Isocapnia can be maintained during hyperventilation by administering carbon dioxide (CO₂) through a modern anesthesia circuit without disconnecting it, a study has found.

The study, performed by a team of anesthesiologists from the Institute for Clinical Sciences, Sahlgrenska Academy, at Gothenburg University in Sweden, used a mechanical lung model set to 50 mL/cm H₂O and fitted with an adjustable CO₂ output. The CO₂ was delivered via an electronic flow controller (VCO₂), and the lung model was ventilated with an S/5 Anesthesia Delivery Unit Carestation (Datex Ohmeda). Hyperventilation was achieved by doubling the minute ventilation and fresh gas flow at three levels: 175, 200 and 225 mL/min and dead-space volume of 44, 92 and 134 mL. CO₂ was added during hyperventilation using the inspiratory limb of the anesthesia circuit with a precision flow meter (DCO₂).

The researchers found that during hyperventilation, alveolar ventilation increased 113%, and there was a linear correlation among end-tidal CO₂ level at baseline ventilation, volume of dead space at baseline ventilation and delivered CO₂ at isocapnic hyperventilation. DCO₂ varied between 113 and 250 mL/min. Low VCO₂and large dead-space volume resulted in a greater DCO₂ flow to achieve isocapnia. The fractional concentration of inspired CO₂ level during isocapnic hyperventilation varied between 2.3% and 3.3%. The team noted that the amount of CO₂ that is required to achieve isocapnic hyperventilation could be determined from the baseline minute ventilation, VCO₂ and dead-space volume.

“[Our] main finding is that it is easy to add carbon dioxide directly into a modern circle system via a flow meter and that it is possible to predict how much carbon dioxide is needed to achieve isocapnia when ventilation is doubled to shorten elimination of inhalation agents [fast wake-up procedure],” said Ola Stenqvist, MD, PhD, Department of Anesthesiology and Intensive Care at Gothenburg University, and one of the study’s co-authors. He presented the findings at Euroanaesthesia 2015 (abstract 1AP9-2).

Isocapnic hyperventilation can potentially increase the elimination rate of anesthetic gases. In animal models, it has been shown to shorten wake-up and postoperative recovery times following inhalation anesthesia.

Vincent J. Kopp, MD, FAAP, professor of anesthesiology and pediatrics, University of North Carolina at Chapel Hill, told Anesthesiology News that the new study “resurrects interest in using exogenously supplied carbon dioxide to an advantage in general anesthesia.”

“The technique employed in this study strikes me as physiologically sound but impractical in the current medical environment,” added Dr. Kopp, who was not involved with the Swedish study.

“Redesign of contemporary anesthesia equipment would have to accommodate the addition of carbon dioxide tanks. It is unlikely wall sources for carbon dioxide would be retrofitted into operating locations to accommodate a change in clinical practice, which is unlikely to substantially affect the ‘economics of anesthesia emergence,’ a small component of case turnover time improvement so valued by hospital administrators and accountants.”