Five-electrode field plethysmography technique for separation of respiration and cardiac signals.

Respiration monitoring by means of electric field plethysmography is influenced by cardiac activity, which represents an artefact although of physiological relevance itself. For a separation of respiration and cardiac signals, a portable plethysmograph was developed, which uses a fifth electrode in addition to the conventional tetrapolar electrode arrangement. The separation is attained by weighted software subtraction of two detected signals. In a comparative way, three mathematical methods are discussed for the effective determination of individual weighting factors. Results of the practical applications indicate the best results for a method based on Fast Fourier Transformation.