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What is SVRI?

Published in Cardiovascular Physiology 2 mins read

Systemic Vascular Resistance Index (SVRI) is a calculated measurement that reflects the afterload the heart must overcome to pump blood throughout the body, adjusted for body surface area.

Here's a more detailed breakdown:

  • Definition: SVRI is an index derived from systemic vascular resistance (SVR) that is normalized to the patient's body surface area (BSA). This normalization allows for comparison of afterload between individuals of different sizes.

  • Calculation: SVRI is typically calculated using the following formula:

    SVRI = SVR / BSA

    Where:

    • SVR (Systemic Vascular Resistance) = [(MAP - CVP) / CO] x 80
      • MAP = Mean Arterial Pressure
      • CVP = Central Venous Pressure
      • CO = Cardiac Output
    • BSA = Body Surface Area (calculated using various formulas, such as the Mosteller formula)

  • Significance: SVRI provides valuable information about the resistance the heart faces when trying to circulate blood. A high SVRI indicates increased afterload, meaning the heart has to work harder to eject blood. Conversely, a low SVRI suggests decreased afterload, making it easier for the heart to pump.

  • Clinical Implications:

    • Increased SVRI: Can be caused by conditions like hypertension, hypovolemia (fluid deficit), or vasoconstrictor medications. An increased SVRI puts more strain on the heart, potentially leading to decreased cardiac output. If the afterload (SVRI) is increased, the heart must pump with more power to eject the same amount of blood as before.

    • Decreased SVRI: Can be caused by conditions like sepsis, vasodilation from certain medications, or anaphylaxis. A decreased SVRI can lead to hypotension (low blood pressure) due to reduced vascular resistance. The lower the afterload, the higher the cardiac output.

  • Monitoring: SVRI is commonly monitored in critically ill patients, especially those with hemodynamic instability or those requiring vasoactive medications. Monitoring trends in SVRI helps clinicians adjust treatments to optimize cardiac function and tissue perfusion.

  • Example: A patient with septic shock might have a very low SVRI due to widespread vasodilation. In this case, a clinician might administer vasopressors to increase SVR and, consequently, SVRI, to improve blood pressure and tissue perfusion.