Med. & Biol. Eng. & Comput., 1981, 19, 638 644 Continuous determination of cardiac output during exercise by the use of impedance plethysmography Y. Miyamoto IVI. Takahashi T. Tamura T. Nakamura T. Hiura M. Mikami Biomedical Control, Research Institute of Applied Electricity, Hokkaido University, 060 Sapporo, Japan Abstract--/l computer-based system has been developed that enables the continuous measurement of cardiac output at rest as well as during exercise. The respiratory and motion artefacts appearing in the first derivative of the transthoracic impedance change (dZ/dt) were eliminated by adopting an ensemble averaging technique. A sufficiently high correlation was observed between cardiac outputs determined by the impedance and CO 2 rebreathing methods. The system may facilitate the physiological investigation of cardiac function during exercise. Keywords--Automatic determination, Cardiac output, Exercise, Impedance plethysmograph 1 Introduction IMPEDANCE plethysmography has a particular value for the assessment of stroke volume, and hence cardiac output, during exercise because of its non-invasive nature. The accuracy and the reproducibility of the stroke volume derived by the impedance method are shown to be reliable in comparison with other standard techniques as will partially be reviewed later. This method, however, requires that the breath be held during the period of measurement to eliminate the influence of respiration on the transthoracic impedance. Although breath holding is easily repeated at rest, it is difficult to perform during exercise. Motion of the body with exercise may also contribute distortion to the measurement. Thus, we have developed a computer-based system that enables a continuous measurement of cardiac output at rest as well as during exercise without breath holding. To improve the signal-to-noise ratio of the impedance information, an ensemble averaging technique was adopted in the present system. Recently, GOLLAN et aL (1978) have successfully measured systolic time intervals during exercise by combining the impedance method and the averaging technique. 2 Methods The transthoracic impedance was measured with a conventional constant current type impedance plethysmograph (Nihon koden, RGA-5). Two pairs of stainless steel strips of 5mm width and 0lmm thickness constituted the tetrapolar electrode system. First received 30th September 1980 and in final form 9th February 1981 0140-0118/81/050638 + 07 $01.50/0 IFMBE: 1981 Two electrodes were placed around the neck, t.he third electrode encircled the thorax at the level of the xyphoid, and the fourth electrode encircled the abdomen between the xyphoid and umbilicus as described by KUBICEK et al. (1966). A constant current of 350/~A r.m.s., 50 kHz was passed between the outer electrodes, and the voltage changes associated with the impedance variation of the thorax was picked up from the inner electrodes. The current and pick-up electrodes were mounted in a pair of rubber bands, which fulfil both the purpose of keeping the distance between the electrodes constant and giving an appropriate tension when placed around the body (see Fig. 1). The tension of the electrode bands was adjusted so that it gave no serious restriction to respiration or body motion during exercise. The conventional disposable electrodes consisting of aluminium strips with adhesive backing were not suitable for the present purpose because these did not contact the skin surface firmly, and hence were prone to produce noises due to body motion during exercise. The electrocardiogram lOcm _ _ ~ lead wires rubbe'r band stainless steel (01ram thickness) Fig. 1 Structure of electrode system. Current and pick-up electrodes at the neck and lower thorax hare fimdamentally the same structure 638 Medical & Biological Engineering & Computing September 1981