Antegrade and retrograde continuous warm blood cardioplegia: A ³¹P magnetic resonance study

Background.: Retrograde normothermic blood cardioplegia has been shown to provide myocardial protection during certain bypass procedures. However, a number of animal studies have shown less than optimal myocardial protection with this technique. Methods.: Isolated, beating porcine hearts were perfused antegradely (aortic root pressure = 75 to 95 mm Hg) for 30 minutes. Arrest was induced and maintained for 60 minutes with high K⁺ blood cardioplegia delivered either antegradely (n = 8) or retrogradely (n = 8) (coronary sinus pressure = 35 to 55 mm Hg). Perfusate was switched to normokalemic blood for recovery of sinus rhythm (30 minutes). Intracellular pH, creatine phosphate, inorganic phosphate, and adenosine triphosphate were monitored continuously and noninvasively with phosphorus 31 magnetic resonance spectroscopy throughout the experiment, and functional variables (rate-pressure product and the positive and negative first derivatives of left ventricular pressure) were assessed concurrently. Results.: Antegrade cardioplegia maintained high-energy metabolites, intracellular pH, and myocardial function. Retrograde normothermic blood cardioplegia resulted in an increase in inorganic phosphate (197% ± 15% of control) and a decrease in creatine phosphate (51% ± 6% of control). There was no significant difference in myocardial function between the two groups (p > 0.05). The magnetic resonance spectroscopy data indicate ischemia occurred within 2 minutes of the initiation of retrograde perfusion. Conclusions.: This study suggests that retrograte normothermic blood cardioplegia causes a transition of the myocardium to ischemic metabolism in the normal porcine heart.