Evaluation of the QA interval as an index of cardiac contractility in anaesthetised dogs: responses to changes in cardiac loading and heart rate

The response of a simple systolic time interval, the interval between the Q wave and the onset of the aortic blood pressure pulse (the QA interval), to changes in cardiac contractility, cardiac loading, and heart rate was evaluated in anaesthetised open chest dogs. The QA interval was found to be sensitive to increases in cardiac contractility, being reduced (maximum reduction 33.7(1.5) ms) in a dose related manner by infusions of isoprenaline, but was unaffected by reductions in cardiac loading (maximum reduction in diastolic blood pressure 39.7(5.9) mmHg), induced by infusion of sodium nitroprusside, or changes in heart rate (125 to 275 beats·min−1), induced by cardiac pacing. This contrasted with the response of a simultaneous alternative measurement of cardiac contractility, the maximum rate of rise of left ventricular pressure (LVdP/dtmax). Like the QA interval, LVdP/dtmax was also found to be sensitive to increased cardiac contractility (maximum increase 3583(401) mmHg·s−1) and was unaffected by changes in heart rate. Unlike the QA interval, however, LVdP/dtmax was consistently reduced (maximum reduction 1146(200) mmHg·s−1), in a dose related manner, by reductions in cardiac loading. The simplicity of the QA interval and its apparently selective response to changes in cardiac contractility suggests that this index could find a wider application in experimental and clinical settings.