Key Points
- Cardiac output (CO) is commonly measured using a pulmonary artery catheter (PAC) via thermodilution or the Fick method.
- Both pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) are calculated as the pressure gradient across the vascular bed divided by the CO.
- SVR is most commonly expressed in dynes•sec•cm-5, while PVR is typically expressed in Wood units (WU = mmHg•min•L-1), which can be converted to dynes•s•cm-5 by multiplying by 80 (e.g., 1 WU = 1 mmHg•min/L = 80 dynes•s/cm5).
Measurement of CO
- Several methods can measure CO, each with its own advantages and limitations. Please see the OA summary on CO monitoring in anesthesia and critical care for more details. Link
- CO is typically measured using a PAC, also known as the Swan-Ganz catheter, with either the thermodilution or Fick method.1
- PAC thermodilution: A known volume of room-temperature fluid is injected into the proximal port of the PAC (usually in the right atrium), and the resulting temperature change is detected by the thermistor at the catheter tip (in the pulmonary artery). The temperature–time curve generated by the thermistor is then used to calculate CO.
- Method of choice for measurement of CO.
- Thermodilution CO measurements may be inaccurate in the presence of intracardiac or extracardiac shunts, low CO, or significant tricuspid or pulmonary insufficiency, because these conditions alter normal right heart to pulmonary artery flow, causing the injected cold saline to disperse irregularly and producing an unreliable temperature–time curve.2
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- The direct Fick method is the gold standard because it directly measures oxygen consumption (VO2) using specialized equipment such as respirometry.
- The indirect Fick method estimates VO2 using a nomogram based on age, sex, height, and weight.
- Both methods calculate arterial (CaO2) and venous (CvO2) oxygen content using the same formula, obtaining arterial blood from an arterial line and mixed venous blood from the distal port of a PAC.
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- Central venous oxygen saturation (ScvO2) obtained from a central venous catheter can be used as an estimate for SvO2 when a PAC is not feasible, but this substitution may introduce significant error.
- The Fick method can be used with cardiac shunts.2
Calculation of PVR
- mPAP and PCWP are obtained through right heart catheterization.
- The mPAP in adults is typically 16 mmHg, with a normal range of 10 to 22 mmHg.1
- PCWP, also referred to as pulmonary artery wedge pressure or pulmonary artery occlusion pressure, is measured by inflating the balloon of a PAC and advancing the catheter until it becomes wedged in a small pulmonary artery branch.
- PCWP serves as a surrogate for left atrial pressure (LAP) in the absence of pulmonary vein obstruction.4
- PCWP is a poor reflection of LAP in patients with severe mitral regurgitation, left atrial enlargement, atrial arrhythmias, left ventricular assist devices, or significant pulmonary disease, including those on mechanical ventilation.5
- PCWP is best measured with the patient in supine position with the tip of the PAC in West zone 3 (dependent regions) of the lung, where the pulmonary capillary pressure exceeds alveolar pressure and ensures true reflection of LAP.1
- Measurements should be taken at least three times and averaged for reliability.
- Normal PCWP ranges from 6 to 15 mmHg, with a mean of 9 mmHg.1
Figure 1. Pulmonary Capillary Wedge Pressure (https://cvphysiology.com/heart-failure/hf008) by Richard E. Klabunde, PhD, (https://cvphysiology.com/author) posted with permission and accessed on Nov 20, 2025
Normal PAC pressure waveform tracing.
RA, right atrium (blue waveform): “a” atrial systole/ contraction, “c” closure of tricuspid valve, “x” atrial relaxation, “v” ventricular systole/ contraction (and atrial diastole), “y” passive filling of RV.
RV, right ventricle (yellow waveform): “S” systole, “D” diastole, “ED” end diastole.
PA, pulmonary artery (red waveform): “S” systole, “D” diastole, “Di” dicrotic notch, mean PA pressure is PAS + (PAD x 2) / 3.
PCW, pulmonary capillary artery wedge (green waveform): “a” atrial contraction, “v” ventricular contraction, mean PCW pressure is mean of “a” descent.
Figure 2. Normal pulmonary artery catheter pressure waveform tracing.
Source: Isseh IN, et al. A critical review of hemodynamically guided therapy for cardiogenic shock: Old habits die hard. Curr Treat Options Cardiovasc Med. 2021.7
- In spontaneously breathing patients, mPAP and PCWP should be measured at end-expiration, when intrathoracic pressure is closest to atmospheric pressure, yielding the most accurate values.1,4
- In patients on controlled mechanical ventilation, averaging the PCWP over several respiratory cycles is advisable, as this can reduce error caused by pressure changes that occur throughout the breath cycle.4,6,7
PCWP tracings during different modes of respiration during
a normal respiration: PCWP measured at end expiration;
b PVV: in a typical patient under mechanical ventilation, positive-pressure ventilation increases intrathoracic pressures. Usual hemodynamic tracings during positive-pressure ventilation rise during inhalation and fall during exhalation, and typical PCWP measurements are made in the troughs of the respiratory cycle.
c PVV: note here that there is more significant negative intrathoracic pressure generated by the patient’s respiratory muscles (Pmus) compared to the contribution from the ventilator positive pressure (Pvent). As a result, the PCWP tracing at end-tidal CO2, which signifies end expiration (top right of the shark fin), is measured similarly in this ventilated patient as it would be in a spontaneously breathing patient just prior to the onset of the negative waveform deflection. Red circles indicate end expiration. Therefore, the accurate measurement of pulmonary wedge pressure is ~ 30.
PCWP, pulmonary capillary wedge pressure; PVV, positive-pressure ventilation.
Figure 3. Pulmonary capillary wedge pressure tracings during different modes of respiration
Source: Isseh IN, et al. A critical review of hemodynamically guided therapy for cardiogenic shock: Old habits die hard. Curr Treat Options Cardiovasc Med. 2021.7
- Transpulmonary gradient is the difference between mPAP and PCWP.8
- A major determinant of PVR is pulmonary vessel constriction, most often secondary to hypoxia.8
- PVR is preferably expressed in Wood units (WU = mmHg•min•L-1), but in research and some clinical contexts it may also be expressed in dynes•s•cm-5, which is calculated by multiplying WU by 80.
- Normal PVR is usually under 2 WU and is typically about one-sixth of the SVR.8
Calculation of SVR
- Mean arterial pressure is measured invasively from a systemic arterial line or calculated using the formula (systolic BP + 2 × diastolic BP) / 3, which can also be rearranged as (diastolic BP + 1/3 Pulse pressure).5
- CVP or RAP (right atrial pressure) is measured with a central venous catheter.
- The major determinant of SVR is arteriolar tone, with additional contributions from blood viscosity and vascular capacitance.8
- SVR is typically expressed in dynes•s•cm-5, with normal values of 900-1200 dynes•s•cm-5 (equivalent to 11-15 Wood units when divided by 80).
References
- Fleitman J, Tetteh ES. Pulmonary artery catheterization: Interpretation of hemodynamic values and waveforms in adults. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate; 2025. Accessed Nov 10, 2025. Link
- Abman SH, Hansmann G, Archer SL, et al. Pediatric pulmonary hypertension: Guidelines from the American Heart Association and American Thoracic Society. Circulation. 2015;132(21):2037-99 PubMed
- Basnet A, Rout P. Calculating FICK cardiac output and input. In: StatPearls (Internet). Treasure Island, FL. StatPearls Publishing; 2025. Accessed Nov 10, 2025. Link
- McLaughlin VV, Archer SL, Badesch DB, et al. ACCF/AHA 2009 expert consensus document on pulmonary hypertension a report of the American College of Cardiology Foundation Task Force on Expert Consensus Documents and the American Heart Association developed in collaboration with the American College of Chest Physicians; American Thoracic Society, Inc.; and the Pulmonary Hypertension Association. J Am Coll Cardiol. 2009;53(17):1573-1619 PubMed
- Connor CW, Conley CM. Commonly used monitoring techniques. In: Cullen BF, Stock MC, Ortega R, et al. Barash, Cullen, and Stoelting’s Clinical Anesthesia. 9th ed. Philadelphia, PA; Wolters Kluwer; 2023: 666-690
- Kovacs G, Avian A, Pienn M, et al. Reading pulmonary vascular pressure tracings. How to handle the problems of zero leveling and respiratory swings. Am J Respir Crit Care Med. 2014;190(3):252-7 PubMed
- Isseh IN, Lee R, Khedraki R, et al. A critical review of hemodynamically guided therapy for cardiogenic shock: Old habits die hard. Curr Treat Options Cardiovasc Med. 2021;23(5):29 PubMed
- Bozkurt B, Das SR, Addison D, et al. 2022 AHA/ ACC key data elements and definitions for cardiovascular and noncardiovascular complications of COVID-19: A report of the American College of Cardiology/American Heart Association task force on clinical data standards. J Am Coll Cardiol. 2022;80(4):388-465 PubMed
Other References
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