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Afterload - Systemic Vascular Resistance Index (SVRI)

The afterload is another determinant of stroke volume/cardiac output. The physiological meaning of SVRI is that of the wall of the left ventricle during ejection. Following Laplace's law, the tension upon the muscle fibers in the heart wall is the product of the pressure within the ventricle and the ventricle's radius, divided by the ventricle wall thickness.

In the clinical context things are often simplified and so the afterload is seen as the resistance the heart has to pump against;  the systemic vascular resistance index (SVRI) is the parameter that represents this.

  • If the afterload (SVRI) is increased, the heart must pump with more power to eject the same amount of blood as before.
  • The higher the afterload, the less the cardiac output.
  • The lower the afterload, the higher the cardiac output.

If the afterload exceeds the performance of the myocardium, the heart may decompensate.

SVRI formula

SVRI graphic

SVRI image