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The optimal rate of chest compression during CPR in man has been debated. Recently, the end-tidal carbon dioxide concentration (Petco2) has been shown to correlate with cardiac output during CPR in experimental animals. Eighteen prehospital cardiac arrest patients were studied to determine the effect of external chest compression rate on the Petco2 and BP in man when ventilation rate, ventilation inspiration time, applied compression force, and a 50:50 downstroke:upstroke ratio were held constant using a microprocessor-controlled CPR Thumper“. Compression rate was increased from 60 to 140/min in 20 beat/min increments.The Petco2 was 1.7 ± 0.2% at a compression rate of 60/min and did not change significantly at increased rates. Systolic BP fell progressively from 59 ± 5 mm Hg at 60/min to 46 ± 4 mm Hg at 140/min. Diastolic BP remained approximately 23 mm Hg at all rates studied.Using a CPR manikin, we found that greater Thumper compression force was necessary to sustain the same sternal displacement and to achieve the same applied sternal pressure when the rate was increased due to a rate-limited fall in the compression duration.