Introduction: Survival from out-of-hospital cardiac arrest (OOH-CA) remains poor, especially when patients are transported with CPR in progress. Previous investigations suggest that CPR quality erodes during transport due to the austere environment. We sought to determine how frequently ambulance personnel are exposed to off-balancing forces during transport of OOH-CA patients and to estimate the potential impact on CPR and coronary perfusion pressure (CPP). Methods: An onboard monitoring system was utilized to record acceleration data during the transport of 50 OOH-CA patients. Acceleration vectors were calculated for every second of drive time (speed >0. m/s). A model was constructed to estimate the potential impact of these vectors upon CPR and CPP. These data were then compared to a case-control cohort of 102 matched non-urgent transports. Results: A total of 5.8. h of drive time was analyzed in the cardiac arrest cohort. Mean transport time was 8. min 53. s with a mean drive time of 6. min 58. s. Critical acceleration threshold was exceeded 60% of transport time (202.42. min, mean 4.05. min/transport) yielding a potential hands-off ratio of 0.42 with a CPP < 15. mmHg 62% of drive time. Ambulance speed was inversely related to the magnitude of off-balancing forces. Comparison to 14.1. h of control cohort yielded similar off-balancing forces and relationships despite lower speeds and no " lights and siren" use. Conclusion: Critical acceleration forces occur frequently during transport of OOH-CA patients and may directly effect CPR quality and thereby CPP. These force vectors are stronger and more frequent at slower speeds, comprising the majority of ambulance drive time. Reducing speed or transporting OOH-CA patients without lights and sirens does little to mitigate these forces. © 2012 Elsevier Ireland Ltd.