Background: Focusing on the efficacy of successful ventilation during cardiopulmonary resuscitation (CPR) with alternative airways, previous reports investigated various parameters such as success rate, tidal volume, incidence of regurgitation, etc. However, there are few investigations of arterial blood gases (ABG) during CPR with alternative airways, especially the laryngeal mask airway (LMA). Methods and Results: A prospective multicenter study, non-randomized control trial compared ABG on hospital admission of patients resuscitated by emergency medical service personnel with a bag-valve-mask (BVM) with those using a LMA in witnessed cardiac-verified out-of-hospital ventricular fibrillation (VF) or pulseless ventricular tachycardia. According to the Utstein template, 173 cases of LMA and 200 of BVM both placed by paramedics were enrolled. The median arterial pH was statistically higher in the LMA group than in the BVM group (7.117 vs 7.075, P=0.02). There was no difference in the median value of PaCO₂ (52.9 vs 55.3, P=0.06) and PaO₂ (64.6 vs 71.9, P=0.56). Conclusions: LMA does not greatly benefit the respiratory status of patients such as in this study population. Delayed placement of a LMA will be recommended to achieve minimally interrupted chest compression in an out-of-hospital CPR protocol for witnessed VF cases following shock. (Circ J 2009; 73: 490 - 496)