20B18: Exam Report

Define afterload (10% marks) and describe the physiological factors that may affect afterload on the left ventricle (90% marks).

53% of candidates passed this question.

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.

G3iii / 20B18: Define afterload & describe the physiological factors that may affect afterload

Definition

Afterload = the sum of all factors required to overcome so that blood may be ejected from the heart to the arterial circulation

  • AfterL gives rise to wall tension in the ventricle → afterL can be thought of as the wall tension required to overcome impedance to eject blood into arterial circulation

∴Factors which determine AFTERLOAD are:

  1. Factors affecting wall tension
  2. Factors affecting outflow

1) Factors Affecting Wall Tension

    • Ventricle shaped like a sphere
    • ∴Obeys Law of Laplace which describes the P-V relationship of spheres
    • \(  \text{Wall Tension = }  \frac{\text{ Transmural P x Radius }}{\text{2 x Thickness}}  \)
    • Ventricle transmural pressure
      • Transmural Pressure = Intraventricular P – Intrapleural P
      • ↑transmural P = ↑wall tension = ↑afterload
      • ↑pressure from: negative intrathoracic P (asthma, obstructed airway)
    • Ventricle wall thickness
      • ↑wall thickness = ↓wall tension = ↓afterL
      • Because ↑ no. myofibres sharing the load i.e. LV hypertrophy
    • Ventricle radius
      • ↑radius = ↑wall tension = ↑afterload
        • i.e. cardiomyopathy
        • Ventricular dilatation
        • Fluid overload

2) Factors Affecting Outflow

  • SVR/PVR
    • SVR is the main determinant of AfterL (LV)
    • Normally 1440 dynes/sec/cm5
    • SVR = R to blood flow by all systemic vessels (excluding pulmonary)
    • Arterioles → largest resistors ∴main determinant of SVR
    • Based on H – P equation
    • \(  \text{R = }  \frac{\text{ 8hL }}{\text{πr4}}  \)

∴radius has biggest influence

    • ↑radius = ↓R = ↓afterL
    • ↑viscosity = ↑R = ↑afterL
    • ↑length of vessel = ↑R = ↑afterL 
  • Outflow tract resistance
    • Any ↑resistance to outflow = ↑afteRL
      • i.e. aortic stenosis/pulmonary stenosis
      • SAM
      • Tension PTX
      • Saddle embolus (RV) 
  • Aortic root compliance
    • C = ∆ volume/pressure
    • Any ↓compliance = ↑afterL i.e. atherosclerosis aortic root