Most regional and general anesthetic techniques result in vasodilatation, and general anesthetic agents may additionally produce cardiac depression. Hypotension may follow the induction of either regional or general anesthesia, owing to the combination of vasodilation and a decrease in myocardial function. Assessment of the prospective volume status is there for, crucial in the evaluation of the emergency patient. Resting the heart rate, orthostatic changes in blood pressure, and the clinical situations are all important considerations in volume assessment. In the emergency patient who is dependent on vasoconstriction to maintain blood pressure, acute vasodilation accompanying induction may precipitate severe hypotension. Preoperative volume loading prior to induction is important in preventing this complication and should be initiated before the anesthetist's preoperative visit. Emergency patients with life threatening surgical disease pose an enormous challenge to the anesthetist. The risks of delaying needed surgery must be balanced against the deterioration in patient status that may occur while optimizing therapy is being administered.
The anesthetist must select an induction agent that minimizes fluctuations in cardiovascular tone and function. It may be necessary to vary the dose of these agents to achieve the desired effect, particularly in hypovolemic patient or the patient with poor cardiac reserve.
Ketamine can be a useful induction agent in volume depleted patients because it has mild elevations in heart rate and mean arterial pressure increasing effect. Ketamine is also a potent bronchodilator that is useful for inducing anesthesia in patients with hyper reactive airways disease, such as asthma. However this drug is also a potent myocardial depressant. In general, the effects of catecholamine release outweigh the myocardial depressive effects.
Etomidate is another intravenous (IV) induction agent. Its desirable property is that it has minimal cardiovascular depressant effects. Thus, it is particularly useful in providing stable hemodynamics in patients with compromised ventricular function. The IV administration of Etomidate has been associated with pain on injection, the development of myoclonus, and transient adrenal suppression.
Thiopentone has been the most commonly used induction drug since Second World War. Despite their association with bronchospasm, myocardial depression and vasodilation, they remain the standard with which most drugs are compared. Their rapid onset action (less than 60 seconds) reflects rapid penetration to the central nervous system. Awakening after a single bolus dose reflects redistribution from the brain to inactive sites. When hypovolemia is suspected the induction dose of thiopental is markedly reduced or other anesthetic agent is used.