Proposal of a new dynamic calibration method for transthoracic admittance cardiogram using an electrical thoracic simulator

One of the most distinctive features of transthoracic admittance (or impedance) cardiography is its capability of non-invasive, continuous and convenient monitoring of stroke volume (SV) or cardiac output (CO) with a very compact design, which allows ambulatory measurement. Although accurate calibration of the time derivative of the admittance (or impedance) signals is required, no appropriate and practical method is available. In the present study, we developed a new electrical thoracic simulator using light-emitting diodes and photoresistors, which is capable of generating a wide range of admittance signals, and proposed a standardized calibration procedure optimized for dynamic calibration using this simulator. We applied the proposed method to a previously developed admittance cardiograph and derived the calibration formula. We then compared this formula with that obtained by conventional method based on calculation using circuit constants. The formulae obtained by the two calibration methods were significantly different. Considering the errors in the actual circuit elements, we recommend using the proposed dynamic calibration method with the novel simulator, rather than the conventional method, to obtain an accurate calibration formula for an admittance (impedance) cardiograph.