Methods: We will conduct a randomized cross-over trial comparing bystander fatigue and CPR quality using two different CPR ratios. Intervention: All participants will be asked to perform two 5-minute sessions of CPR on a recording manikin - one session using the 30:2 ratio, the other using the 15:2 ratio. There will be a supervised practice session in the beginning, and resting periods in between. The order in which the sessions will be executed will be determined in a random fashion. The study population will consist of volunteers aged 55 or older, a group most susceptible to perform CPR on a real victim. The study will take place in a busy public area of The Ottawa Hospital, Civic Campus, between the cafeteria and the emergency department. Participants will have to score 3 or less on the validated Clinical Frailty Scale in order to participate in the study. Participants with physical limitations or disease processes precluding their ability to safely perform CPR will be excluded. Outcome measures: Information on age, gender, height, weight, prior CPR training, and measure of frailty will be collected at the time of enrollment. The number, frequency, and quality of chest compressions (depth and release) will be measured during each CPR session using a recording ResusciAnne manikin. Heart rate and blood pressure will be measured before and after each CPR sessions. The participants will be asked to rate their level of fatigue before and after each CPR session using the validated Borg Rating of Perceived Exertion scale. In addition, serum lactate levels will be determined before and after each CPR session in volunteers from the second group. Sample size: We hope to recruit 42 participants, among which 10 will have their lactate levels measured. Data analysis for Objective#1 and #3 will include descriptive statistics and a paired t-test with 95% confidence interval; Objective#2 will be analyzed using descriptive statistics and WIlcoxon Rank Sum test. In addition, we will perform a stepwise logistic regression analysis to control for the potential confounding effects of variables otherwise associated with bystander fatigue and CPR performance using both CPR ratio. This study protocol will receive ascent form the OHREB before implementation, and will be entered on the OHRI randomized trial registry.