Load-insensitive relaxation caused by hypoxia in mammalian cardiac muscle

What global diastolic function is, what it is not, and how to measure it

Despite Leonardo da Vinci's observation (circa 1511) that “the atria or filling chambers contract together while the pumping chambers or ventricles are relaxing and vice versa,” the dynamics of four-chamber heart function, and of diastolic function (DF) in particular, are not generally appreciated. We view DF from a global perspective, while characterizing it in terms of causality and clinical relevance. Our models derive from the insight that global DF is ultimately a result of forces generated by elastic recoil, modulated by cross-bridge relaxation, and load. The interaction between recoil and relaxation results in physical wall motion that generates pressure gradients that drive fluid flow, while epicardial wall motion is constrained by the pericardial sac. Traditional DF indexes (τ, E/E′, etc.) are not derived from causal mechanisms and are interpreted as approximating either stiffness or relaxation, but not both, thereby limiting the accuracy of DF quantification. Our derived kinematic models of isovolumic relaxation and suction-initiated filling are extensively validated, quantify the balance between stiffness and relaxation, and provide novel mechanistic physiological insight. For example, causality-based modeling provides load-independent indexes of DF and reveals that both stiffness and relaxation modify traditional DF indexes. The method has revealed that the in vivo left ventricular equilibrium volume occurs at diastasis, predicted novel relationships between filling and wall motion, and quantified causal relationships between ventricular and atrial function. In summary, by using governing physiological principles as a guide, we define what global DF is, what it is not, and how to measure it.

Regional diastolic contour abnormalities during contrast stress echocardiography: improved detection of coronary artery disease

BACKGROUND

Use of contrast improves detection of systolic regional wall motion abnormalities (RWMAs) during stress echocardiography. We evaluated regional diastolic contour abnormalities (RDCAs) that were associated with coronary artery disease (CAD).

METHODS

From August of 2003 to September of 2004, we evaluated 89 patients who underwent contrast stress echocardiography (CSE) and coronary angiography within a 3-month period ("invasive" group) and 17 patients with lower CAD risk who underwent CSE only ("reference" group).

RESULTS

RDCAs were present in 73 patients in the invasive group and were associated with higher Framingham risk scores (relative risk, 3.6; 95% confidence interval, 1.9-6.6). RDCAs were present in 1 patient in the reference group. When combined with RWMA, RDCA improved sensitivity of CSE from 78% to 97% and specificity from 26% to 59% (diagnostic threshold for CAD was 70% stenosis).

CONCLUSION

RDCAs were a novel observation associated with higher CAD risk and improved the diagnostic accuracy of CSE.

Biphasic relaxation-frequency relations in patients with effort angina pectoris: a new marker of myocardial demand ischemia

BACKGROUND

Relaxation-frequency relations (RFR) during demand ischemia have not been fully examined in patients with effort angina pectoris (AP). We sought to clarify the effects of pacing and exercise on RFR in patients with AP.

METHODS

We recorded left ventricular (LV) pressures during rapid atrial pacing and symptom-limited supine bicycle exercise. RFR were analyzed in 24 patients with AP and 10 controls.

RESULTS

LV pressure half-time (T(1/2)) in controls was gradually shortened with an increase in heart rate (HR) during pacing (-19% +/- 6% at peak HR). The changes in T(1/2) during pacing were biphasic with initial shortening (-12% +/- 5% at the critical HR) followed by prolongation (-3% +/- 7% at peak HR) in all patients with AP. The critical HR, at which T(1/2) was minimum, preceded the HR at 0.1-mV ST-segment depression, and finally chest pain occurred. The critical HR was correlated negatively with the severity of ischemia as assessed by thallium-201 scintigraphy. T(1/2) was remarkably shortened during exercise in controls (-41% +/- 10% at peak exercise). In patients with AP, 2 distinct patterns of RFR were observed during exercise. T(1/2) was shortened progressively (-37% +/- 8% at peak exercise) in 15 patients, whereas RFR remained biphasic (-21% +/- 10% at the critical HR and -11% +/- 11% at peak exercise) in the other 9 patients. Coronary angiography and exercise scintigraphy suggested more severe ischemia in patients with biphasic RFR during exercise.

CONCLUSIONS

Impaired RFR might be the most sensitive parameter of pacing-induced ischemia. The critical HR was closely related with severity of ischemia. Adverse effects of ischemia on LV relaxation may be alleviated by exercise.

Diastolic response during dobutamine stress echocardiography evaluated by a tissue velocity imaging technique is a sensitive indicator for diagnosing coronary artery disease

BACKGROUND

Tissue velocity imaging (TVI) is a new method that measures regional myocardial velocities on the basis of color Doppler myocardial imaging principles.

METHODS

To diagnose coronary artery disease (CAD) objectively by evaluating left ventricular diastolic responses during dobutamine stress echocardiography (DSE) with TVI, we performed DSE in 22 healthy participants and 28 patients with angina pectoris without wall-motion abnormality at rest. Before and during DSE, we measured the differences of time intervals from the R wave on electrocardiogram to the peak of early diastolic myocardial velocity in the same cardiac cycle between basal segments and midsegments in the septal (dT-S) and inferior (dT-I) walls by TVI.

RESULTS

During DSE, dT-S in patients with left anterior descending CAD and dT-I in patients with right CAD were prolonged compared with that in healthy participants (both P <.01). The localization of the segments with a dT-S or dT-I during low-dose (10 microg/kg/min) dobutamine infusion of >32 milliseconds allowed the correct identification of the stenosed vessel in 87% of 23 patients for whom DSE was performed with the TVI technique before coronary angiography.

CONCLUSIONS

The analysis of regional left ventricular diastolic responses to dobutamine stress using TVI was useful for the objective diagnosis of CAD.

Role of myofilaments and calcium handling in left ventricular relaxation

Myocardial relaxation is governed by the interplay of two macromolecular systems: (1) myofilaments and (2) calcium extruding pumps/exchangers. In myocardium from failing hearts, both systems act more slowly than normal, and cause relaxation to decelerate, which may impede early rapid filling and can often limit cardiac pumping ability--especially during exercise. Gene-based therapy to augment sluggish SERCA pumps is a possibility being currently investigated in research laboratories. In normal myocardium, the rate of dissociation of myosin crossbridges sets the rate of relaxation. In this case, relaxation is characterized by two features: (1) load-dependence and (2) displacement-dependence. Load-dependence derives from cooperative mechanisms acting among ensembles of crossbridges and myofilament regulatory proteins (troponin, tropomyosin); it allows contraction to be prolonged when more crossbridges are attached and mutually support each other. The rate of relaxation can still be rapid, however, as this cooperative system begins to collapse. Displacement-dependence is more important later in contraction, because tenuous crossbridge attachments cannot easily re-form after being disrupted when myofilaments slide along each other. Myofilaments control normal relaxation because the calcium extruding systems reduce calcium to near diastolic levels relatively early; however, when the relative timing of crossbridge dissociation versus calcium sequestration is altered, and calcium uptake is slowed (relative to crossbridges), then removal of calcium can become rate limiting instead. In this case, load- and displacement-dependence are less marked. Both the timing of calcium removal and the sensitivity of the myofilaments to calcium affect relaxation timing.

Diastolic indexes during dobutamine stress echocardiography in patients early after myocardial infarction

The aim of this study was to determine whether Doppler parameters assessed during dobutamine stress echocardiography in the early phase after myocardial infarction could discriminate patients with residual ischemia from those without. Thirty-six patients after a recent myocardial infarction with and without residual ischemia underwent dobutamine stress echocardiography, adenosine sestamibi scintigraphy, and coronary angiography within 2 weeks after the acute event. The only diastolic Doppler parameter discriminating the two groups was the isovolumic relaxation time (IVRT) measured at the peak of the dobutamine infusion. It became shorter in both groups but significantly more in patients without than in those with residual ischemia despite a larger increase in heart rate in the latter group. IVRT at rest was 78 +/- 18 msec and decreased with high-dose dobutamine to 54 +/- 11 msec in the control group and to 69 +/- 16 msec in the ischemic group (p < 0.01). In addition, the rate-corrected IVRT (IVRTc) was calculated: IVRTc = IVRT/sqrtRR. The value of IVRTc = 80 at peak dobutamine infusion is able to discriminate patients with residual ischemia from those without with a sensitivity of 80% and a specificity of 70%.

Pacing-induced left ventricular asynchronies in dogs with critical coronary stenosis: mechanisms and effect of anesthetics

The mechanisms leading to left ventricular (LV) asynchronies are incompletely understood, and reports on the functional significance of asynchronies for the affected segments are conflicting. To characterize LV asynchronies, 16 anesthetized dogs with critical stenosis of the left anterior descending coronary artery (LAD) were instrumented to measure subendocardial contractile function (sonomicrometry) and the ECG in the LAD territory. The subendocardial ECG was also recorded from the anterior basal LV territory. Time of regional S wave arrival (TS) and time of onset of segment shortening were determined. The animals underwent atrial pacing with increasing frequencies until systolic LAD territory contractile dysfunction and eventual LV asynchronies were observed. Six animals without LAD stenosis served as controls to define the normal response (mean +/- 2.SD) to increasing pacing rates of systolic shortening and onset time of segment shortening (time difference between TS and onset of segment shortening). LAD contractile dysfunction was considered as a systolic shortening below the normal range, and LV asynchronies as an onset time of segment shortening above the normal range. When LV asynchronies occurred, onset time of segment shortening in the LAD territory was 80.1 +/- 4.9 ms versus 14.8 +/- 3.7 ms at control (P < 0.01); the time difference between S wave arrival in the LAD and circumflex territories, however, was unchanged. LV asynchronies were associated with marked LAD territory contractile dysfunction (systolic shortening of 9.6 +/- 0.8% v 21.0 +/- 1.9% at control, after systolic shortening of 31.3 +/- 3.8% v 9.0 +/- 2.6% at control; P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for or against the efficacy of calcium channel blockers for management of hypertrophic cardiomyopathy in cats

The positive lusitropic and direct coronary vasodilating properties of the calcium channel blocking agents are beneficial therapeutic effects not provided by the beta-adrenergic blocking agents for the management of feline HCM. Data from cats studied at the University of Tennessee suggest that diltiazem more consistently alleviates clinical signs and more effectively prolongs survival in cats with HCM than either propranolol or verapamil. Orally administered diltiazem appears to have sustained beneficial effects on left ventricular filling and cardiac performance based on its ability to reduce resting heart rate, decrease blood lactate concentration, increase venous oxygen tension, improve echocardiographic parameters, and resolve radiographic abnormalities. Long-term diltiazem administration may also reverse myocardial hypertrophy in some patients. There appear to be few if any side effects of this drug. Diltiazem, therefore, provides a safe and effective approach for the management of feline HCM.

Is stiffness increased during ischemia?

Possible sources of increased ventricular stiffness can be more easily appreciated when pressure and volume patterns are considered as a function of time. A discussion on sources of effective or apparent stiffness or stiffness changes includes viscoelastic properties and active behavior at the muscular level. Chamber geometry and coronary vascular pressure and flow are intrinsic ventricular components. Together with the pressure head and crosstalk as extraventricular components, all these properties are integrated to determine intact heart behavior in late relaxation and diastole.

Assessment of diastolic function of the heart: background and current applications of Doppler echocardiography. Part I. Physiologic and pathophysiologic features

In the past, evaluation of the myocardium has been limited to examining systolic function of the heart. Recently, however, investigators have demonstrated that abnormalities of diastolic function of the heart provide important contributions to the signs and symptoms experienced by patients with heart disease. In addition, abnormalities of diastolic function may precede abnormalities of systolic function in the early stages of disease. Diastolic filling of the heart, however, is a complex sequence of interrelated events. In order to understand diastolic function, each of these factors contributing to filling of the heart must be examined. They include relaxation, passive compliance, atrial contraction, erectile effect of the coronary arteries, viscoelastic properties, ventricular interaction, and pericardial restraint--all of which are interrelated. In addition, diastolic factors are affected by changes in loading conditions and contractility, and they demonstrate nonuniformity in time and space. This report provides an overview of these various factors from the clinical perspective, based on studies involving the isolated papillary muscle and the isolated heart as well as basic clinical studies.

Age-dependent changes of relaxation and its load sensitivity in rat cardiac muscle

The relaxation phase and its load dependence were studied in papillary muscles isolated from the left ventricle of rats of the following ages: 20 days, 2, 8, 18, and 24 months. The myofibrillar ATPase activity and the force-velocity relation were determined in each age group in order to characterize the kinetic properties of the contractile material. Both shortening velocity and myofibrillar ATPase activity showed a progressive reduction with maturation and aging. This observation suggested an age-dependent decrease in cross bridge formation rate. The relaxation phase was characterized by its duration and the maximum rate of tension decline in isometric conditions, and by the speed of relengthening in isotonic conditions. Relaxation became faster and of shorter duration with maturation from 20 days to 2 months and then became slower and of longer duration with further maturation and aging. The sensitivity of relaxation to changes in length or load was evaluated by measuring how much earlier tension declined in the presence of a given length change. An increase in load sensitivity of relaxation was observed during maturation from 20 days to 8 months. This increase was followed by a reduction during aging from 8 to 24 months. Such a biphasic trend of the age-related changes in load sensitivity of relaxation could result from the interplay between the progressive decrease in cross bridge formation rate and a reduction in activation decay rate. The latter was suggested by the prolongation of the relaxation phase and by the maintenance of developed tension during aging.

Major alterations in relaxation during cardiac hypertrophy induced by aortic stenosis in guinea pig

Left ventricular hypertrophy (LVH) was produced in guinea pigs after aortic stenosis (AS). The percentage of LVH in AS was determined by normalizing left ventricular (LV) weight by the mean LV weight of sham-operated controls (n = 12). After 3 weeks of cardiac overload, a mild LVH (30 +/- 3%) was induced in 17 animals and a relatively severe LVH (56 +/- 3%) was induced in 7 animals. LV papillary muscles were rapidly excised for mechanical studies. No significant differences were observed between control and mild hypertrophy groups. In contrast, a marked decrease in myocardial performance was seen in the more severe cardiac hypertrophy group and was expressed as a percentage of sham-operated levels (Vmax, 22%; active isometric force/mm2, 23%; +dF/dt max/mm2, 26%). Relaxation in this group was still more impaired than contraction (peak lengthening velocity, 14%; -dF/dt max/mm2, 19%). Moreover, the load sensitivity of relaxation was present in both sham-operated controls and mild hypertrophy but almost disappeared in more severe hypertrophy. Isometric relaxation was delayed in the latter group, as shown by the 15% increase of the half-time of the decline of isometric relaxation (t 1/2). On the other hand, acute hypoxia (95% N2-5% CO2 for 20 minutes) also induced a fall in contractility and the disappearance of the load sensitivity of relaxation but with a 67% decrease of t 1/2. Thus, the mechanical analysis of relaxation allows the effects of chronic overload in relatively severe cardiac hypertrophy to be separated from those of acute hypoxia. Moreover, in severe cardiac hypertrophy, the impairment of the load sensitivity of relaxation with increased t 1/2 strongly suggests alterations of the sarcoplasmic reticulum, especially since the moderate decrease in the myofibrillar ATPase activity, which has been observed previously in guinea pig pressure overload, cannot account completely for the marked fall in myocardial performance.

Effects of ryanodine on cat papillary muscle and isolated rat heart

The neutral alkaloid, ryanodine, has several actions on cardiac muscle. To delineate better its mode of action, we studied ryanodine's effect upon contracting cat papillary muscles under changing loading conditions and stimulation frequencies. We also studied ryanodine's physiologic and metabolic effects upon isolated rat hearts. The results of our study suggest the following: (1) ryanodine causes both decreased release and decreased uptake of calcium by the sarcoplasmic reticulum; (2) elevation of high-energy phosphates secondary to decreased energy requirements is due to decreased calcium availability to the myofilaments during systole; (3) the slowed or incomplete relaxation caused by ryanodine may be a stimulus for myosin phosphorylation; (4) ryanodine probably decreases calcium movement through the sarcolemma and so increases adenosine and inorganic phosphate and decreased cyclic adenosine monophosphate (AMP) concentration in the myocardium; and (5) the effect of ryanodine on altered loading conditions and contraction velocities can be understood in terms of decreased calcium availability to the myofilaments.

Echocardiographic assessment of left ventricular diastolic function in a normal population and a group of patients with myocardial hypertrophy

In order to study left ventricular diastolic function, digitized M-mode echocardiograms of the free wall endocardium were analyzed. A population of 117 healthy individuals was compared with a group of 22 patients with myocardial hypertrophy due to aortic stenosis (AS), and a group of 21 with hypertrophic cardiomyopathy (HCMP). The time constant Te of endocardial retraction in normals (66 +/- 13 ms) differed significantly from Te in the patients (AS, 105 +/- 23 ms, p less than 0.001; HCMP, 113 +/- 31 ms, p less than 0.001). Peak velocity of endocardial retraction (peak neg. dEnd/dt, in the normals 8.6 +/- 2.7 s-1) did not differ between normals and patients (AS, 7.0 +/- 2.3 s-1; HCMP, 7.3 +/- 2.4 s-1). Fractional shortening (FS) of the internal diameter, a parameter of left ventricular systolic function, tended to be higher in the AS patients than in the normal group, but was on the average significantly increased in the group of HCMP. However, most patients in the hypertrophic groups had FS within normal limits +/- 2 SD. The discrepancy between normal or supernormal systolic function and a pathological prolonged time constant Te in myocardial hypertrophy demonstrates the importance of diastolic parameters in left ventricular characterization. Te is a useful parameter of left ventricular diastolic function which can be provided noninvasively from echocardiographic M-mode curves of the free wall endocardium.

Mechanisms of hypoxia-induced decrease of load dependence of relaxation in cat papillary muscle

Relaxation of mammalian cardiac muscle is very sensitive to the prevailing load, but becomes largely load-independent during hypoxia. This effect was previously ascribed to a delayed removal of activating myoplasmic calcium. To further elucidate the underlying mechanisms of this effect of hypoxia, relaxation was now studied in 26 cat papillary muscles, in which hypoxia-induced decrease of load dependence of relaxation was compared with the effects of low [Ca2+]o (1.0, 0.5, 0.375 mM), verapamil (1 microM) and nifedipine (0.1 microM). Load dependence of relaxation was quantified by comparing force and time coordinates at the onset of the isometric relaxation phase in several after-loaded isotonic twitch contractions with the relaxation of the isometric control contraction. Hypoxia, low [Ca2+]o, verapamil and nifedipine decreased load dependence of relaxation. Although low [Ca2+]o, verapamil and nifedipine had a more marked negative inotropic effect on the contraction phase than hypoxia, the decrease of load dependence with hypoxia was significantly more pronounced and it included two phases: an early and fast drop, followed by a slower and longer-lasting decrease. The early fast phase was neutralized in low [Ca2+]o and also diminished after administration of verapamil or nifedipine. An impaired calcium reuptake by the sarcoplasmic reticulum would underly only the second phase of decreased load dependence. The first phase on the other hand originated mainly from changes in the isometric relaxation of the papillary muscles.

Effects of verapamil on contraction and relaxation of cultured chick embryo ventricular cells during calcium overload

The calcium channel blocking agents verapamil and nifedipine have been reported to lessen abnormalities of left ventricular relaxation in patients with severe left ventricular hypertrophy or coronary artery disease. Whether these effects in human beings are related in part to a direct effect on myocardial calcium metabolism is difficult to determine because of complicating drug influences on ventricular loading via systemic arterial vasodilation, on myocardial blood supply via coronary artery vasodilation and on reflex changes in sympathetic tone. For this reason, the effects of verapamil were investigated in a cellular model of impaired relaxation using spontaneously contracting tissue cultured monolayers of chick embryo ventricular cells exposed to high external calcium ([Ca]o). Under control conditions ([Ca]o, 0.9 mM), verapamil (2 X 10(-8)M) induced a 57 +/- 8% decrease in amplitude of cell contraction monitored with a phase contrast microscope video motion detector system. Elevation of [Ca]o from 0.9 to 8.0 mM resulted in a decrease in amplitude and velocity of contraction and a decrease in velocity of relaxation associated with an upward shift in diastolic cell wall position, suggesting a failure of normal myofilament dissociation. These abnormalities were completely reversible on reperfusion with [Ca]o, 0.9 mM. On re-exposure of the cells to [Ca]o, 8.0 mM, in the presence of verapamil, there was an increase in amplitude of contraction (0.56 +/- 0.11 to 1.03 +/- 0.09 micron, p less than 0.01) and velocity of relaxation (4.97 +/- 0.89 to 9.94 +/- 0.87 micron/s, p less than 0.01) compared with exposure to [Ca]o, 8.0 mM, alone, and an attenuation of the upward shift in diastolic cell wall position.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the effects of nitroprusside and nifedipine on diastolic properties in patients with hypertrophic cardiomyopathy: altered left ventricular loading or improved muscle inactivation?

The calcium channel blocking agent, nifedipine, has been shown to improve indexes of left ventricular relaxation, diastolic filling and compliance in patients with hypertrophic cardiomyopathy. The mechanism of action of nifedipine on diastolic properties in patients with hypertrophic cardiomyopathy is unclear and could result from an improvement in myocardial inactivation or from systemic vasodilation and left ventricular unloading. To distinguish between these mechanisms, the effects of nifedipine and the vasodilator nitroprusside on left ventricular diastolic properties were compared in 10 patients with nonobstructive hypertrophic cardiomyopathy using simultaneous micromanometer left ventricular pressure and echocardiographic measurements. Left ventricular peak systolic pressure was comparable during nitroprusside infusion (132 +/- 38 mm Hg) and after nifedipine (132 +/- 32 mm Hg). During nitroprusside infusion, the decrease in left ventricular end-diastolic pressure (22 +/- 11 to 17 +/- 11 mm Hg, p less than 0.05) was associated with a decrease in left ventricular end-diastolic dimension. In contrast, the decrease in left ventricular end-diastolic pressure after nifedipine (22 +/- 11 to 18 +/- 10 mm Hg, p less than 0.05) was associated with no reduction of left ventricular end-diastolic dimensions, suggesting an increase in left ventricular distensibility. Compared with nitroprusside, nifedipine was associated with less prolongation of the left ventricular isovolumic relaxation time and less depression of the peak left ventricular posterior wall thinning rate and peak left ventricular internal dimension filling rate. These data suggest that the effects of the calcium channel blocker, nifedipine, on diastolic mechanics in hypertrophic cardiomyopathy result not only from systemic vasodilation but also from improved cardiac muscle inactivation.