Heart Rate Myocardial Strain And Performance

There is a direct link between HR and myocardial strain, performance and dysfunction. However, the contractility of the myocardium is also a function of the stroke volume that results from the heart wall tension produced during diastole and the force of contraction during systole. This, together with the rate of contraction, produces a given cardiac output. It is both the rate and force of contraction of the myocardium that determine the oxygen demand or uptake (MVO2) of the heart (Froelicher and Myers, 2000). Hence, the performance of the ventricles is determined by the amount of pressure that can or needs to be created during systole. The systolic pressure therefore provides an indirect means of indicating the force of contraction. A practical index of myocardial strain has thus been described as the product of HR and systolic blood pressure and given the single term rate pressure product (RPP) or double pressure product (Gobel, et al., 1978). In this study, the use of systolic blood pressure in conjunction with HR provided a better index of MVO2 than HR alone.

From a practical perspective, the concept of MVO2 and rate pressure product is best highlighted when comparing upper body and lower body exercise. Miles, et al. (1989) demonstrated in healthy individuals and cardiac patients that for the same oxygen uptake, blood pressure is higher for upper body compared to lower body exercise. This is due to the smaller vascular bed in the arms, compared to the legs, and the added isometric contractions in the thoracic region to provide a stabilising base for the shoulder joints and muscles. Therefore, if an individual exercises at his or her set target HR with the upper body, rather than with the lower body, the result will be a higher systolic pressure, giving rise to a greater rate pressure product or MVO2.

For example, if a patient's target HR is 120 beats • min-1, and during lower body exercise their systolic blood pressure is 150mmHg, then the rate pressure product will equal 18000 (120 x 150). Knowing that upper body exercise will have a greater systolic blood pressure (perhaps 165 mmHg), then at the same target HR of 120 beats • min-1 the rate pressure product would be 19 800 (120 x 165).This represents a 10% increase in myocardial oxygen demand, which is equivalent to raising a person's HR by 7 to 10 beats • min-1 during lower body exercise, such as walking or cycling.

This example shows that it would be wise to reduce the target HR by 5-to-10 beats • min-1 during activities involving the arms. Furthermore, this may also prevent unnecessary muscular fatigue, as it has been shown that, for a variety of reasons, both healthy individuals and cardiac patients are metabolically less efficient during activities that involve the arms compared with the legs (Secher, 1993; Kang, et al., 1997; Buckley, et al., 1999a).

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