Spotlight on Research

Mark Hobson
July 2014

Airways in out-of-hospital cardiac arrest: systematic review and meta-analysisRecently, paramedic advanced airway interventions (AAI) in cardiac arrest have come under increased scrutiny, and supraglottic devices are becoming a more common choice over intubation where clinically appropriate. This meta-analysis specifically aimed to ascertain if patients who have suffered out-of-hospital cardiac arrest have better long (survival to discharge or to one month) and short (return of spontaneous circulation only or survival to hospital admission) term survival with AAI use, when compared to basic airway interventions (BAI).Meta-analyses can be considered useful when the results of individual trials fail to show a definitive answer to a posed research question. By combining the results of multiple trials and then examining the total data for trends, previously unanswerable questions may be answered.The literature search for the systematic review returned 799 papers and ultimately the meta-analysis incorporated 17 studies (publication dates between 1988–2013 for included papers) involving a total of 388 878 patients of 16 years and older, who suffered a cardiac arrest of non-traumatic origin treated in the out-of-hospital environment. For the purposes of the review BAI was defined as bag valve mask ventilation or mouth-to-mouth, with manual manoeuvres and/or the use of an orophayngeal and/or nasopharyngeal airway. AAI included endotracheal intubation (ETI), all supraglottic airways (SGA), double-lumen airways and trans-tracheal or transcricothyroid membrane airways.‘Comparison of long-term outcomes for ETI versus BAI showed a significant decrease in survival’The short-term survival figures showed no significant difference in the overall odds ratio (OR) between the AAI and BAI groups (OR 0.84, 95% CI 0.62–1.13), although there was a non-significant decrease in odds of ROSC when using AAI (OR 0.78, 95% CI 0.60–1.02). Longer-term odds of survival when using AAI were further reduced (OR 0.49, 95% CI 0.37–0.65). However, this is perhaps unsurprising as several confounding factors exist. Patients that have a poorer prognosis initially may be more likely to receive AAI and this is known as confounding by indication. Also, patients that have rapid ROSC are less likely to receive AAI. The authors explain that while AAI has worse outcomes, it may not be that AAI is harmful, but rather that persons receiving AAI might have a poorer prognosis to begin with and are therefore more likely to fall into the AAI group. Six out of 17 studies did not attempt to adjust/match or balance for confounders, which may cause airway interventions to appear better/worse than the true effect. Unfortunately, there is no known way to assess the magnitude of these confounders.Perhaps what is particularly useful for future paramedic practice given the confounding limitations of AAI vs. BAI, is the review's sub-analysis of ETI and SGA. Comparison of long-term outcomes for ETI vs. BAI showed a significant decrease in survival (OR 0.48, 95% CI 0.36–0.64). Corresponding analysis of SGA vs. BAI demonstrated a further decrease in the odds of survival (OR 0.35, 95% CI 0.28–0.44). However, the meta-analysis excluded all studies that included respiratory arrests, mixed arrests and airway obstructions into their patient recruitments. It could be argued that AAI would make a positive difference in survival for this latter small group of patients but these figures are unlikely to significantly alter the overall results.The authors acknowledge that they found no high-quality evidence comparing AAI to BAI in cardiac arrest and no study included in the meta-analysis accounted for other confounders such as interruptions to chest compressions, delays to defibrillation and hyperventilation. Although this meta-analysis suggests decreased survival when using AAI vs. BAI, caution must be applied to these findings as there is potential for bias due to confounding by indication.The authors make a strong recommendation to undertake a prospective controlled trial to address the limitations of previous research.

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