Clinical assessment of diastolic function

Changes in diastolic function during development and correction of chronic LV volume overload produced by mitral regurgitation

BACKGROUND

Mitral regurgitation (MR) causes an augmentation in left ventricular (LV) diastolic function, increasing early diastolic filling rate and decreasing LV stiffness. Whether these changes in diastolic function persist, return to normal, or become abnormal after mitral valve replacement (MVR) is unknown.

METHODS AND RESULTS

Simultaneous LV echocardiography and catheterization studies were performed in six dogs in the baseline state (baseline), 3 months after creation of MR (chronic MR), and 3 months after MVR. Chronic MR caused LV dilation (end-diastolic dimension increased from 4.5 +/- 0.1 cm in baseline to 5.8 +/- 0.1 cm in chronic MR, p < 0.05) and eccentric LV hypertrophy (LV-to-body weight ratio increased from 3.6 +/- 0.2 g/kg in baseline to 4.9 +/- 0.4 g/kg in chronic MR, p < 0.05). Chronic MR caused an increase in LV early diastolic filling rate (peak rate of increase in minor-axis dimension increased from 11 +/- 1 cm/sec in baseline to 18 +/- 1 cm/sec in chronic MR, p < 0.05), did not change the time constant of myocardial relaxation (tau was 31 +/- 4 msec in baseline and 30 +/- 2 msec in chronic MR), and caused a decrease in the modulus of regional chamber stiffness from 7.7 +/- 1.2 in baseline to 2.4 +/- 0.03 in chronic MR, p < 0.05. MVR caused the resolution of LV dilation (end-diastolic dimension returned to normal [4.8 +/- 0.2 cm]), but three months after MVR, regression of LV hypertrophy was incomplete (LV-to-body weight ratio remained elevated [4.4 +/- 0.5 g/kg]). After MVR, LV early diastolic filling rate (8 +/- 1 cm/sec), the relaxation time constant (31 +/- 2 msec), chamber stiffness (7.1 +/- 1.8), myocardial stiffness (11.2 +/- 3.1), and LV end-diastolic pressure (8 +/- 1 mm Hg) returned to normal.

CONCLUSIONS

The enhanced diastolic function seen in chronic MR returned to normal after correction of the chronic volume overload by MVR.

Effects of induced asynchrony on left ventricular diastolic function in patients with coronary artery disease

OBJECTIVES

This study was designed to increase asynchrony with sequential atrioventricular (AV) pacing and to study its effects on left ventricular isovolumetric relaxation, rapid filling and stiffness.

BACKGROUND

Left ventricular nonuniformity is a major determinant of diastolic function.

METHODS

Thirteen patients with coronary artery disease were studied by simultaneous equilibrium radionuclide angiography and cardiac catheterization during atrial and AV pacing. Ejection fraction and peak filling rate were measured by radionuclide angiography. Regional analysis was obtained by analyzing time-activity curves of four left ventricular sectors; systolic and diastolic asynchrony were evaluated as the coefficient of variation of time to end-systole and, respectively, time to peak filling rate in the four sectors. Cardiac index and left ventricular pressure were measured with high fidelity catheters at cardiac catheterization. The time constant of isovolumetric relaxation was derived from left ventricular pressure. Pressure-volume loops were assembled and constants of chamber stiffness were computed.

RESULTS

Atrioventricular pacing led to a decrease in cardiac index (3.7 +/- 0.9 to 3.3 +/- 0.8 liters/min per m2, p = 0.01) and peak filling rate (352 +/- 125 to 287 +/- 141 ml/s, p = 0.03; 2.4 +/- 0.8 to 2.0 +/- 0.8 end-diastolic counts/s, p = 0.02; 4 +/- 1.3 to 3.2 +/- 1.0 stroke counts/s, p = 0.008). The time constant of isovolumetric relaxation increased (57 +/- 10 to 64 +/- 12 ms, p = 0.04) and the global diastolic pressure-volume relation shifted upward.

CONCLUSIONS

Atrioventricular pacing induces left ventricular asynchrony, which is associated with a slower rate of isovolumetric relaxation. The isovolumetric relaxation lasts after the filling phase has begun, thereby reducing the rate of rapid filling.

Effect of chronic supraventricular tachycardia on left ventricular function and structure in newborn pigs

OBJECTIVES

The purpose of this study was to examine the effects of supraventricular pacing tachycardia on left ventricular function and myocardial structure in newborn, immature pigs and to determine whether immature pigs respond to supraventricular tachycardia differently from adults.

BACKGROUND

Previous studies have shown that supraventricular tachycardia causes dilated cardiomyopathy in adult animals; however, in humans, supraventricular tachycardia-induced congestive heart failure occurs most frequently in children and newborns. Because some clinical diseases may cause myocardial failure in adults but rarely do so in children, it was hypothesized that the effects of supraventricular tachycardia in newborns may be different from those in adults.

METHODS

In two groups of newborn swine (3 weeks of age), left ventricular volume, mass and function were assessed with simultaneous echocardiography and cardiac catheterization and myocardial structure was examined with light and electron microscopy. Six piglets underwent 3 weeks of left atrial pacing tachycardia (240 beats/min) and six littermates served as a control group. Both groups were followed up for 3 weeks.

RESULTS

At the end of the protocol, left ventricular dimensions increased in the piglets with supraventricular tachycardia compared with values in the control group, but there were no differences in left ventricular mass. Systolic function, assessed by fractional shortening, peak ejection rate and maximal rate of pressure development, was decreased in the group with supraventricular tachycardia. The fractional shortening-end-systolic stress relation in the piglets with supraventricular tachycardia decreased below normal values. Left ventricular diastolic function assessed by the relaxation time constant was prolonged, the peak filling rate was decreased and left ventricular stiffness was increased in the supraventricular tachycardia group. The morphologic data demonstrated that supraventricular tachycardia did not change total myocyte volume but did decrease total myofibrillar volume.

CONCLUSIONS

Supraventricular tachycardia caused dilated cardiomyopathy in immature pigs. These changes in left ventricular function were associated with a decrease in cellular contractile proteins. Thus, the effects of supraventricular tachycardia on left ventricular function and structure in immature animals were comparable to previous findings in mature animals.

Isovolumic relaxation time varies predictably with its time constant and aortic and left atrial pressures: implications for the noninvasive evaluation of ventricular relaxation

The isovolumic relaxation time (IVRT) is an important noninvasive index of left ventricular diastolic function. Despite its widespread use, however, the IVRT has not been related analytically to invasive parameters of ventricular function. Establishing such a relationship would make the IVRT more useful by itself and perhaps allow it to be combined more precisely with other noninvasive parameters of ventricular filling. The purpose of this study was to validate such a quantitative relationship. Assuming isovolumic relaxation to be a monoexponential decay of ventricular pressure (pv) to a zero-pressure asymptote, it was postulated that the time interval from aortic valve closure (when pv = p(o)) until mitral valve opening (when pv = left atrial pressure, pA) would be given analytically by IVRT = tau[log(p(o))-log(pA)], where tau is the time constant of isovolumic relaxation and log is to the base e. To test this hypothesis we analyzed data from six canine experiments in which ventricular preload and afterload were controlled nonpharmacologically. In addition, tau was adjusted with the use of beta-adrenergic blockade and calcium infusion, as well as with hypothermia. In each experiment data were collected before and after the surgical formation of mitral stenosis, performed to permit the study of a wide range of left atrial pressures. High-fidelity left atrial, left ventricular, and aortic root pressures were digitized, the IVRT was measured from the aortic dicrotic notch until the left atrioventricular pressure crossover point, and tau was calculated by nonlinear least-squares regression.(ABSTRACT TRUNCATED AT 250 WORDS)

A theoretical model for the noninvasive assessment of the transmitral pressure-flow relation

The purpose of this paper is to formulate from the equations of fluid mechanics an equation which describes the transmitral pressure-flow relationship. According to the linear momentum equation applied to the atrioventricular coupling, the left-atrium-left-ventricle pressure difference (Pa-Pv) can be written as Pa-P v = A delta v/delta t + B v 2 + C v, where v is the transmitral blood velocity and A, B, and C are variables related to the geometry of the atrium, ventricle and mitral orifice, respectively. Based on this theory, Pa-Pv is calculated noninvasively in a patient with a nonobstructive mitral valve. Mitral flow and cardiac dimensions recorded by Doppler echocardiography are digitized and analyzed. Calculation shows that Pa-Pv reaches its peak value at the time of flow peak acceleration and has already considerably decreased at the time of peak velocity. The time course of calculated Pa-Pv is in close agreement with the published experimental catherization data. Numerical computation of early diastolic left atrium and left ventricle pressure curves based on the experimental data of others for the time constant of left ventricular relaxation, left atrial and ventricular chambers stiffness constants, combined with sine-waveform-simulated mitral flow, verifies the time course and the magnitude of Pa-Pv as predicted from flow equations. This paper provides a theoretical method for the noninvasive assessment of the transmitral pressure-flow relationship using ultrasound technique and might help to achieve a better understanding of the diastolic function as assessed by Doppler echocardiography.

Cardiac mechanics: basic and clinical contemporary research

This survey of cardiac hemodynamics updates evolving concepts of myocardial and ventricular systolic and diastolic loading and function. The pumping action of the heart and its interactions with arterial and venous systems in health and disease provide an extremely rich and challenging field of research, viewed from a fluid dynamic perspective. Many of the more important problems in this field, even if the fluid dynamics in them are considered in isolation, are found to raise questions which have not been asked in the history of fluid dynamics research. Biomedical engineering will increasingly contribute to their solution.

Numerical modeling of ventricular filling

The fluid dynamical and physiological assumptions underlying general mathematical modeling of ventricular filling are outlined. We then describe the use of a lumped parameter model and computer simulation to study how the early transmitral velocity profile is affected by isolated changes in ventricular compliance and relaxation, atrial pressure and compliance, and valvular morphology. We show that the transmitral velocity is fundamentally affected by two physical determinants: the transmitral pressure difference and the net compliance of the atrium and the ventricle. These physical determinants in turn are specified by the various physiologic parameters of interest. This approach has shown that peak velocity is most strongly affected by initial left atrial pressure, lowered somewhat by prolonged relaxation, low atrial and ventricular compliance, and systolic dysfunction. Peak acceleration is directly affected by atrial pressure and inversely affected by the time constant of isovolumic relaxation, with little influence of compliance, whereas the deceleration rate is almost purely given by mitral valve area divided by instantaneous atrioventricular compliance at the end of the rapid filling wave.

Changes in left ventricular volume, mass, and function during the development and regression of supraventricular tachycardia-induced cardiomyopathy. Disparity between recovery of systolic versus diastolic function

Chronic supraventricular tachycardia causes a dilated cardiomyopathy in man. Terminating this tachycardia appears to result in symptomatic improvement; however, its effects on left ventricular (LV) volume, mass, and function have not been fully examined. Accordingly, hemodynamic studies using simultaneous echocardiography and catheterization were performed in three groups of pigs: 1) those subjected to rapid left atrial pacing (240 beats/min) for 3 weeks (SVT, n = 8), 2) those subjected to supraventricular tachycardia for 3 weeks followed by termination of pacing and a 4-week recovery period (PSVT, n = 9), and 3) sham-operated controls (CTR, n = 10). Systolic pump function was assessed using fractional shortening (FS), peak ejection rate [peak (-)dD/dt], and maximum rate of pressure development [peak (+)dP/dt]. Diastolic function was assessed using the time constant of isovolumic pressure decline (tau), peak early diastolic filling rate [peak (+)dD/dt], the chamber stiffness constant (Kc), and the myocardial stiffness constant (Km). Supraventricular tachycardia caused LV dilation (end-diastolic dimension [EDD] increased from 3.5 +/- 0.4 cm in CTR to 4.9 +/- 0.5 cm in SVT, p less than 0.05) but no change in LV mass (LV weight-to-body weight ratio [LV/BW]) was 2.58 +/- 0.3 g/kg in CTR and 2.66 +/- 0.4 g/kg in SVT), all indexes of systolic function became abnormal (FS fell from 30 +/- 4% in CTR to 13 +/- 5% in SVT, p less than 0.05), and the indexes of relaxation and filling were slowed (tau increased from 36 +/- 3 msec in CTR to 51 +/- 13 msec in SVT, p less than 0.05). There were no significant changes in Kc or Km. After terminating the supraventricular tachycardia, LV volume fell but remained greater than that in CTR (EDD was 4.2 +/- 0.4 cm in PSVT, p less than 0.05 versus CTR) and substantial LV hypertrophy developed (LV/BW was 3.48 +/- 0.5 g/kg in PSVT, p less than 0.05 versus CTR). Systolic function returned to normal (FS was 31 +/- 5% in PSVT) but diastolic function remained abnormal. In PSVT, tau remained prolonged (49 +/- 12 msec, p less than 0.05 versus CTR), Kc increased from 3.7 +/- 1.0 in CTR to 7.4 +/- 1.2 (p less than 0.05), and Km increased from 4.4 +/- 1.5 in CTR to 13.9 +/- 9.7 (p less than 0.05). Thus, the improvement in systolic function that occurs after the termination of supraventricular tachycardia is associated with the development of LV hypertrophy and persistent diastolic dysfunction.

Noninvasive assessment of intrinsic ventricular load dynamics in dilated cardiomyopathy

On the basis of hemodynamic theory, a new noninvasive method is developed to provide improved insights into the significance of depressed Doppler left ventricular ejection variables in patients with dilated cardiomyopathy. The net force (F) associated with intraventricular flow throughout ejection can be written as: F = A.dv/dt + B.v2, where v is the ejection velocity and A and B are variables related to the geometry of the ventricle and its outflow tract. Instantaneous levels of this force were calculated in 9 normal subjects and 10 patients with dilated cardiomyopathy using Doppler, M-mode and two-dimensional echocardiography. The maximal ejection force (Fmax) was 47.5 +/- 8.5 kdyn in normal subjects and 25.5 +/- 6.2 kdyn in those with dilated cardiomyopathy (p = 0.0001). Peak local acceleration and outflow velocity were severely depressed in those with cardiomyopathy compared with normal subjects (1,260 +/- 129 versus 2,671 +/- 430 cm/s2 and 71 +/- 14 versus 109 +/- 7 cm/s, respectively; p = 0.0001). Maximal ejection force was attained very early in ejection. A significant linear correlation was found between peak outflow acceleration and maximal ejection force (n = 19; r = 0.91, p = 0.0001). At the time of peak ejection velocity, the net force had decreased to 64% of its peak value in those with cardiomyopathy, whereas in normal subjects, it had decreased to only 84% of its peak value (p = 0.008). In normal subjects, the ejection force was positive during the first 75% of ejection, but in those with cardiomyopathy, it was positive only during the first 54% (p = 0.0003). Once its peak value was attained, total left ventricular systolic wall stress declined rapidly during ejection in normal subjects (to 33% of its peak value by end-ejection), whereas it remained elevated throughout ejection in patients with cardiomyopathy (at 60% of its peak value by end-ejection, p = 0.0001 versus normal). The maximal ejection force corresponded to a calculated intraventricular peak pressure gradient of 9.8 +/- 1.6 mm Hg in normal subjects and 6 +/- 1.2 mm Hg in those with cardiomyopathy (p = 0.0001). The average contribution of the intrinsic component of the left ventricular systolic load (that is, wall stress associated with the ventricular to aortic pressure gradient) to the total myocardial load was 9.1% (range 7.3% to 11.2%) in normal subjects and 6.2% (range 3.9% to 7.5%) in those with cardiomyopathy (p = 0.0001).(ABSTRACT TRUNCATED AT 400 WORDS)

Clinical assessment of ventricular ejection dynamics with and without outflow obstruction

With the advent of multisensor micromanometric/velocimetric catheterization, digital angiography and Doppler and color echocardiography, extensive fluid dynamic quantitation is now possible in cardiology. Such high fidelity instantaneous measurements offer the clinician the prospect of identifying phasic changes in ventricular ejection dynamics that may disclose contraction abnormalities before overt muscle or pump failure is manifested. Accordingly, this review provides a basis for interpreting these measurements and a conceptual framework for understanding ventricular ejection dynamics with and without outflow obstruction. Necessary terminology and fluid dynamic background, including properties of flows generated by large transient forces, Euler and unsteady Bernoulli equations and local and convective acceleration gradients, are reviewed first. Physiologic aspects of ejection dynamics and transvalvular and intraventricular gradients without obstruction are discussed. Maximal outflow acceleration, rather than ejection velocity, coincides with the attainment of the early peak of the nonobstructive pressure gradients. These gradients are characteristically even more asymmetric than are the associated ejection velocity signals. Clinical correlations are introduced, beginning with obstructive transvalvular and subvalvular gradients in aortic stenosis and the phenomenon of recovery of pressure loss in the poststenotic dilation. The large obstructive gradients tend to be distinctively symmetric, as are the ejection waveforms, whose configuration they track more or less closely, depending on the degree of stenosis and relative preponderance of convective effects throughout ejection. Pitfalls in some unwarranted applications of the "simplified Bernoulli equation" are pointed out. Polymorphic gradients of hypertrophic cardiomyopathy, reflecting dynamically dissimilar intraventricular flow regimes in early, mid and late systole, are examined. Enormous late systolic gradients can be associated with progressive shrinkage of flow passage area and sharp increases in linear velocity while volumetric outflow is diminutive. The concept of ventriculoannular disproportion in dilated ventricles is defined and discussed. The implications of ejection fluid dynamics for systolic ventricular and myocardial loading are examined, and the concept of complementarity and competitiveness between intrinsic and extrinsic load components is introduced. Finally, critical research issues are identified and addressed. The primary emphasis is on using the basic principles of fluid dynamics to better understand ejection in the normal or abnormal human left ventricle and aortic root.(ABSTRACT TRUNCATED AT 400 WORDS)