Role of force--frequency relation during AV-block, sinus node block and beta-adrenoceptor block in conscious animals

Heart muscle contraction is influenced by several mechanisms, including adrenergic stimulation and the force-frequency relationship. While the force-frequency effect is well-documented in isolated and in vivo settings, its role in specific clinical scenarios remains unclear. Prior research has shown that this relationship enhances contractility when heart rate increases. However, it is not fully understood how this effect behaves when heart rate cannot increase, such as in cases of AV block or beta-blocker use. This gap motivated researchers to explore how restricted heart rate affects contractile responses during sympathetic stimulation. Existing knowledge suggests that adrenergic stimulation increases contractility, but the extent of this effect when heart rate is constrained is uncertain. This uncertainty drove the current investigation into the interplay between heart rate and contractility under restricted conditions. No prior work had resolved how the force-frequency relationship operates in the absence of a chronotropic response. This study aimed to clarify the role of heart rate in modulating contractile responses during sympathetic activation.

The goal was to determine how the force-frequency relationship influences contractility during sympathetic stimulation when heart rate is restricted. Researchers focused on three specific conditions: AV block, sinus node block, and beta-adrenoceptor block. These scenarios prevent normal heart rate increases, allowing for a clearer assessment of the force-frequency effect's contribution. The study aimed to test whether contractility can still be enhanced when heart rate is artificially controlled. The motivation stemmed from the need to understand how the force-frequency relationship functions in the absence of a chronotropic response. By manipulating heart rate independently, the researchers sought to isolate the effect of frequency on contractility. This approach allowed them to assess whether the force-frequency relationship alone could drive contractile responses. The study's design aimed to clarify the interdependence between heart rate and contractility during adrenergic stimulation.

The study involved 19 instrumented dogs with implanted devices to monitor left ventricular pressure, aortic and pulmonary artery pressures, and left atrial function. Researchers used pacing leads to control heart rate independently of adrenergic stimulation. AV block was induced surgically in nine dogs, while sinus node block was achieved using UL-FS 49 in another group. Beta-adrenoceptor block was administered via propranolol in ten dogs. Adrenergic stimulation was simulated using isoproterenol infusion or treadmill exercise. Left ventricular pressure development (LVdP/dt) was measured before and after stimulation. Heart rate was manipulated using atrial pacing to test whether contractility could be enhanced independently of chronotropic changes. The experimental setup allowed for precise control of heart rate while measuring contractile responses.

In dogs with permanent AV block, adrenergic stimulation failed to increase LVdP/dt despite isoproterenol infusion or exercise. However, when heart rate was increased via atrial pacing, LVdP/dt rose significantly from 4762 ± 166 to 6485 ± 381 mmHg/s (p < 0.05). In sinus node block cases, heart rate and LVdP/dt responses to isoproterenol were reduced compared to control conditions. When heart rate was matched using pacing, LVdP/dt(max) reached 7310 ± 550 mmHg/s, similar to control values. Beta-adrenoceptor block also reduced heart rate and LVdP/dt responses to sympathetic stimulation. Atrial pacing partially restored LVdP/dt(max) during beta-blockade. These findings suggest that the force-frequency relationship is a key contributor to contractility when heart rate increases. Without a chronotropic response, the inotropic effect is significantly impaired.

The study suggests that the force-frequency relationship plays a major role in the inotropic response during sympathetic stimulation. When heart rate increases are restricted, as in AV block or beta-blockade, contractility is also reduced. The findings indicate that the chronotropic response is essential for the full expression of the inotropic effect. Atrial pacing can partially restore contractility in these conditions, but not completely. These results support the idea that heart rate and contractility are interdependent during adrenergic stimulation. The study highlights the importance of the force-frequency relationship in modulating cardiac function. Without a chronotropic response, the inotropic effect is significantly impaired. The results suggest that heart rate is a critical factor in the full expression of adrenergic-induced contractility.