Controlling Carbon Dioxide Delivery during Mechanical Ventilation

Abstract

Background

Pediatric cardiac patients are sometimes given carbon dioxide (CO₂) postoperatively to manipulate pulmonary vascular resistance and thus reapportion pulmonary and peripheral blood flow. We devised a double-blender system for controlling the delivered CO₂ ( $F_{DC{O}_2}$ ) and oxygen concentrations ( $F_{D{O}_2}$ ) and evaluated the utility of a nomogram to predict blender settings for required gas concentrations.

Description of Device

Two Bird 3800 MicroBlenders were connected in series. The first (α) was supplied by CO₂ and air and fed into the second blender (β), which was also supplied by O₂. System equations were derived and a nomogram constructed to predict settings for desired $F_{DC{O}_2}$ and $F_{D{O}_2}$ expressed as CO₂ % and O₂ %. EVALUATION METHODS: Values predicted using the nomogram were compared to measured values. Analyzers were calibrated with standard gases. Blender calibrations were verified. Using nomogram-derived blender settings, 4 levels of O₂ (21, 40, 60, and 80%) and 2 levels of CO₂ (25 or 30%), in random order, were analyzed in triplicate. The mean and standard deviation of differences (predicted – measured values) were used to estimate inaccuracy intervals that would include 95% of future observations at the 99% confidence level.

Evaluation Results

The inaccuracy interval (mean difference ± 2.97 standard deviations) for CO₂ % was 0.06 ± 1.10 %. For O₂% the interval was 4.0 ± 2.94 %.

Conclusions

The inaccuracy intervals for CO₂ % and O₂% were clinically acceptable. The nomogram should be an efficient bedside tool for the clinician, expediting system setup and changes in the gas mixture.