Systemic Vascular Resistance (SVR) Calculator

How does this systemic vascular resistance calculator work?

This health tool computes the SVR value based on three cardiovascular vital signs and offers information on the resistance the blood flow meets along the systemic circulation, with importance in vascular disease.

The fields in this systemic vascular resistance (SVR) calculator are completed with the values of:

■ Mean Arterial Pressure (MAP) – measured in mmHg by default but also available to input in cmH₂O, kPa, atm or psi;

■ Central Venous Pressure (CVP) – measured in mmHg but with the above pressure measurement units available as well;

■ Cardiac Output (CO) – measured in L/min.

Mean arterial pressure can be defined as an 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. Low MAP reflects decreased blood flow while high MAP is sign of increased cardiac workload.

Central venous pressure or Right Atrial Pressure, is obtained from a central venous and/or a pulmonary artery catheter and is normally between 2 and 6 mmHg. Low values indicate hypovolemia or decreased venous return while high values indicate hyper hydration, increased venous return or right sided cardiac failure.

Cardiac output is a hemodynamic measure of heart function and reflects the volume of blood pumped by the left ventricle during one minute. Normal values are between 4 and 8 L/min. From CO the cardiac index can be derived based on body surface area.

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.

Example of a SVR calculation

Taking the case of a patient with the following vital measurements:

■ MAP = 85 mmHg;

■ CVP = 5 mmHg;

■ CO = 4.5 L/min.

SVR = 80 x (85 – 5) / 4.5 = 1,422.22 dynes-sec/cm⁵

Systemic vascular resistance (SVR)

This is a calculated value that reflects the resistance the blood meets across the entire systemic circulation from the starting point in the aorta to the end point in the right atrium (related to left ventricle afterload).

Normal range for SVR is between 700 and 1600 dynes-sec/cm⁵.

Low values are consistent with:

■ Sepsis;

■ Spinal shock;

■ Anaphylaxis;

■ Hyperthermia;

■ Adrenal insufficiency;

■ AV fistula;

■ Vasodilator medication.

High values are indicative of the following:

■ Hypovolemia;

■ Hypothermia;

■ Cardiogenic shock;

■ Vasopressor medication.

Systemic vascular resistance is controlled mainly by changes to blood vessel circumference through hormones (for example, in low vascular resistance, epinephrine and norepinephrine act on the vessel muscle cells and contract them)

There are three main factors directly affecting SVR:

■ Length of blood vessel [l] – the longer the vessel, the blood will have the tendency to sediment against the walls;

■ Radius of blood vessel [circumference – r] – lower diameters pose greater resistance while dilated vessels pose less resistance. According to the below equation, with a decrease in circumference to a half of the original (i.e in vascular disease), resistance increases 16 fold;

■ Blood viscosity [η] – increased viscosity is directly proportional to increased resistance.

References

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

2) Barratt-Boyes BG, Wood EH. (1958) 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; 51(1):72-90.

3) Otsuki T, Maeda S, Iemitsu M, Saito Y, Tanimura Y, Ajisaka R, Miyauchi T. (2006) Contribution of systemic arterial compliance and systemic vascular resistance to effective arterial elastance changes during exercise in humans. Acta Physiol (Oxf);188(1):15-20.

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