Load-dependent left ventricular relaxation in conscious dogs.

Area of Science:

• Cardiovascular Physiology
• Cardiac Mechanics
• Myocardial Function

Background:

• Understanding myocardial relaxation is crucial for diagnosing and treating cardiac dysfunction.
• The influence of loading conditions on the rate and duration of left ventricular (LV) relaxation requires further elucidation.

Purpose of the Study:

• To investigate the load-dependent nature of myocardial relaxation in conscious dogs.
• To assess the impact of pharmacologic interventions on load-dependent relaxation.

Main Methods:

• Utilized an intra-aortic balloon to manipulate left ventricular (LV) systolic pressure in eight conscious dogs.
• Measured LV pressure transients, including P50, peak negative first time derivative (-dP/dt), and time constant of relaxation (tau).
• Administered propranolol, verapamil, or caffeine to evaluate their effects on LV relaxation and load dependency.

Main Results:

• Late systolic intra-aortic balloon inflation accelerated LV pressure decline and abbreviated contraction duration.
• Propranolol and verapamil slowed LV relaxation but did not alter the qualitative effects of balloon inflation.
• Caffeine attenuated the abbreviation of contraction duration, indicating an influence on load-dependent relaxation.

Conclusions:

• Myocardial relaxation in the intact conscious dog is demonstrably load-dependent.
• Load-dependent relaxation persists despite pharmacologic slowing of relaxation by propranolol or verapamil.
• Caffeine attenuates load-dependent relaxation, likely through modulation of sarcoplasmic reticulum function.