OBJECTIVETo observe the regulation of heart rate to cardiac pump function in the phase of negative force-frequency relationship and their possible mechanisms.

METHODSThe left ventricular pressure, aortic pressure, and cardiac output were measured in isolated working heart of rat from 240 to 300 beats/min of pacing rate.

RESULTSCardiac output of isolated working heart was decreased by a proximally 20% (P < 0.01) with the increase in the pacing rate from 240 to 300 beats/min. Left ventricular end-systolic pressure (LVESP) was declined by 4.8% (P < 0.05), but left ventricular end-diastolic pressure (LVEDP) was elevated by 139% (P < 0.01) with an increase in the pacing rate. Left atrium was enlarged at 300 beats/min of pacing rate. The time from peak to 75% relaxation in left ventricular pressure was shortened with the increased pacing rate. Pressure at aortic valve close was raised (P < 0.01) and ejection duration was shortened with the increased pacing rate (P < 0.01).

CONCLUSIONThose above results suggest that there are different mechanisms between the depressed cardiac output at higher heart rate and negative force-frequency relationship. The frequency-dependent acceleration of relaxation facilitates the decline of left ventricular pressure, and then may elevate the pressure of aortic valve close in the condition that the shape of aortic pressure curve stays the same. Therefore, the ejection duration is shortened at higher pacing rate. The shortened ejection duration may induce a decrease in stroke volume of the left ventricle. The increment of heart rate is not enough to compensate the decreased stroke volume. Finally, cardiac output shows a decrease at higher heart rate.