Mechanical properties of developing swine myocardium

Myocardial contractile function in survived neonatal piglets after cardiopulmonary bypass

Background

Hemodynamic function may be depressed in the early postoperative stages after cardiac surgery. The aim of this study was the analysis of the myocardial contractility in neonates after cardiopulmonary bypass (CPB) and mild hypothermia.

Methods

Three indices of left ventricular myocardial contractile function (dP/dt, (dP/dt)/P, and wall thickening) were studied up to 6 hours after CPB in neonatal piglets (CPB group; n = 4). The contractility data were analysed and then compared to the data of newborn piglets who also underwent median thoracotomy and instrumentation for the same time intervals but without CPB (non-CPB group; n = 3).

Results

Left ventricular dP/dt_max and (dP/dt_max)/P remained stable in CPB group, while dP/dt_max decreased in non-CPB group 5 hours postoperatively (1761 ± 205 mmHg/s at baseline vs. 1170 ± 205 mmHg/s after 5 h; p < 0.05). However, with regard to dP/dt_max and (dP/dt_max)/P there were no statistically significant differences between the two groups. Comparably, although myocardial thickening decreased in the non-CPB group the differences between the two groups were not statistically significant.

Conclusions

The myocardial contractile function in survived neonatal piglets remained stable 6 hours after cardiopulmonary bypass and mild hypothermia probably due to regional hypercontractility.

The determinants of contractility in the heart

The contraction-relaxation cycle of the heart represents the combined action of a variety of different components of the myocytes. For many years an 'index' of contractility has been sought as a means of describing and integrating the large amount of information available from the studies of heart muscle contraction. This review will undertake to show that dF/dt, recorded from the whole heart, and dT/dt, recorded in isometric studies of isolated heart muscle preparations, should not be considered as the 'index' of contractility. Examples will be presented in which an increasing dT/dt is paradoxically accompanied by a lower tension, while a decreasing dT/dt can occur concomitantly with an increased contractile tension. Arguments are further presented in support of the concept that Ca2+, in conjunction with troponin C, is the main determinant of cardiac contractility and that dT/dt reflects a dynamic equilibrium between free and troponin-bound Ca2+. Peak tension is thus the net result of overlapping events competing for Ca2+ during the latter part of contraction, that is, during Phase II of contraction as defined below. These suggestions are based upon the following considerations: (a) The Ca2+ pumps are active even during rest and serve to maintain low cytosolic Ca2+ levels. (b) As cytosolic Ca2+ concentration increases, Ca2+ pump activity also increases. (c) In addition, the Na+/Ca2+ exchange is activated by elevated Ca2+ concentrations and serves to decrease cytosolic Ca2+ levels. (d) The net result is a decline in free Ca2+ concentration during Phase II and a reduction in the rate of cross-bridge formation until peak tension is reached. Thus, the Ca2+ handling elements of the myocyte serve as a finely tuned feedback device, regulating troponin C-Ca2+ interactions controlling the Ca2+ concentration of the cytosol and as a result, the actin and myosin interaction. Factors which influence the function of these elements will change the contractility of the heart.

Effects of glucocorticoids and catecholamines on maturing rat heart

1. The negative force-frequency response of normal rat heart was accentuated when the animals were adrenalectomized. Treatment of adrenalectomized animals with dexamethasone restored the normal force-frequency response. 2. Total adrenalectomy increased the sensitivity of rat heart to calcium. 3. Adrenalectomized-dexamethasone-treated hearts were more responsive to epinephrine and ouabain. 4. Total adrenalectomy caused independent myocardial disturbances in calcium handling elements (glucocorticoid effect) and beta receptors (catecholamine effect).