Non-invasive assessment of myocardial contractility from force-frequency relationship in patients with implanted pacemakers: first results.
In the normal heart, myocardial contractility is augmented at elevated heart rates. This effect is known as force-frequency relationship (FFR). The FFR is altered in failing hearts and may be used to detect myocardial function impairment. Force-frequency relationship measurements usually require invasive techniques and have therefore rarely been used in clinical practice. We studied the feasibility of FFR determination by implanted pacemakers as a non-invasive method in pacemaker patients. During follow-up visits in 30 patients with implanted pacemakers and normal heart function, the intracardiac impedance (IZ) was recorded by the pacemaker during incremental pacing at 80, 100, and 120 bpm. The time interval between the ventricular pacing pulse and IZmax (tIZm) and the maximum acceleration of the IZ change during myocardial contraction (IZacc: d2Z/dt2 max) were determined. Isovolumetric acceleration (IVA) was measured in parallel by tissue Doppler echocardiography at the right free wall, the left free wall, and the septum. With increasing pacing rates, the time interval tIZm shortened significantly and maximum acceleration IZacc increased significantly, consistent with the FFR. The best correlation was achieved between IZacc and IVA septal. Permanent pacemakers offering intracardiac IZ measurements can be used for monitoring FFR changes and, possibly, for low-effort first-line monitoring of myocardial disease.