The isolated working mouse heart: methodological considerations

Our aim was to develop a working isolated murine heart model, as the extens ive use of genetically engineered mice in cardiovascular research requires development of new miniaturized technology. Left ventricular (LV) function was assessed in the isolated working mouse heart perfused with recirculated oxygenated Krebs-Henseleit bicarbonate buffer (37 degrees C pH 7.4) contai ning 11.1 mM glucose and 0.3 mM palmitate bound to 3% albumin. The hearts w orked against an afterload reservoir at a height equivalent to 50 mmHg, and heart rate was controlled by electrical pacing of the right atrium. LV pre ssure was measured with a micromanometer connected to a small steel cannula inserted through the apex of the heart. The experimental protocol consiste d of two interventions. First, following instrumentation and stabilization, the preload reservoir was raised from a pressure equivalent of 7 to 22.5 m mHg, while pacing at 390 beats.min(-1). Thereafter the height of the preloa d reservoir was set to 10 mmHg, and the pacing rate was varied from 260 to 600 beats.min(-1). Aortic and coronary flows were measured by timed collect ions of effluent from the afterload line and that dripping from the heart, respectively [aortic+coronary flow=cardiac output (CO)]. Elevation of LV en d-diastolic pressure (LVEDP) from approximately 5 to 10 mmHg resulted in a twofold increase in average cardiac power [product of LV developed pressure (LVDeVP) and CO], whereas myocardial contractility (first derivative of LV pressure, dP/dt) and LVDevP (LV systolic pressure-LVEDP) increased only mi nimally (5-10%). Measured LVEDP was lower than the equivalent height of the preload reservoir by an amount that was related to the heart rate. Cardiac power, LVDeVP and dP/dt were stable at heart rates up to 400 beats.min(-1) , but declined markedly with higher rates, consistent with the decrease in LVEDP. Thus, cardiac power was reduced to 50% of its maximum value when sti mulated at approximately 500 beats.min(-1), and at even higher rates there was little ejection. By systematic manipulation of the height of the preloa d reservoir and heart rate, we conclude that LV afterload and preload can b e assessed only by high-fidelity measurement of intraventricular pressures. The heights of the afterload column and the preload reservoir are unreliab le and potentially misleading indicators of LV afterload and preload.