Statistical analysis
ventricular and supraventricular couplets, ventricular and supraventricular ectopic beats, ectopy
REGRESSION, EVENTS, EVENT, ECTOPIC BEATS
Observations represent ventricular and supraventricular ectopic beats during five-minute intervals, with ectopy defined as any occurrence of couplets or runs. We inspected data for missing observations, removing six participants who were missing an entire day of observations (five with cardiac conditions and a non-smoker who did not present with a cardiac condition). Measuring ventricular and supraventricular couplets and runs resulted in one observation per measure of ectopy for every five-minute duration. Participants thus had up to 88 measurements available per day for each measure of ectopy. Measurements were recorded before the start of the flight or control condition (pre-condition) and after conditions commenced (post-condition). We examined the presence and rate of ectopy using difference-in-difference regression to compare the change in the odds of at least one event and the change in the rate of events from pre-condition to post-condition between flight and control conditions. The resulting estimated effects represent the ratios of odds ratios (ORs) and rate ratios (RRs), depending on the outcome. The data were correlated because we repeatedly measured ectopy among participants both within exposure day and between; hence, we implemented longitudinal regression techniques to control for within-participant variability. By design, the participants serve as their own controls to account for unmeasured within-participant effects that are stable over the time frame of the study.To examine the effect of exposure on the presence of ectopy we used a Binomial mixed effects model. From these models, we estimated the OR of ectopy during post-experimental condition versus pre- for both the flight and control days. We estimated the effect of exposure using the ratio of the ORs for each day in our primary analysis. For models evaluating the frequency of ectopic beats, we inspected ectopic beats data for extra-Poisson variability, finding evidence of overdispersion in most outcomes. To account for this, we employed a Negative Binomial mixed effects model, which accommodates the overdispersion in the counts relative to that explained by a Poisson distributionWe controlled for repeated sampling by exposure day and accounted for person-level variability. We also controlled for the time of day the participants began monitoring and for the block randomized order of the treatment assignment, hence adjusting for the effect of diurnal variation and for participants growing accustomed to the chamber. We adjusted for blood oxygen at baseline, use of beta blockers, and the presence of a pacemaker. For participant We examined the impact of time in the chamber on ectopy using Binomial mixed effects models for the presence of ectopic beats and Negative Binomial mixed effects models for the rate. 