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- W2055840975 abstract "Most anaesthetists are involved in the care of acutely ill patients, about whom decisions must be made regarding vasoactive drug therapy. Problems can arise due to uncommonly encountered conditions, patients who have common conditions but with unusual characteristics, and ineffective standard therapy. All inotropes must be evaluated as to whether they are effective, ineffective or more damaging in an area of reperfusion in the myocardium. This evaluation is somewhat theoretical, because any inotrope that is effective could also be damaging simply by making a previously ischaemic muscle area move more vigorously. The best inotrope to use after the revascularization of an early evolving infarct has not yet been determined. Stationary vasoactive drug protocols are limited by their very nature when anaesthetists are faced with a constant state of mechanical and metabolic flux. We cannot hope to help unless we base part of our judgement on physiology with its increasing dependence on our knowledge of cellular or receptor function. Many of our manipulations take place in a state of physiological imbalance, especially when the oxygen supply is limited by coronary artery disease. As clinicians, we can modify the contractile activity of the heart as well as these receptors and channels responsible for normal function. The clinical choice of a vasoactive drug is conditioned not only by the well-studied physiological features such as mean arterial pressure (MAP) versus SVR but also by the need to balance the various competing side-effects. This picture of patient physiology is limited by a few crude measurements and yet we have to derive a solution that is reasonable and efficacious. The need for a therapeutic end-point is important to the physician who treats emergent cardiovascular problems. Defining a threshold gives us a place in our dose-response from which we can easily recognize this relationship. The effect of circulatory impairment on drug disposition and elimination must be considered. Both the rate and amount of drug administered may need to be modified. When a dilatory or constrictive drug is used to improve the circulation, certain principles must be remembered. The venous capacitance system has a much greater volume and, therefore, effect on the distribution of the cardiac output. The property of a drug (i.e. whether it is simply an inotropic agent such as dobutamine or whether it has α-constrictive properties such as dopamine) must be considered. Vasoconstriction itself is a potent and often misused property of many vasoactive agents. The pressor effect of a drug should be used only if organ damage is imminent, as with a low perfusion pressure. The other properties of a vasoactive drug can often be anticipated by analysing its receptor-binding properties. The main drugs used in anaesthesiology today have not changed much in the past several years. The principles of use are consistent with certain pathophysiological states. The main effects of an inotrope are on β- or α-receptors. The total effect of a drug is often the physiological sum of these opposing receptor actions. Modifications of the receptor capability is evident when vasoactive drugs are administered for a long time; thus the effect of tolerance or down-regulation of drugs, especially the β-effects, must be determined. When one component of a β-system has become insensitive, then the second receptor in that system can often be used to achieve an inotropic effect on the myocardium. Vasodilatory clinical situations are treated with a predominant α-drug. If inotropy is also needed, then a balance between β-inotropy and α-vasoconstriction must be achieved. If the renal vasculature needs to be considered in any vasoactive protocol, then the dopaminergic properties of dopamine or dopexamine should be considered. If inotropy alone with vasodilatation is considered the optimal choice then dobutamine or even isoprenaline in non-ischaemic hearts could be beneficial. If the β-system has become ineffective, then drugs such as phosphodiesterase inhibitors can be extremely valuable in the failing heart. Phosphodiesterase inhibitors are particularly beneficial in the hypervolaemic patient who has poor cardiac output because they reduce after-load at the same time that they increase cardiac contractility. Vasodilators are also important to the failing heart. Perhaps none is so vital at present than nitroglycerine. This drug produces a graded vasodilatation depending on dosage in the venous system, thus reducing preload strain on the heart. It also improves coronary circulation and arteriole compliance. No other dilator is as safe to administer with so few side-effects. A major therapeutic advance in the past decade has been the use of calcium antagonists, with therapeutic effect ranging from antiarrhythmic to coronary and arteriole dilatory properties. Because the influx of calcium is part of ischaemic injury events, both in the heart and the central nervous system, these agents are now used to attenuate an uncontrolled gain in calcium and to stabilize membrane and metabolic events in the area of ischaemic injury. Beta-blockers have become more selective and, in the case of esmolol, shorter acting. It is now possible to titrate β-blockers against unwanted increases in heart rate and to use them in component therapy against sympathetic stress in general. The ability to choose β-blockade with vasodilatation, as with labetalol, or as an ultra-short acting agent, esmolol, has increased the therapeutic options for anaesthetists. There is a clinical decision which is informed by academic pharamcology and chemistry but which necessarily includes intangibles and experience. It is not the case that the complete body of pharmacological knowledge can be reduced here to a cookbook for haemodynamic manipulation of the critically ill patient under anaesthesia. Rather, the clinical decision is informed by this rationale and needs to be influenced by it, but not in a mechanical or sterile way. Knowledge of these drugs and their effects is most of all first hand, the changes are so rapid and the situations so varied that the clinician will never be able to derive a course of action solely from a group of references. The circulatory emergent situations encountered by anaesthetists treating patients during circulatory failure must deal with systems that are in mechanical, physiological and metabolic flux. Vasoactive drug protocols are designed to anticipate these changes and to help the anaesthetist determine which drugs may be best suited for a particular situation. This chapter has presented numerous options for the anaesthetist to consider when treating patients having cardiac problems during anaesthesia." @default.
- W2055840975 created "2016-06-24" @default.
- W2055840975 creator A5076569612 @default.
- W2055840975 date "1991-12-01" @default.
- W2055840975 modified "2023-09-27" @default.
- W2055840975 title "Pharmacokinetics and clinical uses of vasoactive drugs" @default.
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- W2055840975 doi "https://doi.org/10.1016/s0950-3501(05)80048-0" @default.