Systemic Vascular Resistance (SVR) Calculator
Systemic Vascular Resistance (SVR) Calculator
SVR and SVRI in both unit systems at once, because the two differ by exactly 80 and quoting the wrong one is the commonest eighty-fold error in haemodynamics.
Systemic vascular resistance and its index
80 × (MAP − CVP) ÷ COMAP 85 mmHg, CVP 6 mmHg, cardiac output 5.5 L/min, BSA 1.9 m²
Two unit systems, and the 80 between them
SVR (Wood units, mmHg·min/L) = (MAP − CVP) ÷ CO
SVRI (dyn·s·cm⁻⁵·m²) = 80 × (MAP − CVP) ÷ cardiac index = SVR × BSA
- where the 80 comes from
- it is a pure unit conversion and nothing physiological. One mmHg is 1333.22 dyn/cm², and one litre per minute is 1000 ÷ 60 = 16.667 cm³/s. So one mmHg·min/L is 1333.22 ÷ 16.667 = 79.99 dyn·s·cm⁻⁵, which every published formula rounds to 80
- Wood units
- mmHg·min/L, also written HRU (hybrid resistance units). Named after Paul Wood. Divide a dyn·s·cm⁻⁵ figure by 80 to get them
- the trap
- systemic resistance is conventionally quoted in dyn·s·cm⁻⁵ and pulmonary resistance in Wood units. Read one convention’s number against the other convention’s range and you are eighty-fold out — a systemic resistance of 14 Wood units looks catastrophically low beside 800 to 1200, and is in fact 1149 dyn·s·cm⁻⁵ and normal
- MAP − CVP
- the driving pressure across the systemic circulation. The CVP is subtracted, not ignored: at a MAP of 70 and a CVP of 18 the driving pressure is 52, not 70
- indexed is not a smaller number
- SVRI uses the cardiac INDEX, not the cardiac output. Since the index is the output divided by surface area, dividing by it is multiplying by surface area — so SVRI = SVR × BSA and is LARGER than SVR in any adult. That is why the published index range, 1970 to 2390, sits above the non-indexed 800 to 1200
- what resistance is not
- it is not measured. It is a quotient of a pressure difference and a flow, and it inherits the error of both. It also assumes steady laminar flow through a rigid tube, which the arterial tree is not — pulsatility, compliance and wave reflection are all discarded by the time this single number is reached
Worked example
MAP 85 mmHg, CVP 6 mmHg, cardiac output 5.5 L/min, BSA 1.9 m²
Driving pressure = 85 − 6 = 79 mmHg
In Wood units: 79 ÷ 5.5 = 14.36 mmHg·min/L
In dyn·s·cm⁻⁵: 80 × 14.36 = 1149 dyn·s·cm⁻⁵ — inside the quoted 800 to 1200
Both figures describe the same resistance. 1149 ÷ 80 = 14.36, exactly
Cardiac index = 5.5 ÷ 1.9 = 2.89 L/min/m²
SVRI = 80 × 79 ÷ 2.8947 = 2183 dyn·s·cm⁻⁵·m², which is also 1149 × 1.9. Inside the quoted 1970 to 2390 — and LARGER than the non-indexed figure, which is the sign that the index was taken the right way round
Take the index the wrong way round — multiply by the surface area in the denominator instead of dividing — and you get 605 rather than 2183. At a BSA of exactly 1.00 m² those two agree, which is why a transposed index is invisible in a test on an average-sized patient
Forget the CVP entirely and the resistance becomes 80 × 85 ÷ 5.5 = 1236, 8% high. At a CVP of 18 the same omission would overstate it by 27%
The same resistance in both unit systems
| Wood units (mmHg·min/L) | dyn·s·cm⁻⁵ | Reads as, against 800–1200 |
|---|---|---|
| 5.0 | 400 | Low |
| 10.0 | 800 | At the lower bound |
| 14.36 | 1149 | Within the range — this page’s example |
| 15.0 | 1200 | At the upper bound |
| 20.0 | 1600 | High on the Edwards range, at the top of the wider one |
| 25.0 | 2000 | High on both |
Published ranges, and the two sources that disagree
| Figure | Edwards and WVU cards | The wider published range | In Wood units |
|---|---|---|---|
| SVR | 800 to 1200 dyn·s·cm⁻⁵ | 700 to 1600 dyn·s·cm⁻⁵ | 10.0 to 15.0 against 8.75 to 20.0 |
| SVRI | 1970 to 2390 dyn·s·cm⁻⁵·m² | Not stated by the wider source | 24.6 to 29.9 ·m² |
| CVP | 2 to 6 mmHg | Not stated | — |
| Driving pressure, MAP − CVP | Not published as a range | — | — |
One resistance, two unit systems, and the factor of exactly 80
Systemic vascular resistance is a quotient, not a measurement: the pressure drop across the systemic circulation divided by the flow through it. The arithmetic has exactly one trap. In the units a bedside monitor works in — millimetres of mercury and litres per minute — the quotient comes out in mmHg·min/L, which are Wood units. In the CGS units the critical-care literature settled on, it comes out in dyn·s·cm⁻⁵. The two differ by a factor of 80, and that 80 contains no physiology: one mmHg is 1333.22 dyn/cm², one litre per minute is 16.667 cm³/s, and 1333.22 divided by 16.667 is 79.99.
The reason it matters is a convention split. Systemic resistance is almost always quoted in dyn·s·cm⁻⁵, where normal is a four-figure number. Pulmonary resistance is almost always quoted in Wood units, where normal is a single digit and the 2022 ESC/ERS guidelines define pulmonary hypertension around a threshold of 2. A reader who carries one convention’s instincts into the other’s report is eightyfold out, in whichever direction. This page prints both figures on every calculation for that reason, and the Wood-unit value is the one the raw division gives — the 80 is applied afterwards.
The indexed figure has its own transposition trap. SVRI divides the driving pressure by the cardiac INDEX rather than by the cardiac output, and since the index is the output divided by body surface area, dividing by it multiplies by the surface area. So SVRI is larger than SVR in any adult, and the published index range, 1970 to 2390, sits above the non-indexed 800 to 1200 rather than below it. An indexed figure that came out smaller means the multiply and the divide have been swapped — and at a surface area of exactly 1.00 m² the two versions agree, which is why that error survives being tested on an average patient.
Two cautions about meaning. The driving pressure is MAP minus CVP and the subtraction is not optional: at a MAP of 70 and a CVP of 18 it is 52, and ignoring the venous pressure overstates the resistance by a third. And because the cardiac output sits in the denominator, a high calculated resistance is produced just as readily by a low output as by vasoconstriction — cardiogenic shock, hypovolaemia and a mismeasured output all land there. Underneath it all the calculation assumes steady laminar flow through rigid tubes, discarding pulsatility, compliance and wave reflection entirely. Every figure here is derived from other measurements, so it carries their errors as well as its own. A derived index is never more reliable than the least reliable number that went into it. A derived haemodynamic number is read alongside the patient — the history, the perfusion, the lactate, the trend across serial measurements — and never instead of them. It supports a clinician’s judgement rather than replacing it.
Frequently asked questions
What is the formula for systemic vascular resistance?
SVR in dyn·s·cm⁻⁵ is 80 × (MAP − CVP) ÷ cardiac output, with pressures in mmHg and output in L/min. Without the 80 the same division gives Wood units, mmHg·min/L. At a MAP of 85, a CVP of 6 and an output of 5.5 L/min that is 14.36 Wood units, or 1149 dyn·s·cm⁻⁵.
Where does the factor of 80 come from?
It is a unit conversion. A pressure of 1 mmHg is 1333.22 dyn/cm², and a flow of 1 L/min is 1000/60 = 16.667 cm³/s. Dividing one by the other gives 79.99 dyn·s·cm⁻⁵ per mmHg·min/L, which published formulas round to 80. There is no physiology in it.
What is a normal systemic vascular resistance?
Two widely used sources disagree and this page prints both. The Edwards Lifesciences and West Virginia University SICU reference cards give 800 to 1200 dyn·s·cm⁻⁵, which is 10.0 to 15.0 Wood units. The most widely circulating general reference gives 700 to 1600 dyn·s·cm⁻⁵, 8.75 to 20.0 Wood units. A resistance of 1400 is therefore high on one and normal on the other. Ranges are population- and method-dependent and your own unit’s values take precedence.
Why is SVRI bigger than SVR?
Because the index puts the cardiac index in the denominator instead of the cardiac output, and the cardiac index is the output divided by body surface area. Dividing by a smaller denominator gives a larger answer, so SVRI = SVR × BSA. The published ranges reflect it: 1970 to 2390 indexed against 800 to 1200 non-indexed. An indexed figure smaller than the non-indexed one means the multiply and divide have been transposed.
Does a high SVR mean the patient is vasoconstricted?
Not on its own. The cardiac output is in the denominator, so a low output raises the calculated resistance just as effectively as a narrowed vasculature does — cardiogenic shock and hypovolaemia both produce a high SVR, and so does an underestimated cardiac output. Resistance is a quotient and has to be read beside the two numbers it was made from.
Related calculators
References
- Edwards Lifesciences. Normal Hemodynamic Parameters and Lab Values (EU master reference card). Cardiac output 4–8 L/min, cardiac index 2.5–4 L/min/m², stroke volume 60–100 mL/beat, stroke volume index 33–47 mL/m²/beat, SVR 800–1200 and SVRI 1970–2390 dyn·s·cm⁻⁵ (·m²), PVR 100–250 dyn·s·cm⁻⁵, MAP 70–105 mmHg, CVP 2–6 mmHg, PAWP 6–12 mmHg; prints MAP as “[SBP + (2 x DBP)]/3” and SVR as “MAP-RAP x 80/CO”.
- West Virginia University Medicine, Surgical Intensive Care Unit. Normal Hemodynamic Parameters — Adult. An independent reference card that agrees row for row with the Edwards figures used here, and prints the resistance formulas with the 80 written out: “80 x (MAP – RAP)/CO” and “80 x (MPAP – PAWP)/CO”.
- Vascular resistance. Wikipedia. The widest-circulating statement of the unit relation — “dyn·s/cm^5” against “mmHg·min/L or HRU, aka Wood units”, with the 80:1 ratio derived from 1 mmHg = 1333.22 dyn/cm² and 1 L/min = 16.67 cm³/s. Quotes SVR as 700–1600 dyn·s/cm⁵ (9–20 Wood units) and PVR as 20–130 (0.25–1.6 Wood units), which do not agree with the reference cards — hence both are printed here.
- Humbert M, Kovacs G, Hoeper MM, et al. 2022 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Heart J. 2022;43(38):3618–3731. PH “is defined by a mean pulmonary arterial pressure (mPAP) >20 mmHg at rest”; “the upper limit of normal PVR and the lowest prognostically relevant threshold of PVR is 2 Wood units”; the diastolic pressure gradient is “no longer used” to separate the two post-capillary patterns “because of conflicting data”.
- Vincent J-L, De Backer D. Circulatory shock. N Engl J Med. 2013;369(18):1726–34. Sets out the four mechanisms of shock and why a single haemodynamic number does not identify which one a patient is in.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/