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Cardiac Output: Fick And Thermodilution

Systemic cardiac output (CO) is the product of left ventricular stroke volume and the heart rate (CO = SV x HR). I see CO measured with thermodilution (Td) and the Fick equation… two techniques which often give very different values.

In the Td technique, cold saline of a known volume and temperature is injected into a proximal catheter port (ie, in the right atrium) at end-expiration (since CO changes during the respiratory cycle) thereby cooling the surrounding blood. A thermistor located downstream along the catheter (ie, in the pulmonary artery) measures the resulting temperature change. In low CO states, there is more time for heat to transfer and a greater resulting drop in temperature and vice versa. However, because these measurements involve only right heart structures (RA, RV, PA), intracardiac shunts and tricuspid regurgitation can certainly affect their accuracy in addition to low CO states.

The Fick technique uses a simple physiologic relationship – the ratio of oxygen consumption (VO2) to the difference in arterial and venous oxygen content is equal to CO. In other words: CO = VO2 / (arterial oxygen content – venous oxygen content). Although O2 content can be readily measured, determining VO2 is much more difficult especially in critically ill or obese patients. In the ICU, imagine how many variables can affect oxygen consumption (critical illness, recovery, nutrition, sedation, etc.)

As an intensivist and cardiothoracic anesthesiologist, I stick to measuring CO through echocardiographic estimates (especially when challenging patients with volume), but use thermodilution for more continuous monitoring.

What technique do you use? Drop me a comment with your thoughts below! 🙂

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3 Comments

  1. Hi Rishi! Thank you for this wonderful info.

    May I ask also and I am really curious. I noticed that whenever I checked fick and thermo in patients who have VA ECMO. There’s a huge difference. 1.7 index in thermo then 2.1 in fick.

    Fick is always higher. Is it because when I am drawing the mixed venous blood, most of the flood are coming from the pump (ecmo) ? What is the mechanism of it when pt have an ecmo?

    1. Honestly I’m skeptical about using these cardiac output monitors in patients on VA-ECMO because a.) as with any Fick calculation, you need to assume a metabolic component which might not as predictable for these critically ill patients and b.) depending on the way the cannulas are arranged, blood flow that transits past the thermodilution catheter may not be representative of the ECMO flows (ie, if you had a PA catheter positioned properly and central VA ECMO cannulation… you’d have very little blow flow measured by the PA catheter). I look more at ECMO flows and resulting pressures/signs of perfusion throughout the body.

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