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According to the ERC guidelines of 2005 and 2010, two rescuers basic life support (BLS) with a compression-ventilation ratio of 30:2 is recommended. It is suggested to switch chest compressors approximately every 2 minutes to prevent fatigue leading to decreases in CPR efficacy. Otherwise, interruptions of chest compressions by switching compressors, delivering ventilations or diagnostic or therapeutic interventions should be minimized to reduce coronary and cerebral no-flow time during the resuscitation. The goal of this study was to determine the optimal time to switch compressors in various climatical conditions. 41 professional health care providers trained in CPR performed eight minutes of BLS in a randomized simulation study using an interactive manikin, ResusciAnne® Simulator (Laerdal™) in normal, warm and cold climatic conditions. Primary endpoints were changes in total number of chest compressions, depth of compressions, hands-off-time, number of correctly applied chest compressions minute-byminute as well as the compressor’s subjective sensation of fatigue. Using the Laerdal PC SkillReporting Systems™ the chest compressions applied were recorded and analysed. Data were analysed using Bonferroni’s univariate analysis of variance (ANOVA) or Friedman’s analysis with Dunn’s post test where appropriate. Statistical significance was defined as a P value less than 0,05. In addition, test persons answered questions regarding their formal knowledge of current resuscitation guidelines and their CPR experience. Before and after the simulation study in the cold condition, blood lactate testing was performed in 34 test persons. The observed increase in blood lactate after the simulation in the cold condition is equivalent to average effort exercise. The declared wishes to switch compressors declined during the study intervals. In the first setting, normal climatic conditions, there were ten test persons, in the second setting, warm climatic conditions, five test persons and in the last setting, cold climatic conditions, four test persons who wished to switch during the study period of eight minutes. Simulated BLS over eight minutes without switching compressors resulted in less compressions and increases in hands-off-time after one minute as well as inadequate depth of chest compressions after two minutes. These effects were more pronounced in the warm and cold climatic conditions without reaching statistical significance every minute. There was a noticeable training effect during the study period: the third BLS simulation in the cold climatic condition resulted in statistically significant more correctly applied chest compressions. There was discrepancy in test persons’ self-assessment on physical capacity and total number of wishes to switch with the quality of chest compressions recorded. Overall, BLS simulation over eight minutes in all three climatic conditions was consistent with average ERC 2005 standards. According to the updated ERC guidelines of 2010, depth of compressions as well as frequency of compressions during simulation were in the lower minimum range. The results of the study confirm the ERC recommendations of 2005 and 2010 that lack evidence to switch compressors after two minutes independent of personal fitness and self-assessment. In extreme climatic conditions, compressor’s switch may be reasonable even after one minute. During resuscitation in conditions when switching compressors is challenging, this switch after two minutes is not mandatory. The results of this study are consistent with the demand to train professional health care providers in CPR on a routine basis.