Theoretical analysis of the isovolumic phase of left ventricular contraction in terms of cardiac muscle mechanics

Myocardial contractile function in survived neonatal piglets after cardiopulmonary bypass

Background

Hemodynamic function may be depressed in the early postoperative stages after cardiac surgery. The aim of this study was the analysis of the myocardial contractility in neonates after cardiopulmonary bypass (CPB) and mild hypothermia.

Methods

Three indices of left ventricular myocardial contractile function (dP/dt, (dP/dt)/P, and wall thickening) were studied up to 6 hours after CPB in neonatal piglets (CPB group; n = 4). The contractility data were analysed and then compared to the data of newborn piglets who also underwent median thoracotomy and instrumentation for the same time intervals but without CPB (non-CPB group; n = 3).

Results

Left ventricular dP/dt_max and (dP/dt_max)/P remained stable in CPB group, while dP/dt_max decreased in non-CPB group 5 hours postoperatively (1761 ± 205 mmHg/s at baseline vs. 1170 ± 205 mmHg/s after 5 h; p < 0.05). However, with regard to dP/dt_max and (dP/dt_max)/P there were no statistically significant differences between the two groups. Comparably, although myocardial thickening decreased in the non-CPB group the differences between the two groups were not statistically significant.

Conclusions

The myocardial contractile function in survived neonatal piglets remained stable 6 hours after cardiopulmonary bypass and mild hypothermia probably due to regional hypercontractility.

Validation of continuous wave Doppler-determined right ventricular peak positive and negative dP/dt: effect of right atrial pressure on measurement

OBJECTIVES

The present study aimed to validate the peak positive and negative values of the first derivative of right ventricular pressure (dP/dt) using Doppler echocardiography and to determine the impact of right atrial pressure on the measurements.

BACKGROUND

A pressure gradient between the right ventricle and the right atrium can be obtained by continuous wave Doppler-derived tricuspid regurgitant velocity using the simplified Bernoulli equation. If right atrial pressure fluctuation during systole and isovolumic diastole were small compared with right ventricular pressure changes, right ventricular pressure could be evaluated, and maximal positive and negative dP/dt could also be determined with Doppler echocardiography.

METHODS

We investigated 11 patients with a wide range of right atrial pressure with tricuspid regurgitation using simultaneous examination by Doppler ultrasound and catheterization. Hemodynamic conditions were altered by the Valsalva maneuver, and a total of 40 beats were analyzed.

RESULTS

There was good correlation between Doppler-derived and catheterization-derived peak positive dP/dt (y = 1.0x - 15.4, r = 0.98, n = 40), irrespective of the level of right atrial pressure. Doppler-derived peak negative dP/dt also showed good correlation with that determined by catheterization (y = 0.9x + 58.2, r = 0.93, n = 40). However, in patients with high right atrial pressure (v wave pressure > or = 10 mm Hg), Doppler-derived peak negative dP/dt tended to show lower values than those from catheterization measurements, except in patients with pulmonary hypertension.

CONCLUSIONS

We conclude that right ventricular dP/dt can be estimated by the Doppler method accurately and noninvasively. However, when right atrial pressure is relatively high compared with corresponding right ventricular pressure changes during isovolumic diastole, Doppler-derived peak negative dP/dt might underestimate catheter-derived measurements.

The effect of changes in afterload on systolic bulging

It has been previously shown that after acute coronary occlusion, the extent of systolic bulging is dependent on preload and the function of the remote nonischemic myocardium is influenced by the motion of the ischemic myocardium as well as by the loading conditions. To examine the isolated effects of changing afterload on the movement of acutely ischemic and nonischemic myocardium, seven open-chest, anesthetized dogs were paced from the left atrium at a rate of 100 beats/min after crushing of the sinus node. The pulmonary artery was perfused artificially and the left ventricular end-diastolic pressure (LVEDP) was carefully controlled with a right heart bypass system. Twenty minutes after occlusion of the left anterior descending artery, the peak left ventricular pressure (LVP) was adjusted to three levels (70, 90, and 110 mm Hg) by blood withdrawal or aortic constriction, while the LVEDP was kept constant (8.3 +/- 2.3 mm Hg). Segment length in the ischemic (IZ) and nonischemic zones (NZ) were measured with sonomicrometers and total, isovolumetric, and ejection systolic shortening (% delta L) were calculated. Changes in left ventricular minor-axis diameter were measured with diameter crystals. Increasing the peak LVP increased the LVP both at aortic valve opening and closing. To keep the LVEDP constant as peak LVP was increased, the cardiac output had to be decreased (p less than .0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of myocardial function in the horse. 1. Theoretical and technical considerations

The paper discusses the various parameters which are currently used in attempts to assess the contractile efficiency of the myocardium in various species. These procedures depend upon accurate recording of intracavity pressure. The response of a catheter-mounted microtransducer compared with a fluid-filled catheter manometer system is illustrated to show the advantage of the former in providing a true representation of pressure changes.

Assessment of myocardial function in the horse. 2. Experimental findings in resting horses

The paper describes observations on 8 horses at rest using catheter-mounted microtransducers to record intracavity pressures. In 5 of them records were obtained from both left and right sides and in 3 horses from the right heart only. Twelve indices were measured. Variations in left ventricular end-diastolic pressure seemed to influence the results on many of the parameters. It is suggested that left ventricular end-diastolic pressure itself may be a useful guide to left ventricular functional efficiency but more work is necessary to further evaluate these indices as possible measures of myocardial function in the horse.

Left ventricular contractile function in aortic stenosis evaluated by isovolumic and ejection phase indexes

This study is to reappraise the usefulness of isovolumic as compared to ejection phase indexes for detecting abnormal left ventricular contractile function patients with a common hemodynamic abnormality, namely, chronic left ventricular pressure overload. In 41 subjects with pure or predominant aortic stenosis left ventricular pressure measurements were performed by micromanometry. Single-plane left ventricular cineangiograms were carried out in the right anterior oblique (RAO) and the A-P position. The isovolumic contractile indexes we used in this study were peak measured velocity of shortening of the contractile elements (Vpm) and Vmax obtained from linear extrapolation of total pressure-velocity curves. The end-diastolic and end-systolic RAO cineventriculograms served for the calculation of the ejection phase parameters mean velocity of circumferential fiber shortening (VCF) and mean normalized systolic ejection rate (MNSER). Of the 41 patients, Vpm was depressed in 16 (39 per cent), Vmax in 17 (42 per cent), VCF in 12 (29 per cent), and MNSER in 14 (34 per cent). When the isovolumic and the ejection phase parameters were combined, 24 patients (59 per cent) were found to have at least one of the four contractile indexes below normal. In 26 of the 41 patients the isovolumic and the ejection phase indexes provided the same conclusions as to normality of left ventricular function. In contrast, 15 patients showed discordant isovolumic and ejection phase indexes. An increased left ventricular end-diastolic pressure was only inconsistently related to an abnormal left ventricular function because in 7 of 28 patients with an end-diastolic pressure above 14 mm. Hg all contractile indexes were normal. Furthermore, a normal end-diastolic pressure was present in three of 24 patients with depressed myocardial function. It is concluded that in chronic left ventricular pressure overload from aortic stenosis neither the isovolumic nor the ejection phase indexes are superior in sensitivity for assessing contractile function. In this clinical setting the combination of both types of indexes appears to be the most reliable way for identifying patients with depressed contractile function of theleft ventricle in the basal state.

Left ventricle contractile function in trained dogs with cardial hypertrophy

Eight mongrel dogs exercised for 8 weeks by treadmill running at 20 per cent incline 20 to 25 minutes twice daily, 4-5 days/week. Another eight dogs which were kept in the cages for a similar period served as controls. The exercise program was effective in inducing myocardial hypertrophy since the ratio left ventricular weight/body weight was significantly (P less than 0.001) higher in the trained dogs (5.04 g/kg) than in the sedentary animals (3.83 g/kg). In morphine-chloralose anesthesia the dogs were studied by left heart catherization and cineangiography at spontaneous heart rate (run I), at paced heart rate (run II), at paced heart rate following cardiac autonomic nervous blockade by bilateral vagotomy and the administration of propranolol (run III) and during acute pressure loading with methoxzmine at constant heart rate (run IV). Intergroup comparison yielded no significant difference in any hemodynamic or volumetric parameter throughout the entire study. However, with intragroup comparisons between run III and run IV a less significant increase in left ventricular end-diastolic pressure (from 5 to 15 mm Hg; P less than 0.05) was observed in the trained animals than in the control dogs (from 6 to 25 mm Hg; P less than 0.001). Left ventricular end-diastolic volume increased significantly only in the control dogs during acute pressure loading. Mean aortic pressure and left ventricular peak dP/dt increased to a similar extent in both groups. Since in the trained dogs the left ventricle encroaches less on the Frank-Starling mechanism than in normal animals for overcoming an acute pressure burden it is concluded that the development of hypertrophy concomitant with chronic exercise represents an adaptive mechanism with evidence of beneficial consequences for the intrinsic contractile function of the myocardium.

A comparison of methods of evaluating myocardial contractility

An investigation was carried out in dogs to determine how the acceleration of blood in the aorta (dV/dt), as a new index of myocardial contractility compared with existing indices and how they correlated with each other. It was found that the indices derived from velocity and flow of blood, dV/dt max and dQ/dt max, and from intraventricular pressure, dP/dt max, correlate well with each other but there is less agreement between them and the reciprocal of the Pre-Ejection Period, l/PEP. The ratio dP/dt max divided by I.P. correlated well with dQ/dt max divided by I.Q. and dQ/dt max divided by Q. max but not so well with dV/dt max divided by I.V. (instantaneous velocity) of dV/dt max divided by V. max and the ratios dQ/DT DIVIDED BY I.Q. and dQ/dt divided by Q max as well as l/PEP. In view of the lack of agreement of quantitative definition of myocardial contractility, the ratio dV/dt divided by V. max would have several practical advantages as an indicator of the inotropic state of the heart; these are that the probe used to establish descending aortic blood velocity does not require calibration, and the signal can be obtained by a relatively non-invasive technique that is suitable for patient care and yet agrees with other established indices of myocardial contractility.

A noninvasive technique for the determination of velocity of circumferential fiber shortening in man

Velocity of circumferential fiber shortening (V as determined by analysis of left ventricular cineangiocardiograms has been proposed as a measure of left ventricular myocardial contractility. In this study, a noninvasive ultrasonic technique was used to measure the rate of shortening of the left ventricular internal minor axis throughout ejection in 61 patients. These measurements permitted calculation of mean Vcr. In 23 patients with normal left ventricular function, mean V CF averaged 1.45 ± 0.08 circumferences/sec (mean SE). Tn contrast, mean V in 38 patients with impaired left ventricular function was significantly depressed (0.91 ±0.09 circumferences/see, P <0.001). In 17 patients, adequate resolution of septal and posterior wall thicknesses permitted measurement of midwali radius at 50-msec intervals throughout the cardiac cycle. These measurements permitted computation of instantaneous and peak V Values for peak V CF at the midwall were generally similar to those observed for mean V at the internal axis. In patients with normal left ventricular function, peak V CF averaged 1.58 ± 0.23 circumferences/sec. This study has demonstrated the feasibility of measuring the velocity of left ventricular circumferential fiber shortening in man by a noninvasive method. Initial studies indicate that this approach may prove useful in the detection and serial evaluation of left ventricular performance in patients with heart disease.