Motion illness is a common aeromedical problem that may occur in pilots making rapid and frequent head movements while being exposed to increased gravitational (G) forces in the head-to-foot direction (+Gz). We have previously reported that motion illness affects autonomic nervous functions, particularly cardiovascular control (Mekjavic et al. 2001). We reasoned therefore, that motion illness might interfere with cardiovascular responses to high G-forces, thereby decreasing G-tolerance. The present study tested this hypothesis.

The experiments were approved by the Human Ethics Committee at Karolinska Institutet. G-intensity tolerance and cardiovascular responses to increased +Gz-load were studied in nine healthy males in a centrifuge in two conditions. In the motion illness (MI) condition the subject was exposed to a motion illness provocative stimulation (MIPS) in the centrifuge comprising repeated rapid changes in G-load (between +1.4 and +2.5 Gz) in combination with a regimen of head movements. In the control condition (C) the subject was exposed to the same cumulative G-stress, but without the MIPS. Mean arterial pressure (MAP) was measured at the level of the eyes using a volume-clamp technique. An index of peripheral vascular resistance was achieved by measuring the difference in skin temperature between the forearm and fingertip (▓Dgr│T_{forearm-fingertip}) (Sessler et al. 1988). Data were examined using analyses of variance.

MIPS decreased (P < 0.01) gradual onset rate (GOR; 0.1 G s^-1) G-tolerance by 10 % from 5.1 ± 1.0 G (mean ± S.D.) to 4.6 ± 0.9 G. There was no change in GOR tolerance in the C condition. Likewise, rapid onset rate (3.5 G s^-1) G-tolerance was 15 % lower (P < 0.001) in the MI (2.9 ± 0.5 G) than in the C (3.4 ± 0.3 G) condition. MIPS reduced (P < 0.01) MAP by 10-12 mmHg at any given G-load. MIPS induced a change (P < 0.05) in ▓Dgr│T_{forearm-fingertip} of -4.2 ± 4.1 °C, whereas in the C condition ▓Dgr│T_{forearm-fingertip} was unaffected. This suggests that the MIPS substantially reduced the tone of peripheral resistance vessels.

It is concluded that motion illness may reduce the tone of peripheral resistance vessels to the extent that it results in a drop in arterial pressure large enough to significantly diminish the capacity of an individual to withstand increased G-forces in the head-to-foot direction. This may have implications for pilots flying high-performance fighter aircraft or involved in civilian aerobatic sports.