Cardiac Output — Afterload

Afterload

• Load against which the muscle exerts its contractile force (Guyton)
• It is represented by the gradient of the line connecting the end-diastolic volume, to the end-systolic point
• pressure which the ventricle has to contract against (Power & Kam)

Determinants of Afterload

Modified Laplace Equation

• where
  • T represents afterload
  • P represents aortic pressure
    • Therefore, afterload increases as aortic (or pulmonary arterial) pressure increases
  • R represents ventricular radius
    • Afterload increases as ventricular radius increases
  • H represents ventricular thickness
    • Afterload decreases as the thickness of the ventricular wall increases (hypertrophy secondary to chronic hypertension and cardiac remodelling)

Modified Hagen-Poiseuille Equation

• Afterload is affected by resistance to cardiac output
  • Afterload increased by reduced radius of systemic vasculature
  • Afterload increased by increasing viscosity of blood

Resistance in parallel

• Afterload is affected by addition, or loss of large capillary networks in parallel
  • Systemic vascular resistance is increased significantly by loss of placenta, with parallel vascular networks
  • Pulmonary vascular resistance is decreased significantly by inflation of lung, causing creation of vast capillary network in parallel

Factors affecting Right Ventricular Afterload

• Pulmonary vascular resistance increases afterload
  • Hypoxic vasoconstriction increases PVR
  • Lung volumes
    • Pulmonary vascular resistance minimal at FRC
• Increased pulmonary artery pressure increases RV afterload
  • Left heart failure
  • Critical mitral stenosis, mitral regurgitation
  • Pulmonary emboli
• Right ventricular outflow tract obstruction increases afterload
  • Pulmonary stenosis
  • Saddle pulmonary embolism
• Right ventricular dilation
  • Susceptible due to thin RV wall
  • Acute PE
    • RV spiral of death
      • Increased intraventricular pressure decreases blood flow à ischaemia à decreased contractility à further increase in ventricular volume

Factors affecting Left Ventricular afterload

systemic vascular resistance, aortic impedance and ventricular radius

According to La Place Equation

• Aortic pressure
  • Afterload increases as aortic pressure increases (increases with hypertension)
• Ventricular radius
  • Afterload increases as ventricular radius increases (increases with ventricular dilation)
• Ventricular wall thickness
  • Afterload decreases and ventricular wall thickness increases (decreases with ventricular hypertrophy)

Determinants of aortic pressure indirectly affect afterload

• Aortic compliance
  • Afterload decreases with increased compliance
• Arterial blood volume
  • Given set compliance, afterload will increase with arterial volume
• From Modifed Poisuille-Hagen Equation
  • Radius of aorta
    • Autonomic vasomotor tone
    • Increased intrathoracic pressure
    • Physical compression
  • Viscosity of blood
• From equation MAP = CO / SVR
  • Tissue flow autoregulation
    • Myogenic
    • Metabolic

Other

• Left ventricular outflow tract obstruction
  • Aortic stenosis
  • HOCM
  • Coarctation of aorta

Sakurai 2016

Answer by Sakurai