Study characteristics and quality assessment
According to the search strategy, a total of 996 related articles were retrieved. A total of 292 articles were excluded after duplicates were removed and 673 articles were firstly included. After reading titles, abstracts and full texts, 667 articles were excluded, of which 35 were reviews. A total of 14 reports, 532 non-pre-hospital emergency studies, 40 that did not use laryngeal masks, 36 with non-randomized data, 7 with insufficient data and 9 randomized manikin studies were excluded. Finally, 31 randomized human studies [8-38] were included. The included studies focused on cases of cardiac arrest, respiratory failure, coma and other critical illnesses, and adverse effect mainly included laryngeal edema and mucosal hemorrhage, bucking, tooth loss and aspiration. All endotracheal intubations in the included studies were performed with direct laryngoscopes. Flow diagrams for the literature selection are shown in Figure 1. The study characteristics are shown in Table 1.
Meta-Analysis of laryngeal mask vs endotracheal intubation
Ventilation efficiency rates
Meta-analysis of 17 randomized studies [8-22, 18-20, 23 25, 27, 29-31, 33] showed that the ventilation efficiency rates of the laryngeal mask groups were higher than those of the endotracheal intubation groups [RR=1.20, 95% CI (1.06, 1.35), P<0.001] without heterogeneity (P = 0.941, I2 = 0%) (Figure 2).
Success rate of overall intubation s and first intubation
A total of 19 [8, 11-14, 16, 19-20, 24-28, 30-33, 35-36] and 3 randomized studies [26,32, 37] reported the success rates of first intubation and overall intubation success rates, respectively. The results suggested that the success rates of first intubation for the laryngeal mask were higher than those for endotracheal intubation [RR=1.29, 95% CI (1.18, 1.40), P<0.001] without heterogeneity (P = 0.840, I2 = 0%). However, no statistical differences in overall intubation success rates were observed [RR=1.11, 95% CI (0.88, 1.39), P<0.001] without heterogeneity (P = 0.979, I2 = 0%) (Figure 3).
Insertion time and SpO2 rise time
A total of 27 [8, 11-22, 24-28, 30-38] and 3 studies [8, 11, 24] reported insertion and SpO2 rise times, respectively. The results suggested that both were shorter in the laryngeal mask groups compared to endotracheal intubation [SMD=-3.48, 95% CI (-4.17, -2.80), P < 0.001; -2.19, 95% CI (-3.06, -1.32), P < 0.001] but with significant heterogeneity (P = 0.000, I2 = 96.8%; P = 0.001, I2 = 85.5%) (Figure 4).
Blood gas index
Three studies [10, 23, 28] reported PaCO2, PaO2 and pH, and 8 studies [10, 14, 16-17, 23, 28, 35, 38] reported SpO2. The pooled results showed that laryngeal mask ventilation increased pH [SMD=3.74, 95% CI (0.68, 6.80), P < 0.001, I2 = 97.4%], PaO2 [SMD=2.93 95% CI (1.43, 4.42), P < 0.001, I2 = 91.4%] and SpO2 [SMD=1.07, 95% CI (0.11,2.04), P < 0.001, I2 = 95.8%] and lowered PaCO2 [SMD=-4.03, 95% CI (-5.32, -2.73), P < 0.001, I2 =83.1%] (Figure 5).
Overall adverse events and aspiration
Thirteen [8,11-14,19,22-25,27-29,31,34] and five studies [8,11,22,28,31] reported the incidence rates of overall adverse events and aspiration, respectively. These suggested that laryngeal mask ventilation could reduce the occurrence of adverse events compared to endotracheal intubation [RR=0.41, 95% CI (0.30, 0.57, P<0.001, I2=0]. No differences in aspiration were observed (Figure 6).
Due to the existence of heterogeneity, we performed subgroup analysis on the first success rates of the first insertion and insertion time. As shown in Table 2, we failed to identify any source of heterogeneity in insertion times across the included studies.
Sensitivity analysis and publication bias
The results of sensitivity analysis and publication bias are shown in Table 3. Funnel plots of included studies are shown in Figure 6. Sensitive analysis suggested that all the results were stable. Egger's tests showed the presence of publication bias in the ventilation efficiency rates and insertion times, but metatrim suggested that the bias had no influence on the final results.