Define afterload and describe the physiological factors that may affect afterload

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College Answer

Definitions for afterload vary slightly amongst common physiology textbooks, and 
candidates were expected to mention any one commonly accepted definition. Essentially 
afterload is the resistance to ventricular ejection - the "load" that the heart must eject blood 
against and is related to ventricular wall stress (Law of Laplace, T=Pt.r/u). Candidates were 
expected to mention aortic valve and systemic vascular resistance, aortic impedance, blood 
viscosity, intathoracic pressure and relationship of ventricular radius and volume. 
Candidates generally did well, but few substantially good answers, with a lack of detail being 
the biggest limiting factor.

Discussion

  • Afterload can be defined as the resistance to ventricular ejection - the "load" that the heart must eject blood against. It consists of two main sets of determinant factors:
    • Myocardial wall stress, which represents intracardiac factors
    • Input impedance, which represents extracardiac factors
  • Wall stress is described by the Law of Laplace ( P × r / T)
    and therefore depends on: 
    • P, the ventricular transmural pressure, which is the difference between the intrathoracic pressure and the ventricular cavity pressure.
      • Increased transmural pressure (negative intrathoracic pressure) increases afterload
      • Decreased transmural pressure (eg. positive pressure ventilation) decreases afterload
    • r, the radius of the ventricle
      • Increased LV diameter increases wall stress at any LV pressure
    • T,  the thickness of the ventricular wall
      • A thicker wall decreases wall stress by distributing it among a larger number of working sarcomeres
  • Input impedance describes ventricular cavity pressure during systole and receives contributions from:
    • Arterial compliance
      • Aortic compliance influences the resistance to early ventricular systole (a stiff aorta increases afterload)
      • Peripheral compliance influences the speed of reflected pulse pressure waves (stiff peripheral vessels increase afterload)
    • Inertia of the blood column
    • Ventricular outflow tract resistance (increases afterload in HOCM and AS)
    • Arterial resistance
      • Length of the arterial tree (the longer the vessels, the greater the resistance)
      • Blood viscosity (the higher the viscosity, the greater the resistance)  

References

Norton, James M. "Toward consistent definitions for preload and afterload."Advances in physiology education 25.1 (2001): 53-61.

ROTHE, CARL. "Toward consistent definitions for preload and afterload—revisited." Advances in physiology education 27.1 (2003): 44-45.

Vest, Amanda R. "Afterload." Cardiovascular Hemodynamics. Humana, Cham, 2019. 23-40.

Milnor, William R. "Arterial impedance as ventricular afterload." Circulation Research 36.5 (1975): 565-570.

Vlachopoulos, Charalambos, Michael O'Rourke, and Wilmer W. Nichols. McDonald's blood flow in arteries: theoretical, experimental and clinical principles. CRC press, 2011. (Specifically, Chapter 12 is gold)

Moriarty, Thomas F. "The law of Laplace. Its limitations as a relation for diastolic pressure, volume, or wall stress of the left ventricle.Circulation research 46.3 (1980): 321-331.

Covell, J. W., H. Pouleur, and Jr J. Ross. "Left ventricular wall stress and aortic input impedance." Federation proceedings. Vol. 39. No. 2. 1980.