The use of indirect indices of myocardial oxygen consumption in evaluating angina pectoris

Regional myocardial oxygen consumption determined noninvasively in humans with [1-11C]acetate and dynamic positron tomography

Experimental studies of animals have previously demonstrated the validity of [1-11C]acetate as a tracer of oxidative metabolism for use with positron emission tomography. The present study was undertaken to define in normal human volunteers the relation between myocardial clearance kinetics of [1-11C]acetate, and the rate-pressure product as an index of myocardial oxygen consumption. Twenty-two studies were performed of 12 volunteers. The rate-pressure product was increased with continuous supine bicycle exercise in six studies. Of the 16 resting studies, seven were performed in the fasted state and nine following an oral glucose load, to define possible effects of substrate availability on the tracer-tissue kinetics. Myocardial tissue time-activity curves were biexponential. Clearance of activity was homogeneous throughout the myocardium. The rate constants k1, obtained from biexponential fitting, and kmono, obtained by monoexponential fitting of the initial linear portion of the time-activity curves, correlated well with the rate-pressure product. Although the correlation coefficient was higher for k1 than for kmono (0.95 vs. 0.91), analysis on a sectorial basis showed less regional variability in kmono. This suggests that kmono, which is more practical than k1 because it requires shorter acquisition times, may be more clinically and experimentally useful for detection of myocardial segments with abnormal oxygen consumption. Overall, changes in myocardial substrate supply were without significant effect on the relation between the rate constants (k1 and kmono) and the rate-pressure product, although a small decrease in kmono/rate-pressure product was observed following oral glucose by paired analysis in four subjects. It is concluded that [1-11C]acetate can be used for the noninvasive measurement of myocardial oxygen consumption in humans with positron emission tomography, and, thus, has clinical and experimental potential as a tool for the understanding and diagnosis of myocardial disease.

A beat-to-beat calculator for the diastolic pressure time index and the tension time index

We have developed a beat-to-beat calculator which can calculate in real-time the ratio of the diastolic pressure time index (DPTI), and the tension time index (TTI) as an index of the myocardial oxygen supply/demand balance. Physicians set up presumed value for the left ventricular endodiastolic pressure, a search area for the dicrotic notch, a threshold for the onset of the up-slope and the corresponding value of the calibration signal on the digital switches of the calculator. Next, the arterial pressure analog signal is input into the calculator. The calculator searches automatically for both the onset of the up-slope and the dicrotic notch. The arterial pressure curve is displayed beat-to-beat with the recognized onset and the dicrotic notch on the CRT to be confirmed by physicians. When physicians do not agree with the automatic recognition they can fit the automatic recognition to the observation. If the recognition of the onset is inadequate, the threshold can be re-adjusted to trigger the onset. If recognition of the dicrotic notch is inadequate, the physician can adjust the search-area. Therefore, physicians who operate the calculator can rely on the calculated DPTI/TTI. This calculator can continuously monitor the myocardial oxygen supply/demand balance in patients with acute myocardial infarction or just after open-heart surgery.

Effect of coronary vasodilating drug on myocardial work

The purpose of this paper is to show, by noninvasive measurements, previously undocumented effects on cardiac work of the nitrate coronary vasodilator Pentaerythritol Tetranitrate (Peritrate). The chew-and-swallow 40-mg tablets of this drug were given to 10 patients of type II AHA classification. The Diastolic Time Tension Index/Systolic Time Tension Index Ratio increased significantly from 1.15 (+/- 0.11 SEM) to 1.46 (+/- 0.13), p less than 0.05. The Double Product decreased significantly from 9813 (+/- 606) to 8336 (+/- 497), p less than 0.01. The Triple Product also decreased significantly from 3223 (+/- 235) to 2425 (+/- 206), p less than 0.01. The percent diastole of the R-R interval increased significantly from 65% (+/- 2.44) to 70% (+/- 1.73) p less than 0.05, while no significant change in heart rate occurred (63.2 bts/min to 62.4 bts/min). Correspondingly, there was a decrease of systolic time interval, from 0.33 second (+/- 0.2) to 0.29 second (+/- .01) p less than 0.01. A significant decrease in blood pressure was also noted. Systolic BP dropped from 155 (+/- 8.0) to 134 (+/- 7.2), p less than 0.01. Diastolic BP dropped from 89.2 (+/- 2.8) to 79.2 (+/- 2.2), p less than 0.01. This study shows that Peritrate produced significantly favorable changes in all the cited indicators of cardiac work.

Respiratory alkalemia during exercise reduces angina threshold

The effect of hyperventilation-induced alkalemia on angina threshold was evaluated in nine subjects who had a consistent pattern of chest pain and ST segment depression during exercise. For this study, the subjects performed graded bicycle exercise to angina during normal breathing and during hyperventilation. The maximum workload achieved was not significantly different between normal breathing and hyperventilation exercise. However, in five subjects who had arterial alkalemia during hyperventilation exercise (mean pH = 7.52), the heart rate X blood pressure product (HR X BP) at angina was 224 X 10(2) compared with 240 X 10(2) during normal breathing exercise (P less than 0.05). Four subjects appeared to hyperventilate, but were not alkalemic (mean pH = 7.40). Their HR X BP at angina was not significantly different between the two exercise periods (288 X 10(2) vs 284 X 10(2). In conclusion, the threshold for angina during exercise fell in the five patients in whom hyperventilation caused alkalemia. This finding suggests that the alkalemia interfered with myocardial oxygen supply.

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.

On-line computer processing of pressure data from cardiac catheterizations

A flexible program system for on-line analysis of pressure data from cardiac catheterizations is described. The programs are implemented on an IBM 1800 computer, equipped with remote oscilloscope/keyboard terminals. The current computer system can handle any combination of up to 4 pressure signals. During catheterization, measurement specifications (i.e. calibration levels or sites of pressure recordings) are entered via the keyboard immediately before each recording. As an option the whole expected measurement sequence may be stored on disk before the catheterization starts. This method will minimize the necessary interaction with the computer when the same catheterization procedure is used on several occasions. Changes from the predetermined scheme may, however, be undertaken before each recording to meet with unexpected events that may arise during the catheterization. After computer detection of calibration levels, the recorded signals are digitized during 20 seconds and analysed beat-by-beat. Calculated values are averaged and presented on the terminal oscilloscope in tabular and/or graphic form. The waveform analysis performed by the program system is validated in a statistical comparison between manually and automatically computed values.

Constancy of pressure-rate product in pacing-induced angina pectoris

The pressure-rate product during anginal pain produced by right atrial pacing was studied in 12 patients before, during, and after an angiotensin infusion sufficient to produce a significant rise in blood pressure. During the infusion the pain occurred at a significantly lower heart rate (P less than 0.001). However, the pressure-rate product was similar during anginal pain before and during the angiotensin-induced hypertension and after it wore off. Our studies support the concept that in each individual there is a constant level of myocardial oxygen consumption, as expressed by the pressure-rate product, at which anginal pain occurs.

Oral nitroglycerin as a prophylactic antianginal drug: clinical, physiologic, and statistical evidence of efficacy based on a three-phase experimental design

With the use of a three-phase experimental design, the efficacy of oral nitroglycerin has been evaluated in a total of 53 patients with documented angina pectoris due to coronary artery disease. The study were a double-blind, randomized, and cross-over comparison of controlled-release nitroglycerin (2.6 mg. tablets administered three times daily) and an indistinguishable placebo. Sixteen patients recorded anginal symptoms by the diary method over a 6 month trial of randomly sequenced 1 month periods of drug or placebo. In 15 patients, ST segments were monitored with a Holter dynamic electrocardiograph for periods of 10 to 12 hours under normal life style and evaluated by matching activities during periods of drug and placebo. In 22 patients, a multistage treadmill exercise test was conducted to an endpoint of anginal pain. The three phases of the investigation were run in succession; each phase was completed before the next one was begun. Oral nitroglycerin reduced the incidence and severity of anginal attacks by 47.2 and 49.4 per cent, respectively, and decreased the number of sublingual nitroglycerin tablets used by 51.1 per cent in comparison to placebo (p less than 0.001). Eleven of 16 patients (69 per cent) decreased their need for sublingual nitroglycerin by over 50 per cent. Based on a polynomial trend analysis over a period of 8 weeks, no tolerance to the therapeutic effects of the drug was found. With DCG monitoring, drug decreased the ST segment depression from 1.76 mm. on placebo to 1.12 mm, with a significant difference of 0.64 mm. (p less than 0.001). ST segment depression was decreased more than 0.5 mm. by drug in comparison to placebo in 10 of 15 patients (66 per cent). Larger depressions of the ST segment noted with placebo at heart rates greater than 80 beats per minute were prevented by administration of the drug. During treadmill exercise, drug delayed the onset of pain by 83 seconds (64 per cent) over placebo (p less than 0.001) and decreased the duration of pain by 70 seconds (49 per cent) in comparison to placebo (p less than 0.001). Drug did not affect heart rate or systolic blood pressure at rest or after exercise, as well as rate-pressure product for production of angina following exercise (p less than 0.05). There was no side effects reported caused by the drug. The data demonstrate that oral nitroglycerin, given as controlled-release tablets, was absorbed from the gastrointestinal tract in quantities sufficient to provide statistically significant clinical improvement of angina pectoris.

Haemodynamic effects of intravenous verapamil at rest and during exercise in subjectively healthy middle-aged men

Verapamil, 0.1 mg/kg body wt, was injected i.v. over 2 minutes in 8 subjectively healthy middle-aged men, followed by a continuous infusion of 0.007 mg/kg body wt per minute. Prior to the injection several of the subjects had raised pulmonary or systemic arterial pressures. At rest, the central pressures increased slightly, which was taken as a sign of a moderate negative inotropic effect, but there was no change in pre-ejection period or maximal dp/dt of the aortic pressure. The heart rate increased and there was a small decrease in systemic arterial pressure, probably due to a fall of systemic vascular resistance. The PQ time was prolonged. During exercise, with its positive inotropic stimulation, the moderate negative inotropic effect of verapamil disappeared, whereas the increase in heart rate and decrease in aortic pressures persisted. Some variables that reflected the oxygen demand of the heart decreased. The slight negative inotropic effect does not appear to be a particular contraindication to the use of verapamil, but it should be employed cautiously in conditions with a compensatory rise in systemic vascular resistance, or if atrioventricular conduction is impaired.

Hemodynamic predictors of myocardial oxygen consumption during static and dynamic exercise

Hemodynamic predictors of myocardial oxygen consumption (MVO 2 ) during static and dynamic exercise were examined in ten normal subjects. Studies were done under the following circumstances: 1) during upright bicycle exercise at an average heart rate of 147 beats/min, 2) during static exercise with an isometric load in the left hand equal to 17% of the maximal voluntary contraction (MVC), and 3) during combined dynamic exercise (average heart rate 147 beats/min) and static exercise using 17% MVC of the left hand. Mean myocardial blood flow (MBF) was 181 ml/100 gm LV/min during dynamic exercise, 98 ml/100 gm LV/min during static exercise, and 201 ml/100 gm LV/min during combined static and dynamic exercise. Addition of a static load to the dynamic load resulted in a higher blood pressure (average 12 mm Hg), MVO 2 and MBF than during dynamic exercise alone. MVO 2 correlated best with products of heart rate and blood pressure regardless of whether the blood pressure was obtained by a central aortic catheter ( r = 0.88) or by a blood pressure cuff ( r = 0.85).

When the current data were combined with previous data, 82 determinations of MVO 2 and MBF in 29 normal subjects during several levels of upright exercise were available for analysis. Forty-four determinations were done during dynamic upright exercise, 18 during exercise after propranolol, ten during combined static and dynamic work, and ten during static work alone. MVO 2 correlated best with the product of heart rate and blood pressure ( r = 0.86). Heart rate alone correlated better with MVO 2 ( r = 0.82) than did the tension time index ( r = 0.65) or the product of systolic blood pressure, heart rate, and ejection time ( r = 0.68). The readily measured variables of heart rate and of heart rate x blood pressure correlated well with MVO 2 in normal young men during exercise under a wide variety of circumstances.