[Ionic bases of cardioplegic solutions. II. Influence of the ionic composition of a cardioplegic solution on the metabolic and functional preservation of ischemic myocardium. Experimental evaluation with phosphorus 31 nuclear magnetic resonance and applications to cardiac surgery].

The object of this study was to establish whether the protective effects of a cardioplegic solution could be improved by ionic or pharmacological intervention aimed at reducing cellular Ca++ overload resulting from myocardial ischaemia. The experimental model was the isolated perfused working heart of the rat submitted to 60 or 120 minutes of hypothermic ischaemia (15 degrees C) followed by 30 minutes of reperfusion at 37 degrees C. The high energy phosphates were measured every 2,5 or 5 minutes by Phosphorus 31 (P31) nuclear magnetic resonance and correlated with haemodynamic data. Our results showed that the best metabolic (75,5 +/- 9,7 p. 100 preservation of ATP after 60 minutes ischaemia) and functional protection (91,8 +/- 4,7 p. 100 recovery of aortic output after 30 minutes reperfusion) was obtained with a solution with the following ionic properties: 1) high Mg++ concentration (13 mM); 2) low Ca++ concentration (0,25 mM); 3) high Na+ concentration (100 mM). The protective effects of this solution were further improved by the addition of a calcium blocking agent (nifedipine 0,2 micrograms/ml). This preserved 85,5 +/- 3,2 p. 100 of basal ATP values after 120 minutes of ischaemia and was associated with a 92,9 +/- 2,8 p. 100 recovery of aortic output at the end of reperfusion. We conclude that: 1) limitation of cellular Ca++ overload is one of the major objectives to be considered when making up cardioplegic solutions; 2) the use of P 31 nuclear magnetic resonance on the isolated working heart is the technique of choice for comparing methods of myocardial protection because it provides a non-invasive, sequential and simultaneous assessment of the parameters of metabolic and haemodynamic function.