🫀 Fick Cardiac Output / Cardiac Index
Estimates cardiac output (CO) and cardiac index (CI) by the indirect Fick principle from oxygen consumption and the arteriovenous oxygen content difference.
The indirect Fick relies on an assumed VO2, which can err by 25% or more — direct measurement of VO2 is preferred when accuracy is critical (e.g., shunt or PVR calculation). (original synthesis · not guideline verbatim)
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When to use
Enter VO2 (or an estimate), hemoglobin, and arterial/mixed-venous oxygen saturations; the tool returns CO and CI. Useful at the bedside or in the cath lab when thermodilution is unavailable or unreliable.
How it works
CaO2−CvO2 = 1.34 × Hb × (SaO2 − SvO2) × 10 (mL/L). CO = VO2 / (CaO2 − CvO2). CI = CO / BSA. Estimated VO2 commonly uses 125 mL/min/m² × BSA (or 110 in the elderly).
Key points
- The indirect Fick relies on an assumed VO2, which can err by 25% or more — direct measurement of VO2 is preferred when accuracy is critical (e.g., shunt or PVR calculation). (original synthesis · not guideline verbatim)
- Accuracy degrades at high cardiac output (the narrow a-v difference amplifies measurement error) and with intracardiac shunts.
- Use true mixed-venous (pulmonary artery) saturation; a central-venous (SVC) sample is a substitute that introduces additional error.
References
- LaFarge CG, Miettinen OS. The estimation of oxygen consumption. Cardiovasc Res. 1970;4(1):23-30.
- Narang N, et al. Inaccuracy of estimated resting oxygen uptake in the clinical setting. Circulation. 2014;129(2):203-210.
Worked calculation
The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.
| Oxygen consumption VO₂ | 250 mL/min |
|---|---|
| Hemoglobin | 130 g/L |
| Arterial oxygen saturation SaO₂ | 98 % |
| Mixed venous oxygen saturation SvO₂ | 70 % |
| Body surface area (optional, for CI) | 1.7 m² |
→Cardiac output CO5.13L/min
- Arteriovenous O₂ difference：48.8 mL/L
- Cardiac index CI：3.01 L/min/m²
Frequently asked questions
- What is Fick Cardiac Output / Cardiac Index?
- Estimates cardiac output (CO) and cardiac index (CI) by the indirect Fick principle from oxygen consumption and the arteriovenous oxygen content difference.
- How is Fick Cardiac Output / Cardiac Index calculated? What is the core formula?
- CaO2−CvO2 = 1.34 × Hb × (SaO2 − SvO2) × 10 (mL/L). CO = VO2 / (CaO2 − CvO2). CI = CO / BSA. Estimated VO2 commonly uses 125 mL/min/m² × BSA (or 110 in the elderly).
- When is Fick Cardiac Output / Cardiac Index used?
- Enter VO2 (or an estimate), hemoglobin, and arterial/mixed-venous oxygen saturations; the tool returns CO and CI. Useful at the bedside or in the cath lab when thermodilution is unavailable or unreliable.
- What are the key clinical points for Fick Cardiac Output / Cardiac Index?
- The indirect Fick relies on an assumed VO2, which can err by 25% or more — direct measurement of VO2 is preferred when accuracy is critical (e.g., shunt or PVR calculation). (original synthesis · not guideline verbatim) Accuracy degrades at high cardiac output (the narrow a-v difference amplifies measurement error) and with intracardiac shunts. Use true mixed-venous (pulmonary artery) saturation; a central-venous (SVC) sample is a substitute that introduces additional error.
- What are the limits and cautions when using Fick Cardiac Output / Cardiac Index?
- For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
- How is Fick Cardiac Output / Cardiac Index calculated in practice? Can you show a worked example?
- Inputs: Oxygen consumption VO₂ 250 mL/min, Hemoglobin 130 g/L, Arterial oxygen saturation SaO₂ 98 %, Mixed venous oxygen saturation SvO₂ 70 %, Body surface area (optional, for CI) 1.7 m² → Result: Cardiac output CO 5.13 L/min（Arteriovenous O₂ difference: 48.8 mL/L, Cardiac index CI: 3.01 L/min/m²）