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Changes in heart rate and tissue blood volume induced by inspiration and expiration

Respiration induces fluctuations in heart rate and arterial blood pressure, originating from either mechanical effect of respiration or from changes in autonomic nervous system activity. In order to study the relationship between cardiovascular hemodynamics and the two phases of respiration, inspiration and expiration, we used the photoplethysmographic (PPG) signal, which reflects the cardiac-induced changes in the tissue blood volume. The time of inspiration and expiration was determined by a novel depth-of-breath sensor. The PPG and the depth-of-breath signals were simultaneously recorded during long breathing period of 12 s duration per breath. In most examinations the finger blood volume increased during inspiration, probably due to higher sympathetic activity during expiration. In a minority of examinations tissue blood volume decreased during inspiration probably due to the mechanical effect of low pressure in the thoracic cavity during inspiration.