Tachypnea and hypocapnia are induced by ‘buffeting’ in vehicles

Respiratory vulnerability to vehicle buffeting

OBJECTIVE

Buffeting in a jerky ride in a bus or ambulance normally provokes a sustained tachypnoea driven by vibration and sensory mechanisms including vestibular signals. Tachypnoea reinforces the torso against mechanical shocks but results in overbreathing, causing a mild fall in CO(2). However, normal CO(2) is rapidly restored by a reduction in depth of breathing. We test the hypothesis that vulnerable subjects, exemplified by elderly individuals and patients with vestibular disorders, may fail to adapt to buffeting.

METHODS

Respiratory and cardiovascular functions were recorded from five elderly subjects, two patients with bilateral loss of vestibular function and five patients with 'BPPV,' while being exposed to 15-min buffeting in a flight simulator which simulated transport in an ambulance over rough pavement. Results were compared with published norms.

RESULTS

Some subjects sustained overbreathing during motion, through either tachypnoea or deep breathing, causing a marked reduction in CO(2) levels (3/5, 2/2 avestibular, 4/5 elderly, 4/5 BPPV). Others failed to raise breathing frequency which would render them susceptible to mechanical shock (4/5 elderly, 1/2 avestibular). Overbreathing was particularly evident in three anxious subjects.

INTERPRETATION

Overbreathing during buffeting could be caused by (1) resetting of CO(2) rest levels lower; (2) change in receptor sensitivity; (3) adjustment of central drive to breathing; and (4) stiffening of posture because of motion discomfort reduced the ability to modulate breathing. The buffeting experienced was moderately violent. More profound hypocapnia and mechanical shock are likely to result in vulnerable individuals failing to adapt to severe buffeting in transport on unpaved roads, in war zones or by sea ambulance.

Effect of temporal relationship between respiration and body motion on motion sickness

BACKGROUND

This study investigated the effect controlling the phase of respiration on the development of nausea provoked by periodic motion at 0.2 Hz which is maximal for provocation of motion sickness.

METHODS

Subjects were exposed to 60 degrees peak-peak, pitch backwards from upright motion while viewing a video of the environment with 180 degrees phase delay. Motion duration was a maximum of 30 min and frequency was set to match individuals' spontaneous respiration. Conditions were: A, spontaneous breathing; B, inspiration cued to begin when head-down; C, inspiration cued to begin when upright; D, inspiration cued with a +/-18 degrees desynchronizing phase drift with respect to the tilt cycle. Nausea was rated and ventilation was recorded.

RESULTS

Magnitudes of nausea ratings were ordered D<C<B<A (p=0.008) and speed at which nausea developed were ordered A<B<C<D (p=0.001).

DISCUSSION

The lower sickness ratings and prolonged times to develop nausea in B, C, D confirm that controlled breathing gives some protection against motion sickness. The differences between B, C and D in the development of nausea support the hypothesis of Von Gierke and Parker [von Gierke HE, Parker DE. Differences in otolith and abdominal viscera graviceptor dynamics: implications for motion sickness and perceived body position. Aviat Space Environ Med. 65:747-51, 1994.] that motion sickness can be provoked by a conflicting mismatch between visceral and otolithic signals of orientation to the vertical. The mismatch is greatest in the more provocative condition B because the viscera are mechanically unloaded due to exhalation when the body attains uprightness whereas mismatch is lessened by the mechanical reinforcement afforded by inspiration in (C) and by inconstant relationships between visceral and otolithic signals in (D), both of which afford better protection against sickness.