be a controlled, benign and reversible process. Unfortunately, the anesthetic drugs produce their effects primarily by limited depression of vital processes. The inherent dangers of anesthesia and the debilitation of injuries and illness that require anesthesia and surgery predispose the patient to risks of serious complications and emergencies. Most anesthetic complications and emergencies can be related to human errors, equipment problems, ventilatory problems, or circulatory problems. Most anesthetic emergencies and complications can be prevented or adequately managed.
Human error is ultimately responsible for the majority of problems encountered with anesthetic management. The importance of vigilance in anesthetic care cannot be overemphasized. It has been noted that hundreds of errors are made due to not looking for every one error made due to not knowing.
It should be recognized that there is a significant degree of safety with familiarity. Errors are more common when the anesthetist is not familiar with either the drugs or equipment being used. Miscalculation of anesthetic drug doses is a common error. The narrow therapeutic index of most anesthetic drugs makes correct dose determination or titration crucial. An absolute or relative overdose of anesthetic can cause every problem from minor excess physiologic depression to death.
An overdose with barbiturates should be managed with physiologic support of ventilation, continuous monitoring of cardiopulmonary function, and IV fluid therapy to speed recovery and improve cardiopulmonary function. In the context of cumulative overdoses from repeated injections of barbiturates to prolong anesthesia, the intravenous administration of bicarbonate at 0.5 to 1.0 mEq/kg can speed recovery from barbiturate overdose by favoring elimination. The non-specific stimulant-antagonist drug, doxapram, can be helpful in treating depression due to barbiturate overdose. It is administered intravenously at 1.0 to 5.0 mg/kg. This stimulant can result in very deleterious stress and should not substitute for good care and proper dosing of anesthetics. Overdoses with other anesthetics are also managed with supportive care which is often adequate in mild to moderate overdose situations.
Fortunately there are specific antagonist drugs available to counteract the effects of some anesthetic drugs. For narcotics, the pure antagonist agent, naloxone, will reverse effects of an overdose. With a large overdose or a long lasting narcotic, renarcotization can occur with a return to the effects of the narcotic agent. For the tranquilizer/sedatives xylazine and medetomidine, and other alpha-2 agonists, there are specific antagonists available. One of these, yohimbine, was approved for use in dogs years ago to reverse the effects of xylazine. Atepamezole is a better antagonist for medetomidine and is often effective by titration of reduced doses (approved for SC administration) to secure prompt recovery with less excitement and stress than would result from the administration of a higher dose.
Non-specific partial reversal of anesthetic depression is possible by administration of the respiratory stimulant doxapram but this is usually not an appropriate replacement for positive pressure ventilation and other supportive care. Although the net effect can be life saving, non-specific reversal has been associated with residual undesirable effects related to CNS stimulation and even deaths! Other stimulants have been advocated to correct excessive effects of various anesthetics but the benefits are usually very limited.
Anesthetics administered by an incorrect route can have very adverse effects. The extravascular injection of barbiturates can cause severe irritation and sloughing of surrounding tissue. Extravasation should be treated immediately with generous infiltration of the site with lidocaine and saline, followed by warm compresses. Errors in the administration of anesthetics also include the misidentification of drugs and accidental use of the wrong medication.
Among the most serious anesthetic complications is the failure to deliver oxygen to the patient. This can be caused by respiratory obstruction or misused or defective anesthetic equipment. Empty tanks or misconnected gas lines and breathing circuits prevent the delivery of oxygen. Such problems must be recognized and corrected immediately. Empty anesthetic vaporizers, vaporizers filled with the wrong agent or overfilled are common problems. Delivery of nitrous oxide in combination with too little oxygen should be carefully avoided and is not always prevented by "fail-safe" systems incorporated in modern machines.
Kinked or plugged endotracheal tubes cause respiratory obstruction. Improper cuff inflation can result in obstruction, tracheal injury, or allow for aspiration pneumonitis. Improper placement of endotracheal tubes is very common, even in species that are easily intubated. Correct placement should always be verified.
An inability to adequately fill the rebreathing bag or to provide positive pressure ventilation by squeezing the bag often indicates major leaks or disconnections. These can result in a failure to deliver anesthetics and oxygen and substantially contribute to anesthetic gas pollution of the veterinary hospital. Stuck valves in the anesthesia machine or circuit can cause difficulty in ventilation. Inappropriate rebreathing of exhaled gases or the accumulation of excessive pressure results. Patients that consistently seem to be too deep or too light may indicate that the vaporizer is out of calibration due to wear and tear, the accumulation of deposits within the vaporizer, or other factors. These common problems emphasize the importance of regular inspection and maintenance of equipment.
Electrical problems with monitoring or supportive equipment risk injury to personnel as well as to patients. Inadequately grounded or protected equipment can cause electrical burns, electrocution, or fires. Unsafe or sub-standard equipment should be repaired or replaced.
Hypoventilation due to anesthetic overdose is one of the most frequently encountered and serious complications in anesthesia. Inadequate breathing occurs with either relative or absolute overdoses of many anesthetics. Weakened debilitated animals are more susceptible to the ventilatory depression which may occur secondary to circulatory depression and inadequate perfusion of CNS respiratory centers, electrolyte imbalances, muscle relaxant drugs or thoracic injury. Support of ventilation requires endotracheal intubation and positive pressure breathing, preferably with oxygen. Identification and correction of the primary problem is then undertaken.
Hyperventilation is often due to inadequate anesthetic depth and represents an excessive response to surgical stimulation. It is important to rule out the possibility of carbon dioxide accumulation, due to exhausted absorber granules or improper connection of the breathing circuit, as the cause of hyperventilation. Panting can occur with narcotics and thereby decrease the effective ventilation. Most often this represents an inconvenience to the surgeon. A less common cause of panting is actual hyperthermia. Erratic or jerky breathing patterns also usually indicate improper anesthetic depth. As before, airway obstruction and various causes of carbon dioxide accumulation should be ruled out.
Palor of mucous membranes is a complex sign in that it may occur as a compensatory response to either excessively light or deep planes of anesthesia. Reduced cardiac output due to anesthetic depression or increased sympathetic tone due to pain can cause palor. It is important to identify the cause in order to appropriately treat the problem. Incorrect management may compound the problem and cause decompensation and immediate deterioration.
Cyanosis rarely occurs in anesthetized patients breathing oxygen. In order for cyanosis to develop, hemoglobin must be present in sufficient quantities and in the reduced (non-oxygenated) state. Hypoxemia that accompanies anemia therefore will not become evident through cyanosis. When cyanosis of either mucous membranes or blood in the operative field does occur, oxygen should be administered and adequate ventilation and pulse quality assured.
Bradycardia is often associated with procedures or drugs which cause increases in vagal parasympathetic nervous system tone. Difficult endotracheal intubations, deep abdominal surgical procedures, intraocular surgeries, some surgeries on the neck or in the thorax can all cause vagal-mediated bradycardia. Atropine or glycopyrrolate administration is effective in prevention of most vagal effects. Treatment after the vagal effects become evident is often less rewarding.
Non-vagal bradycardias may result from excessive anesthetic depth, hypoxia or hypothermia. Bradycardia can be a very serious sign of a significant anesthetic emergency. Administration of atropine and attention to possible causes is imperative.
Every member of a veterinary hospital staff should be prepared to constructively contribute in an emergency resuscitation.
Hypotension is caused by either decreased cardiac output, increased capacitance of the vasculature, or inadequate blood volume. Intraoperative fluid therapy at 10 ml/kg/hr is often appropriate for replacement in many surgical patients but increased volumes can be necessary. Clinical evaluation to distinguish between hypovolemia and reduced cardiac output states as causes of hypotension can be based on patient history and evaluation, including central venous and arterial pressures.
Vasodilatation is a very common side effect of many anesthetic drugs. The tranquilizer, acepromazine, is a hypotensive drug, particularly at higher doses. The volatile anesthetics also cause significant vasodilatation. Most anesthetics also are potent cardiac depressants, again particularly at higher doses. Hypotension under anesthesia is therefore most appropriately managed by reduction of anesthetics and fluid administration as primary management.
Heart rates above 180/min in dogs and 200/min in cats are associated with decreased efficiency and increased work load. Tachycardia can be due to fear, pain, inadequate anesthetic depth, pre-anesthetic excitement or a rough induction of anesthesia. Hypotension causes a compensatory tachycardia. These causes of supra-ventricular tachycardia should be recognized and treated.
Compensatory tachycardia in response to hypovolemia and hypotension results in decreased coronary artery blood flow and increased myocardial work load. If other conditions contribute to hypoxia there is significant risk of development of more serious arrhythmias. Fluid therapy for hypovolemia, adjustment of anesthetic plane, and support measures to avoid cardiovascular deterioration are necessary.
Ventricular tachycardias are a much more serious emergency. An occasional ventricular ectopic beat is cause for concern but not necessarily indicative of patient distress. When ventricular arrhythmias become frequent or progress to ventricular tachycardia immediate treatment is required. Ventricular arrhythmias indicate an irritated, hypoxic, or diseased myocardium.
Ventricular tachycardia should be treated with intravenous bolus injection of 2% lidocaine at a dose of 1, 2, or 3 cc in small, medium, or large size dogs respectively. This rule-of-thumb will allow for immediate therapy without an accurate dose calculation which could contribute to a life-threatening delay. It has been recommended that propranolol is the drug of choice for treating ventricular arrhythmias in cats. Lidocaine is also effective in cats. Total dose limitation is more important in cats due to their smaller body size and blood volume.
Success in emergency management of ventricular arrhythmias is evaluated by continuous ECG monitoring. Bolus injections of lidocaine can be repeated to a total accumulated dose of about 10 mg/kg without significant risk of overdose. When two or three injections are required over a period of 15-20 minutes it is necessary to convert to a continuous IV infusion of lidocaine at 30-80 micrograms/kg/min. Refractory arrhythmias may require conversion to therapy based on alternative antiarrhythmic medication.
Delayed recovery from anesthesia is managed by recognition of differential causes and a rule out of individual possibilities. Systematic approach to potential causes will provide for balanced care with correction of often multiple factors including such factors as hypothermia, inadequate fluid support, reduced metabolism or clearance of drugs, and debilitation associated with the stress of anesthesia and surgical trauma. Deterioration due to an hypoxic episode must be considered.
Hypothermia is among the most common of anesthetic complications. Body heat is lost with preparation of the surgical site, contact with cool surfaces such as surgical tables, breathing of dry anesthetic gases, and evaporation from the airways and the surgical field. Moderate hypothermia is a frequent problem even with attention to each of these factors. Body temperatures down to approximately 92 F increase oxygen and energy requirements during recovery, but most patients can tolerate this level of hypothermia. More extreme hypothermia causes delayed recovery and reduces tissue perfusion and increases morbidity and mortality.
The risks of thermal injury are so great with older styles of electric heating pads that their use in anesthetized, sedated, or depressed (many critically ill) patients is considered extremely hazardous. The new "Hot Dog" warming system, designed by Dr. Dan Augustine, is a very different dispersed electrical heating blanket to avoid thermal injury and safely warm the patient. Warm water bottles or surgical gloves filled with warm water have been shown to be rather ineffective in raising the body temperature of hypothermic patients and at the same time constituting a significant risk of causing thermal burns at the site of contact. Circulating warm water blankets are a much better alternative to warm water bottles or gloves. Forced warm air heating systems are more effective than circulating warm water blankets and can also be used to cool hyperthermic patients when set to deliver unheated ambient air. Proper use of forced air systems must include dispersive blankets to envelop the patient in warmed air and avoid hot spots by distributing the warmed air.
Many other complications and emergencies can occur during or be associated with anesthesia. These include anaphylactic-like reactions, hyperthermia, biochemical imbalances, gastroesophageal reflux, regurgitation, vomiting, aspiration, and many surgical complications such as hemorrhage and pneumothorax. Avoidance of complications and effective management of emergencies requires continued vigilance and immediate appropriate actions.