Response to exercise and mechanical efficiency in non-ischaemic stunning, induced by short-term rapid pacing in dogs: a role for calcium?

Aim: Rapid pacing (RP) is a regularly used model to induce heart failure in dogs. The aim of the study was to evaluate Ca handling, left ventricular (LV) contractile response during Ca administration compared to exercise, as well as oxygen consumption and mechanical efficiency after 48 h of RP.

Methods: Fifty-three mongrel dogs were instrumented to measure LV pressure, LV fractional shortening, regional wall thickening and coronary blood flow. Contractile reserve was measured with isoproterenol and intravenous (IV) Ca administration. To assess the function of the sarcoplasmic reticulum (SR), post-extrasystolic potentiation (PESP) and SR Ca uptake were measured. A graded treadmill test was performed in baseline and after RP (n = 14). In a separate group of animals (n = 5), myocardial performance and oxygen consumption were measured using a wide range of loading conditions.

Results: Left ventricular contractility was significantly decreased upon cessation of pacing. The contractile response to isoproterenol was blunted compared to a preserved response to IV Ca . Post-extrasystolic potentiation was slightly increased after RP. Maximal velocity (V ) of SR Ca uptake was unchanged. Contractile response during exercise is attenuated after RP. External work is reduced, whereas oxygen consumption is preserved, provoking a reduced mechanical efficiency.

Conclusion: Forty-eight-hours RP provokes a reversible LV dysfunction, while the SR function and response to exogenous Ca are preserved. This is compatible with an intracellular functional remodelling to counteract Ca overload provoked by RP. Left ventricular dysfunction is accompanied by a reduced contractile reserve, but an unchanged oxygen consumption, illustrating an alteration in oxygen utilization.