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Biomedical subjects

D H Glaister

Publications and source records attributed to D H Glaister.

At least 19 recordsLinked to original sources
PubMed1990

Cerebral tissue oxygen status and psychomotor performance during lower body negative pressure (LBNP).

Cerebral oxygen sufficiency was studied noninvasively, using multiwavelength near-infrared spectrophotometry, in eight subjects exposed to lower body negative pressure (LBNP) of up to -90 mm Hg to induce presyncopal symptoms and signs. LBNP caused only small changes in the forebrain measures until the last 60 s of the exposures, whereupon oxyhemoglobin (HbO2) and oxidised cytochrome c oxidase fell, reduced hemoglobin (Hb) rose slightly, and the tissue blood volume (HbO2 + Hb) fell. In subjects showing presyncope, these changes anticipated the onset of a terminal bradycardia by some 20 s and may provide the trigger for cardiovascular decompensation, while the cessation of LBNP led to an overshoot in cerebral blood volume suggestive of a reactive hyperemia. Psychomotor testing showed a significant slowing of reaction time with LBNP, but only for the easiest component of a complex task, while saccadic latencies were found to be shortened following LBNP exposure.

Adult↗
PubMed1988

A near-infrared spectrophotometric method for studying brain O2 sufficiency in man during +Gz acceleration.

A technique for the noninvasive monitoring of cerebral oxygen status was evaluated on volunteer subjects on the USAF School of Aerospace Medicine centrifuge. By using multiwavelength near-infrared spectrophotometry, the instrumentation measured changes in the quantities of reduced and oxygenated hemoglobin (and their sum, an indicator of cerebral blood volume), and the quantity of oxidized cytochrome c oxidase within the forebrain. Tests used acceleration of up to 9 G with onset rates from 0.1 to 5.0 G.s-1, anti-G suits and straining maneuvers, and hyperoxic and hypoxic breathing mixtures. In general, +Gz acceleration produced a fall in blood volume within the cerebral microcirculation with a relative increase in the content of reduced hemoglobin and a tendency towards reduction of cytochrome c oxidase. These findings are discussed in relation to accepted changes in arterial blood pressure, cerebral blood flow, and arterial oxygen saturation caused by acceleration exposure.

Acceleration↗
PubMed1988

Current and emerging technology in G-LOC detection: noninvasive monitoring of cerebral microcirculation using near infrared.

G-induced loss of consciousness (G-LOC) has emerged as an important operational problem of high-performance aircraft. Since it appears that G-LOC will continue to be a problem, a requirement exists to detect its occurrence in pilots so that the aircraft may be placed on autopilot. One excellent method of detecting G-LOC physiologically, one would assume, would be based on the oxidative status of the brain. This determination can be made noninvasively with an Oxidative Metabolism Near-Infrared monitor using 4 wave lengths (OMNI-4). The OMNI-4 is capable of measuring the relative quantities in the brain of hemoglobin (Hb), oxygenated hemoglobin (HbO2), blood volume (BV), and oxidative status of cytochrome c oxidase. This instrument was tested on subjects in the USAFSAM human-use centrifuge at +3, 4, and 5 Gz with onset rates of 1 G.s-1. Results showed changes within the brain, as expected, during increased G with reductions in Hb, BV, and HbO2. Cytochrome c oxidase measurements were inconclusive. Immediately following G exposure, Hb, BV, and HbO2 "overshoots" occurred suggesting vasodilation of the cerebral microcirculation. The use of OMNI-4 in the laboratory and its possible role as a detector of G-LOC in pilots are discussed including suggestions for future developments.

Aircraft↗
PubMed1987

Induction and prevention of acceleration atelectasis.

Acceleration atelectasis is the absorptional collapse of alveoli in the dependent lung due to increased accelerative forces. It is exacerbated by breathing 100% oxygen and, during +Gz exposure, by the use of an anti-G suit. Experiments were conducted on 12 subjects using simulated aerial combat maneuvers (SACM) with G profiles having peak exposures of either 4.5 G or 9 G. Decreases in vital capacity (VC) measurements were used as quantification of atelectasis, two types of reduction being identified and described. Labile reductions in VC were readily restored by a deep breath or cough. Such reduction approximated 28% following the 4.5-G SACM and 25% following the 9-G SACM. More persistent (so called) stable reductions were of lesser degree, values of -20% being seen following both 9 G and 4.5 G maneuvers. Acceleration atelectasis causes symptoms of chest pain, coughing, and shortness of breath. Subjective ratings of the severity of these symptoms were obtained from the subjects, and these were much greater following the 4.5-G SACM exposures than after the 9-G runs. Acceleration atelectasis was reduced by dilution of the inspired oxygen concentration by argon and nitrogen (evaluated at 95, 82.5, 70, 50, and 20% oxygen); the addition of unassisted positive pressure at 30 mm Hg (4 kPa) to the breathing mask; or the performance of the anti-G straining maneuver (AGSM).

Acceleration↗
PubMed1987

+Gz-induced loss of consciousness and aircraft recovery.

Aircrew incapacitation resulting from very high onset sustained +Gz stress has resulted in significant losses of aircraft and aircrew. Enhanced protection and training toward prevention of +Gz-induced loss of consciousness (G-LOC) will continue to be vital. Techniques for reduction of the time of incapacitation, should G-LOC occur, must also be explored and developed. Current capability of aircraft autorecovery as demonstrated by the Advanced Fighter Technology Integration F-16 (AFTI/F-16) promises to enhance safety from the acute incapacitation resulting from G-LOC (and spatial disorientation). Physiologic monitoring for determining G-LOC has certain advantages especially in the aerial combat arena. The optimum physiologic monitoring technique would be direct determination of failure of brain cell function at the cellular or subcellular level. Complete investigation of G-LOC is necessary to understand the phenomenon and to develop methods for enhancing recognition and recovery. This paper discusses aircraft autorecovery technology and potential methods for physiologic monitoring of G-LOC. Integration of physiologic monitoring techniques into aircraft autorecovery systems requires a broad approach for optimal development.

Aerospace Medicine↗
PubMed1987

The effect of head and neck suction on G tolerance.

A device was constructed which allowed subatomospheric pressures of up to -50 mm Hg to be applied to the neck, or to the head plus neck, of volunteer subjects riding on the USAFSAM centrifuge. Breathing pressures were always atmospheric. The G tolerance was measured during gradual (0.1 G.s-1) and rapid onset (1.0 G.s-1) runs and heart rate was monitored. Neck suction decreased tolerance to an extent explicable by carotid sinus activation (about -0.8 G at -50 mm Hg), while combined head and neck suction of -25 mm Hg increased tolerance to an extent greater than predicted from pressure effects on the eye and carotid sinus. Neck suction of -50 mm Hg induced less bradycardia when applied at +3 Gz than under 1-G conditions. These results were discussed in relation to intraocular tension, baroreceptor responsiveness, and jugular venous siphon effect.

Acceleration↗
PubMed1981

Effects of beta blockers on psychomotor performance--a review.

There is a possibility that beta-receptor blocking drugs may modify psychomotor performance, either peripherally or centrally. The inherent problems in evaluating the effects of drugs on performance are mainly due to interpretation of what is meant by "psychomotor" as the term embraces many mental and physical skills. In addition, statistical validation is often difficult. The current literature is reviewed with these provisos in mind and 24 studies of the effects of beta blockade are discussed. Of these 15 could be assessed as showing "no effect" and, of the remaining 9, improved performance was reported in 6 and impairment in 3. There is a wide variation in the results of such studies but, in general, it would appear that while the clinical use of beta-blocking drugs in aviation may produce an adverse effect on performance, it should be no greater than ordinary day-to-day variation.

Adrenergic beta-Antagonists↗
PubMed1978

Human tolerance to impact acceleration.

Tolerance to brief impact is related to the imposed velocity change, whilst in longer impacts it is the level of acceleration which is critical. This behaviour results from the dynamic characteristics of the body which, for practical purposes, can be represented as a simple mass spring system. For any given acceleration vector, and for any given system of body restraint, whole body (primary) tolerance can be defined by two parameters--a critical velocity change and a critical plateau acceleration level. Appropriate figures are given for 14 different posture and restraint conditions. It is stressed that these are best estimates (or in some cases, best guesses) and that secondary or tertiary mechanisms of injury, in particular head strikes, must not be ignored in any real-life conditions of impact.

Acceleration↗