Thoracic epidural anesthesia for cardiac surgery

Abstract

The coronary arterial bed is diffusely and heavily innervated by the sympathetic nervous system. The anatomic arrangement of both the vagal and sympathetic at the level of the cardiac plexus and below are also sufficiently complex as to make clear distinctions between these nerves difficult. At the level of the coronary artery however, the situation seems clearer. The normal arterial smooth muscle responds to sympathetic stimulation with vasodilation; however, the atherosclerotic coronary bed responds with vasoconstriction. The exact etiology of this response is either a reflection of the loss of endothelial cell integrity and, hence, vasodilatory capacity or the release of mediators which are not normally found at sympathetic terminals. This latter response is probably related to the release of serotonin. The source for the serotonin is thought to follow its nonspecific uptake at platelet activation sites. These sites are typically found at areas of high turbulence and constriction in the coronary bed. The successful ablation of this sympathetic constriction is accomplished most thoroughly with the use of thoracic epidural blockade or anesthesia (TEA). There is extensive experience in both animals and man with the use of TEA around the time of myocardial ischemia. In animals the use of TEA is capable of decreasing the incidence and type of ventricular arrythmias following an ischemic insult. In man the use of TEA can in most patients reduce and in many totally supplant the need for antiischemic medications during unstable anginal episodes. Further, the outcome of these patients may be improved compared with cohorts not receiving TEA. The use of TEA for cardiac surgery is associated with an improvement in pulmonary and myocardial function following surgery. While the release of catecholamines is reduced during surgery, glucocorticoid response remains unimpaired. The incidence of ischemia is reduced by 50% in the postoperative period following TEA activation. Finally, patients receiving TEA more rapidly resume normal body temperatures and are extubated more quickly than their conventionally-treated counterparts. While there is much we do not know about the use of TEA during cardiac surgery, we do know that the risk of neuraxial hematoma formation is not excessively high. As of this writing almost 4,000 patients worldwide have received TEA during cardiopulmonary bypass and there are no reports of hematoma formation. This suggests that the previously postulated mechanisms for hematoma formation may be in error.