Intramyocardial pressure and distribution of coronary blood flow during systole and diastole in the horse.

This research article focuses on understanding the distribution of coronary blood flow and the measurement of intramyocardial pressure during systole and diastole in horses, demonstrating the potential of the horse as a study model due to its thick ventricular walls and slow heart rate.

Measurement of Transmural Myocardial Blood Flow & Intramyocardial Pressure

• The study was conducted on seven anaesthetised horses, where both the intramyocardial pressure and transmural myocardial blood flow were measured during the systole and diastole phases of the cardiac cycle.
• They used a mini manometer implanted at the tip of a 16-gauge needle to measure the intramyocardial pressures.

Findings: Systole and Diastole

• In terms of systole, the peak systolic intramyocardial pressure decreased from subendocardium to subepicardium and never exceeded intraventricular pressure. This means that the pressure within the heart muscle is less than the pressure inside the ventricle and decreases as you move from the inner to the outer layer of the heart muscle.
• The blood flow during systole decreased from epicardium (outer layer of the heart muscle) to endocardium (inner layer), where it did not differ from zero. Essentially, this denotes that no significant blood flow took place from the epicardium to the endocardium during the systolic phase.
• Unlike systole, diastolic phase showed an increase in blood flow from epicardium to subendocardium but then dropped off in the endocardial layer, reaching a level similar to the immediate subepicardial layer. This indicates that during diastole, blood flow was predominant towards the inner layers of heart muscle, but decreased significantly at the most inner layer.

Validity of Horse as a Model

• These findings underscore the feasibility of using horses as a study model for examining these parameters. The thick ventricular walls of the horse’s heart and slow heart rate allow for easier and more accurate assessments during various phases of the cardiac cycle.
• While the measurements of transmural myocardial blood flow and intramyocardial pressure obtained were consistent with most other findings, a notable difference was the reduction in blood flow at the subendocardial level compared to those layers nearer the epicardium.