Afterload

Afterload is another vital component of stroke volume and therefore blood pressure. Afterload is most often equated with ventricular wall tension. Wall tension is also considered as the pressure the ventricle must overcome to eject a volume of blood past the aortic valve. In most normal clinical situations, afterload is assumed to be proportional to systemic vascular resistance. Wall tension is greatest at the moment just before opening of the aortic valve. Wall tension is described by LaPlace's law:

Circumferential stress = Pr/2H

where circumferential stress is the wall tension, P is the intra-ventricular pressure, r is the ventricular radius, and H is the wall thickness.

An increase in afterload requires ventricular pressure to increase during isovolumic contraction before the aortic valve opens (Fig. 7). Because of the increase in afterload, the ability of the ventricle to eject blood is decreased. This results in decreased stroke volume (SV* in Fig. 7A) and increased end-systolic volume (B* in Fig. 7B). If afterload remains increased, the myocardium establishes a new steady state that is shifted to the right, and stroke volume is restored. A patient with severe

Increased Afterload Heart Graphics

Fig. 7. (A) Effect of acutely increasing afterload on the pressure-volume loop. An increase in afterload while maintaining normal contractility and preload results in decreased stroke volume (SV*). A higher pressure is also required before the aortic valve opens (C*). (B) Restoration of stroke volume after increasing afterload. An increase in afterload while maintaining normal contractility and preload results in decreased stroke volume (SV*). A higher pressure is also required before the aortic valve opens (C*). e, contractility line; SV, stroke volume.

Fig. 7. (A) Effect of acutely increasing afterload on the pressure-volume loop. An increase in afterload while maintaining normal contractility and preload results in decreased stroke volume (SV*). A higher pressure is also required before the aortic valve opens (C*). (B) Restoration of stroke volume after increasing afterload. An increase in afterload while maintaining normal contractility and preload results in decreased stroke volume (SV*). A higher pressure is also required before the aortic valve opens (C*). e, contractility line; SV, stroke volume.

aortic stenosis will likely elicit a pressure-volume loop as in Fig. 7. The myocardium usually compensates by increasing contractility to maintain adequate stroke volume. Patients with hemodynamically significant aortic stenosis often develop left ventricular hypertrophy.

Afterload may be inversely related to cardiac output. In a dysfunctional myocardium, such as with congestive heart failure, stroke volume decreases with increases in afterload. Increases in afterload also requires the myocardium to expend more energy to eject blood during systole.

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • Azzeza
    Why does heart failure increase afterload?
    5 years ago
  • matta
    Is afterload increased by vascular resistance and changes in ventricular wall tension?
    5 years ago
  • sven bauer
    Is afterload increased or decreased in left systolic heart failure?
    4 years ago

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