out - of - hospital cardiac arrest is still a major public health issue , claiming hundreds of thousands of lives worldwide yearly . bystander - initiated cardiopulmonary resuscitation ( cpr ) is essential to increase the chance of survival and neurological recovery . despite huge efforts to train laypeople to recognize and treat cardiac arrest , incidence of bystander reluctance to perform mouth - to - mouth ventilation is one of the major reason . whereas cpr including ventilation is still considered the gold standard approach before advanced life support can be instituted , a growing number of studies compared a simplified form of cpr , based on chest compression alone versus standard cpr including ventilation . animal studies showed no difference in survival   or even worse outcomes   when ventilation was added to chest compressions ; nevertheless , in animal models of cardiac arrest due to respiratory causes a positive effect of ventilations was demonstrated . in humans , observational studies of bystander - initiated cpr comparing standard and compressions - only cpr reported similar survival rates ; however , interpretation of the results is made difficult due to the high heterogeneity of the causes of cardiac arrest and of the rescue characteristics . chest compression - only cpr is simpler than standard cpr to teach ( during courses but even by dispatchers under real conditions ) , and likely a higher percentage of bystanders would accept to perform it while avoiding mouth - to - mouth contact : the demonstration that it is ( at least ) as effective as standard cpr can be crucial to improve survival rate in out - of - hospital cardiac arrest .   with the underlying hypothesis that out - of - hospital cardiac arrest bystander - initiated compression - only cpr is equivalent to cpr including ventilation ( standard cpr ) , we performed a comprehensive systematic review and meta - analysis of randomized controlled trials , focusing on survival at hospital discharge . pertinent studies were independently searched in biomedcentral , central , and pubmed ( updated september 1 , 2010 ) by several trained investigator . ( cpr or resuscitation ) and compression and breath * and cardiac and arrest and survival )  . further hand or computerized searches involved the recent ( 2008 - 2010 ) conference proceedings from the international anesthesia research society , american heart association , american college of cardiology , american society of anesthesiology and european society of cardiology congresses . in addition , we employed backward snowballing ( ie scanning of reference of retrieved articles and pertinent reviews ) and contacted international experts for further studies . no language restriction was enforced , and non - english - language articles were translated when appropriate . references obtained from database and literature searches were first independently examined at the title / abstract level by several investigators with divergences resolved by consensus , and then , if potentially pertinent , retrieved as complete articles . the following inclusion criteria were employed for potentially relevant studies : a. random allocation to treatment ; b. comparison of chest - compression - only versus standard cpr . the exclusion criteria were : a. non - parallel design ( ie cross - over ) randomized trials , b. duplicate publications ( in this case only the article reporting the longest follow - up was abstracted ) ; c. non - human experimental studies ; two investigators independently assessed compliance to selection criteria and selected studies for the final analysis , with divergences finally resolved by consensus ( table 1 ) . design features and appraisal of the internal validity of included studies . * data abstraction and study characteristics 
  baseline and outcome data were independently abstracted by several investigators with divergences resolved by consensus ( table 2 ) . the primary end - point of the present review was survival ( hospital discharge ) . overall characteristics of 3737 patients who received either compression - only ( 1852 patients ) or standard - cpr ( 1895 patients ) for out of hospital cardiopulmonary resuscitation . internal validity assessment 
  the internal validity of included trials was appraised according to the cochrane collaboration methods , ie judging the risk for selection bias ( ie the bias due to the unbalanced enrolment of specific patient subsets in one of the groups ) , performance bias ( ie the bias due to differences in the management of patients or ancillary treatment , beyond the intervention object of randomized allocation ) , attrition bias ( ie the bias due to incomplete follow - up or different length of follow - up ) , or difference in number of withdrawals ) , and reporting bias ( difference between reported and unreported findings ) , and expressed as low risk of bias ( a ) , moderate risk of bias ( b ) , high risk of bias ( c ) , or incomplete reporting leading to inability to ascertain the underlying risk of bias ( d ) . in addition , allocation concealment explicitly distinguished as adequate ( a ) , unclear ( b ) , inadequate ( c ) , or not used ( d ) ( table 1 ) . two independent and experienced reviewers ( gl , gb - z ) appraised study quality , with divergences resolved by consensus . data analysis and synthesis 
  binary outcomes from individual studies were analyzed in order to compute individual risk ratios ( rr ) with pertinent 95% confidence intervals ( ci ) , and a pooled summary effect estimate was calculated by means of a fixed effects model . statistical heterogeneity and inconsistency was measured using , respectively , cochrane q tests and i. the risk of small study bias was not assessed given the inclusion of 3 studies only . computations were performed with revman 5.0 ( the cochrane collaboration , copenhagen , denmark ) . database searches , snowballing and contacts with experts yielded a total of 32 citations ( figure 1 ) . excluding 29 non - pertinent titles or abstracts , we retrieved in complete form and assessed according to the selection criteria 3 studies . which were included in the final analysis . study characteristics the 3 randomized controlled studies included 3737 patients ( 1895 to chest - compression - only and 1842 to the standard cpr group ) ( table 2 ) . all studies were performed in non - traumatic out of hospital patients and stated that the updated international basic life support and advanced life support guidelines were strictly followed . all studies were of high quality ( table 1 ) as testified by the details on the method used for randomized sequence generation and allocation , adequate allocation concealment and low risk of selection , performance , attrition and detection bias . one study employed a multicenter design , a feature which does not strictly impact on internal validity , but usually increases external validity of a trial . quantitative data synthesis overall analysis showed that , in comparison to standard cpr , chest - compression - only was associated to increased survival at hospital discharge ( 211/1842 [ 11.5% ] vs 178/1895 [ 9.4% ] , rr=1.24 [ 1.01 - 1.54 ] , p=0.04 ) ( figure 2 ) . forest plot for the comparison of standard cpr vs compression - only cpr on hospital survival after cardiopulmonary resuscitation . similar results were obtained at sensitivity analyses using random - effect methods or risk differences ( all p<0.05 ) . only one study   considered the favourable neurological outcome suggesting better outcome in clinical subgroups of patients receiving chest - compression - only . available evidence from randomized controlled trials strongly supports the superiority of bystander - initiated compression - only cpr , given that patients who experienced out - of - hospital cardiac arrest could be saved by limiting cpr to chest compression . these results are crucial to significantly improve the first response to out - of - hospital cardiac arrests , a worldwide major public health problem . previous findings from observational studies in humans   documented that spontaneously performed ( i.e. , not dispatcher instructed ) compression - only cpr was as effective as standard cpr . in the sos - kanto study , including witnessed cardiac arrests , compression - only cpr resulted in a higher proportion of patients with a favourable neurological outcome than standard cpr in patients with apnoea , shockable rhythm and resuscitation started within 4 minutes ; ventilations did not add benefits in any subgroup . on the contrary , two recent nationwide observational studies   conducted in japan concluded that standard cpr should be preferred in out - of - hospital cardiac arrests of noncardiac origin , both in adults and in children ; in this group , the two cpr techniques were similarly effective for arrests of cardiac origin . among the tree randomized controlled studies included in this meta - analysis , only one study   included paediatric patients < 8 years old , but results of this subgroup were not separately reported ; the study by rea et coworkers reported a tendency towards a worse efficacy of compression - only cpr than standard cpr in cardiac arrests of noncardiac origin , and a tendency towards a better efficacy in shockable rhythm and in rapid ( < 6 minutes ) response by the emergency medical system .   the results of the present meta - analysis are consistent with the most recent observational study on 4415 cardiac arrests not due to trauma or asphyxia , drug overdose or drowning , in which a 5-years data collection was accompanied by a statewide public education campaign aimed to increase bystander compression - only cpr . in this study , the incidence of bystander - initiated cpr increased every year , as did the proportion of compression - only cpr ; more importantly , overall survival increased significantly over time . overall survival to hospital discharge was equal between the no bystander cpr and the standard cpr groups , while survival and neurological outcome were significantly better in the compression - only cpr group ; compression - only cpr resulted particularly effective when the cardiac arrest was witnessed and presented with a shockable rhythm . in 1128 patients with cardiac arrest of presumed noncardiac origin , not included in the study , survival was lower and not different among the two technique and no bystander cpr . it is well documented that both interruptions of chest compressions during ventilation and positive -pressure ventilations have detrimental effects on survival rate . oxygenation and ventilation could be allowed - at least initially- by passive ventilation during chest compressions , by spontaneous gasping and by the lungs capacity to act as a reservoir   in addition , compression - only cpr is easier to teach , to remember and to perform , and it does not require mouth - to - mouth contact , so resulting in a better willingness to start cpr by bystanders . it is worth noting that in two of the three analyzed studies   bystanders randomly assigned to standard cpr were significantly more likely to withhold cpr than callers assigned to compression - only group . based on our findings , compression - only cpr should be considered as the preferred bystander cpr technique , even if ventilations still have a crucial role in cardiac arrests of presumed noncardiac origin , in children   and when resuscitation is started more than 4 minutes after the arrest . . however , it should be considered that most victims of cardiac arrest are adults , and the cause is cardiac in about 2/3 of cases . the importance of ventilations in cardiac arrests lasting more than 3 - 4 minutes is more controversial , as two recent studies in a porcine models reported contradictory results . likely laypersons training on cpr should be simplified to privilege compression - only cpr ; in 2008 the american heart association already recommended that untrained bystanders should provide compression - only cpr for adults with sudden cardiac arrest . however , fatigue can be a relevant problem decreasing the quality of compressions and a change of cpr providers every one minute instead of every two minutes has been suggested . available evidence from randomized controlled trials suggests that compression - only cpr is superior to standard cpr at least when performed by untrained bystander . these results have relevant implications on teaching , and if adequately publicized they should favour a crucial increase in the rate of bystanders performed cpr .
introductionout - of - hospital cardiac arrest has a low survival rate to hospital discharge . 
 recent studies compared a simplified form of cpr , based on chest compression alone versus standard cpr including ventilation . 
 we performed systematic review and meta - analysis of randomized controlled trials , focusing on survival at hospital discharge.methodswe extensively searched the published literature on out - of hospital cpr for non traumatic cardiac arrest in different databases.resultswe identified only three randomized trials on this topic , including witnessed and not - witnessed cardiac arrests . 
 when pooling them together with a meta - analytic approach , we found that there is already clinical and statistical evidence to support the superiority of the compression - only cpr in terms of survival at hospital discharge , as 211/1842 ( 11.5% ) patients in the chest compression alone group versus 178/1895 ( 9.4% ) in the standard cpr group were alive at hospital discharge : odds ratio from both peto and dersimonian - laird methods = 0.80 ( 95% confidence interval 0.65 - 0.99 ) , p for effect = 0.04 , p for heterogeneity = 0.69 , inconsistency = 0%).conclusionsavailable evidence strongly support the superiority of bystander compression - only cpr . 
 reasons for the best efficacy of chest compression - only cpr include a better willingness to start cpr by bystanders , the low quality of mouth - to - mouth ventilation and a detrimental effect of too long interruptions of chest compressions during ventilation . 
 based on our findings , compression - only cpr should be recommended as the preferred cpr technique performed by untrained bystander .