The thermodilution cardiac output measurement technique is a well-established, albeit invasive, method often considered a clinical gold standard, particularly in intensive care units (ICUs). This page explains the principles behind thermodilution, how it’s used for cardiac output measurement, and provides a conceptual understanding of its calculation. While a direct online “calculator” for raw thermodilution curves is complex (usually done by dedicated monitors), we’ll detail the inputs and outputs involved.
Thermodilution Cardiac Output Calculator
Results:
Cardiac Output (CO): – L/min
Cardiac Index (CI): – L/min/m²
Thermodilution Formula:
CO = [V × (Tb – Ti) × K] / AUC
Where:
- V = Injectate Volume (mL)
- Tb = Blood Temperature (°C)
- Ti = Injectate Temperature (°C)
- K = Computation Constant
- AUC = Area Under the Temperature-Time Curve
Cardiac Index (CI) = CO / BSA (using standard 1.73 m²)
Principle of Thermodilution
Thermodilution works on the indicator dilution principle. A known volume of an indicator (cold saline solution at a known temperature) is injected into the bloodstream (typically the right atrium or superior vena cava via a proximal port of a pulmonary artery catheter – PAC). This cold bolus mixes with blood and travels through the right heart to the pulmonary artery.
A thermistor near the tip of the PAC in the pulmonary artery measures the change in blood temperature over time. The degree and duration of temperature drop are inversely proportional to the blood flow (cardiac output): higher flow results in a smaller, quicker temperature change, while lower flow results in a larger, more prolonged change. This relationship is described by the Stewart-Hamilton equation. Understanding how to calculate cardiac output via various methods puts thermodilution into perspective.
The Stewart-Hamilton Equation (Conceptual Basis)
The modified Stewart-Hamilton equation used for thermodilution CO is:
![\[ \text{CO} = \frac{V_i \times (T_b - T_i) \times K_1 \times K_2}{\int \Delta T_b(t) \, dt} \]](https://cardiacoutputcalculator.com/wp-content/ql-cache/quicklatex.com-5bebd8ff3df5d19ade37feebc49fa550_l3.png "Rendered by QuickLaTeX.com")
Where:
- CO = Cardiac Output
- Vi = Volume of injectate (cold saline, e.g., 10 mL)
- Tb = Initial blood temperature
- Ti = Temperature of injectate
- K1 = Density factor (specific gravity and specific heat of injectate and blood)
- K2 = Computation constant (accounts for catheter dead space, injectate warming, etc. Often specific to the catheter and monitor system)
- ∫ΔTb(t)dt = Area under the thermodilution curve (change in blood temperature over time).
Modern cardiac output monitors automatically calculate CO by integrating the area under the curve. You can learn more about different CO formulas on our cardiac output formulas page.
Clinical Procedure and Measurement
- Preparation: Ensure PAC is correctly positioned. Prepare injectate (e.g., draw 10 mL of normal saline into a syringe, may be cooled).
- Connect: Attach syringe to the proximal (RA/CVP) port of the PAC. Connect temperature probe if using cooled injectate.
- Baseline: Monitor notes baseline blood temperature.
- Injection: Inject the saline smoothly and rapidly (e.g., over <4 seconds) at end-expiration to minimize respiratory variation.
- Recording: The monitor records the temperature change in the pulmonary artery and plots the thermodilution curve.
- Calculation: The monitor computes the CO.
- Repeat: Perform 3-5 injections, allowing blood temperature to return to baseline between each. Discard measurements with irregular curves. Average the acceptable CO values.
For a look into other advanced techniques, see our article on advanced cardiac output monitoring.
Clinical Significance and Target Audience (ICU)
Thermodilution CO is primarily used in critically ill patients in the ICU for:
- Managing complex shock states (septic, cardiogenic, hypovolemic).
- Guiding fluid resuscitation and vasopressor/inotropic therapy. More on clinical significance here.
- Assessing right ventricular function (with continuous CO/SvO2 catheters).
- Diagnosing and managing pulmonary hypertension.
- Perioperative monitoring in high-risk cardiac or major non-cardiac surgeries.
It offers more precision than non-invasive methods like Doppler echo in certain complex patients, but is being challenged by less invasive technologies. You can read a comparison of methods here.
Comparison with Fick and Doppler Methods
| Feature | Thermodilution | Fick Method | Doppler Echo |
|---|---|---|---|
| Invasiveness | Invasive (PAC required) | Invasive (PAC & arterial line) | Non-invasive |
| Accuracy | Good, considered clinical gold standard by many | Excellent (true gold standard if VO2 measured) | Operator-dependent, good for trends |
| Continuity | Intermittent (bolus) or Continuous (with specialized PACs) | Intermittent | Intermittent (can be repeated frequently) |
| Common Setting | ICU, Cardiac Cath Lab | Cardiac Cath Lab, Research, specific ICU cases | Ward, Clinic, ICU, ER |
| Key Challenge | PAC risks, tricuspid regurgitation effect | VO2 measurement, stable state required | Operator skill, acoustic window |
For more detailed comparisons, check out Doppler vs Fick method.
Limitations
- Invasiveness & Risks: PAC insertion carries risks (arrhythmias, pneumothorax, infection, PA rupture).
- Tricuspid Regurgitation: Significant TR can lead to underestimation of CO due to loss/recirculation of indicator.
- Intracardiac Shunts: L-to-R shunts cause premature appearance of indicator and CO overestimation. R-to-L shunts cause CO underestimation.
- Arrhythmias: Irregular heart rates can affect accuracy.
- Technique Dependent: Injection speed, volume, and timing with respiration matter.
For comprehensive guidelines on hemodynamic monitoring, resources from the Society of Critical Care Medicine (SCCM) are invaluable. If you’re new to these concepts, start with What is Cardiac Output?