Method and apparatus for respiration monitoring

An apparatus for monitoring the expansion and contraction of the thoracic cavity of a patient caused by respirations comprises, generating means for generating an AC examination current signal, coupling means for applying the examination current signal to the patient so that the examination current passes through the thoracic cavity of the patient, but at least a portion of the current does not pass through the thoracic cavity of the patient, detecting means coupled to the coupling means for detecting an amplitude modulated voltage signal developed across the patient in response to the application of the examination current and variation of the transthoracic impedance of the patient due to respiration, and current modifying means coupled with the generating means for modifying the AC examination current during application to the patient so that the portion of the examination current which does not pass through the patient is at least partially compensated.