Question 7

College Answer

Many definitions of afterload were accepted. The main factors affecting left ventricular
afterload are systemic vascular resistance, aortic impedance and ventricular radius. Other
factors include blood viscosity and positive intrathoracic pressure. Good answers expanded
on the points above. Candidates who failed this question did not have enough facts.
Syllabus C1c C2c
Reference: Bray 4th edition p 342-344 and p 360-361

Discussion

"Many definitions of afterload were accepted" because no standard definition has been agreed upon after over a hundred years of physiologists debating each other. They all seem to agree on the sort of factors that are involved - just not on how these factors are related. The following attempt should not be viewed as a definitive expression of afterload - instead, it represents a Tetris-like attempt to stack these determinant factors together into a logical-looking rectangular array.

• 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.