Force-frequency relationship as a predictor of long-term prognosis in patients with heart diseases

Failing Left Ventricles Have an Enhanced Post-Stimulation Potentiation Despite Their Impaired Force Frequency Relationship

The left ventricular contractile force (LV dP/dtmax) of patients with left ventricular systolic dysfunction does not increase effectively with an increase in heart rate. In other words, their force-frequency relationship (FFR) is impaired. However, it is unknown whether a longer coupling interval subsequent to tachycardia causes a stronger contraction (poststimulation potentiation, PSP) in a rate-dependent manner.In 16 patients with idiopathic dilated cardiomyopathy (DCM) (48 ± 2 years old, LVEF 30 ± 10%) and 6 control patients (58 ± 4 years old, LVEF 70 ± 7%), FFR was assessed by right atrial pacing using a micro-manometer-tipped catheter. At each pacing rate, the increase of LV dP/dtmax over basal LV dP/dt (ΔFFR) and the increase of LV dP/dtmax of the first beat after pacing cessation over LV dP/dtmax during pacing (ΔPSP) were evaluated.Patients with DCM had smaller LV dP/dtmax at baseline (872 ± 251 versus 1370 ± 123 mmHg/second, P = 0.0002) and developed smaller ΔFFR (eg, at 120/minute, 77 ± 143 versus 331 ± 131 mmHg/second, P = 0.0011). In contrast, they showed a rate-dependent increase of LV dP/dtmax of PSP and had greater ΔPSP (eg, at 120/minute, 294 ± 173 versus -152 ± 131 mmHg/second, P < 0.0001).Failing left ventricles develop little contractile force during tachycardia despite their rate-dependent enhancement in post-stimulation potentiation, suggesting that refractoriness of contractile force underlies impaired FFR.

Ultrasound assessment of the force-frequency relationship from the law of conservation of momentum in patients with left ventricular dysfunction

Measurement of force-frequency relationship (FFR) is useful in the evaluation of heart rate-dependent contractile dysfunction. The purpose of this study was to evaluate a new Doppler-derived method for assessing FFR. Doppler velocity spectra at the left ventricular (LV) outflow tract was used to estimate mean blood flow velocity (mBFV), ejection time (ET) and velocity-time integral. LV ejection force (LVF) was then calculated according to the law of conservation of momentum: 1060 kg/m³ × (mBFV[m/sec]/ET [s]) × Stroke volume (mL). A symptom-limited, graded, bicycle semi-supine exercise test was performed in 56 patients with LV dysfunction (LV ejection fraction = 27 ± 6%). Measurements were obtained at baseline and serially during the test. The change in FFR was defined as up-sloping when the peak LVF was higher than the baseline value. The change was biphasic when the trend was initially up-sloping, then down-sloping; it was flat or negative when peak LVF was less than the baseline value. LVF was 30 ± 12 mN in patients with up-sloping FFR (n = 39) and 15 ± 6 mN in those with biphasic or flat FFR (n = 17; p < 0.0001). The ultrasound assessment of the FFR was highly concordant with a previously validated method based on pressure-end-systolic volume index ratio (κ = 0.75; 95% confidence interval, 0.55-94.0). The evaluation of the LVF using Doppler is an alternative method for the assessment of FFR during stress echocardiography in patients with LV dysfunction.

Mechanical alternans in human idiopathic dilated cardiomyopathy is caused with impaired force-frequency relationship and enhanced poststimulation potentiation

Mechanical alternans (MA) is frequently observed in patients with heart failure, and is a predictor of cardiac events. However, there have been controversies regarding the conditions and mechanisms of MA. To clarify heart rate-dependent contractile properties related to MA, we performed incremental right atrial pacing in 17 idiopathic dilated cardiomyopathy (DCM) patients and in six control patients. The maximal increase in left ventricular dP/dt during pacing-induced tachycardia was assessed as the force gain in the force-frequency relationship (FG-FFR), and the maximal increase in left ventricular dP/dt of the first post-pacing beats was examined as the force gain in poststimulation potentiation (FG-PSP). As a result, MA was induced in 9 DCM patients (DCM MA(+)) but not in the other 8 DCM patients (DCM MA(-)), and not in any of the control patients. DCM MA(+) had significantly lower FG-FFR (34.7 ± 40.9 vs 159.4 ± 103.9 mmHg/s, P = 0.0091) and higher FG-PSP (500.0 ± 96.8 vs 321.9 ± 94.9 mmHg/s, P = 0.0017), and accordingly a wider gap between FG-PSP and FG-FFR (465.3 ± 119.4 vs 162.5 ± 123.6 mmHg/s, P = 0.0001) than DCM MA(-) patients. These characteristics of DCM MA(+) showed clear contrasts to those of the control patients. In conclusion, MA is caused with an impaired force-frequency relationship despite significant poststimulation potentiation, suggesting that MA reflects ineffective utilization of the potentiated intrinsic force during tachycardia.