A 24-year-old healthy woman participates in a pharmacologic clinical study. Various cardiovascular parameters are measured while she is resting in an upright position. These baseline values are plotted on the cardiac and vascular function graph shown below.
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She receives a moderate dose of intravenous nitroglycerin by continuous infusion, and the measurements are repeated. Which of the following changes will most likely be observed?
The cardiovascular system is a closed circuit: Blood ejected from the left ventricle (cardiac output) returns to the right atrium (venous return) to pass through the lungs and be ejected once again. Cardiac function–vascular function curves demonstrate the interdependent relationship between cardiac output and venous return. These curves can be described as follows:
The cardiac function curve is essentially a Frank-Starling curve, representing constant left ventricular contractility (inotropy) along the curve. As right atrial pressure (or preload) increases, cardiac output increases due to myocardial fiber stretching and a corresponding increase in stroke volume. Changes in contractility shift the curve to the left or the right.
The vascular function curve represents venous return to the right atrium. As right atrial pressure increases, venous return decreases as there is a reduced pressure gradient driving blood return to the right atrium. Changes in blood volume, venous capacitance, or systemic vascular resistance (SVR) shift the curve to the left or the right.
The intersection point of the curves represents the steady-state right atrial pressure and cardiac output for a given set of cardiac and vascular parameters.
This study participant was given nitroglycerin, which primarily causes venodilation and increased venous capacitance. The result is additional blood pooling in the capacitance veins and decreased venous return, represented by a shift in the venous return curve to the left. The cardiac function curve is unchanged as contractility is not affected. A new steady-state point develops at a lower right atrial pressure and cardiac output.
(Choices A and B) A shift in the cardiac function curve to the left represents an increase in contractility. This change, with no change in the vascular function curve, is expected with administration of a positive inotrope that has no effect on the vasculature (eg, digoxin). A shift in the cardiac function curve to the right represents a decrease in contractility, as is expected with administration of a negative inotrope (eg, verapamil).
(Choice C) A shift in the venous return curve to the right represents an increase in venous return. This change, with no change in the cardiac function curve, is expected with the increase in blood volume that occurs with infusion of normal saline.
(Choice E) A leftward shift in the cardiac function curve combined with a rightward shift in the vascular function curve can be seen with the increase in contractility and decrease in SVR that occur with exercise.
Educational objective:
Cardiac function–vascular function curves demonstrate the interdependent relationship between cardiac output and venous return. Nitroglycerin is a venodilator that increases venous capacitance to reduce venous return and shift the vascular function curve to the left.