Page 36 - JSOM Summer 2024

Evaluation of a Rebreathing System
for Use with Portable Mechanical Ventilators

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Thomas Blakeman, MSc, RRT *; Maia Smith, MS, PhD ;
Richard Branson, MSc, RRT 3

ABSTRACT
Introduction: Maximizing the capabilities of available low- over $150,000 and typically do not have advanced ventila-
flow oxygen is key to providing adequate oxygen to pre- tor modes and monitoring. These devices use a circle system
vent/treat hypoxemia and conserve oxygen. We designed a that allows rebreathing of the patient’s exhaled gases while
closed-circuit system that allows rebreathing of gases while eliminating exhaled carbon dioxide (CO ) via a CO absor-
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scrubbing carbon dioxide (CO ) in conjunction with portable bent while recirculating anesthetic gases and oxygen. The CO
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mechanical ventilators in a bench model. Methods: We evalu- absorbent used is typically composed of soda lime and small
ated the system using two portable mechanical ventilators cur- amounts of other chemicals that remove CO by chemically
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rently deployed by the Department of Defense—Zoll 731 and converting it to calcium carbonate. Heat and water are the
AutoMedx SAVe II—over a range of ventilator settings and byproducts of these reactions. The absorbent granules change
lung models, using 1 and 3L/min low-flow oxygen into a res- color when saturated with CO , providing a visual indicator
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ervoir bag. We measured peak inspired oxygen concentration that the absorbent’s ability to capture CO has reached its
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(FiO ), CO -absorbent life, gas temperature and humidity, and capacity. 7
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the effect of airway suctioning and ventilator disconnection on
FiO on ground and at altitude. Results: FiO was ≥0.9 across Many early rebreathing system configurations are attributed to
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all ventilator settings and altitudes using both oxygen flows. designs by Mapelson. These systems were simple to operate
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CO -absorbent life was >7 hours. Airway humidity range was but were inefficient and required fresh gas flows of 1–3 times
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87%–97%. Mean airway temperature was 25.4°C (SD 0.5°C). the patient’s minute ventilation in order to prevent rebreathing
Ten-second suctioning reduced FiO 22%–48%. Thirty- second of CO . Modifications of these early systems to mitigate the
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ventilator disconnect reduced FiO 29%–63% depending on dangers CO rebreathing and excessive fresh gas use led to the
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oxygen flow used. Conclusion: Use of a rebreathing system advent of circle or closed systems. This innovation allowed
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with mechanical ventilation has the potential for oxygen con- for much lower fresh gas flows while producing a higher FiO .
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servation but requires diligent monitoring of inspired FiO and In anesthesia, lower fresh gas flows allow conservation of ex-
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CO to avoid negative consequences. pensive anesthetic agents. We designed a closed-circuit system
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that allows rebreathing of gases while scrubbing CO in con-
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Keywords: mechanical ventilation; oxygen; rebreathing; hy- junction with mechanical ventilation in a bench model.
poxemia; transport
Methods
Introduction We used two portable mechanical ventilators currently em-
ployed by the Department of Defense (DoD)—the 731 (Zoll
Under normal hospital conditions, oxygen is abundant. Under Medical, Chelmsford, MA) and SAVe II (AutoMedx, Addison,
far forward conditions and in resource-poor areas, oxygen may TX)—for the evaluation. The experiment’s design is shown in
be scarce. Supplying oxygen in austere/resource-constrained Figure 1. Figure 2 shows the rebreathing system as it would be
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environments presents significant logistical challenges. In mil- connected to a patient. We evaluated the system over a range of
itary applications, oxygen is a finite resource, and methods ventilator settings that represent the likely range of respiratory
for conservation include targeted oxygen delivery, closed-loop rates (RRs) and tidal volume (V ) required by most patients
T
control of inspired oxygen, and use of chemical oxygen gen- (Table 1) and two lung conditions representing normal lung
erators and oxygen concentrators. In situations where the compliance and low lung compliance that may be required
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use of pressurized oxygen cylinders is logistically difficult or for patients with acute respiratory distress syndrome (ARDS)
not permitted due to potential hazards including fire and pro- (Table 2). An engineering group (Sparx Engineering, Manvel,
jectile risks, low-flow oxygen from alternative sources is the TX) designed and 3D printed the CO absorber canister in-
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next available option. Maximizing the capabilities of low-flow corporated into the rebreather system. A soda lime–based 3L
oxygen is key to providing adequate oxygen to prevent/treat absorbent (Sodasorb, Molecular Products Inc., Louisville, CO)
hypoxemia and conserve oxygen. was used for the evaluation. The canister was placed in the
inspiratory limb. We introduced oxygen flows of 1 and 3L/min
Anesthesia workstations have been used in the operating room into a 3L reservoir bag attached to the ventilator inlet and
for decades and are well understood. These workstations cost made the following measurements:
*Correspondence to Thomas Blakeman, University of Cincinnati, 231 Albert Sabin Way, Cincinnati OH 45267 or thomas.blakeman@uc.edu
1 Thomas Blakeman is affiliated with the University of Cincinnati, Cincinnati OH. Dr. Maia Smith is affiliated with Cape Fox Federal Integrators,
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Chantilly, VA. Richard Branson is affiliated with the University of Cincinnati, Cincinnati OH.
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