6.2 Cardiac Output: Fick & Thermodilution
Key Takeaways
- Fick cardiac output: CO (L/min) = VO₂ (mL/min) ÷ [arterial O₂ content − mixed venous O₂ content], using vol% difference × 10 or the Hgb-based content formula
- Worked example: VO₂ 250 mL/min, aortic O₂ 21 vol%, PA O₂ 16 vol%, BSA 1.8 m² → CO ≈ 5.0 L/min and CI ≈ 2.8 L/min/m²
- Thermodilution CO injects cold saline into the RA and integrates the downstream temperature change — fast but invalidated by severe TR, shunts, and low-output states
- Fick requires steady-state oxygen consumption and accurate mixed venous sampling from true PA blood — not RA blood when significant shunt or TR exists
- Normal CO is 4–8 L/min; normal cardiac index (CO ÷ BSA) is 2.5–4.0 L/min/m²; CI below 2.2 with elevated PCWP suggests cardiogenic shock physiology
6.2 Cardiac Output: Fick & Thermodilution
Quick Answer: Cardiac output (CO) quantifies how much blood the heart pumps per minute. The two methods RCIS candidates must master are the Fick principle (oxygen consumption divided by arteriovenous oxygen difference) and thermodilution (cold saline indicator dilution via PA catheter). Each has distinct assumptions and failure modes — knowing when to trust one method over the other is a high-yield RCIS exam topic.
Cardiac output drives nearly every hemodynamic calculation downstream: cardiac index, pulmonary vascular resistance (PVR), systemic vascular resistance (SVR), valve area (Gorlin), and shunt ratio (Qp:Qs). Accurate CO measurement separates meaningful hemodynamics from artifact.
Normal Values
| Parameter | Normal Range | Formula |
|---|---|---|
| Cardiac output (CO) | 4–8 L/min | — |
| Cardiac index (CI) | 2.5–4.0 L/min/m² | CI = CO ÷ BSA |
| Stroke volume (SV) | 60–100 mL/beat | SV = CO ÷ HR |
Body surface area (BSA) is typically calculated from height and weight (Mosteller formula on many exam items). A patient with CO 5.0 L/min and BSA 1.8 m² has CI = 2.78 L/min/m² — within normal limits.
The Fick Principle
The Fick method applies conservation of mass: in steady state, oxygen consumed by the body equals oxygen delivered by blood flow minus oxygen returned in mixed venous blood.
Vol% Method (Common on RCIS Exams)
When oxygen contents are expressed as volume percent (vol%):
CO (L/min) = VO₂ (mL/min) ÷ [(Ao vol% − PA vol%) × 10]
Where:
- VO₂ = oxygen consumption (mL/min), often assumed 250 mL/min for resting adults or measured via metabolic cart
- Ao vol% = arterial oxygen content as volume percent (≈20–21 vol% on room air)
- PA vol% = mixed venous oxygen content from true pulmonary artery blood (≈14–17 vol%)
The factor ×10 converts vol% difference to the proper denominator units.
Worked Example (Exam-Style)
Given:
- VO₂ = 250 mL/min (assumed resting consumption)
- Aortic O₂ content = 21 vol%
- PA (mixed venous) O₂ content = 16 vol%
- BSA = 1.8 m²
Step 1 — Calculate arteriovenous difference: 21 vol% − 16 vol% = 5 vol%
Step 2 — Calculate CO: CO = 250 ÷ (5 × 10) = 250 ÷ 50 = 5.0 L/min
Step 3 — Calculate cardiac index: CI = 5.0 ÷ 1.8 = 2.78 L/min/m² (normal)
This matches the canonical RCIS sample calculation: VO₂ 250, Ao 21 vol%, PA 16 vol%, BSA 1.8 → CO ≈ 5 L/min.
Hemoglobin-Based Fick (Alternative Form)
When saturations and hemoglobin are given instead of vol%:
CO = VO₂ ÷ [1.36 × Hgb × (SaO₂ − SvO₂)]
Where SaO₂ and SvO₂ are expressed as decimals (0.95 and 0.65, not 95 and 65). The constant 1.36 mL O₂/g Hgb is the Hüfner number.
Example: VO₂ 250, Hgb 12 g/dL, SaO₂ 95%, SvO₂ 65%:
- Difference = 0.30
- Denominator = 1.36 × 12 × 0.30 = 4.896
- CO = 250 ÷ 4.896 ≈ 5.1 L/min
Both methods converge near 5 L/min — exam items may use either format.
Fick Assumptions and When Fick Fails
| Assumption | Why It Matters |
|---|---|
| Steady-state VO₂ | Acute changes in work of breathing, fever, or sedation alter VO₂ and invalidate assumed 250 mL/min |
| True mixed venous sample | PA blood is mixed venous; RA blood is NOT mixed venous if shunt or significant TR causes streaming |
| No left-to-right shunt at sampling site | Shunt blood with higher O₂ content dilutes the true C(a-v)O₂ difference, overestimating CO |
| Accurate arterial sample | Must be true systemic arterial (aortic or high-quality radial/femoral) |
When to prefer Fick: Low cardiac output states, severe tricuspid regurgitation (thermodilution recirculation), significant intracardiac shunts, and when thermodilution and Fick disagree — Fick with measured VO₂ is often the reference standard.
Thermodilution Method
Thermodilution uses a PA catheter thermistor. The RCIS or operator injects a known volume of cold saline (typically 10 mL at 0°C) into the RA port. The downstream temperature change at the PA thermistor is integrated over time; cardiac output is inversely proportional to the area under the temperature-time curve.
Conceptual formula: CO ∝ (Volume of injectate × Temperature difference) ÷ ∫ΔT dt
Computerized systems apply correction constants (K1, K2) for catheter and injectate characteristics. RCIS candidates rarely calculate manually — but must know technique and limitations.
Proper Thermodilution Technique
- Confirm PA catheter position (PA waveform, not wedge)
- Inject briskly at end-expiration (reduces respiratory variation)
- Use consistent injectate volume and temperature
- Discard highest and lowest of 3–5 injections; average the remainder
- Document TR severity before trusting results
When Thermodilution Fails
| Condition | Mechanism | Result |
|---|---|---|
| Severe tricuspid regurgitation | Indicator recirculates to RA before full washout | Falsely high CO |
| Intracardiac shunt | Indicator lost across shunt; non-linear dilution | Unreliable |
| Low CO (<2.5 L/min) | Small temperature change → poor signal-to-noise | Underestimates or fails |
| Poor injection technique | Slow injection, wrong volume, intracardiac placement | Erratic curves |
| Cardiac arrhythmia | Variable cycle length between injections | Inconsistent readings |
RCIS exam tip: Thermodilution is inaccurate in severe TR and shunts — use Fick instead. This phrase appears repeatedly on RCIS items.
Comparing Methods: Decision Table
| Scenario | Preferred Method | Reason |
|---|---|---|
| Routine PA catheter CO, no TR/shunt | Thermodilution | Fast, repeatable |
| Severe TR | Fick | Recirculation artifact |
| Atrial septal defect (left-to-right) | Fick with oximetry | Shunt invalidates indicator dilution |
| Low output / cardiogenic shock | Fick (measured VO₂) | Thermodilution loses accuracy |
| No PA catheter available | Fick | Only invasive option with A-line + oximetry |
Derived Calculations Using CO
Once CO is established, the RCIS should recognize downstream formulas:
PVR (Wood units) = (mean PAP − mean PCWP) ÷ CO
Example: mean PAP 30, PCWP 14, CO 4.5 L/min → PVR = 16 ÷ 4.5 = 3.6 Wood units (normal <2–3 WU at rest; multiply by 80 for dynes·sec/cm⁵).
SVR = (mean MAP − mean RA) ÷ CO × 80 (for dynes·sec/cm⁵)
Normal SVR ≈ 800–1200 dynes·sec/cm⁵.
Clinical Scenario: Conflicting CO Values
Thermodilution reports CO 7.5 L/min in a patient with known severe TR and RA enlargement on echo. Fick calculation (VO₂ 250, Ao 21 vol%, PA 16 vol%) yields CO 5.0 L/min. Trust Fick — thermodilution is falsely elevated by TR recirculation. Document both values and communicate the discrepancy to the operator.
RCIS Responsibilities During CO Determination
- Prepare and label syringes for thermodilution injectate (verify volume and temperature per protocol)
- Time injections at end-expiration when requested
- Record oximetry samples in correct order (SVC, RA, RV, PA, Ao) for Fick and shunt studies
- Never inject air — catastrophic embolism risk
- Chart CO, CI, and method used; note TR, shunt, or arrhythmia that may affect accuracy
Using the Fick vol% method, calculate cardiac output: VO₂ = 250 mL/min, aortic O₂ = 21 vol%, PA O₂ = 16 vol%. What is the CO?
Thermodilution cardiac output reads 8.2 L/min in a patient with severe tricuspid regurgitation, while Fick CO is 4.8 L/min. Which explanation is MOST accurate?
A patient has CO 5.0 L/min and BSA 1.8 m². What is the cardiac index, and is it within normal limits?