Impact of cardiopulmonary bypass and cardioplegic arrest on myocardial efficiency

Objective

Mechanical efficiency of the heart, defined as cardiac work over myocardial oxygen consumption, is an important parameter of cardiac function. This study investigated the impact of cardiopulmonary bypass (CPB) and cardioplegic arrest (CPA) on myocardial efficiency (ME).

Methods

After initiation of CPB 10 dogs were submitted to 60min of CPA with continuous, normothermic blood cardioplegia. 30min after CPA all animals were weaned from CBP. Stroke work (micromanometry and sonomicrometry), contractility (preload recruitable stroke work and dP/dt _max ), cardiac output (thermodilution), coronary blood flow (Doppler flow), and myocardial oxygen extraction (O₂-Sat difference aorta-coronary sinus) were determined at baseline and at 1 and 2h after CPB. ME was calculated as the relation of stroke work over myocardial oxygen consumption. Two dogs with identical instrumentation and experimental time, but without CPB and CPA served as controls.

Results

Contractility, cardiac output, and myocardial oxygen extraction after CPB and CPA showed no significant difference to baseline. However, myocardial oxygen consumption, determined 1 and 2 h after CPB increased to 151, and 167% of baseline, respectively (P < 0.05). At the same time points ME decreased to 58, and 49% of baseline, respectively (P < 0.05). In controls all measurements were within 15% of baseline values.

Conclusions

Although contractility and cardiac output remained at baseline level after CPB and CPA, ME decreased significantly after CPB and CPA. The systemic inflammatory response to CPB, myocardial edema formation during CPB, or the Gregg phenomenon may be considered as possible mechanisms for this finding. ME indicated a significant change in cardiac function when hemodynamic parameters such as contractility and cardiac output were still unchanged. ME may be a valuable parameter for the assessment of post CPB cardiac function.