Respiratory vulnerability to vehicle buffeting

Abstract

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. 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. 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. 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.