Cardioplegia: relation of myocardial protection to infusion volume and duration

Clinically, initial infusion volumes of crystalloid cardioplegic solution are relatively low (500-1000 ml or 2-4 ml/g myocardium) compared to those used experimentally. In particular, rat hearts (in which many clinical solutions have been developed and evaluated) commonly use 20-30 ml/g myocardium (equivalent to 5.0-7.5 l in human heart). We used the isolated working rat heart to characterise the relationships between myocardial protection and (a) infusion duration, and (b) infusion volume of St. Thomas' Hospital cardioplegic solution (STH), Hearts were aerobically perfused (20 min) and subjected to varying durations of STH infusion (0-300 s) prior to normothermic global ischaemia (30 min). During reperfusion, maximal recovery of cardiac output occurred when infusion durations exceeded 30 s and infusion volumes exceeded 5.0 ml/g myocardium. To assess infusion volume rather than duration, hearts were infused with 1.0, 1.5 or 2.0 ml of STH for 120 s. Optimal recovery of cardiac output required 2.0 ml/g myocardium for 120 s. To assess infusion duration with low infusion volumes, 2.0 ml STH/g myocardium was infused for 10, 30, 60 and 120 s; optimal recovery of cardiac output occurred with infusions of 30 s or longer. Thus, even in the rat heart, optimal protection with STH can be achieved by infusion at a volume of 2.0 ml/g myocardium for a duration of not less than 30 s, similar to that now in current clinical use.