Cardiac responses to isometric exercise: comparative effects with different postures and levels of exertion.

Abstract

The time course of cardiocirculatory responses to different levels of isometric handgrip (IHG) in sitting position were studied noninvasively in normal males and compared to supine IHG results in the same subjects. Control Comparisons. Resting postural differences were as expected: sitting heart rates (HR), pre-ejection phase (PEP) and PEP/LVET were higher while left ventricular ejection time (LVET) and ejection time index (ETI) were lower than those supine (P < 0.05 to P < 0.001). The following trends, while not statistically significant, were also noted: systolic and diastolic blood pressure were lower while pressure-rate product (PRP), the interval from appearance of the Q wave to the first major "mitral" component of the first heart sound (Q-I M ), and isovolumic contraction time (IVCT) tended to be higher than those supine. Isometric Exercise. The supine IHG results are consistent with data reported in the literature. During sitting IHG at 15% maximum voluntary contraction (MVC) there was no statistical change in any parameter measured. At 30%, 50%, and 100% MVC there were increases in HR, systolic and diastolic pressures, and PRP. At 30% MVC, Q-I M and pulse transmission time (PTT) decreased, ETI increased and IVCT, PEP, LVET, and PEP/LVET did not change significantly. At 50% MCV, Q-I M , IVCT, PEP, LVET, PEP/LVET and PTT shortened while ETI increased. At 100% MCV, Q-I M , IVCT, PEP, and LVET shortened but no significant changes occurred in PEP/LVET or ETI. When sitting IHG was compared to supine IHG the end points of exercise yielded statistically similar values for systolic and diastolic pressures, PRP, Q-I M , IVCT, PEP, and PTT. LVET and ETI were consistently lower ( P < 0.02) while HR and PEP/LVET were higher for sitting vs supine IHG. Standard errors were smaller supine, a finding that indicates greater homogeneity of response in this posture and probably explains why some changes in cardiac parameters at lower levels of IHG reached statistical significance in the supine but not the sitting position. The changes in systolic intervals are consistent with the increased contractility resulting from IHG. Lack of uniform decreases in PEP/LVET with higher levels of IHG, however, were believed to reflect decreased myocardial adaptability to higher pressure (vs volume) loads. Decreases in pulse transmission time appeared to be influenced by diastolic pressure increases which were directly related to the levels of IHG. Once resting postural differences are accounted for, however, the effect of posture on IHG is not very great and certainly does not present the disparity which exists between sitting and supine rhythmic exercise. Most responses were directly related to the level of exertion, with greater changes occurring at the higher levels of IHG. For noninvasive studies, 50% or 100% MVC are the recommended levels of exertion since significant cardiocirculatory responses are greater than at 15% and 30% MVC and occur within one minute.