Systemic Vascular Resistance (SVR) Calculator

SVR variables and formula

The SVR calculator accounts for three cardiovascular vital signs in order to provide information about the resistance the blood flow encounters in the systemic circulation.

■ Mean arterial pressure (MAP) is measured in mmHg by default but also available to input in cmH₂O, kPa, atm or psi.

MAP is the average between systolic and diastolic blood pressure readings off an arterial catheter or a cuff.

Normal values range between 70 and 100 mmHg for values of systolic BP of 90 to 140 mmHg and of diastolic BP of 60 to 90 mmHg.

■ Central venous pressure is also known as right atrial pressure and is measured in mmHg but can also be expressed in cmH₂O, kPa, atm or psi.

CVP is obtained from a central venous and/or a pulmonary artery catheter and normally ranges between 2 and 6 mmHg.

■ Cardiac output is measured in L/min and reflects the volume of blood pumped by the left ventricle during one minute. Normal values are between 4 and 8 L/min.

The formula used is:

SVR measured in dynes-sec/cm⁵ = 80 x (MAP in mmHg – CVP in mmHg) / CO in L/min

The simplification for SVR measured in mmHg-min/L is (MAP in mmHg – CVP in mmHg) / CO in L/min.

Similarly, pulmonary vascular resistance can be calculated.

About systemic vascular resistance

SVR reflects the resistance met by the blood across the entire systemic circulation from where it starts in the aorta to where it finishes, in the right atrium (related to left ventricle afterload).

Normal values range between 700 and 1600 dynes-sec/cm⁵.

This type of resistance is controlled by changes to the blood vessel circumference through hormones. Epinephrine and norepinephrine contract vessel muscle layer in order to increase resistance when it’s too low and determine a more rapid blood flow.

Two characteristics of blood vessels affect SVR: their length [l] where the longer the vessel, the higher the tendency of blood to sediment against vascular walls and their radius/circumference [r] where vessels with greater diameters pose less resistance.

Blood viscosity [η] is another factor that can affect SVR as increased viscosity is directly proportional to increased resistance.

SVR values are of relevance in a series of conditions, most of them related to the cardiovascular function. Low systemic vascular resistance values are consistent with:

■ Sepsis;

■ Spinal shock;

■ Hyperthermia;

■ AV fistula;

■ Adrenal insufficiency;

■ Anaphylaxis;

■ Vasodilator medication.

High values are indicative of the following:

■ Hypovolemia;

■ Cardiogenic shock;

■ Hypothermia;

■ Vasopressor medication.

References

1. Barratt-Boyes BG, Wood EH. Cardiac output and related measurements and pressure values in the right heart and associated vessels, together with an analysis of the hemo-dynamic response to the inhalation of high oxygen mixtures in healthy subjects. J Lab Clin Med. 1958; 51(1):72-90. 

2. Skimming JW, Cassin S, Nichols WW. Special Article: Calculating Vascular Resistances. Clin. Cardiol. 1997; 20, 805-808.

3. Lang RM, Borow KM, Neumann A, Janzen D. Systemic vascular resistance: an unreliable index of left ventricular afterload. Circulation. 1986; 74(5):1114-23.