Systolic-dominant coronary flow in rats and mice: Challenging the diastolic paradigm across conscious and anaesthetized states.

Extensive research in humans, dogs, rabbits, rats, mice and other mammals has consistently demonstrated that coronary blood flow (CBF) peaks during ventricular diastole. For example, studies using transthoracic Doppler echocardiography in anaesthetized rats and mice, isolated blood-perfused rat hearts and Doppler probes sutured to the myocardial surface have reported diastolic-dominant CBF. In contrast, while evaluating the effects of dietary salt on coronary vascular resistance in rats, we unexpectedly observed that left CBF peaked during ventricular systole. This observation prompted two follow-up protocols to test the hypothesis that left coronary flow in rats and mice peaks during systole. In Protocol 1, chronically instrumented conscious male Sprague-Dawley rats were implanted with telemetry pressure sensors and pulsed Doppler flow probes around the ascending aorta and left main coronary artery. Coronary and aortic flow waveforms exhibited nearly identical timing, indicating that CBF peaked during systole. In Protocol 2, anaesthetized, open-chest, mechanically ventilated rats and mice (both sexes) were studied. Doppler probes and ECG electrodes were used to compare the time from the R wave to the peak of both aortic and coronary flow. Student's paired t-test showed no significant difference between the two, confirming that coronary and aortic flow occur synchronously during systole. These findings demonstrate that, in rats and mice, left coronary blood flow peaks during ventricular systole - not diastole - challenging the widely accepted paradigm. This may reflect structural and haemodynamic features unique to small mammals, such as low ventricular wall tension and high heart rates.