Loading determinants of isovolumic pressure fall in closed-chest dogs

Late systolic central hypertension as a predictor of incident heart failure: the Multi-ethnic Study of Atherosclerosis

BACKGROUND

Experimental studies demonstrate that high aortic pressure in late systole relative to early systole causes greater myocardial remodeling and dysfunction, for any given absolute peak systolic pressure.

METHODS AND RESULTS

We tested the hypothesis that late systolic hypertension, defined as the ratio of late (last one third of systole) to early (first two thirds of systole) pressure-time integrals (PTI) of the aortic pressure waveform, independently predicts incident heart failure (HF) in the general population. Aortic pressure waveforms were derived from a generalized transfer function applied to the radial pressure waveform recorded noninvasively from 6124 adults. The late/early systolic PTI ratio (L/E(SPTI)) was assessed as a predictor of incident HF during median 8.5 years of follow-up. The L/E(SPTI) was predictive of incident HF (hazard ratio per 1% increase=1.22; 95% CI=1.15 to 1.29; P<0.0001) even after adjustment for established risk factors for HF (HR=1.23; 95% CI=1.14 to 1.32: P<0.0001). In a multivariate model that included brachial systolic and diastolic blood pressure and other standard risk factors of HF, L/E(SPTI) was the modifiable factor associated with the greatest improvements in model performance. A high L/E(SPTI) (>58.38%) was more predictive of HF than the presence of hypertension. After adjustment for each other and various predictors of HF, the HR associated with hypertension was 1.39 (95% CI=0.86 to 2.23; P=0.18), whereas the HR associated with a high L/E was 2.31 (95% CI=1.52 to 3.49; P<0.0001).

CONCLUSIONS

Independently of the absolute level of peak pressure, late systolic hypertension is strongly associated with incident HF in the general population.

Heart failure-associated alterations in troponin I phosphorylation impair ventricular relaxation-afterload and force-frequency responses and systolic function

Recent studies have found that selective stimulation of troponin (Tn)I protein kinase A (PKA) phosphorylation enhances heart rate-dependent inotropy and blunts relaxation delay coupled to increased afterload. However, in failing hearts, TnI phosphorylation by PKA declines while protein kinase C (PKC) activity is enhanced, potentially augmenting TnI PKC phosphorylation. Accordingly, we hypothesized that these site-specific changes deleteriously affect both rate-responsive cardiac function and afterload dependence of relaxation, both prominent phenotypic features of the failing heart. A transgenic (TG) mouse model was generated in which PKA-TnI sites were mutated to mimic partial dephosphorylation (Ser22 to Ala; Ser23 to Asp) and dominant PKC sites were mutated to mimic constitutive phosphorylation (Ser42 and Ser44 to Asp). The two highest-expressing lines were further characterized. TG mice had reduced fractional shortening of 34.7 ± 1.4% vs. 41.3 ± 2.0% ( P = 0.018) and slight chamber dilation on echocardiography. In vivo cardiac pressure-volume studies revealed near doubling of isovolumic relaxation prolongation with increasing afterload in TG animals ( P < 0.001), and this remained elevated despite isoproterenol infusion (PKA stimulation). Increasing heart rate from 400 to 700 beats/min elevated contractility 13% in TG hearts, nearly half the response observed in nontransgenic animals ( P = 0.005). This blunted frequency response was normalized by isoproterenol infusion. Abnormal TnI phosphorylation observed in cardiac failure may explain exacerbated relaxation delay in response to increased afterload and contribute to blunted chronotropic reserve.

Differential effects of heart rate reduction and beta-blockade on left ventricular relaxation during exercise

Left ventricular (LV) relaxation is crucial for LV function, especially during exercise. We compared the effects of increasing doses of ivabradine, a selective inward hyperpolarization-activated current inhibitor, and atenolol on the rate and extent of LV relaxation (best fit method: time constant tau(BF), pressure asymptote P(BF)) at rest and during exercise. Eight dogs were chronically instrumented to measure LV pressure and LV wall stresses. During exercise under saline, heart rate increased from 108 +/- 5 to 220 +/- 6 beats/min and tau(BF) was significantly reduced from 22 +/- 1 to 14 +/- 2 ms. At rest, atenolol but not ivabradine increased tau(BF). For similar heart rate reductions during exercise, atenolol impeded the shortening of tau(BF) (23 +/- 2 ms) whereas ivabradine had no effect (15 +/- 2 ms). The extent of the relaxation process (P(BF)) at peak exercise was increased by ivabradine, and to a greater extent by atenolol, compared with saline. Thus, for a similar reduction in heart rate at rest and during exercise, ivabradine, in contrast with atenolol, does not exert any negative lusitropic effect.

Different left ventricular relaxation parameters in isolated working rat and guinea pig hearts. Influence of preload, afterload, temperature, and isoprenaline

In isolated ejecting rat and guinea pig hearts, the sensitivity of the time constant tau of left ventricular isovolumic pressure fall, the maximum pressure fall velocity min LVdP/dt, and the relaxation time to different hemodynamic conditions, temperature, and isoprenaline were investigated. Tau was obtained by fitting the isovolumic pressure fall three-parametrically to the exponential p(t) = p infinity + (p0-p infinity) exp (-t/tau) which was found to be superior to semilogarithmic estimation. The influence of different working conditions on the relaxation parameters was tested by a rank correlation test and quantified by calculating standardized regression coefficients. Hemodynamic conditions were altered by changing left ventricular end-diastolic pressure (increasing inflow to the heart) and peak pressure (max LVP, varying aortic outflow resistance), and by atrial pacing (variation of interbeat interval). Lusitropic sensitivity was investigated by changing temperature and by applying isoprenaline. All regression parameters were only moderately sensitive to changes in end-diastolic pressure, max LVP, or heart rate, with the exception of a considerable afterload dependence of min LVdP/dt in rat hearts. This dependence, however, can be overcome to a large extent by dividing min LVdP/dt by mean aortic pressure. Isoprenaline strongly influenced all relaxation parameters, and so did temperature, except for relaxation time in guinea pig hearts. We conclude that tau serves as a reliable relaxation parameter, also in the hearts of small animals with heart rates up to 450 beats/min. In isolated hearts, min LVdP/dt, corrected for afterload dependence, is also suitable as a complementary index of the early relaxation phase.