Comparing three wearable accelerometers to measure early activity after cardiac surgery

Steps on postoperative day 2 were lower in patients with 3 adverse hospital outcomes.

Figure 1. Activity and mobility on postoperative days 1-10, as measured by 3 different wearable accelerometers (StepWatch, ActiGraph, Fitbit), and 1clinical observation scale (Johns Hopkins-Highest Level of Mobility [JH-HLM]). The distributions of activity on all postoperative days 1-10 are shown for StepWatch Ambulation Monitor (A), ActiGraph (B), Fitbit (C), and the JH-HLM scale (D). A similar pattern of increasing activity from postoperative days 1 through day 5 is observed, regardless of the method of measuring activity. Note that the boxplots reflect only patients in the hospital, and discharges of healthier patients began to occur at approximately day 5, and so the activity of the remaining hospitalized patients decreases after postoperative day 5. The lower and upper borders of the box represent the lower and upper quartiles (25th and 75th percentile). The middle horizontal line is the median value, and the diamond in the box is the mean value. The lower and upper whiskers represent 1.5 times the interquartile range. Open circles represent outliers. “N” represents the number of patients wearing the monitor on each day.

Figure 2. Differences in 30-day readmission, discharge location, and duration of hospitalization based on postoperative day 2 activity and mobility. The distributions of postoperative day 2 activity and mobility are shown for patients stratified by 3 outcomes (30-day readmission, discharge to a nonhome location, and duration of hospitalization >7 days). Postoperative day 2 activity is greater in patients with favorable outcomes, regardless of the method of measurement: StepWatch Ambulation Monitor (A), ActiGraph (B), Fitbit (C), and the Johns Hopkins-Highest Level of Mobility (JH-HLM) scale (D). However, the strength of the association between early activity and each outcome varies depending on the method of measurement. The lower and upper borders of the box represent the lower and upper quartiles (25th and 75th percentile). The middle horizontal line is the median value, and the diamond in the box is the mean value. The lower and upper whiskers represent 1.5 times the interquartile range. Open circles represent outliers.

Figure 3. Receiver operating curves (ROCs). ROCs comparing (A) duration of hospitalization, (B) discharge location, and (C) 30-day readmission by postoperative day 2 activity and mobility as measured by 3 accelerometry devices (StepWatch, Fitbit, ActiGraph) and one clinical observation scale (Johns Hopkins-Highest Level of Mobility [JH-HLM]). Taken as a whole, StepWatch provided the highest discrimination (area under the curve [AUC] 0.71-0.76 for all outcomes).

Figure 4. Physical activity and mobility were measured after cardiac surgery using wearable accelerometry devices. Steps on postoperative day 2 were lower in patients with 3 adverse hospital outcomes, with 250 to 500 steps/day appearing to be an important cutoff.

Figure E1. Patient flow diagram. The numbers of screened, enrolled, and analyzed patients are described.