Oxygen consumption of a pneumatically controlled ventilator in a field anesthesia machine

Area of Science:

• Anesthesiology and Respiratory Care
• Biomedical Engineering
• Remote and Austere Medicine

Background:

• Field anesthesia machines (FAM) are designed for remote locations lacking reliable electricity or compressed medical gases.
• Pneumatically controlled ventilators in FAMs utilize compressed gas for pneumatic timing and valve actuation, replacing electrical power.
• Assessing oxygen (O2) and ventilator gas consumption is vital for FAMs operating in resource-limited environments.

Purpose of the Study:

• To quantify the total O2 consumption of a FAM's pneumatically controlled ventilator.
• To determine ventilator gas consumption, including drive gas (DG) and pneumatic control (PC) gas.
• To evaluate gas consumption under varying total thoracic compliance (high [HC] and low [LC]) and tidal volumes (V(T)).

Main Methods:

• The Magellan-2200 FAM's O2 consumption was calculated using the ideal gas law and measured O2 mass from E cylinders.
• Drive gas (DG) consumption was measured using a Wright Respirometer Mk 8.
• Pneumatic control (PC) gas consumption was derived by subtracting DG and fresh gas flow (FGF) from total O2 consumption. Tidal volume (V(T)) was measured with a pneumotach, tested at 500, 750, and 1000 mL in HC and LC lung models.

Main Results:

• Total O2 consumption demonstrated a direct relationship with V(T) and an inverse relationship with compliance.
• The lowest O2 consumption rate (9.3 L/min) occurred in the HC-500 model, while the highest (15.9 L/min) was in the LC-1000 model (P < 0.001).
• Mean PC circuitry consumption was 3.9 L/min (390 mL/breath).

Conclusions:

• Estimating cylinder duration is critical for patient safety, particularly in low total thoracic compliance scenarios during potential loss of central drive gas supply.
• In the LC-1000 model, a full O2 E cylinder would last less than 42 minutes, highlighting the need for careful resource management.
• A full H cylinder, however, could provide approximately 433 minutes of oxygen in the same low compliance, high tidal volume scenario.