Unconstrained heart-rate monitoring during bathing.

An unconstrained electrocardiograph (ECG) and heart-rate monitor was designed to record ECG measurements in the bathtub. The apparatus consists of analog and digital parts. In the analog part, electrocardiographic signals obtained from electrodes placed inside the bathtub were amplified approximately 4,000 times. The electrodes were made of stainless steel and were 104 x 74 x 0.5 mm in size, including the absorbent sponge covers. Each electrode fitted into a plastic case 110 x 79 x 23 mm in size. Working electrodes were placed 20 cm from the bottom of the long bathtub wall, close to the position of the immersed trunk, and a reference electrode was placed 5 cm from the bottom of the side wall close to the toe position. In the digital part of the system the heart rate was calculated from the R-R interval of each ECG pulse and the calculated data were stored in a random-access memory. These stored data were transferred to a personal computer through an RS 232C interface and analyzed. The heart rates obtained from the bathtub ECG agreed with those obtained from a body-surface ECG. During measurements in bathwater with various concentrations of sodium chloride, the amplitude of the electrocardiographic waveform decreased as the electrical conductivity increased. The frequency characteristics of the bathtub ECG varied with the bathtub dimensions and the electrical conductivity of the water. Using this apparatus, heart-rate measurements can be obtained easily and noninvasively, but interpretation of the ECG signal requires great care. This monitor could be used as a home health heart-rate monitor.