Procedure and design The study was structured according to a test–retest within subjects design, using HRV and RR as dependent variables and time as an independent variable. Between March and July 2006, all subjects underwent evaluations of HRV and RR on two occasions, with
an interval of 3–4 days between assessments. Two to 3 days before the first assessment of HRV and RR, the subjects completed three questionnaires to measure the extent of their fatigue complaints, subjective health complaints and functional impairment. The questionnaires were completed under the guidance of the test leader. A diagram of the procedure is presented in Fig. 1. Fig. 1 Schematic presentation of the protocol On both assessment days selleck the participants
visited the outpatient clinic. The protocol (Guijt et al. 2007) was performed in a separate room, starting at approximately the same STA-9090 supplier time of day on each occasion. The protocol took 30 min. After the explanation, the subjects were see more seated in a resting position for 5 min for adaptation purposes, after which they reclined in a supine position for 10 min (reclining). They subsequently performed light exercise for 12 min (cycling), cycling on a bicycle ergometer using a single load of 50 W with a pedal frequency between 60 and 65 min−1 (the posture of the subjects was the same on both occasions). Parameters Variation in heart rate, HRV, was evaluated by means of time-domain measures. In a continuous electrocardiographic record (ECG) QRS complexes are shown. The R wave peaks of the QRS
complex were detected and the so called normal-to-normal (NN) intervals were determined. Time-domain measures were calculated from these NN intervals and differences between adjacent NN intervals. HRV was assessed as the standard deviation of the NN intervals (SDNN) and the square root of the mean squared differences of successive NN intervals (RMSSD). RR was assessed by means of chest extension, defining the breath frequency per minute. Measurement device Heart rate variability and RR were recorded using the Co2ntrol (Decon Medical Systems, Weesp, the Netherlands). The Co2ntrol uses a Polar HR “detection board” (PCBA Vasopressin Receptor receiver) to register RR intervals. The QRS detection timing accuracy and detection reliability of the detector system were tested with an artificially generated ECG signal. The tests indicated that timing errors of less than 1 ms can be detected in real measurements, even under noisy conditions (Ruha et al. 1997). The device is attached to an elastic belt. The belt contains a stable case with heart rate electrodes and a polar HR transmitter (Polar T31™ transmitter, Polar Electro, Almere, the Netherlands). The Co2ntrol is built to detect QRS complexes and to determine RR during normal activities. ‘Normal-to-normal’ (NN) intervals (i.e. intervals between adjacent QRS complexes) are defined with an accuracy of 1 ms.