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.