Age For the critical outcome of favorable neurologic outcome, we identified very-low-quality evidence from 11 observational studies (downgraded for bias inconsistency, indirectness, and imprecision) comprising 4054 patients.(Frates RC Jr 1981, 1006; Nagel 1990, 422; Quan 1990, 586; Anderson 1991, 27; Niu 1992, 81; Mizuta 1993, 186; Kyriacou 1994, 137; Al-Mofadda 2001, 300; Blasco Alonso 2005, 20; Nitta 2013, 1568; Quan 2014, 790) Of the 7 pediatric studies, 6 found that young age, variably defined as less than 3, 4, 5, or 6 years, was not associated with favorable neurologic outcome.(Frates RC Jr 1981, 1006; Nagel 1990, 422; Quan 1990, 586; Niu 1992, 81; Mizuta 1993, 186; Al-Mofadda 2001, 300) A single pediatric study including 166 children aged less than 15 years reported better outcomes in children aged less than 5 years (RR, 0.12; 95% CI, 0.03–0.44).(Kyriacou 1994, 137) Four studies considered drowning victims of all ages; 3 found no relationship between age and outcome.(Anderson 1991, 27; Blasco Alonso 2005, 20; Nitta 2013, 1568) One reported worse outcomes among children aged greater than 5 years (RR, 0.66; 95% CI, 0.51–0.85).(Quan 2014, 790) For the critical outcome of survival, we identified very-low-quality evidence (downgraded for risk of bias, inconsistency, indirectness, and imprecision) from 6 observational studies, which included 1313 patients.(Orlowski 1979, 176; Mosayebi 2011, 187; Claesson 2012, 1072; Dyson 2013, 1114; Claesson 2014, 644; Vahatalo 2014, 604) Three studies found that age was not associated with outcome.(Mosayebi 2011, 187; Dyson 2013, 1114; Vahatalo 2014, 604) Two showed better outcomes associated with younger ages (less than 58 years: RR, 0.27; 95% CI, 0.08–0.96(Claesson 2012, 1072); less than 46 years: RR, 0.98; 95% CI, 0.99–0.99),(Claesson 2014, 644) and 1 favored older age (3 years or older: RR, 1.51; 95% CI, 1.19–1.9).(Orlowski 1979, 176) EMS Response Interval No studies were identified that addressed the critical outcome of favorable neurologic outcome. For the critical outcome of survival, we identified very-low-quality evidence (downgraded for risk of bias, indirectness, and imprecision) from 2 observational studies, including 746 patients in the Swedish EMS OHCA registry.(Claesson 2008, 381; Claesson 2012, 1072) EMS response intervals of less than 10 minutes were associated with better survival: RR of 0.29 (95% CI, 0.13–0.66)(Claesson 2008, 381) and reported OR of 0.44 (95% CI, 0.06–0.83).(Claesson 2012, 1072) Salinity For the critical outcome of favorable neurologic outcome, we identified very-low-quality evidence (downgraded for risk of bias, indirectness, and imprecision) from 4 observational studies,(Mizuta 1993, 186; Blasco Alonso 2005, 20; Forler 2010, 14; Quan 2014, 790) which included 1842 drowning victims, of which 370 occurred in salt water and 1427 in fresh water. Two showed salt water was associated with better outcomes (RR, 1.3; 95% CI, 1.12–1.5(Blasco Alonso 2005, 20); RR, 1.2; 95% CI, 1.1–1.4(Mizuta 1993, 186)), and 2 showed water type was not associated with outcome (RR, 1.1; 95% CI, 0.95–1.2(Forler 2010, 14); RR, 1.14; 95% CI, 0.9–1.4(Quan 2014, 790)). For the critical outcome of survival, we identified very-low-quality evidence (downgraded for risk of bias imprecision, inconsistency, indirectness, and imprecision) from 3 studies.(Orlowski 1979, 176; Bierens 1990, 1390; Dyson 2013, 1114) One reported better outcomes for salt water (RR, 1.34; 95% CI, 1.19–1.52),(Bierens 1990, 1390) 1 showed no difference (RR, 1.22; 95% CI, 0.95–1.56),(Orlowski 1979, 176) and 1 showed worse survival in cases with salt water drowning (RR, 0.18; 95% CI, 0.03–1.43).(Dyson 2013, 1114) Submersion Duration For the purposes of this review, we considered studies in 3 groups. We defined those with short submersion intervals (less than 5–6 minutes), those with intermediate duration (less than 10 minutes), and those with prolonged submersion intervals (less than 20–25 minutes). Short Submersion Intervals (Less Than 5–6 Minutes). For the critical outcome of favorable neurologic outcome, we identified moderate-quality evidence from 12 observational studies (downgraded for bias and indirectness, upgraded for dose response), which included 2409 cases.(Kruus 1979, 315; Frates RC Jr 1981, 1006; Quan 1990, 586; Anderson 1991, 27; Niu 1992, 81; Mizuta 1993, 186; Kyriacou 1994, 137; Graf 1995, 312; Al-Mofadda 2001, 300; Torres 2009, 234; Quan 2014, 790; Kieboom 2015, h418) All studies noted worse outcomes among patients with submersion durations exceeding 5 minutes (RRs ranged between 0.05(Quan 2014, 790) and 0.61(Kyriacou 1994, 137)). The 713/826 patients (86.3%) who had outcome information available and were submerged for short durations had good outcomes compared to 128/1150 (11%) with longer submersion durations. For the critical outcome of survival, we identified low-quality evidence (downgraded for risk of bias, indirectness, and imprecision; upgraded for dose response) from 5 observational studies comprising 317 cases.(Orlowski 1979, 176; Kaukinen 1984, 34; Bierens 1990, 1390; Veenhuizen 1994, 906; Mosayebi 2011, 187) All studies noted worse outcomes among patients with prolonged compared to short submersion durations (RRs ranged between 0.27(Kaukinen 1984, 34) and 0.83(Veenhuizen 1994, 906)). The 159/170 patients (93.5%) submerged for short durations had good outcomes compared to 45/84 (53%) with longer submersion durations. Intermediate Submersion Intervals (Less Than 10 Minutes). For the critical outcome of favorable neurologic outcome, we identified moderate-quality evidence (downgraded for bias, indirectness, and imprecision; upgraded for dose response) from 9 observational studies that included 2453 cases.(Kruus 1979, 315; Quan 1990, 586; Quan 1992, 909; Mizuta 1993, 186; Kyriacou 1994, 137; Graf 1995, 312; Suominen 1997, 111; Quan 2014, 790; Kieboom 2015, h418) All studies noted worse outcomes among patients with prolonged submersion durations compared with intermediate submersion durations (RRs ranged between 0.02(Quan 2014, 790) and 0.45(Kyriacou 1994, 137; Graf 1995, 312)). The 787/1019 patients (77.2%) submerged for intermediate durations had good outcomes compared to the 36/962 (3.7%) with longer submersion durations. For the critical outcome of survival, we identified low-quality evidence (downgraded for bias, indirectness, and imprecision; upgraded for dose response) from 2 observational studies(Bierens 1990, 1390; Panzino 2013, 178) comprising 121 cases. The first study(Bierens 1990, 1390) reported 56/73 (77%) of those submerged for less than 10 minutes survived compared with none of the 7 patients who were submerged for more prolonged periods survived (RR not estimable; absolute difference, 76.7%; 39.7%–94.9%). The second study(Panzino 2013, 178) also noted better survival among those submerged for less than 10 minutes (46/50 [96%] survived) compared with those submerged for more than 10 minutes (2/5 [40%] survived).(Panzino 2013, 178) Prolonged Submersion Intervals (Less Than 15–25 Minutes). For the critical outcome of favorable neurologic outcome, we identified low-quality evidence (downgraded for bias and imprecision, upgraded for dose response) from 3 observational studies that included 739 cases.(Quan 1990, 586; Mizuta 1993, 186; Kieboom 2015, h418) In 1 study (n=398),(Mizuta 1993, 186) submersion less than 20 minutes was associated with improved survival (289/370 [78%] good outcome versus 1/27 [4%] good outcome; RR, 0.05; 95% CI, 0.01–0.31). The second study(Quan 1990, 586) reported better outcomes if submersion duration was less than 25 minutes (68/101, or 67%) versus submersion duration longer than 25 minutes (0/4, 0%). The third study, which included hypothermic children in cardiac arrest, observed 12/66 (18%) survivors who were submerged for less than 25 minutes compared with 0/39 who were submerged for more than 25 minutes.(Kieboom 2015, h418) For the critical outcome of survival, we identified very-low-quality evidence (downgraded for bias, indirectness, and imprecision) from a single study(Kaukinen 1984, 34) comprising 49 patients. Cases with a submersion interval of less than 15 minutes had an overall survival rate of 82% (33/39) compared with none of the 2 victims whose submersion duration exceeded 15 minutes (RR not estimable; absolute difference, 84.6%; 17.3%–92.8%). Water Temperature For the critical outcome of favorable neurologic outcome, we identified very-low-quality evidence (downgraded for bias, inconsistency, indirectness, and imprecision) from 2 studies(Quan 2014, 790; Kieboom 2015, h418) of 1254 cases. The largest study (n=1094) included all unintentional drownings in open waters (lakes, ponds, rivers, ocean) in a single large region, collected from medical examiners, EMS systems, and all regional hospitals.(Quan 2014, 790) Water temperatures were measured within a month of the drowning incident. Univariate analysis according to temperatures less than or greater than 6°C or less than or greater than 16°C found no difference in neurologic survival: RR, 1.11 (95% CI, 0.9–1.37); RR, 1.02 (95% CI, 0.81–1.27); absolute difference, −0.5% (−7.5% to 6.1%), respectively. Multivariate analysis also showed that water temperature was not associated with outcome. The second study included 160 hypothermic children who required resuscitation after submersion. Water temperatures were estimated based on the season. Submersion in the winter (water temperature estimated as 0°C–8°C) was associated with better outcomes than submersion in spring or summer (water temperature 6°C–28°C) (univariate OR, 4.55; 95% CI, 1.37–15.09). For the critical outcome of survival, we identified very-low-quality evidence (downgraded for risk of bias, indirectness, and imprecision) from a single study(Claesson 2012, 1072) that included 250 patients. This study included only drowning victims who had an OHCA and received EMS care, and it included those with intentional (suicide and homicide) drowning. This study showed no relationship between water temperature less than or greater than 15°C and outcome (RR, 0.94; 95% CI, 0.34–2.62; absolute difference, 0.36%, −6.4% to 6.5%). Witnessed Status The definition of witnessed versus unwitnessed was inconsistently defined in the studies reviewed. It was often unclear if witnessed related to the submersion or time of cardiac arrest. For the critical outcome of favorable neurologic outcome, we found very-low-quality evidence (downgraded for indirectness and imprecision) from 1 observational study(Nitta 2013, 1568) involving 1737 patients. Univariate analysis reported by the study authors indicates an unadjusted OR for good outcomes in the group where submersion was witnessed (OR, 16.33; 95% CI, 5.58–47.77). In multivariate analysis, witnessed status was related to favorable outcome (adjusted OR, 11.8; 95% CI, 2.84–49.08); however, the analysis did not include submersion duration, which several studies have reported is an independent predictor. For the critical outcome of survival, we found low-quality evidence (downgraded for risk of bias, indirectness, and imprecision) from 4 studies(Claesson 2008, 381; Dyson 2013, 1114; Nitta 2013, 1568; Claesson 2014, 644) involving a total of 2857 victims. Two studies(Claesson 2012, 1072; Claesson 2014, 644) were from the same EMS system, and both used multivariate analysis. The smaller study (255 victims) showed that witnessed status was not significantly associated with improved survival (RR, 0.55; 95% CI, 0.17–1.75; absolute difference, 3%; −3.1% to 11.2%).(Claesson 2012, 1072) However, in the larger subsequent study from that same EMS system, witnessed status predicted better outcome (reported univariate analysis P=0.05; adjusted OR, 2.5; 95% CI, 1.38–4.52).(Claesson 2014, 644) A further study(Dyson 2013, 1114) showed no association of witnessed status with improved survival (RR, 0.82; 95% CI, 0.26–2.59). A large observational study from Japan(Nitta 2013, 1568) reported an unadjusted OR of 7.38 (95% CI, 3.81–14.3) and an adjusted OR of 6.5 (95% CI, 2.81–15.02), although the unusual population of much older victims, most drowning in bathtubs, with very low favorable outcomes limited the generalizability of these findings.