Figure 7
Pacing stress. External programming of permanent PM induces a controlled change in heart rate which is independent of the patient capability to exercise. Echocardiographic assessment is highly feasible during PM stress and technically easier than during dynamic exercise. Heart rate increase is achieved with no adrenergic stimulation which may inflate the inotropic response through a mechanism different from Bowditch treppe. Lower panels. The ventricular force is expressed as SP/ESV assessed by echocardiography in the stress echo lab. Left, residual contractile reserve; the programmed increase in heart rate is accompanied by no changes in systolic pressure with a pronounced decrease of the end-systolic volume (up sloping force-frequency relation). The Δ rest-peak SP/ESV index is >2 mmHg/ml/m2 (2 mmHg/ml/m2 = cut-off value for positive vs. negative contractile reserve in pacing stress). Middle, force-frequency curve with stress echo in a subject with previous MI and stress induced heterozonal ischemia. The force-frequency relation is biphasic, with an initial up-sloping trend followed by a later down-sloping trend at ischemia; an intermediate situation between normal and chronic failing hearts is effort induced ischemia, in which the normal up sloping force-frequency relation is abruptly interrupted by the hypoxic dysfunction in calcium homeostasis, resulting in reduced contractile performance at ischemia. Right, force-frequency curve with stress echo in a subject with LV dysfunction and dilation (previous MI). The force-frequency relation is biphasic, with an initial up sloping trend followed by a later down sloping trend. The Δ rest-peak SP/ESV index is = 0.1 mmHg/ml/m2 (abnormal contractile reserve). Upper panels. Sensor built force-frequency relation simultaneously recorded in the same patients in which standard echo force-frequency relation was built. Left, normal up sloping force-frequency relation: the Δ rest-peak force is >7 g * 10-3 (normal contractile reserve cut-off value for pacing stress). Middle, the same biphasic pattern at ischemia found with stress echo. Right, abnormal biphasic force-frequency relation; the critical heart rate occurs at 110 bpm. The critical heart rate (or optimum stimulation frequency) is the human counterpart of the treppe phenomenon in isolated myocardial strips; the optimal heart rate is not only the rate that would give maximal mechanical performance of an isolated muscle twitch, but also is determined by the need for diastolic filling.