Cardiac Function Curve
- The Cardiac Function Curve describes how the cardiac output varies with the right atrial pressure.
- The shape of the curve is essentially explained by the Frank-Starling Relationship operating within its physiological range. This is based on the notion that in the normal heart the quantity of venous return determines both the right atrial pressure and the Ventricular End Diastolic Volume (VEDV). As more blood returns to the heart, the diastolic atrial pressures increase which in turn enhance VEDVs or in other words cardiac preload. The Frank-Starling Relationship states the increased venous return to the heart boosts the heart's cardiac output through intrinsic mechanical mechanisms of the myocardium. This is reflected in the graph above in which increased right atrial pressures, indicating larger venous returns, boost the heart's cardiac output.
- The primary modulator of the cardiac function curve is the autonomic nervous system as described in autonomic cardiac regulation. Increased SNS activity boost the heart's contractility and heart rate, resulting in more cardiac output for any given amount of right atrial pressure. Conversely, reduced SNS activity and increased PNS tone reduce the heart's contractility and the heart rate, resulting in less cardiac output for any given amount of right atrial pressure. This is reflected in the graph above with increased and decreased slopes of the cardiac function curve occurring in response to increased or decreased nervous stimulation of the heart.