Increased Myocardial Oxygen Consumption and Contractile State Associated with Increased Heart Rate in Dogs

Abstract

The effects of increasing the frequency of contraction on myocardial oxygen consumption per minute (MVo 2 ) were examined in eight dogs using an isovolumic left ventricular preparation. MVo 2 was determined at two to four levels of heart rate in each animal. Peak wall stress was maintained constant in each animal so that changes in it would not influence the effects of heart rate on oxygen consumption per beat. As heart rate was increased, there was a highly significant linear increase in MVo 2 . Oxygen consumption per beat was shown to be a negative linear function of the reciprocal of heart rate. Thus, as heart rate increased there was a significant increase in oxygen consumption per beat; when basal oxygen consumption was subtracted from total oxygen consumption, there was a much larger increase in oxygen consumption per beat. Myocardial contractile state, defined as the maximum observed contractile element velocity at the lowest common level of wall stress, was significantly increased by increasing heart rate. The data suggest that the increased M"v"o 2 associated with augmented heart rate is secondary to augmentation of contractile state, as well as to the increase in stress development per minute. ADDITIONAL KEY WORDSmyocardial metabolism inotropic state stress-velocity relation peak myocardial wall stress isovolumic left ventricular contractions• There is general agreement that increasing the frequency of contraction augments the inotropic or contractile state of the myocardium, as reflected by a shift of the force-velocity relationship, both in isolated cardiac muscle (1-3) and the intact heart (4, 5). Recently, it has been shown that the contractile state of the heart is an important determinant of myocardial oxygen consumption (6-9), and therefore, it might be expected that increasing the frequency of cardiac contraction would be

Extent of Regulation of the Heart's Contractile State in the Conscious Dog by Alteration in the Frequency of Contraction

“…When these increases were compared statistically over similar ranges of tachycardia (100 beats/min increments), the augmentation of peak dP/dt, and dP/dt per P were greater (P < 0.01) in the anesthetized state than in the conscious state. Positive inotropic effects of this magnitude have also been observed by other investigators (8)(9)(10)(11) when the frequency of contraction of the intact hearts of anesthetized preparations was increased over the same range of frequencies as in the present study. In these previous studies, the control levels of peak dP/dt in anesthetized dogs were considerably less than those measured in the conscious animals in the present study, suggesting that the ventricular myocardium in the animals in these earlier studies was considerably depressed by the anesthetic agent.…”

“…Since the classical studies of Bowditch (1) and Woodworth (2), numerous investigators have documented the positive inotropic effect of increasing the frequency of contraction in excised myocardial strips (3)(4)(5), isolated hearts (6), and the in situ hearts of anesthetized preparations (7)(8)(9)(10)(11). Although it has been assumed that the positive inotropic effect of increasing frequency is applicable to the intact, conscious state, there are relatively few observations relative to this point.…”

Beneficial Effects of Heart Rate Reduction on Cardiac Mechanics and Energetics in Patients With Left Ventricular Dysfunction

“…40 In contrast, Boerth et al observed a slight increase in MVO2 per beat as HR was increased from 100 to 200 beats/min. 41 In the present study, MVO2 per beat remained unchanged Japanese Circulation Journal Vol.63, December 1999 whereas EW decreased during atrial pacing. Consequently, mechanical efficiency (EW/MVO2) decreased significantly regardless of the per beat or per min basis.…”

The Effect of Atrial Pacing-Induced Tachycardia on Left Ventricular Contractile Function in Conscious Newborn and Adult Sheep

“…The standard errors of the heart rates are given in Table 2. groups directly regarding the relative magnitude of the inotropic effect of a heart rate increase. The positive inotropic effect of atrial pacing has been demonstrated previously in adults by: (1) an increase of myocardial oxygen consumption per beat in an isovolumic adult dog preparation (4), and (2) from measurements of pre-ejection and ejection phase induces of contractility in conscious adult humans and dogs (2, 9, 17-19, 21, 28, 33). It is noteworthy that the % change in dP/dt for a given heart rate increase in our adult sheep (Fig.…”