GENDER AND AGE-DIFFERENCES IN TRANSTHORACIC BIOIMPEDANCE

Thoracic impedance consists of a constant baseline component Z(0) and a time-variable component Delta Z which represents the impedance chang e related to the cardiac cycle. The maximum part of Delta Z [(dZ/dt)(m ax)] represents the peak of the ascending aortic blood flow. Measureme nts of basal thoracic impedance are affected by structural and anatomi cal differences in the thorax related to sex and ageing. This componen t is a variable in the denominator of Sramek's formula which is used f or calculating stroke volume. The aim of this study was to elucidate t he question as to whether the age-and sex-related variation in basal i mpedance may affect bioimpedance measurements of stroke volume. The st udy comprised 111 healthy subjects (55 males and 56 females) of ages b etween 20 and 69 years, divided according to age decades into five gro ups each of males and females. Stroke volume index (SI), Z(0) and (dZ/ dt)(max) were measured in every subject, using transthoracic bioimpeda nce cardiography. Z(0) and (dZ/dt)(max) had significantly higher value s in females than in males in every age group except the oldest one in the case of Z(0) and the oldest two groups in the case of (dZ/dt)(max ). Stroke index showed no significant sex difference, although the hig her Z(0) in females may underestimate the values of stroke index. Elev ation of (dZ/dt)(max) in females may therefore reflect a positive rela tion to Z(0) rather than higher flow rates. Since Z(0) and (dZ/dt)(max ) are variants in opposite positions in Sramek's formula (denominator and numerator, respectively), this functional relationship may keep th e bioimpedance measurements from being affected by the sex- and age-re lated changes in Z(0).