REBEL Cast Ep126: Should We Not Be Recommending Small Adult BVMs in OHCA?

REBEL EM – Emergency Medicine Blog

Background: The holy grail of outcomes in OHCA is survival with good neurologic outcome. The only interventions proven to increase this outcome are high quality CPR and defibrillation in shockable rhythms. Ventilation is also an important component of resuscitation in OHCA. Excess minute ventilation can adversely affect hemodynamics due to increased intrathoracic pressure (i.e. decreased venous return). Additionally, low CO2 levels from hyperventilation can lead to cerebral vasoconstriction which could lead to worsened secondary brain injury.

Most organizations recommend adults to be ventilated with tidal volumes of 500 to 600mL/breath during ongoing CPR. Large adult BVMs can have maximum tidal volumes of ≈1500mL and deliver about 750mL per one handed ventilation. Simulation studies have shown that health care professionals often provide minute ventilation well above these recommended ranges.

One of the recommendations from many experts to mitigate the perceived risk of large adult BVMs is using smaller adult BVMs. This change would result in decreasing the maximum volume from 1500 to 1000mL and an expected delivered tidal volume from 750 to 450mL/breath (much more inline with recommended ranges). However, evidence that this approach makes is difference is lacking.

REBEL Cast 126: Should We Not Be Recommending Small Adult BVMs in OHCA?

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Paper: Snyder BD et al. Association of Small Adult Ventilation Bags with Return of Spontaneous Circulation in Out of Hospital Cardiac Arrest. Resuscitation 2023. PMID: 37805062

Clinical Question: Is large adult BVM or small adult BVM associated with more ROSC in adult patients treated with advanced airway placement for nontraumatic OHCA?

What They Did:

• Retrospective, observational cohort analysis of prospectively obtained data from a single urban EMS system
• Evaluating adults treated with advanced airway placement for nontraumatic OHCA
• Jan 2015 to Dec 2021
• Changed from large adult BVMs to small adult BVMs in summer of 2017 (3 month crossover period was allowed and excluded from analysis)
• Used a Mercury medical CPR-2 small ventilation bag
• Compared rates of ROSC, ventilation rate, and mean end tidal carbon dioxide (ETCO2) by minute before and after small adult BVM implementation

Outcomes:

• Primary: ROSC at the end of EMS care (i.e. Arrival to ED or terminated efforts in the field)
• Secondary:
  • Ventilation rate
  • Mean end-tidal CO2 (ETCO2) during CPR

Inclusion:

• Adult patients with nontraumatic OHCA
• Treated with an advanced airway (i.e. Endotracheal intubation or iGel)

Exclusion:

• Age <18 years
• Received basic life support only
• Termination of resuscitation due to advanced directives
• ALS interventions prior to EMS arrival
• Insufficient capnography data
• Cricothyrotomy
• Advanced airway placed while patient had spontaneous circulation
• Airway was managed with BVM only
• Did not receive CPR while under EMS ALS care

Results:

• 1994 Patients included in analysis
  • 1331 (67%) treated with small adult BVM
  • 663 (33%) treated with large adult BVM
  • 21% had an initial shockable rhythm
• ROSC
  • Small Adult BVM: 33%
  • Large Adult BVM: 40%
  • uOR 0.74; 95% CI 0.61 to 0.90; P = 0.003
  • After adjustment for age, sex, witnessed arrest, bystander CPR, and initial rhythm this finding remained statistically significant (aOR 0.74; 95% CI 0.61 to 0.91)
• Ventilation rates did not differ between cohorts (≈12BPM)
• ETCO2
  • Small Adult BVM: 36.9 +/- 19.2mmHg
  • Large Adult BVM: 33.2 +/- 17.2mmHg
  • P <0.01

Strengths:

• Written records are compared to cardiac monitor files and audio recordings to adjudicate differences before integrating information into the registry
• Intubations confirmed with ETCO2
• Took into account the COVID-19 pandemic time period
• Also took into account the potential for trends over time by visualizing the incidence of ROSC by month over a seven year period and found no significant change in the slope before and after the implementation of the small adult BVM

Limitations:

• Only included patients that were intubated with an endotracheal tube or iGel (these results may not apply in patients without these devices)
• There were some confounding baseline differences (explained more in discussion)
• Unclear what other interventions were performed in terms of ACLS medications or what the specific causes of the cardiac arrest were from
• This was a before and after study not allowing for a control group. Before and after studies can introduce numerous biases particularly if other pieces of care changed between the two time periods. (Can also go in the discussion)
• The actual tidal volume delivered was not measured in this trial and therefore the delivered minute ventilation is unknown
• As this is a retrospective study, we can only show association, BUT NOT causation of the size of the adult BVM affecting ROSC outcomes

Discussion:

• There are some key BASELINE DIFFERENCES that could account for the results of this trial (i.e. confounders):
  • More patients in the small adult BVM cohort received bystander CPR (64% vs 59%). This would favor more ROSC in the small adult BVM cohort
  • Unwitnessed arrest was slightly greater in the large adult BVM cohort (58% vs 53%)…This would favor more ROSC in the small adult BVM cohort
  • Fewer patients in the small adult BVM cohort arrested in public (22% vs 27%…Unclear how this would impact ROSC
  • The interval from 911 call to start of CPR (10 vs 9min) and advanced airway placement (20 vs 18min) were longer in the small adult BVM cohort…Not sure 1 to 2min of difference would result in more ROSC in the large adult BVM cohort
  • Adherence to guideline recommended ventilation rates of 10 BPM was more common in the small adult BVM cohort (28.4% vs 31.2%)…This would favor more ROSC in the small adult BVM cohort
  • It would appear most things at baseline favored the small adult BVM cohort (Although the authors did account for most of these in adjusted analyses)
• The end of this trial took place during the COVID-19 PANDEMIC:
  • Anyone who took care of cardiac arrest patients during the COVID-19 pandemic knows that there were significant delays in care
  • According to the authors any cases of OHCA that occurred after the start of the pandemic (Feb 2020) were censored from the analysis and the results were evaluated again
  • When looking at cases of OHCA that occurred prior to Feb 2020 the small adult BVM cohort had a similarly lower odds of ROSC (OR 0.75; 95% CI 0.60 to 0.93; p = 0.008) as the entire time period this intervention was implemented
  • This remained the case even after adjusting for initial rhythm, age, sex, witnessed arrest and bystander CPR (aOR 0.76; 95% CI 0.61 to 0.95; p = 0.018)
• While I would imagine during a code most people are bagging faster than 10BPM, in this study 6 to 18 BPM were delivered in 82.5% of the measured ventilations. Is this a result of Hawthorne effect or the implementation of a metronome to guide chest compression and ventilation rates (implemented June of 2015) or simply a well trained EMS system? This addition would seem to favor the small adult BVM group
  • This EMS organization appears to be very high functioning with lots of training and education which may not be the standard at other agencies. The fact that the medics are providing a good RR and good TV throughout a 7-year period would suggest this and in doing so a simple change from a large adult BVM to a small adult BVM may have resulted in the association of lower ROSC whereas an agency that does not get as much training or high functioning may actually still be causing harm with the large adult BVM
• Finally, there was a higher ETCO2 in the small adult BVM cohort compared to the large adult BVM cohort. As ventilatory rate was essentially similar between groups, this most likely means a smaller tidal volume was delivered with each breath. This smaller tidal volume could have lead to physiologic changes that are potentially harmful:
  • Hypoventilation
  • Increased dead space fraction
  • Alveolar decruitment
  • Atelectasis causing shunt physiology

Author Conclusion: “Use of small adult bag during OHCA was associated with lower odds of ROSC at the end of EMS care. The effects on acid base status, hemodynamics, and delivered minute ventilation remain unclear and warrant additional study.”

Clinical Take Home Point: This is a really messy trial, with lots of methodological and confounding issues that make it difficult to interpret. It does show that when experts recommend an intervention it is important to study it. Until better evidence shows us differently it is probably best to stick with a large adult BVM but use one hand for bagging and maintain a rate of 10BPM.

References:

1. Snyder BD et al. Association of Small Adult Ventilation Bags with Return of Spontaneous Circulation in Out of Hospital Cardiac Arrest. Resuscitation 2023. PMID: 37805062

Post Peer Reviewed By: Anand Swaminathan, MD (Twitter/X: @EMSwami)

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