Evaluation of a rebreathing system for use with portable mechanical ventilators

SESSION TITLE: Mechanical Ventilation: Beyond the basics SESSION TYPE: Original Investigations PRESENTED ON: 10/09/2023 08:30 am - 09:30 am PURPOSE: Under normal conditions oxygen is plentiful. Under far forward conditions and pre-hospital environments, oxygen is a scarce resource. Supplying oxygen in austere, constrained environments represents significant logistical challenges. Maximizing the capabilities of available low flow oxygen is key to providing adequate oxygen to prevent/treat hypoxemia and conserve oxygen. We designed a closed-circuit system which allows rebreathing of gases while scrubbing carbon dioxide (CO2) in conjunction with portable mechanical ventilators in a bench model. METHODS: We used two portable mechanical ventilators currently in the strategic national stockpile, (731, Zoll Medical, and SAVe II, AutoMedx) for the evaluation. We evaluated the system over three respiratory rate/tidal volume combinations, two PEEP settings and two lung models, using 1 L/min and 3 L/min oxygen bleed-in to a reservoir bag on the ventilator inlet, at ground level and two altitudes. We measured peak inspired oxygen concentration (FiO2), CO2 absorbent life, gas temperature and humidity, volume of condensate in the ventilator circuit, and the effect of airway suctioning and ventilator disconnection on FiO2. RESULTS: FiO2 during use of the 731 was not impacted by ventilator settings (p>0.99). Mean FiO2 was 0.96 ± 0.25 for all settings after 30 minutes. FiO2 was significantly (p=0.04) associated with altitude and were lower at higher altitudes although differences were not clinically important (FiO2 0.95-0.97). FiO2 was significantly (p<0.0001) higher with 3 L/min oxygen bleed-in, vs 1 L/min (0.98 ± 0.01 and 0.94 ± 0.02 respectively). FiO2 during use of the SAVe II was not associated with ventilator settings (p=0.73) or altitude (p=0.97). FiO2 was significantly (p<0.0001) higher with 3 L/min oxygen, vs 1 L/min (0.97 ± 0.01 and 0.92 ± 0.01 respectively). CO2 absorbent life range was 456-557 minutes across all ventilator settings (p=0.005). FiO2 at baseline, after 5 and 10 seconds of simulated airway suctioning, and after 15 and 30 seconds of a ventilator disconnect, and recovery times back to baseline FiO2 resulted in lower FiO2 and longer recovery times with 1 L/min oxygen bleed-in vs 3 L/min bleed-in (p<0.05). Ventilator circuit condensate volume range was 25-63 mL over the duration of all tests. CONCLUSIONS: The study results show that using low oxygen flow ≤ 3 L/min with a rebreathing system attached to a portable ventilator can provide FiO2 ≥ 90% across a range of ventilator settings, lung models, and altitudes. Suctioning and disconnection from the ventilator should be used sparingly due to hypoxemia risk. CO2 absorbent life was at least 7 hours at all conditions which would be adequate for most aeromedical and ground transports and anywhere oxygen is scarce. Use of a rebreathing system has the potential for oxygen conservation but requires diligent monitoring of inspired FiO2 and CO2 to avoid untoward consequences. CLINICAL IMPLICATIONS: The use of a rebreathing system attached to a portable ventilator can provide high FiO2 while using 更多