Intraoperative protection of the heart: topical myocardial hypothermia

It is apparent from the cyclical recrudescence of the subject of intraoperative myocardial preservation over the entire history of modern cardiac surgery that no perfect and universally accepted method has yet evolved. I would like, in this chapter, simply to describe our experience at Stanford with topical hypothermia for myocardial protection. At the outset, it probably should be pointed out that the term ‘myocardial protection’ represents at the present state of development of cardiac surgery a euphemism for minimizing myocardial injury during cardiac operations that involve interruption of coronary flow and arrest of ventricular contractions. Under experimental conditions nearly complete preservation of myocardial structure and function during cardiopulmonary bypass can be achieved by continuous normothermic or mildly hypothermic perfusion of a beating, vented heart that is performing minimal external work under conditions of physiological perfusion pressures and arterial oxygen content. Most cardiac operations, however, are facilitated greatly by a quiet and relatively bloodless field provided by both mechanical cardiac arrest and interruption of coronary perfusion. During periods of coronary non-perfusion ischaemic damage to the myocardium occurs in a time-dependent fashion as a function of the disproportion between metabolic demand and substrate supply, and negative imbalance in this relationship is expressed in depletion of intracellular myocardial levels of high energy phosphate compounds, obligatory anaerobic metabolism, and evidence of membrane injury and contractile dysfunction after restoration of coronary blood flow. This has been summarized elegantly in the presentation of Dr Hearse (Chapter 33 in this book). The magnitude of ischaemic damage during aortic cross-clamping can be decreased significantly by reduction of myocardial temperature and by sustained membrane depolarization with a hyperkalaemic extracellular fluid. In this chapter, however, 1 would like to describe our clinical experience with myocardial hypothermia induced by simple topical, or epicardial, cooling during periods of coronary perfusion.