Background: Recently, a new device has been developed to measure cardiac output noninvasively using partial carbon dioxide (CO(2)) rebreathing. Because this technique uses CO(2) rebreathing, the authors suspected that ventilatory settings, such as tidal volume and ventilatory mode, would affect its accuracy: they conducted this study to investigate which parameters affect the accuracy of the measurement.
Methods: The authors enrolled 25 pharmacologically paralyzed adult post-cardiac surgery patients. They applied six ventilatory settings in random order: (1) volume-controlled ventilation with inspired tidal volume (V(T)) of 12 ml/kg; (2) volume-controlled ventilation with V(T) of 6 ml/kg; (3) pressure-controlled ventilation with V(T) of 12 ml/kg; (4) pressure-controlled ventilation with V(T) of 6 ml/kg; (5) inspired oxygen fraction of 1.0; and (6) high positive end-expiratory pressure. Then, they changed the maximum or minimum length of rebreathing loop with V(T) set at 12 ml/kg. After establishing steady-state conditions (15 min), they measured cardiac output using CO(2) rebreathing and thermodilution via a pulmonary artery catheter. Finally, they repeated the measurements during pressure support ventilation, when the patients had restored spontaneous breathing. The correlation between two methods was evaluated with linear regression and Bland-Altman analysis.
Results: When VT was set at 12 ml/kg, cardiac output with the CO(2) rebreathing technique correlated moderately with that measured by thermodilution (y = 1.02x, R = 0.63; bias, 0.28 l/min; limits of agreement, -1.78 to +2.34 l/min), regardless of ventilatory mode, oxygen concentration, or positive end-expiratory pressure. However, at a lower VT of 6 ml/kg, the CO(2) rebreathing technique underestimated cardiac out-put compared with thermodilution (y = 0.70x; R = 0.70; bias, -1.66 l/min; limits of agreement, -3.90 to +0.58 l/min). When the loop was fully retracted, the CO(2) rebreathing technique overestimated cardiac output.
Conclusions: Although cardiac output was underreported at small VT values, cardiac output measured by the CO(2) rebreathing technique correlates fairly with that measured by the thermodilution method.