Which formula defines Systemic Vascular Resistance (SVR)?

Which formula defines Systemic Vascular Resistance (SVR)?

• A. ((MAP / CO) × 80)
• B. ((MAP - CVP) / CO) × 80
• C. ((Mean PAP - PCWP) / CO) × 80
• D. (MAP - LVEDV) × CO

Answer: (B)

Systemic Vascular Resistance represents the afterload the left ventricle has to overcome and is determined by the pressure driving flow through the systemic circulation relative to the actual flow. The driving pressure is the gradient across the systemic circulation, approximated by mean arterial pressure minus the venous preload pressure, which is best represented by CVP. This pressure difference is then divided by cardiac output to reflect how much flow is produced for that pressure, and the 80 is a conversion factor to express SVR in the standard units of dynes·s·cm⁻⁵. So the form that captures this is to take (MAP minus CVP), divide by CO, and multiply by 80. This yields SVR. The other forms aren’t correct for SVR: using MAP alone ignores the venous pressure portion of the gradient; the formula with mean pulmonary artery pressure and wedge pressure corresponds to pulmonary vascular resistance, not systemic; and subtracting a volume term like LVEDV from pressure and then multiplying by CO mixes incompatible quantities and doesn’t describe vascular resistance.

Systemic Vascular Resistance represents the afterload the left ventricle has to overcome and is determined by the pressure driving flow through the systemic circulation relative to the actual flow. The driving pressure is the gradient across the systemic circulation, approximated by mean arterial pressure minus the venous preload pressure, which is best represented by CVP. This pressure difference is then divided by cardiac output to reflect how much flow is produced for that pressure, and the 80 is a conversion factor to express SVR in the standard units of dynes·s·cm⁻⁵.

So the form that captures this is to take (MAP minus CVP), divide by CO, and multiply by 80. This yields SVR.

The other forms aren’t correct for SVR: using MAP alone ignores the venous pressure portion of the gradient; the formula with mean pulmonary artery pressure and wedge pressure corresponds to pulmonary vascular resistance, not systemic; and subtracting a volume term like LVEDV from pressure and then multiplying by CO mixes incompatible quantities and doesn’t describe vascular resistance.