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! 🙂