Evaluating Activity Monitors in COPD Patients

Each patient wore 6 activity monitors simultaneously which were selected as a result of a systematic review of the literature. These were two uniaxial activity monitors [Kenz Lifecorder (Kenz), Actiwatch (Actiwatch)], three triaxial activity monitors [RT3, Actigraph GT3X (Actigraph), DynaPort MiniMod (MiniMod)] and one multisensor activity monitor combining a triaxial accelerometer with different sensors [SenseWear Armband (SenseWear)]. More details about software, type, body location and outputs of these monitors can be found in Table 1.
Patients also wore a portable metabolic system (Jaeger Oxycon Mobile), an oxygen saturation finger probe and a Polar T31 (Polar) coded transmitter belt for heart rate monitoring. The portable metabolic system was attached to the upper chest with a harness and due to its low weight (950 g), caused minimal discomfort. A face mask with a dead space of <30 mL (Hans Rudolph Inc, Kansas City MO/USA) was used. Location of attachment for the Oxycon Mobile together with the six activity monitors is shown in Figure 1. A two-point gas calibration was completed prior to each test. Oxygen consumption (VO₂), carbon dioxide production (VCO₂), heart rate, respiratory rate and tidal volume were measured continuously. Breath-by-breath measurements were averaged over one-minute intervals. After the experiment, stored data were downloaded from the portable metabolic device to a personal computer. VO₂ values were divided by participants’ body weight and converted to Metabolic Equivalents of Task (METs) [24] . Energy expenditure estimates from the portable metabolic system (METs) were used as a criterion measure for energy expenditure and were compared with the following activity monitor outputs: Kenz - arbitrary units (AU); Actiwatch - activity counts (AC); Actigraph and RT3 - vector magnitude units (VMU); MiniMod and SenseWear - METs.
Patients were instructed to perform a strict schedule of activities lasting 59 minutes (Table 2) which were chosen to be representative of everyday tasks (such as walking, stair climbing and sweeping the floor) that are reported as problematic by COPD patients [25] (link). Time was kept with both a stopwatch and a laptop computer clock so that activities were completed in whole minutes.