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Background The primary goal of Basic Life Support (BLS) is to maintain oxygen supply to vital organs. In the current guidelines of the European Resuscitation Council (ERC) a combination of 30 chest compressions with two rescue breaths is rec-ommended. The optimum ratio of compression and ventilation is currently the subject of a worldwide debate. In our study we compared the influence of two different compression/ventilation ratios on gas exchange and return of sponta-neous circulation (ROSC) in a porcine model of cardiac arrest. Methods After approval of the animal welfare commission the study was performed on 16 pigs under general anaesthesia with endotracheal intubation. Arterial and central venous lines were inserted. After three minutes of cardiac arrest (ventricular arrest) the animals were treated with a compression/ventilation ratio of 30:2 or 100:5. The ventilation was performed with 17% O2 and 4% CO2. Subsequently 10min BLS, Advanced Life Support (ALS) was performed (100%O2, 3 defibrillations, 1mg adrenaline iv). Data were analyzed with 2-factorial ANOVA. Results During BLS, PaCO2 increased to 55mmHg (30:2) and 68mmHg (100:5; p=0,0001). PaO2 decreased to 58mmHg (30:2) and 40mmHg (100:5; p=0,15). ROSC was achieved in 4/8 (30:2) and 5/8 (100:5) animals. Conclusion During BLS, a compression/ventilation ratio of 100:5 seems to be equivalent to 30:2. This method may simplify resuscitation and the training of lay rescuers.