Friday May 26, 2006
Cardiac Output Pitfalls
Determination of cardiac output by thermodilution has several technical pitfalls. Any deviation in technique can produce inaccurate and inconsistent results.
Basis of thermodilutional cardiac output: The method relies on an injection of a known volume of fluid (5-10 mL) into the right atrium. This fluid, either normal saline or D5W is at room (or iced at known *) temperature and therefore cooler than blood. The cooler injectate mixes with blood, thus lowering its temperature. The cooled blood is ejected into the pulmonary artery and flows past a thermistor located in the distal end of the PA catheter. The thermistor generates a change in temperature to time curve. The area under this curve is calculated by integration and is inversely proportional to the flow past the thermistor. In other words, the longer it takes for this change in temperature to “wash out,” the slower the flow past the thermistor. The converse is also true. The greater the flow, the faster the temperature “wash out,” and therefore the smaller the area under the curve. Anything that can disrupt the “washout” of this temperature change can affect the accuracy of this measurement.
* iced saline has been said to provides a better "signal-to-noise" ratio but controversy continues in literature regarding iced vs room temperature solution.
1. Severe tricuspid regurgitation causes the injectate to recycle back and forth across the valve falsely lowering cardiac output.
2. An injectate volume that is too large will also falsely lower cardiac output.
3. Intracardiac shunts can falsely elevate cardiac output. In a right-to-left shunt, part of the injectate escapes through the shunt and decreases the amount of time required for washout of the temperature change. An injectate volume that is too small will cause an abbreviated washout and therefore falsely elevate the cardiac output.
Also see Pulmonary Artery Catheter Primer from American Thoracic Society (About 100 MCQ questions covering almost all aspects of PAC).
Related previous pearl: Arterial pressure-based continuous cardiac output