Changes in vagal tone with posture in asthma are related to changes in ventilatory pattern

Posturally induced changes in minute ventilation in patients may be autonomically driven. This study aimed to test whether changes in autonomic tone with posture differed between normal and asthmatic subjects and whether this related to changes in indices of ventilation.

Ten patients with type 1 brittle asthma (BA), 10 with non-brittle, severe asthma (SA), 10 with mild asthma (MA) and 10 normal individuals were studied lying flat, at 60 degrees head up tilt and flat again each for 30min, assessing end-tidal CO(2) (ETCO(2)), respiratory rate and autonomic tone by heart rate variability.

Parasympathetic tone (as high-frequency [HF] power) fell on tilt in all four groups, all showing increases in LF (low frequency)/HF ratio (sympatho-vagal balance) on tilt (BA, 2.03-4.9; SA, 3.3-8.2; MA, 1.5-4.9; normals, 2.3-6.6). ETCO(2) on tilt fell significantly in all 3 asthma groups (BA, DeltaETCO(2) -0.4, 95% CIs -0.18 to -0.75, P=0.005; SA, -0.7, 95% CIs -0.27 to -1.16, P=0.004; MA, -0.5, 95% CIs -0.14 to -0.78, P=0.01) but not in normals (-0.1, 95% CIs +0.23 to -0.49). The fall in ETCO(2) on tilt correlated with the fall in HF power in all three asthma groups but not in normal subjects.

Changes in vagal tone posture are seen on tilt in both normal and asthmatic subjects which relate to changes in ETCO(2) only in asthmatic subjects. This provides support for the hypothesis that hyper-ventilatory response to postural change in asthma is autonomically influenced.