The effect of increased apparatus dead space and tidal volumes on carbon dioxide elimination and oxygen saturations in a low-flow anesthesia system.

STUDY OBJECTIVE

To determine if a large tidal volume (VT), with an unchanged end-tidal carbon dioxide partial pressure (PETco2), could improve arterial carbon dioxide elimination, oxygen saturation (Spo2), and arterial blood oxygenation.

DESIGN

Prospective, randomized, clinical study.

SETTING

Single university hospital.

PATIENTS

60 ASA physical status I and II patients scheduled for elective urologic or general surgery.

INTERVENTIONS

Patients were randomly assigned to one of two treatments: patients in group 1, nondead space (NDS), received a fresh gas flow of 1 L/min without added apparatus dead space volume. Patients in group 2, dead space (DS), received ventilation using an added dead space volume between the Y-piece and tracheal tube. In both groups, patients' lungs were ventilated to a fixed PETco2 value of 33.8 mmHg. Patients in the DS group were ventilated with VTs to maintain an airway plateau pressure (Pplateau) of 0.04 cm H2O/kg over initial plateau pressure. The corrugated tube was then adjusted to maintain a fixed PETco2.

MEASUREMENTS

Dead space volumes, PETco2, arterial CO2 tension (Paco2), SpO2, arterial O2 tension (Pao2), VTs, and airway pressures were measured.

MAIN RESULTS

Arterial CO2 tension was significantly lower in the DS group, 36 +/- 2.3 mmHg, compared with the NDS group, 37.5 +/- 2.3 mmHg (P < 0.05), and the difference between PETco2 and Paco2 was lower in the DS group than in the NDS group (P < 0.001). Oxygen saturation was 99% +/- 1.0% in the DS group compared with 98.5% +/- 1.5% in the NDS group (P < 0.05). Arterial O2 tension was 13.2 +/- 25.5 mmHg in the DS group and 119.1 +/- 30.2 mmHg in NDS group (not significant).

CONCLUSION

Larger VTs, with an unchanged PETCO2 concentration created by an added apparatus dead space volume, improved arterial carbon dioxide elimination.