Ejection has both positive and negative effects on left ventricular isovolumic relaxation

Area of Science:

• Cardiovascular Physiology
• Cardiac Mechanics

Background:

• Left ventricular (LV) relaxation speed in isovolumic beats is determined by peak active stress.
• The relationship between ejection and LV relaxation is less understood, though ejection is generally believed to hasten relaxation.

Purpose of the Study:

• To investigate how cardiac ejection modifies the relationship between LV relaxation and peak active stress.
• To develop a model predicting LV relaxation during ejection beats.

Main Methods:

• Isolated rabbit hearts were subjected to varied loading conditions.
• Left ventricular pressure and volume were measured to calculate active stress.
• Relaxation time (Tr) was defined as the time for stress to fall from 75% to 25% of peak stress.
• A multiple regression model was developed using steady-state and transient isovolumic and ejection beat data.

Main Results:

• Peak active stress, current beat ejection, and previous beat ejection history were identified as independent predictors of relaxation time (Tr).
• The model accurately predicted relaxation in isovolumic beats (r2 > 0.98) and showed predictive capability for transient ejection beats (r2 = 0.30).
• Current ejection hastened relaxation (negative coefficient), while ejection history slowed it (positive coefficient), resulting in a net hastening effect.

Conclusions:

• Cardiac ejection significantly impacts LV relaxation dynamics, with both immediate and historical effects.
• The observed effects suggest non-mechanical mechanisms, possibly related to ejection-mediated alterations in myofilament calcium sensitivity.