Effects of thoracic epidural anaesthesia on the coagulation system

The coronary arterial bed is diffusely and heavily innervated by the sympathetic nervous system. The anatomical arrangements of both the vagal and the sympathetic at the level of the cardiac plexus and below are also sufficiently complex 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 aetiology 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. The latter response is probably related to the release of serotonin. The source for the serotonin is thought to follow its non-specific 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 anaesthesia (TEA). There is extensive experience in both animals and humans with the use of TEA around the time of myocardial ischaemia. In animals the use of TEA is capable of decreasing the incidence and type of ventricular arrythmias following an ischaemic insult. In humans the use of TEA can reduce in most patients and totally supplant in many the need for anti-ischaemic medications during unstable anginal episodes. Further the outcome of these patients may be improved when 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 ischaemia is reduced by 50% in the post-operative period following TEA activation. Finally, patients receiving TEA resume normal body temperatures more rapidly 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 haematoma formation is not excessively high. As of this writing almost 4000 patients world wide have received TEA during cardiopulmonary bypass and there are no reports of haematoma formation. This suggests that the previously postulated mechanisms for haematoma formation may be in error.