Association of partial pressure of carbon dioxide in expired gas and arterial blood at three different ventilation states in apneic chickens (Gallus domesticus) during air sac insufflation anesthesia.

Objective: To test whether partial pressure of CO2 in expired gas (PĒCO2) predicts the partial pressure of CO2 in arterial blood (PaCO2) in apneic chickens during air sac insufflation anesthesia at three different ventilation states.

Secondary Objective: To determine the PĒCO2 at which apnea occurs during air sac insufflation anesthesia.

Study Design: Randomized cross-over study.

Animals: Twenty-three healthy male white leghorn chickens.

Methods: Chickens were anesthetized via mask with isoflurane in oxygen and an air sac cannula was placed in the right abdominal air sac. Delivery of isoflurane in O2 was transferred from the mask to the air sac cannula. The birds were maintained at a surgical plane of anesthesia and apnea was induced by adjusting gas flow; the PĒCO2 at apnea was recorded. The birds were then paralyzed and gas flow was adjusted to achieve three different PĒCO2 s in random order: 43 mmHg (5.6 kPa) [hypoventilation]; 33 mmHg (4.3 kPa) [normoventilation]; and 23 mmHg (3.0 kPa) [hyperventilation]. After maintaining the target expired isoflurane concentration (EIso; 1.85 or 1.90%) and PĒCO2 for 15 minutes, arterial blood gas analysis was performed to determine the PaCO2 . The chickens were euthanized at the end of the experiment.

Results: Based on Bland-Altman comparisons, PĒCO2 was not strongly associated with PaCO2 during the three ventilation states. The PĒCO2 at which apnea occurred varied {median (minimum, maximum): 35 (30, 48) mmHg [4.6 (3.9, 6.2) kPa]}.

Conclusions: Measured PĒCO2 cannot be used in a simple linear fashion to predict PaCO2 in birds during air sac insufflation anesthesia. The PĒCO2 at which apnea occurs during air sac insufflation anesthesia is not predictable.

Clinical Relevance: Arterial blood gases should be used to monitor CO2 during air sac insufflation anesthesia to verify appropriate patient ventilation.