The spherical dynamics of the heart (myocardial tension, oxygen consumption, coronary blood flow and efficiency)

The common characteristics and mutual effects of heart failure and atrial fibrillation: initiation, progression, and outcome of the two aging-related heart diseases

Atrial fibrillation (AF) and heart failure (HF) are common chronic diseases noted in humans. AF and HF share several risk factors, such as age, hypertension, obesity, diabetes, and dyslipidemia. They can interact with each other, while both their morbidity and mortality have been considerably increased. And AF and HF often occur together, suggesting a strong association between the two. However, the underlying mechanism behind this association is not well understood. Among them, aging is the most significant common risk factor, which represents an aging heart and is characterized by fibrosis and decreased number of cardiomyocytes, known as senescence-related cardiac remodeling for both atria and ventricles. Finally, it is proposed that cardiac remodeling is the key link between AF and HF.

A mathematical model for estimating muscle tension in vivo during esophageal bolus transport

We present a model of esophageal wall muscle mechanics during bolus transport with which the active and "passive" components of circular muscle tension are separately extracted from concurrent manometric and videofluoroscopic data. Local differential equations of motion are integrated across the esophageal wall to yield global equations of equilibrium which relate total tension within the esophageal wall to intraluminal pressure and wall geometry. To quantify the "passive" (i.e. inactive) length-tension relationships, the model equations are applied to a region of the esophagus in which active muscle contraction is physiologically inhibited. Combining the global equations with space-time-resolved intraluminal pressure measured manometrically and videofluoroscopic geometry data, the passive model is used to separate active and "passive" components of esophageal muscle tension during bolus transport. The model is of general applicability to probe basic muscle mechanics including the space-time stimulation of circular muscle, the relationship between longitudinal muscle tension and longitudinal muscle shortening, and the contribution of the collagen matrix surrounding muscle fibers to passive tension during normal human esophageal bolus transport and in pathology. Example calculations of normal esophageal function are given where active tone is found to extend only over a short intrabolus segment near the bolus tail and segmental regions of active muscle squeeze are demonstrated.

LEFT VENTRICULAR OXYGEN CONSUMPTION, BLOOD FLOW, AND PERFORMANCE IN MITRAL STENOSIS

The restriction imposed by mitral stenosis on the cardiac output response to exercise provides a unique opportunity in man to evaluate determinants of coronary flow and myocardial oxygen consumption, since one or several variables may be limited. Coronary and systemic hemodynamic patterns were therefore evaluated in 12 patients with mitral stenosis and compared with normal subjects.

While cardiac output and left ventricular work were lower than normal at rest, coronary flow and myocardial oxygen consumption did not differ from normal. During exercise, despite subnormal cardiac output and left ventricular work responses, coronary blood flow and myocardial oxygen consumption increased normally in the group of patients in whom arterial pressure rose. However, coronary flow fell in four patients who experienced exertional hypotension, while myocardial oxygen consumption was reduced or unchanged. In both groups, myocardial oxygen extraction was above normal at rest and rose further during exercise. Changes in myocardial oxygen consumption followed alterations in the pressure-time index, but the excessive heart rate response to exercise resulted in a greater oxygen expenditure per unit of tension developed per beat than that found in normal subjects.

It is concluded that the primary determinant of coronary blood flow in mitral stenosis is the arterial perfusion pressure while myocardial oxygen consumption is primarily dependent on the pressure-time index. In addition, heart rates inappropriately high for a given state result in energy expenditures above that required for maintenance of pressure and cardiac output.

Cardiac efficiency

Efficiency is defined as the ratio of the energy delivered by a system to the energy supplied to it. Depending on the particular question being addressed, there exist a plethora of definitions of efficiency in medical texts, thus hampering their comparison. If only the ventricular work seen by the arterial system is under investigation, pressure-volume work will serve as a useful numerator. If, on the other hand, external and internal work together, i.e. the total mechanical work, is of interest, the pressure-volume area might be employed. Total myocardial oxygen consumption (MVO2) will be a useful denominator in the case of aerobic energy production. The MVO2 for the unloaded contraction must be assessed if, as in other energy transfer systems, net efficiency is to be addressed. If even smaller steps in the chain of energy transfer are to be investigated MVO2 for the arrested heart must be assessed. With an appropriate therapy, hemodynamic determinants can be varied, to improve cardiac efficiency. Nonetheless, measurement of all variables necessary for the calculation of efficiency remains a challenge, in particular in the clinical setting. Separation of the direct effects of drugs on efficiency is even more difficult, since hemodynamic conditions can hardly be controlled throughout the observation period, and changes in efficiency might be secondary to changes in hemodynamics. Whether the heart by itself employs mechanisms to improve its efficiency is still a matter of discussion: there is evidence that when oxygen supply decreases, the heart can switch from one substrate to a less costly one, or possibly can improve efficiency through better use of oxygen. Moreover, the heart seems to "sense" an even more decreased oxygen supply and reduce function in response. Myocardial stunning could be regarded as a protective mechanism as well, with function remaining depressed and the oxygen supply being normal or close to normal. One may conclude from the decreased efficiency that the excess oxygen consumption is used up for repair processes. The improved efficiency found in hypertrophied hearts represents another adaptive process. The underlying mechanism is unclear: a shift towards isomyosin V3 or some undefined shift in metabolic pathway is discussed. It is also still a moot question towards which objective the efficiency of the heart is adjusted. It has been described that under physiologic conditions, the efficiency of both the left and the right ventricle ought to be maximized.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative analysis of regional systolic function with left ventricular aneurysm

Left ventricular aneurysm (LVA) remains a poorly understood entity, often resulting in congestive failure that is not consistently improved by standard resection with linear closure. Although other surgical approaches have been proposed, current methods to assess their effect on left ventricular function are not adequate. The purpose of the present study was to quantitatively define regional systolic function in patients with LVA and to assess acute changes in regional function after standard repair. Seven patients underwent resection of an anteroapical LVA. Intraoperative two-dimensional echocardiography was performed off cardiopulmonary bypass immediately before and after resection. In all patients, short-axis views at the papillary muscle (apex) level showed anteroseptal paradox and distorted geometry, whereas at the mitral valve (base), symmetric wall motion and geometry were preserved. Videotaped echo images were divided into octants by a floating axis fitted to internal landmarks. Myocardial area and midwall perimeter were obtained for each octant, and wall thickness was calculated at end diastole (ED), isovolumetric systole (IS), and end systole (ES). Wall thickening (delta t) for each segment was calculated as the percent increase in thickness from ED and averaged for all seven patients. At the apex level before resection, isovolumetric thinning occurred in the aneurysm as well as bordering segments, with delta t ranging from -17 +/- 5% (+/- SEM) in the anteroseptal segment to 12 +/- 6% posterolaterally (p less than 0.05). The isovolumetric bulge was followed by late-systolic thickening, however, with delta t ranging from 13 +/- 7% to 27 +/- 8% (NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Reconsideration of normalization of Emax for heart size

We previously proposed Emax Vd as a normalized form of Emax for heart size relatively independent of wall volume Vm, where Emax is the slope of the end-systolic pressure-volume line and Vd is its volume axis intercept. When Emax Vd remains constant, average circumferential stress for a specified average circumferential strain in the ventricular wall also remains relatively constant, despite changes in Vd/Vm around its normal value. Because accurate determination of Vd is difficult and stress for a given Emax Vd changes slightly with Vd/Vm, we investigated whether Vd could be replaced with Vm in a normalized Emax in this analysis. As the result, we obtained a function of Vd/Vm as the coefficient by which to multiply Emax Vd or Emax Vm to yield stress for a specified strain. Using this coefficient, one can easily calculate stress for any strain from Emax, Vd, and Vm in order to compare myocardial contractility among left ventricles of different sizes. The present study confirms the importance of Vd as an indispensable reference volume for normalization of Emax, as well as the low sensitivity of Emax Vd as a normalized Emax to changes in Vd/Vm. Only when Vd/Vm remains constant is Emax Vm proportional to Emax Vd and can replace Emax Vd.

Effects of denopamine (TA-064), a new positive inotropic agent, on myocardial oxygen consumption and left ventricular dimension in anesthetized dogs

We compared the effects of denopamine (TA-064) and isoproterenol on hemodynamics, myocardial oxygen consumption and the left ventricular (LV) dimension in halothane-N2O anesthetized dogs. Denopamine (0.25-1 micrograms/kg/min, i.v., infusion X 15 min) produced a maximum increase in LV dp/dtmax by 64% of the control, without affecting aortic pressure significantly. Doses of isoproterenol (0.01-0.04 micrograms/kg/min, i.v., infusion X 15 min) were selected to produce a positive inotropic action similar to that of denopamine. Denopamine produced significantly less increasing effects in heart rate, cardiac output and myocardial oxygen consumption and had more reducing effects in LV internal diameter than isoproterenol, while isoproterenol tended to produce a more potent increase in coronary blood flow, but a smaller decrease in LV end-diastolic pressure than denopamine. PQ interval was similarly reduced. Denopamine caused no substantial increase in myocardial oxygen consumption at a lower dose, at which LV dp/dtmax was significantly increased. A weak effect of denopamine on myocardial oxygen consumption may result partly from a weak positive chronotropic effect and partly from a reduction of preload and cardiac size.

Double-blind randomized crossover trial of verapamil and propranolol in chronic stable angina

Propranolol (240 mg daily) and verapamil (360 mg daily) were objectively compared for their respective efficacy in the treatment of chronic stable angina pectoris. Twenty-two patients were studied in a randomized placebo controlled, double-blind crossover trial with 4 weeks on each active drug treatment. Multistage treadmill exercise with computer-assisted ECG analysis was performed after 2 weeks on placebo and at the end of each 4-week active drug treatment. The mean exercise time to produce angina was 5.5 minutes (SEM +/- 0.4 minutes) on placebo and this increased to 7.8 (+/- 0.5) minutes on propranolol and 9.1 (+/- 0.5) minutes on verapamil. The improvement in exercise time of verapamil over propranolol was statistically significant (p less than 0.01). Ten patients became free of angina with verapamil and four with propranolol. Resting and maximal exercise heart rates were significantly reduced by propranolol; verapamil did not reduce the maximal heart rate but reduced the resting heart rate slightly. However, the heart rate increase per minute of exercise was significantly diminished (p less than 0.001). ST segment changes showed improvement with both drugs despite marked differences in heart rate profile. The overall efficacy of the slow calcium channel blocker, verapamil, compares favorably with that of a standard beta-adrenoreceptor blocking drug (propranolol), thus providing a new perspective in the management of angina pectoris. These two classes of drugs seem to act by different mechanisms and it is suggested that if patients are resistant or intolerant to one of these drugs, the other can be used to yield a beneficial response.

Improvement of resting myocardial asynergy with cessation of upright bicycle exercise

Exercise generally aggravates ischemic myocardial dysfunction, presumably by increasing tissue oxygen demand out of proportion to the increase in supply. Nevertheless, resting left ventricular (LV) wall motion abnormalities can improve dramatically after upright exercise. To investigate this "paradoxical" phenomenon, we performed upright bicycle exercise equilibrium radionuclide ventriculography in 93 patients with angiographic coronary artery disease. Immediately after exercise, LV end-diastolic volume was similar to the resting level (1 +/- 22% of rest value), but end-systolic volume (ESV) was significantly below (p less than 0.05) that at rest (-11 +/- 32%) and LV ejection fraction increased significantly compared with rest (0.57 +/- 0.16 vs 0.51 +/- 0.13, p less than 0.05). Improvement in resting myocardial asynergy was frequent (115 of 330 abnormal segments), and was observed more commonly in patients without pathologic Q waves and in segments manifesting mild rather than severe asynergy. In 60 additional patients with resting asynergy who were also studied after nitroglycerin (NTG), there was 89% concordance of wall motion response in asynergic segments after exercise and NTG: 71 of 85 segments manifesting improvement with NTG also improved after exercise, and 157 of 172 segments without improvement with NTG also failed to improve after exercise. Despite the similar wall motion response, the mechanism of improvement is probably different from that produced by NTG. With NTG, preload (end-diastolic volume) and afterload (systolic blood pressure) were significantly lower than their resting control levels (p less than 0.05). These changes did not occur after exercise. Instead, an isolated, significant reduction in ESV was noted. These data support the hypothesis that catecholamine stimulation is responsible for paradoxical wall motion improvement after upright exercise.

Validity of myocardial oxygen consumption parameters

The purpose of this study was to examine any reported indices for estimating myocardial oxygen consumption (MVO2) under uniform experimental conditions at maximal variation of hemodynamics and MVO2. One hundred sixty-two steady states were analyzed in 10 closed-chest dog experiments. Myocardial blood flow was directly measured by a different pressure catheter in the coronary sinus. The indirect values of MVO2 calculated from 24 indices were compared with directly measured MVO2. Throughout a wide range of hemodynamic states, the best correlate with MVO2 was found to be the additive parameter Et (r = 0.96). Any indices that do not incorporate potentially important changes of MVO2 related to both myocardial contractility and ventricular dimensions show unsatisfactory correlations with MVO2 at extreme changes of hemodynamics. Tension-time index (TTI) correlates poorly with MVO2 (r = 0.63). This result is due to the neglect of contractility. Pressure-heart rate product (P X HR) correlates with MVO2 with r = 0.86. Better results for TTI and P X HR, as reported in previous works, are reproducible by dividing our data into two groups of different inotropic states. At normal and moderate inotropic stimulation the correlation for TTI rises to r = 0.96, and for P X HR to r = 0.91. This augmentation is to be referred to the close relationship (r = 0.92) of peak ventricular pressure to maximum rate of pressure rise in this group. The additive parameter E1 is the best, both at moderate (r = 0.97) and at maximal inotropic stimulation (r = 0.87), and is to be preferred for indirect estimation of MVO2. Results are discussed with regard to the clinical application of MVO2 indices.

Heat production and oxygen consumption of the isolated rabbit heart: their relation to mechanical function

A method is described for simultaneous determination of the heat production and the oxygen consumption of the isolated, isovolumetrically beating rabbit heart. The perfusion of the heart was performed via the aorta at a constant flow rate with carbogen saturated Tyrode's solution at a temperature of 37.0 degrees C. Heart function was varied by stepwise augmentation of the left intraventricular volume (LVV) by means of a balloon catheter. The following mechanical parameter of heart function were determined: enddiastolic pressure (EDP), peak pressure (PP), developed pressure (DP), max. contraction and relaxation velocity (dP/dt-A and dP/dt-B), enddiastolic tension (Tens-EDP), peak tension (Tens-PP), developed tension (Tens-DP), and circumferential tension (Tens-Cir). DP, dP/dt-A and dP/dt-R showed a maximum response to changes of the LVV at 2.0 ml LVV and 19.3 mm Hg EDP. Heat production (H) and oxygen consumption (Q) were correlated closely to mechanical function and to each other (r = 0.89, n = 8). The ratio H/Q was 4.9 cal/ml O2 and remained constant during the experiment. The myocardial energy consumption was significantly correlated toall contraction parameters with the best fit to DP and Tens-Cir (r = 0.934 and 0.933 resp.). On the basis of the calculated mean regression lines, the function-independent and the function-dependent energy consumption were calculated.

Clinical and circulatory effects of isosorbide dinitrate. Comparison with nitroglycerin

In order to resolve current controversies on isosorbide dinitrate (ISDN), we employed a particularly sensitive testing protocol to evaluate effects of sublingual ISDN and nitroglycerin on the exercise capacity of patients with angina. Ten minutes after ISDN 21 of 23 patients exercised longer (average 2.7 minutes, P < 0.001) than after placebo. Benefit was evident in only a minority of patients tested one hour and in none tested two hours after either ISDN or nitroglycerin. A given amount of exercise resulted in lower mean blood pressure (average 13 mm Hg, P < 0.001), higher heart rate (average 10 beats/min, P < 0.001), and shorter ejection time (average 0.04 second, P < 0.001) after ISDN. Similar changes were seen after nitroglycerin. The product of blood pressure, heart rate, and ejection time, an index of myocardial O 2 consumption, was unchanged at angina after ISDN or nitroglycerin despite the increased exercise capacity, suggesting that clinical improvement after these drugs may be due to circulatory changes causing decreased myocardial O 2 demand. We conclude that sublingual ISDN closely resembles nitroglycerin in its alteration of circulatory responses to exercise and in the duration of the resultant improvement in exercise capacity.

Augmentation of myocardial oxygen consumption in hyperthyroid cats

To clarify the effects of hyperthyroidism on myocardial oxygen consumption (VO 2 ), a polarographic method was employed to compare the VO 2 of isolated papillary muscles from 13 normal euthyroid cats with that of 11 hyperthyroid cats. Basal VO 2 was greater in the hyperthyroid group (3.03±0.20 vs. 2.36±0.19 SE filter-ing dry wrr 1 · hour 1 , P < 0.05). In muscles studied under afterloaded isotonic conditions, hyperthyroidism shifted the forcevelocity curve upward and to the right, with an increase in both extent and velocity of shortening at equivalent loads. These changes in myocardial behavior in hyperthyroidism were associated with an increase in myocardial VO 2 . Isometrically contracting muscles from hyperthyroid animals demonstrated significant increases in both developed tension (6.3±0.7 vs. 4.7±0.4 g/mm 2 , P < 0.05) and rate of tension development (32.6±3.5 vs. 19.4±1.5 g/mm 2 · second- 1 , P < 0.01), as compared to the euthyroid group. Myocardial VO 2 , expressed per g/mm 2 isometric developed tension, was significantly greater in the hyperthyroid group (0.641±0.09 vs. 0.42±0.04 milliter · mg dry wt- 1 · beat-, P < 0.02). Thus, experimental hyperthyroidism augments myocardial VO 2 whether measured in resting or contracting cardiac muscle. This increase can be attributed, at least in part, to $ie altered contractile function of the heart in hyperthyroidism.

The influence of changes in blood volume on angina pectoris. A study of the effect of phlebotomy

Angina pectoris was precipitated in 15 patients with coronary artery disease by the technic of right atrial pacing. This was accompanied by hemodynamic evidence of impaired left ventricular function. In eight patients, while pacing was continued, a phlebotomy averaging 276 ml was carried out and in all but one patient angina was relieved and ventricular function returned to normal. Phlebotomy was accompanied by a decline in cardiac index (9.6%) and left ventricular filling pressure, but no change in brachial artery mean pressure, tension-time index, or dp/dt. With reinfusion of blood, angina returned in all but one patient and there was a rise in left ventricular end-diastolic pressure but no change in cardiac index, brachial artery pressure, tension-time index, or dp/dt. In seven patients alterations of blood volume were carried out between three 9-minute periods of atrial pacing and similar observations were obtained. The observations indicate that phlebotomy exerted its beneficial effect through a reduction in left ventricular volume, and it is suggested that this may be the primary mode of action of nitroglycerin.

he effect of acute digitalization on the hemodynamic response to exercise in coronary artery disease

Hemodynamic observations are made at rest and during exercise before and following the administration of 0.5 mg of ouabain in patients with coronary artery disease and in normal subjects. Patients with coronary artery disease and exertional angina showed evidence of reversible cardiac failure during exercise. The patients with coronary artery disease who did not have angina and the normal subjects had normal hemodynamic responses to exercise. Acute digitalization did not prevent angina in any of the symptomatic patients, although there was hemodynamic evidence of improved ventricular performance. In the patients with coronary disease without angina minor hemodynamic changes at rest and during exercise were suggestive of improved left ventricular function.

Effect of ouabain on the energy output of rabbit cardiac muscle

The effect of ouabain, 0.3 µg/ml, on the energy output of rabbit papillary muscles has been examined by a myothermic technique. The experiments were conducted at two temperature ranges, 19.2° to 22.8°C and 29.0° to 32.2°C, and both isometric and afterloaded isotonic contractions were studied. Temperature differences alone caused pronounced physiological changes, the higher temperature being associated with lower tension-independent heat and markedly higher active efficiency, external work /(external work + active heat production). The heat versus tension curve was rectilinear at higher temperatures but showed upward curvature at lower temperatures. At 19.2° to 22.8°C, ouabain increased the tension-independent heat by 23%, maximum tension development by 23%, and mean work output by 39%. Ouabain did not significantly alter the slope of the heat versus tension curve and increased mean efficiency only slightly. At 29° to 32.2°C, ouabain did not cause any significant change in the slope of the heat versus tension curve or in mean muscle efficiency. Ouabain produced significant increases in maximum tension development, mean work output, and the tension-independent heat. The effects of ouabain at the higher temperature were examined at two different calcium levels, 2.5 and 1.25 mM. In the isometric studies the effects of ouabain were independent of the calcium level, and the calcium level itself had no significant effect on the heat-tension relationship. In the isotonic studies, ouabain increased work output but more so at the 2.5 mM calcium level. Ouabain did not affect mechanical efficiency at either calcium level but muscle efficiency was higher at the 2.5 mM calcium level. It is concluded that any effects of the cardiac glycosides on energy expenditure are consequences of their inotropic actions and do not represent changes in the energy cost of contraction.

The effect of calcium depletion upon the tension-independent component of cardiac heat production

Rabbit papillary muscle has been exposed to calcium concentrations ranging from 2.5 mM to zero. Its mechanical and electrical activity has been monitored and its heat production measured using a myothermic technique. Calcium depletion decreased the magnitude of the tension-independent heat per contraction from a mean of 0.45 mcal/g muscle to 0.31 mcal/g muscle at room temperature (18 degrees to 22 degrees C). Calcium-chelating agents did not abolish action potential conduction under the experimental conditions used but they further reduced the magnitude of the tension-independent heat. Raising the temperature from room level to 32 degrees C decreased the tension-independent heat from a mean of 0.52 to a mean of 0.24 mcal/g muscle. Calcium depletion at 32 degrees C further decreased this heat and it was calculated that the energy now liberated in activating the muscle was about 2% of the total energy normally liberated in the working heart. The results are interpreted in terms of current biochemical and myothermic data.

Hemodynamic effects of artificial pacing in complete heart block complicating acute myocardial infarction

The hemodynamic changes associated with complete heart block complicating acute myocardial infarction and the effects of artificial pacing at various rates on these measurements were studied in 13 patients. Studies were performed on the first day of complete block in all patients and were repeated on subsequent days in six.

With two exceptions cardiac output was increased by pacing. In patients paced at progressively increasing ventricular rates, the rate associated with the maximal cardiac output exceeded 100/min, with one exception. However, an adequate response usually occurred between 80 and 90/min. Stroke volume fell as the rate was increased with pacing in most patients, but rose in some, suggesting improved myocardial performance. Systemic blood pressure increased, and the change paralleled cardiac output changes fairly closely as pacing rates were increased. The blood pressure did not, however, reflect reliably the absolute level of cardiac output either in heart block or after the return of sinus rhythm. Systemic vascular resistance was high in most patients with low cardiac outputs and fell with pacing at maximal output rates. There was no consistent change in mean right atrial or mean pulmonary arterial pressure with pacing during the period of study. Tension time index was low in heart block, and increased considerably with pacing, implying a corresponding increase in myocardial oxygen requirements.

Pacing improved signs of depressed mental function and poor skin circulation which were invariably associated with a severe reduction in cardiac output. These signs were found to be superior to heart rate and blood pressure in clinical evaluation of the adequacy of the cardiac output in heart block and the response to pacing.

Mechanochemistry of cardiac muscle. II. The isotonic contraction

The aim of this study was to evaluate utilization of chemical energy in relation to myocardial mechanics in variably afterloaded contractions of cardiac muscle by determining total energy utilization (∼P) in the absence of energy production. Right ventricular papillary muscles of cats were equilibrated at 26°C in a myograph in Krebs' solution while contracting isometrically (12/min). Following treatment with iodoacetate and N 2 to inhibit completely ATP production from glycolytic and aerobic metabolism, the muscles were stimulated to contract isotonically 20 to 75 times with varying after loads. They were then rapidly frozen in liquid N 2 -cooled isopentane, and concentrations of ATP, creatine phosphate, inorganic phosphate and creatine were measured.

The efficiency of energy utilization for the performance of internal work was 0.0067 µmoles ∼P/g-cm of work and for external work was 0.0031 µmoles ∼P/g-cm. In addition, resting energy utilization was 0.662 µmoles/g/min and activation energy was estimated to be 0.040 µmoles/g/contraction. These findings provide a demonstration of the Fenn effect in cardiac muscle and explain the well-known discrepancy in energy cost when cardiac work is increased by increasing pressure load as opposed to increasing volume load.

Control of myocardial oxygen consumption: relative influence of contractile state and tension development

Myocardial oxygen consumption was measured in 11 anesthetized, open-chest dogs in order to compare in the same heart the relative influence on oxygen usage of tension development and the contractile or inotropic state, as reflected in V(max.) the maximum velocity of shortening of the unloaded contractile elements. The isovolumetrically contracting left ventricle was studied with left ventricular volume, heart rate, and systemic perfusion rate controlled. Wall tension, contractile element velocity, and V(max) were calculated. Peak developed tension was increased at a constant V(max) by increasing ventricular volume, and the effect on oxygen consumption was determined. Oxygen utilization was then redetermined at an increased V(max) but at a constant peak developed tension by infusing norepinephrine (0.76 to 7.6 mug/min) and decreasing ventricular volume to match the tension existing before norepinephrine infusion. Oxygen consumption consistently increased with increases in both developed tension and V(max) with the following multiple regression equation relating these variables: myocardial oxygen consumption (mul/beat per 100 g in LV) = K + 0.25 peak developed tension (g/cm(2)) + 1.43 V(max) (cm/sec). These data indicate that the oxygen cost of augmentation of contractility is substantial, can be independent of any change in fiber shortening, and is similar in order of magnitude to the effect of alterations in tension development

Myocardial oxygen consumption in acute experimental cardiac depression

Myocardial oxygen consumption (MV · o 2 ) during drug-induced cardiac depression was measured in 8 anesthetized, open chest dogs in which myocardial wall tension was controlled. The right side of the heart was bypassed and myocardial contractility was reduced with procaine HCl, propranolol, or pronethalol. MV · o 2 consistently fell during cardiac depression (avg = 1.52 ml/min per 100 g left ventricle or - 11.6%). These reductions occurred despite small increases in developed tension. Changes in the tension-time index, contractile element work, and contractile element power did not correlate invariably with ΔMV · o 2 , while reductions in velocity of the contractile elements at isotension, maximum left ventricular dp/dt, and the extent of shortening of the contractile elements and circumferential fibers were associated in every experiment with the reductions in MV · o 2 . The finding that negative inotropic influences are associated with a reduction in myocardial energy utilization, when considered with earlier observations showing that positive inotropic influences induce an augmentation of MV · o 2 , provides evidence that the inotropic state and its mechanical correlates are coupled with myocardial energy utilization by a mechanism that is independent of tension development.

A comparative study of nitroglycerin and propranolol

The action of sublingual nitroglycerin (0.6 mg) and intravenous propranolol (0.1 mg/kg) on shortening of right ventricular fibers was investigated in patients with and without coronary artery disease. This was accomplished with a newly devised strain gauge catheter. Hemodynamic parameters and changes in the oxidation-reduction potential of heart muscle were determined. Nitroglycerin results in decrease in shortening and velocity of shortening of ventricular fibers, heart rate, left ventricular end-diastolic and systemic pressure, dp/dt, tension-time index, stroke index, peripheral resistance, and left ventricular minute work in normal and arteriosclerotic subjects. Intravenous propranolol leads to a fall in velocity of shortening of myocardial fibers, heart rate, dp/dt, stroke index, and left ventricular minute work. However, there is an increase in tension-time index and peripheral resistance with no change in systemic pressure. Left ventricular end-diastolic pressure rises significantly in arteriosclerotic patients. The myocardial oxidation-reduction potential increases after nitroglycerin whereas it falls after propranolol in normal and arteriosclerotic patients.

Energetics of cardiac contractions

1. The average resting heat production of a muscle under zero tension is 24.8 mcal/g muscle .min at 20 degrees C. In the majority of muscles examined the resting heat production increases when the resting tension and muscle length are increased.2. The relation between actively developed tension and heat produced is similar to that existing in skeletal muscle. The plot of heat against developed tension can be obtained either by altering muscle length or by varying the stimulus frequency.3. The mean maximum total efficiency work/(work + heat) in the work experiments was 11.6%. The total energy produced (work + heat) correlates with the load rather than with the work done.4. In isotonic contractions more heat is liberated than the heat versus tension plot predicts. This extra heat is load-dependent.

Studies on digitalis. XVI. Effects on myocardial oxygen consumption

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