A 44-year-old man is hospitalized with multiple injuries following a motor vehicle collision. An arteriovenous fistula created by an injury would most likely result in which of the following changes to the left ventricular pressure-volume loop?Solid line = preinjury
Dashed line = postinjury

Question

A 44-year-old man is hospitalized with multiple injuries following a motor vehicle collision.  An arteriovenous fistula created by an injury would most likely result in which of the following changes to the left ventricular pressure-volume loop?Solid line = preinjury
Dashed line = postinjury

Quizplus · Accepted Answer

11ec160b_3c89_8724_9249_3f2a007e044f_MD0001_00 The left ventricular (LV) pressure-volume loop follows the cardiac cycle. At point 1, the left ventricle is full of blood and begins isovolumetric contraction until the aortic valve opens at point 2, causing blood to eject into the aorta and LV volume to decrease. LV contraction ends at point 3, and LV pressure rapidly decreases to point 4, where the mitral valve opens and LV diastolic filling begins again. An arteriovenous fistula, which can be congenital (eg, arteriovenous malformation) or acquired (eg, traumatic injury, surgical placement for hemodialysis), allows blood to shunt from the arterial circulation to the venous circulation without passing through the high resistance of the systemic arterioles. This has the following effects on the LV pressure-volume loop: The low-resistance fistula pathway creates overall reduced systemic vascular resistance (ie, reduced afterload). Blood quickly and easily passes through the reduced peripheral resistance, resulting in increased venous return to the right atrium (ie, increased preload). Both the reduced afterload and increased preload facilitate increased stroke volume, which increases cardiac output and helps maintain blood pressure. Contractility is unchanged initially but may eventually increase via the baroreceptor reflex if there is sustained hypotension. With a large arteriovenous fistula, the left ventricle may be unable to keep up with the increased venous return despite a sustained increase in cardiac output, leading to high-output heart failure. (Choice A) This pressure-volume loop shows increased contractility with a resulting increase in stroke volume and slight increase in afterload. Such changes are expected with administration of a positive inotropic agent such as digoxin. (Choice C) This pressure-volume loop shows increased afterload with a resulting decrease in stroke volume. Contractility is unchanged. Such changes are expected with the obstruction to LV outflow that occurs with aortic stenosis. (Choice D) This pressure-volume loop shows decreased preload with a resulting decrease in stroke volume and slight decrease in afterload. Such changes are expected with administration of a primarily venous vasodilator such as nitroglycerin. Educational objective: An arteriovenous fistula allows blood to shunt from the arterial circulation to the venous circulation, bypassing the high resistance of the systemic arterioles. This leads to reduced systemic vascular resistance (ie, reduced afterload), increased venous return (ie, increased preload), and increased stroke volume. Left ventricular contractility is unchanged.

A 44-Year-Old Man Is Hospitalized with Multiple Injuries Following a Motor