Gas exchange dynamics with sinusoidal work in young and elderly women

The objective of this study was to model the dynamics of, and interrelationships among, gas exchange, ventilation, and heart rate responses to sinusoidal forcing in young and elderly women. Nineteen females, 22-28 years (n = 10) and 62-73 years (n = 9) volunteered for the study. All experiments were conducted at work rates below the ventilatory threshold (TVE). A sine wave test consisted of 4 min of cycling (60 rpm) at a work rate equal to the mean of the limits of sinusoid (60% TVE) followed by 16-20 min of a sinusoidally varying work rate (30-90% of TVE) and ending with 4 min of cycling at 60% TVE. The periods were 0.75, 1.0, 1.5, 3.0, 6.0, 10.0 min, assigned randomly. The averaged data were used to determine amplitude and phase lag of the sinusoidal response of VO2, VCO2, VE, fH, PETCO2, and PETO2. Bode plots demonstrated that the dynamics of the cardiorespiratory responses were all well-described by a first-order exponential equation with a delay for both young and elderly subjects. The time constants were much longer in the elderly. Nevertheless, there appeared to be a strong link between the relative slowing of the four components of the gas transport system (VO2, VCO2, VE, and fH). This may suggest one single factor is reflected in the slowing of all components in older subjects.