Question 18

College Answer

Afterload can be defined as factors resisting ventricular ejection and contributing to myocardial wall stress during systole. Most answers utilised the law of Laplace to expand upon factors affecting ventricular wall tension. Systemic vascular resistance was commonly mentioned, but less frequently defined. Aortic and left ventricular outflow tract impedance were commonly referred to. Effects of preload and neurohumoral stimuli were less well outlined. Description of factors affecting right ventricular afterload and depictions of left ventricular pressure volume loops earned no extra marks unless directly referenced to the question.

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
  • Input impedance
• 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)  
    • Vessel radius (the smaller the radius, the greater the resistance)

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.

Sonnenblick, Edmund H., and S. Evans Downing. "Afterload as a primary determinant of ventricular performance." American Journal of Physiology-Legacy Content 204.4 (1963): 604-610.

Mills, C. J., et al. "Pressure-flow relationships and vascular impedance in man." Cardiovascular Research 4.4 (1970): 405-417.