A 46-year-old woman comes to the emergency department due to progressive dyspnea over the last 2 days. The patient's exercise tolerance has decreased dramatically and she had to sleep in a sitting position last night. She has no history of cardiovascular disease. The patient does not use tobacco or alcohol. Family history is unremarkable. Blood pressure is 110/65 mm Hg and pulse is 105/min and regular. The apical impulse is hyperdynamic. Cardiac auscultation reveals a diminished S1 and an apical holosystolic murmur radiating to the axilla. Diffuse pulmonary crackles are heard bilaterally. There is no peripheral edema. ECG shows sinus tachycardia but is otherwise unremarkable. Which of the following would most likely increase the ratio of forward flow volume to regurgitant flow volume in this patient?
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This patient's dyspnea, orthopnea, and crackles on lung auscultation are consistent with decompensated left-sided heart failure; the apical holosystolic murmur radiating to the axilla suggests mitral regurgitation (MR) as a contributing factor. In patients with MR, some of the blood in the left ventricle is pumped forward through the aortic valve (forward stroke volume), and some is forced backward through the incompetent mitral valve (regurgitant stroke volume). The amount of blood flowing down each pathway is determined by the relative contribution of the resistance of each pathway to the total left ventricular afterload:
Resistance to forward flow is primarily determined by the pressure in the aorta (systolic blood pressure)
Resistance to regurgitant flow is determined by the mitral valve orifice size during systole and the degree of left atrial compliance. In chronic MR, the left atrium becomes more compliant and the lower left atrial pressures facilitate greater regurgitant flow.
Left atrial pressure remains relatively constant over the short term, but aortic pressure can vary significantly with changes in systemic vascular resistance. A reduction in systemic vascular resistance leads to reduced systemic blood pressure and an increase in the ratio of forward to regurgitant blood flow. Pharmacologic vasodilators (eg, nitroprusside) therefore help to increase forward cardiac output and reduce pulmonary congestion in patients with MR.
(Choices A and E) A decrease in heart rate leads to an increase in venous return to the left ventricle (preload) due to increased diastolic filling time. This causes an absolute increase in both forward and regurgitant blood flow (stroke volume increases in accordance with the Frank-Starling mechanism), but the ratio of forward to regurgitant flow is unlikely to significantly change.
(Choice C) A reduction in venous return to the left ventricle decreases stroke volume with absolute reductions in both forward and regurgitant flow, but is unlikely to significantly affect the ratio of forward to regurgitant flow.
(Choice D) An increase in left ventricular contractility increases stroke volume and leads to an increase in both forward and regurgitant blood flow. However again, as the relative resistance of each pathway is unchanged, the ratio of forward to regurgitant blood flow remains essentially the same.
Educational objective:
In patients with mitral regurgitation, left ventricular afterload is determined by the balance of resistance between forward flow (aortic pressure) and regurgitant flow (left atrial pressure). A reduction in systemic vascular resistance increases the ratio of forward to regurgitant blood flow and improves cardiac output.