Muller MD, Ryan EJ, Kim C-H, Bellar DM, Blankfield RP, Glickman EL. Reliability of the measurement of stroke volume using impedance cardiography during acute cold exposure. Aviat Space Environ Med 2010; 81:120–4. Introduction: It is well documented that cardiovascular alterations occur during acute cold exposure (ACE). Interindividual variability is present, due mainly to body size differences, gender, and age. However, no study has evaluated stroke volume in the same individual twice in the same ambient conditions (i.e., test-retest reliability). Impedance cardiography (ICG) has become a popular method to acquire hemodynamic data in both clinical and applied physiology settings. Further, ICG does not interfere with other dependent variables such as oxygen consumption. Therefore, based on the uniqueness of the methodology, we sought to test reliability in this technology at 5°C for 65 min on two separate occasions. Methods: Nine young men underwent two 65-min trials of resting ACE, separated by at least 72 h. Volunteers were clothed in approximately one layer of clothing. Core and skin temperatures, oxygen consumption, and central hemodynamics were measured. Results: As expected, core and skin temperature decreased while oxygen consumption showed a modest increase over time. In both trials, stroke volume significantly increased over time as heart rate decreased. There was similarity within subjects and between trials for all variables, as assessed via bivariate correlations. Conclusion: Cold increased stroke volume and decreased heart rate when subjects were pooled together, but each subject retained his individuality (minimal interindividual differences). Results suggest that impedance cardiography may be a reliable technique to use during acute cold exposure.