Partial CO₂ Rebreathing Fick Cardiac Output Monitoring

Another method of cardiac output monitoring that does not require pulmonary artery catheterization is the partial CO₂ rebreathing Fick technique.^{[772] [773] [774]} Because of the difficulty encountered in measuring oxygen consumption or mixed venous hemoglobin saturation with the standard Fick method, this technique is based on a restatement of the Fick Equation for carbon dioxide elimination rather than oxygen uptake.





where = cardiac output
V̇CO₂ = rate of carbon dioxide elimination
Cv̄CO₂ = carbon dioxide content of mixed venous blood
CaCO₂ = carbon dioxide content of arterial blood

This method uses the change in CO₂ production and end-tidal CO₂ concentration in response to a brief, sudden change in minute ventilation. ^{[775] [776]} With a specially designed breathing system and monitoring computer, this measurement is easily performed in any tracheally intubated patient. Every 3 minutes, a computer-controlled pneumatic valve intermittently increases dead space for a 50-second period, thereby causing partial rebreathing of exhaled gases. Changes in CO₂ production and end-tidal CO₂ in response to the rebreathing are used to calculate cardiac output by a differential version of the Fick equation for carbon dioxide. The attractive features of this method are that it is entirely noninvasive, it can be performed every few minutes, and the brief episodes of rebreathing pose no substantial risk to most patients, with end-tidal CO₂ values increasing by less than 3 mm Hg. However, as currently designed, accurate measurements with this technique require tracheal intubation for precise measurement of exhaled gases. Furthermore, changing patterns of ventilation may have an unpredictable influence on the measurement. As with all Fick-based techniques, the partial CO₂ rebreathing method measures pulmonary capillary blood flow as an indicator of total cardiac output and thus requires correction for pulmonary shunting.

The initial clinical trials suggest reasonably good agreement between the partial rebreathing CO₂ cardiac output method and other techniques such as thermodilution. However, as with most of these newer monitoring methods, the clinical trials are small and mainly focused on specific patient groups, particularly coronary artery bypass patients.^{[773] [774]} At present, the clinical role for this technique is focused mainly on short-term intraoperative applications or uncomplicated mechanically ventilated postoperative patients.