Oxygen conservation during long-distance transport of ventilated patients: assessing the Modified Circle System

Introduction: The Modified Circle System was developed to reduce the amount of oxygen required for long-distance transportation of ventilated patients. Formal testing of the system has never been documented. Various aspects of the system were tested, with the study broken into four parts.

Methods: Bench testing was performed with a test lung that simulated two different lung compliances. Four ventilators were tested with the system. The four parts to the study were oxygen flow rates, position of oxygen inflow, pressure drop across the system, and specific testing with the LP 10 ventilator.

Results: The minimum oxygen flows required to maintain forced inspiratory oxygen (FiO₂) higher than 0.93 over a range of ventilator and lung settings was 1.5 L/min. The oxygen inflow is best connected directly to the CO₂ absorber unit. A significant pressure drop occurs across the system, which increases progressively with higher ventilator pressures. The LP 10 does not allow improved oxygen conservation when recycling gases via its air inlet port.

Conclusion: The study provides previously unknown, clinically important information regarding the performance of the Modified Circle System.