R wave duration as a measure of body position changes during sleep

Any change in body position alters the relative angle between the ECG electrodes and the electric axis of the heart. Thus it influences the time interval during which the projection of the electric dipole, on any ECG lead is positive. We used the R wave duration (RWD) as representing this positive projection and as indicator of body position changes. Two experimental setups were used: (1) Simultaneous ECG recordings from leads I, II, and III during a sequence of well-defined body positions, (controlled conditions). (2) Lead II and video recordings from healthy subjects, who underwent whole night sleep studies. Using data of any of the leads, we could identify over 90% of the changes in body position, in both experimental setups. RWD calculated from lead II was the most reproducible upon reassuming the same body position. Measuring RWD from ECG may help to reduce the number of sensors required in full sleep studies, and to obtain better interpretation of non-attended ones.