Multifunction transducer for obtaining digital volume pulse, skin-resistance response and electrocardiogram

A multifunction transducer for obtaining the digital photoplethysmogram, skin-resistance response and electrocardiogram is described. The importance of the time constant in photoplethysmography is illustrated and discussed. Simultaneous digital photoplethysmographic recordings of the pulsatile volume and vasomotor changes were obtained using direct coupling and various time constants. Results indicate that, when compared to direct-coupled recordings, only a 5% loss in amplitude occurs in the pulsatile volume and a 25% loss in the amplitude of the vasomotor response recorded with a 3 s time constant. A 25 s time constant results in only a 5% loss in the vasomotor amplitude and a faithful reproduction of pulsatile volume. Obviously, the clearest reproduction of vasomotor change occurs with a direct-coupled recording. Although the use of a 3 s time constant prevents faithful reproduction of the vasomotor response, it permits the identification of changes in pulsatile volume. It is recommended that investigators interested in both pulsatile-volume and vasomotor changes use separate recording channels with time constants of 3 s and one with direct coupling. If direct coupling cannot be used for recording vasomotor responses, a time constant in excess of 25 s is required.