Dynamic Ventilatory Response to Acute Isocapnic Hypoxia in Septuagenarians

This study compared the ventilatory response to 20 min of acute isocapnic hypoxia (end‐tidal P O2 , 50 mmHg) using the technique of dynamic end‐tidal forcing in young (Y) and old (O) men. Two groups of non‐smoking male subjects (mean ± s.d. age: Y, 29.8 ± 6.9 years; O, 73.4 ± 2.8 years) with similar body size, normal age‐predicted spirometry, and normal moderate levels of physical activity were studied. Compared with baseline ventilation in euoxia (10.79 ± 1.99 and 11.88 ± 0.91 l min −1 ) both groups responded to the abrupt onset of isocapnic hypoxia with peak ventilatory responses of 22.58 ± 2.60 and 24.56 ± 2.54 l min −1 for Y and O, respectively (not significant, n.s.). Both groups demonstrated a significant increment in neuromuscular drive (i.e. tidal volume ( V T )/inspiratory time ( T I ); 0.46 ± 0.06 to 0.91 ± 0.15 and 0.48 ± 0.06 to 0.91 ± 0.12 l s −1 for Y and O, respectively) with a small (but also significant) change in central timing ( T I /total ventilation time ( T tot ); 0.38 ± 0.02 to 0.41 ± 0.02 and 0.42 ± 0.02 to 0.45 ± 0.02 for Y and O, respectively). Oxygen sensitivity was assessed using Weil's equation, and gave a hyperbolic factor ( A ) of 282 ± 75 and 317 ± 72, and using the linear equation: change in expiratory minute volume (ΔV E )/change in arterial O 2 saturation (Δ S a,O2 ) which gave ‐1.17 ± 0.57 and ‐1.17 ± 0.42 l min −1 % −1 (n.s.) for Y and O, respectively. After 20 min of sustained isocapnic hypoxia, ventilation declined to 14.29 ± 1.92 and 16.85 ± 2.34 l min −1 for Y and O, respectively (n.s.). The acute response to hypoxia was characterised by similar time constants (16.0 ± 5.4 and 18.5 ± 6.7 s) and time delays (4.8 ± 2.1 and 4.6 ± 1.9 s) for Y and O, respectively. Thus, the dynamic ventilatory response to acute isocapnic hypoxia is maintained into the eighth decade in a group of habitually active elderly men.