Cardiac Input - Measuring Cardiac Output - Pulmonary Artery Thermodilution (Trans-right-heart Thermodilution)

Pulmonary Artery Thermodilution (Trans-right-heart Thermodilution)

The indicator method was further developed with replacement of the indicator dye by heated or cooled fluid and temperature change measured at different sites in the circulation rather than dye concentration; this method is known as thermodilution. The pulmonary artery catheter (PAC), also known as the Swan-Ganz catheter, was introduced to clinical practice in 1970 and provides direct access to the right heart for thermodilution measurements. Continuous invasive cardiac monitoring in the Intensive Care Unit has been all but phased out in an age of hospital acquired infection. Use of the PAC is still useful in right heart study in the cardiac catheterization laboratory today.

The PAC is balloon tipped and is inflated, which helps "sail" the catheter balloon through the right ventricle to occlude a smaller branch of the pulmonary artery system. The balloon is deflated. The PAC thermodilution method involves injection of a small amount (10ml) of cold glucose at a known temperature into the pulmonary artery and measuring the temperature a known distance away (6–10 cm) using the same catheter with temperature sensors set apart at a known distance.

The historically significant Swan-Ganz multi-lumen catheter allows reproducible calculation of Cardiac Output from a measured time/temperature curve (The "thermodilution curve"). Enabled Thermistor technology allowed the observation that low CO registers temperature change slowly, and inversely, high CO registers temperature change rapidly. The degree of change in temperature is directly proportional to the cardiac output. Under this unique method, three or four repeated measurements or passes are usually averaged to improve accuracy. Modern catheters are fitted with a heating filament which intermittently heats and measures the thermodilution curve providing serial Q measurement. However, these take an average of measurements made over 2–9 minutes, depending on the stability of the circulation, and thus do not provide continuous monitoring.

PAC use is complicated by arrhythmias, infection, pulmonary artery rupture, and right heart valve damage. Recent studies in patients with critical illness, sepsis, acute respiratory failure and heart failure suggest use of the PAC does not improve patient outcomes. This clinical ineffectiveness may relate to its poor accuracy and sensitivity which has been demonstrated by comparison with flow probes across a sixfold range of Qs. PAC use is in decline as clinicians move to less invasive and more accurate technologies for monitoring hemodynamics.

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