Relationships of left side systolic time intervals to beat-by-beat heart rate and blood pressure variables in some cardiac arrhythmias of the horse.

Systolic time intervals (STIs), isovolumic contraction time (ICT) and left ventricular ejection time (LVET) were recorded from seven horses with supraventricular arrhythmia. The STIs were measured over a number of beats (33 to 100) directly from the left ventricular (LV) and aortic (Ao) pressure contours which were recorded simultaneously using two catheter-mounted transducers. ICT was significantly (P less than 0.01) and directly related to beat-by-beat heart rate (HR = 60/pulse interval) and LVET was significantly (P less than 0.01) and inversely related to heart rate in each of five horses. In two horses with atrial fibrillation, LVET declined much more rapidly at heart rates greater than 70 to 80 beats minute-1 which suggested the possibility of impaired ventricular filling above this heart rate range in this arrhythmia. ICT, LVET, LVET/ICT and mechanical systole (MS = ICT + LVET) were regressed against a number of pressure parameter and also against heart rate. ICT was directly related to aortic end diastolic pressure (AoEDP) and inversely related to left ventricular end diastolic pressure (LVEDP) and LV dP/dtmax. LVET was directly related to aortic systolic pulse pressure (SPP = Peak AoP-AoEDP) and inversely related to AoEDP. It was related to heart rate in only two out of the seven horses. LVET/ICT was most strongly and inversely related to AoEDP. It was directly related to LVEDP, SPP and to LV dP/dtmax but was not consistently related to heart rate. The relationship of these variables to LVET/ICT reflected largely their separate relationships to ICT and LVET.(ABSTRACT TRUNCATED AT 250 WORDS)