Page 53 - JSOM Winter 2024

The available data demonstrate that medical providers expel PEEP in conjunction with an LMA for elective surgery pa-
42%–73% of tidal volume from the BVM resuscitator bag. tients. In a similar study, 80 healthy anesthetized adults were
Given AHA and ERC guidelines, a BVM with a resuscitator bag ventilated on a spontaneous ventilation setting in conjunction
capacity of 1,000mL is most appropriate for TCCC patients. with an LMA. The study found that PEEP did not improve
Level of Evidence: Level A and B-NR (Multiple Randomized oxygenation with a PEEP setting of 7cmH O versus a ZEEP
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Trials or Meta-Analyses and Nonrandomized) control group (ZEEP 97.2 [SD 1.8%] versus PEEP 97.2 [SD
1.9%]). Additionally there was no difference in oxygen satu-
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What is the efficacy of Positive End­Expiratory ration in recovery between the PEEP and ZEEP groups (ZEEP
Pressure (PEEP) in the battlefield? 96.0 [SD 1.8%] vs. PEEP 96.1 [SD 2.0%]). The study’s authors
concluded that the application of a PEEP of +7cmH O with a
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Extrinsic positive end-expiratory pressure (PEEP) provides im- LMA under spontaneous ventilation cannot be recommended.
proved lung compliance by the prevention of alveolar collapse PEEP does not appear to improve mortality, hypoxemia, or
and atelectasis. The increase in lung recruitment from PEEP length of stay (LOS) in hospital in patients without ARDS. In
increases surface area leading to better gas exchange and de- a 2021 meta-analysis of 28 studies including 2,712 patients,
creases the patient’s respiratory effort. The following eval- evaluating PEEP on non-ARDS patients, the authors found
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uates the suitability of PEEP in the prehospital environment. that among four different PEEP levels (including ZEEP) there
was no significant impact on LOS in hospital, LOS in the ICU,
Common injuries predicted in future conflicts that would in-hospital mortality, 28-day mortality, ICU mortality, occur-
require possible use of a BVM with PEEP during the initial rence of ARDS, as well as atelectasis and hypoxemia. 57
phases of care and transport include acute lung injuries (ALI), Level of Evidence: Level C-LD (Limited Data)
acute respiratory distress syndrome (ARDS), and primary blast
lung injury (PBLI). Blast lung is caused by a shockwave from What are the concerns associated with using
explosives causing widespread inflammation and intrapulmo- Positive End­Expiratory Pressure (PEEP) in the
nary hemorrhage that impedes pulmonary gas exchange and
can worsen over several hours with the possibility of advanc- battlefield environment?
ing to ARDS. Significant inflammation can occur 3–12 hours PEEP can increase intrathoracic pressure (ITP), reducing ve-
post-incident and can worsen up to 24 hours after the initial nous preload and cardiac output. While there is a decrease in
injury. PBLI patients are likely at a higher risk of developing preload due to ITP, it has been observed that PEEP may cause
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ARDS, given the traumatic injury associations. Additionally, an increase in mean systemic pressure, helping preserve the
ALI/ARDS incidences in modern combat range from 3% to return blood flow and reducing ITP impacts. 58
33% with a mortality rate from ALI/ARDS of 12.8%–33%. 52
If attempting to provide PEEP to a patient through a BVM, it
A U.S. Military study revealed that ALI has an association with is important that the user maintains a proper seal of the mask
injury and possible blood product transfusion. Patients who to prevent lung de-recruitment. To provide effective PEEP ven-
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developed ALI were more severely injured with a significantly tilations, a complete mask seal is required throughout respira-
higher ISS and required significantly more blood products, in- tions, likely requiring two-person BVM ventilation.
cluding packed red blood cells, fresh frozen plasma, and plate-
lets, during their initial 24-hour resuscitation. Patients who • PEEP of 5–10cmH O: Increases airway pressure improving
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developed ALI also received more colloids than those who did alveolar recruitment, it increases functional residual capac-
not develop ALI. Gunshot wounds were the cause of injury in ity (FRC) while reducing the incidence of ventilator-induced
32% of patients who developed ALI and 45% of those who lung injury by keeping more alveoli open, thereby prevent-
did not, whereas explosion injuries occurred in 68% of those ing mechanical trauma from repeated collapse, and shifting
who developed ALI and in 55% of those who did not. There lung water from the alveoli to perivascular spaces. A PEEP
was no difference on the development of ALI based on the value of 5cmH O was found to deliver the optimal oxygen
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mechanism of injury. levels during CPR in a porcine model, while not inhibiting
cardiac output due to increased intrathoracic pressure. 59
Currently, the globally recognized best practice for mechanical • PEEP 10–20 cmH O: An abstract on the effects of PEEP
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ventilation in casualties with ALI and ARDS (including PBLI) and blood pressure on cerebral oxygen saturation in swine
is the approach advocated by the ARDS network (ARDSnet) models showed that increased levels of PEEP resulted in ce-
group. The “protective ventilation” approach recommends low rebral hypoxia with lower mean arterial pressure (MAP).
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tidal volumes and relatively high PEEP to minimize mechanical Additionally, a study in 2010 found that in patients with a
volutrauma and limit atelectasis. It advocates using the lowest low potential for recruitment, high PEEP levels may be in-
acceptable inspired concentration of oxygen (generally not ap- effective, potentially resulting in high levels of intrathoracic
plicable in the TCCC setting) to prevent oxygen toxicity and pressure, thus producing a profound decrease in preload.
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tolerates hypercapnia within the limits of cardiovascular sta- During CPR, increasing PEEP pressures to 10 or above re-
bility (assuming the patient has not suffered a head injury). 54 sulted in significant declines in cardiac output and oxygen
delivery. 59
In a short-term anesthesia study examining PEEP used with
an LMA on elective surgery patients, a PEEP of 8cmH O pa- Dynamic airway collapse or obstruction, a condition character-
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tients had modestly higher oxygen saturation compared to the ized by significant narrowing of larger airways during breathing
zero end-expiratory pressure (ZEEP) group (98.5% vs. 98.0% due to airway inflammation or mucus is a relative contraindica-
SpO ) after 25 minutes of ventilation, despite the PEEP group tion for PEEP. The application of extrinsic PEEP will generate
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beginning surgery with an average SpO 0.5% higher than back pressure that will prevent air from getting out of the lungs,
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the ZEEP group. This study’s authors could not recommend worsening air trapping. High PEEP pressures are also relatively
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