Response of human cerebral blood flow to +Gz accelerations.

Intolerance symptoms associated with high sustained +Gz (head to foot) accelerations are attributed to lack of cerebral perfusion. To determine the response of cerebral circulation to +Gz stress, cerebral blood flow (CBF) was measured in humans with the transcranial Doppler method while cephalic arterial blood pressure was calculated simultaneously using a photoplethysmographic technique. Nine volunteers performed four randomized centrifuge runs at +2 to +5 Gz with a 0.4-G/s onset rate for 30 s. Compared with the control values, for +2-, +3-, +4-, and +5-Gz profiles, CBF was reduced by 19 +/- 7, 26 +/- 8, 49 +/- 26, and 61 +/- 29% (SD), respectively, at the end of the onset and by 18 +/- 4, 21 +/- 11, 27 +/- 7, and 47 +/- 29%, respectively, in the last 20 s of the plateau of acceleration. At the end of the onset and during the plateau of +Gz acceleration, CBF was less reduced than cephalic arterial blood pressure, suggesting that some mechanisms would occur to maintain cerebral perfusion under +Gz stress. These protective mechanisms are likely due to a siphon effect and/or an autoregulatory compensation.

A comparison of the nauseogenic potential of low-frequency vertical versus horizontal linear oscillation.

The aim of this study was to compare the nauseogenic potential of low-frequency linear motion in the Earth-vertical versus the Earth-horizontal plane, delivered through the same Z-axis of the head and body. Twelve subjects were challenged with linear motion (0.3 Hz, 1.8 ms-2 rms) through the same head and body Z-axis in the Earth-vertical (sitting upright) versus horizontal (lying on the back), while either performing a continuous visual search task or with their eyes closed. Each subject completed the four conditions on a Latin square design with sessions spaced 1 week apart at the same time of day. Vertical motion was clearly more provocative than horizontal motion, and nauseogenicity of motion was exacerbated by a visual search task. Motion sickness impaired performance of the search task. Motion Sickness Susceptibility Questionnaire (MSSQ) scores correlated with individual susceptibility to the motion challenge. Mean sickness ratings for vertical motion showed some correspondence with those predicted by mathematical models of motion sickness dose response relationships.

Transport aircraft crew and decompression hazards: study of a positive pressure schedule.

The protection of the transport aircraft crew against cabin decompression hazards at high altitude (Z less than 45,000 ft) (13,700 m) is achieved by positive pressure breathing (PPB). Currently, many PPB schedules are used. Our research was performed to propose a PPB schedule, using the hypothesis of a decompression at high altitude, including a stay at the flight level and an emergency descent at the rate of 15,000 ft.min-1. The measures were arterial oxygen saturation, heart rate, speech capabilities, and psychomotor performance. The tests were conducted up to 45,000 ft. They show that the best protection at 45,000 ft is afforded when the PPB is included between 2 and 2.5 kPa (15 to 18.75 mm Hg).

Increase of LBNP tolerance using elastic compression stockings.

BACKGROUND: Astronauts returning from spaceflight are affected by reduced orthostatic tolerance resulting from exposure to weightlessness. There are some countermeasures currently in use to improve cardiovascular performance of returning astronauts, while there are others that are being tested in flight and in ground-based investigations. This paper presents a study on the use of elastic compression stockings to reduce leg blood capacity (LBC) which is believed to be one of the determinants of orthostatic tolerance. METHODS: The data are from 6 healthy men with a mean age of 36 +/- 5 (SE) yr. Assessment of the effectiveness of stockings in improving orthostatic tolerance is based on a presyncopal-limited lower body negative pressure (LBNP) test, consisting of successive 3 min exposures to negative pressures of -20 hPa (-15 mmHg), -40 hPa (-30 mmHg), and decrements in steps of 10 hPa (7.5 mmHg) from then on until termination of the test. RESULTS: Results show an increase in the maximal level of LBNP tolerated (88 hPa or 66 mmHg for control vs. 108 hPa or 81 mmHg for stockings; p = 0.018) as well as in the cumulative stress index (CSI) (1122 hPa-min or 842 mmHg-min for control vs. 1734 hPa-min or 1300 mmHg-min for stockings; p = 0.029). CONCLUSIONS: The improvement of LBNP tolerance with elastic compression stockings coupled with their ease of use support the need for further experimental studies for evaluating their potential as a countermeasure for astronauts after return from spaceflight.

Cerebral blood flow velocity response induced by a 70-hPa Valsalva manoeuvre associated with normo- and hypergravity in humans.

Anti-G straining manoeuvres, derived from the Valsalva manoeuvre (VM), are physiological methods for protecting fighter pilots against positive accelerations (+Gz). The aim of this study was to investigate the effects of a standard VM on cerebral haemodynamics, in normo- and hypergravity. In six healthy male volunteers, we investigated the cerebral blood flow velocity response induced by a 10-s, 70-hPa (52.5 mmHg) VM, under normogravity, +2, +3 and +4 Gz acceleration plateaus. Mean blood flow velocity [formula: see text] in middle cerebral artery was monitored by transcranial Doppler velocimetry. In normogravity, no significant variation in [formula: see text] was observed at the onset of VM. After a maximal period of 1.2 s, while VM was sustained, [formula: see text] decreased significantly (P < 0.05). Following the end of the manoeuvre [formula: see text] did not change significantly. When the expiratory pressure had returned to the control value, [formula: see text] was transiently increased (P < 0.05) before returning to control values. During hypergravity, [formula: see text] was significantly decreased at +3 and +4 Gz (P < 0.05) before the onset of VM. While performing VM under +Gz, the main difference compared to the normogravity condition was a significant increase of [formula: see text] (P < 0.05) at the onset of the manoeuvre. Our findings would suggest that when performed under +Gz stress, a 70-hPa VM can transiently improve cerebral haemodynamics. However, when VM is sustained for more than 1.2 s it results in a lasting decrease of cerebral perfusion which may lower +Gz tolerance.