Page 39 - JSOM Summer 2024

TABLE 3 FiO and Recovery Time After 5- and 10-Second Lowes and Sharley evaluated the Modified Circle System
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Suctioning and 15- and 30-Second Ventilator Circuit Disconnect (MCS), an adaptation of the closed-circuit anesthesia system,
Using 1 and 3L/min Oxygen in a bench model. The evaluation used two V /RR combina-
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T
O test, mean (SD) tions, two PEEP settings, and normal and stiff lung settings.
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1L/min 3L/min The majority of the testing was accomplished with an LP10
Baseline FiO 2 0.96 (0.003) 0.99 (0.01) ventilator (Puritan Bennett, Medtronic, Minneapolis, MN).
5-second suction FiO 0.9 (0.01) 0.93 (0.001) Across all settings, the authors found that the oxygen flow
2 required to maintain a stable FiO >0.93 was 0.75–1.5L/min.
Recovery time, s 5.3 (0.4) 4.0 (1.4) The findings were similar to the results of a portion of our
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10-second suction FiO 0.47 (0.09) 0.77 (0.04)
2 study, although the goal of our testing was to determine the
Recovery time, s 1125.5 (140.7) 334.5 (14.8) highest FiO using two oxygen flows over a range of ventilator/
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15-second disconnect FiO 0.96 (0.001) 0.99 (0.01) lung compliance settings and three altitudes. The setup config-
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Recovery time, s 0 0 urations were similar with the MCS and our system with two
30-second disconnect FiO 2 0.32 (0.05) 0.69 (0.02) exceptions. The CO absorber in the MCS model was placed
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Recovery time, s 1446 (178) 443 (5.7) on the ventilator air intake side versus the ventilator output
side with our configuration. Additionally, the LP10 has an ex-
Discussion ternal PEEP valve versus the PEEP being controlled internally
with the ventilators we used for testing. We initially attempted
This study showed that a rebreathing system can be adapted to place the CO absorber on the ventilator air intake, but
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to selected portable ventilators and provides FiO ≥90% with this configuration interfered with ventilator PEEP controls
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1–3L/min oxygen introduced into the system over a range of and resulted in alarms, necessitating placement of the CO
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ventilator settings and lung conditions, while scrubbing CO absorber in the inspiratory limb. Placement in the inspiratory
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from the inspired gas. The study also showed the effects of limb where the ventilator provides the power to overcome the
airway suctioning and ventilator disconnect on delivered FiO 2 resistance is also an advantage. Using the external PEEP valve
as well as the volume of condensate produced and capacity of likely allowed for the CO absorber attachment to the LP10
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the CO absorbent. without an impact on PEEP.
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Full-size rebreathing anesthesia machines are too cumber- Duration of mechanically ventilated patient transports can
some and weight- and cube-prohibitive for use in transport vary widely depending on the transportation method. Ground
and/or austere environments. Oswald and DeBoer in the early and rotor wing transports are relatively short in duration.
1990s described a closed-circuit anesthesia device developed Buchanan et al. reported that transport time for trauma pa-
for transport use with off the shelf components. The potent tients to the referring hospital from the scene via ground or
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agent portable apparatus (PAPA) was intended to be used for rotor wing transport was <30 minutes. Transport time would
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anesthesia gas delivery but could also be used without anes- obviously depend on the distance from the referring facility.
thesia, potentially extending the life of an oxygen cylinder. The Aeromedical fixed wing transports can range from 30 minutes
device was much smaller and lighter (30lbs) than a typical an- to 16 hours. Duration of transcontinental aeromedical trans-
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esthesia machine and could be mounted to a hospital bed, but ports from Iraq and Afghanistan to Germany were approxi-
it required fresh gas flows up to 6L/min, albeit an improve- mately 7 hours. 19–20 The rebreather system CO -absorbent life
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ment over its predecessors. Pollock and Natoli conducted a in our study was 7.6 hours (SD 56 minutes) which would sat-
pilot study with 6 normal subjects, evaluating the performance isfy the requirement for operation for most of these transports.
of a closed-circuit emergency medical oxygen (REMO(2)) sys- For longer transports and prolonged field care, the absorbent
tem designed for field use in a laboratory setting. Subjects must be replaced or another full absorbent canister available
breathed spontaneously on the device via oronasal mask for to replace the canister and exhausted absorbent.
8 hours. The device provided 0.93–0.98 peak FiO using 1.0
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(SD 0.17) L/min oxygen flow. However, this device did not One of the most important aspects when discussing these
provide positive pressure ventilation, instead relying on users closed-circuit systems is the effect of breaking the system,
to generate their own minute ventilation. 13 potentially resulting in decreased FiO and patient hypoxia.
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Mechanically ventilated patients often require suctioning via
In the hospital setting, oxygen is generally abundant and inex- in-line suction catheters or disconnection from the ventilator
pensive, but this is often not the case in combat and aeromed- for open suctioning. Our study showed that utilizing in-line
ical evacuation settings. Oxygen containing and/or generating suction for 10 seconds or disconnection from the ventilator
equipment occupy 15%–30% of the available footprint for a for 30 seconds can result in low FiO and recovery times back
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given setting 14,15 and represent substantial weight. Because of to baseline from 5 to 24 minutes, depending on oxygen flow
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these logistical constraints, efforts have been made to reduce used. These results suggest that with patients requiring high
oxygen usage primarily by the automatic titration of oxygen FiO to maintain oxygenation airway suctioning and ventila-
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delivery to a target oxygen saturation (SpO ). 14–16 Barnes et tor circuit, disconnection should be limited because of the risks
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al. found in an observational study that 68% of mechanically of hypoxemia and resulting sequelae.
ventilated aeromedical transport combat casualties required
<3L/min oxygen. Although this was a small observational Limitations
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study with important findings, a substantial number of casu- There are a number of limitations with this study. This was a
alties required higher FiO , prompting a search for a potential bench study conducted under controlled settings including room
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solution to providing higher FiO while using ≤3L/min oxygen, temperature, close control of CO production, oxygen delivery
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the maximum flow provided by portable oxygen concentra- and simulated oxygen consumption. We cannot be certain the
tors deployed by the DoD (Saros, Caire Inc, Ball Ground, GA). rebreathing system would perform the same if used with patients
Evaluation of a Rebreathing System | 37