Effect of orthostatic stress on systolic time intervals in rabbits.

Systolic time intervals (STI) are sensitive indices of myocardial function. The present study was conducted to evaluate the effect of orthostatic stress on STI before and after 70 degrees head up tilt (HUT) in rabbits. Electrocardiogram (ECG), phonocardiogram and carotid arterial pulse were recorded simultaneously on four channel polyrite (INCO). Within 1 second of tilt there was a significant rise (P<0.001) in electromechanical systole (QS2), left ventricular ejection time (LVET), pre-ejection period (PEP), and ratio of PEP/LVET (P<0.01), however, there was no change in heart rate and mean arterial blood pressure insignificantly. QT/QS2 ratio and ejection fraction (EF) were also elevated (P<0.05) on tilt. After 20 seconds of HUT, QS2 showed reduction (P < 0.001), LVET elevation (P < 0.001), PEP and ratio of PEP/LVET reduction (P < 0.001) from control level. QT/QS2 and EF declined gradually towards basal values. These changes in STI (raised LVET, reduced PEP and PEP/LVET ratio) and increased QT/SQ2 ratio and EF pointed towards rise in contractility of heart.