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Biomedical subjects

P B Bennett

Publications and source records attributed to P B Bennett.

At least 91 records · Page 5Linked to original sources
PubMed1984

Soleus H reflex studies in humans at 40-70 bar in helium-oxygen-nitrogen (trimix).

Excitability of the Hoffmann (H) reflex was studied in 6 subjects at pressures of 40 to 70 bar during two trimix exposures (10% N2, 0.5 bar O2, the remainder He; Atlantis II and III simulated dives, Duke University Medical Center). In Atlantis II (12 h to 47 bar, 1 + 3 days to 66 bar), the H reflex recruitment ratio (Hmax/Mmax) was increased in 2 out of 3 subjects and reduced in the 3rd, while the usual facilitatory effect on the H reflex of a hand-grip reinforcement maneuver was reduced in all subjects. These results are similar to those previously seen in He-O2 dives and suggest a slight hyperreflexia mediated by an increase in alpha motoneuron excitability. In Atlantis III (1.5 days to 47 bar, 7 days to 66 bar) H recruitment ratios were normal in 2 subjects, and reduced in 1 subject (P less than 0.001). Early (III) and late (V) inhibitory phases of the H reflex recovery cycle were not significantly altered, while a 50-ms delay in the recovery cycle rebound (phase IV) in 5 out of 6 subjects at 66 bar (P less than 0.01) is attributed to the slowed muscle contraction. However, the increased amplitude of the rebound and the presence of clonic electromyographic potentials in Atlantis III are evidence of stretch receptor hyperexcitability that apparently persisted when H reflex excitability was normal. It is concluded that increased reflex responsiveness at depth is not promoted exclusively by either enhanced alpha motoneuron excitability or stretch receptor sensitivity but is mediated via both systems. Slower compression and the presence of N2 appear to preserve normal spinal inhibitory feedback and alpha motoneuron excitability but do not prevent increases in stretch receptor sensitivity.

Atmospheric Pressure↗
PubMed1983

Anesthetic antagonism of the effects of high hydrostatic pressure on locomotory activity of the brine shrimp Artemia.

The locomotory activity of small groups of brine shrimp (Artemia salina) was studied under conditions of high hydrostatic pressure, varying temperatures and exposure to several gaseous anesthetics. Both compression and exposure to anesthesia reduced the animal's swimming activity, while temperature increased or decreased activity as it was raised or lowered from ambient. The effect of the anesthetics was less during periods of simultaneous exposure to high hydrostatic pressure. It is concluded that pressure antagonism of anesthesia is demonstrable in invertebrate organisms and may represent a fundamental interaction of these parameters in biological systems.

Anesthetics↗
PubMed1983

Liquid ventilation in dogs: an apparatus for normobaric and hyperbaric studies.

A liquid-breathing apparatus is described for remote surface studies and for use in experiments of near-hydraulic compression in dogs. It consists of a flexible tank sealed against chamber gas, containing a supply of clean warmed (38 degrees C) fluorocarbon (FC-80) equilibrated with 1 bar O2 and an electronically controlled means of delivering the liquid to the dog. Each breath (tidal volume 290 ml) was "weighed" into the animal by the signal from a force platform supporting the dog and a digital control unit that automatically actuated inspired-and expired-line solenoid valves. The apparatus was successfully used to remotely maintain liquid ventilation in awake dogs for 2 h during surface studies (5 dogs) and in dives to 1,000 m seawater (5 dogs). During liquid breathing, mean arterial O2 partial pressure was always adequate (congruent to 300 Torr) and mean arterial CO2 partial pressure was normal (less than or equal to 40 Torr). An uncompensated metabolic acidosis was indicated by low pH values and a decrease in arterial base excess to--4.5 meq x 1(-1). O2 uptake and CO2 output appeared to be significantly lower (42 and 35%, respectively) during liquid ventilation.

Animals↗
PubMed1983

Force and duration of muscle twitch contractions in humans at pressures up to 70 bar.

The contraction peak force (CPF) and contraction time (CT) of the soleus Hoffmann (H) reflex and motor (M) response muscle twitch were measured in three subjects during the 686-m seawater Atlantis III simulated dive at Duke University. The mean CPF of both H and M twitches was observed to be 40-60% larger during compression than in predive control studies (P less than 0.001 for M response). This effect was sustained during the first 300 m of decompression, while postdive values were normal. Rate of contraction, as measured by changes in CT, was slowed significantly but only during compression (by 23% in the H reflex at 66.4 bar, P less than 0.001). These marked effects in CPF and CT cannot be explained by the small increases observed in the muscle action potential latency and duration, or other neuronal mechanisms. It is concluded that the twitch contraction of skeletal muscle is significantly slowed and augmented at pressure due to changes in excitation-contraction coupling or in the contractile process itself. These findings may be of importance in interpreting the etiology of many neuromuscular signs of the high-pressure neurological syndrome.

Atmosphere Exposure Chambers↗
PubMed1983

Latency changes in the human somatosensory evoked potential at extreme depths.

Somatosensory evoked potentials to lower limb stimulation were recorded from the three participants in a recent chamber dive to a world depth record of 686 m (2250 ft). Because of the 10% nitrogen breathing mixture, both inert gas narcosis and high pressure nervous syndrome were factors in the dive. Latencies of evoked potential peaks were measured relative to the peak of the first cortical component, P1, in order to separate central from peripheral effects. Several peak latencies changed systematically during compression. The several components of the somatosensory evoked potential appeared differentially sensitive to either inert gas narcosis or the high pressure nervous syndrome with increasing depth. The latencies of all peaks following the initial cortical P1 were shorter at depth than in surface control recording. This is consistent with a state of hyperexcitability in the brain that is felt to characterize the high pressure nervous syndrome. Changes in peak latency were consistent across subjects and in some cases achieved statistical significance despite the small number of subjects available for testing. Although these findings are preliminary, they suggest that evoked potential latencies may have value as a means of following the effects of extreme pressure on the central nervous system.

Atmosphere Exposure Chambers↗
PubMed1982

Central nervous system reactions during heliox and trimix dives to 31 ATA.

UNLABELLED: Two groups of divers were compressed to 300 msw (984 fsw) with heliox (n = 3) and trimix (n = 3). Neuropsychological/neurological testing was performed repeatedly during the compression and on reaching 250 (820 fsw) and 300 msw. On the second day the trimix group was tested before and after a gas change to heliox. For the heliox group there was a marked increase in tremor and EEG slow waves, and reduction of alpha band and in hand-grip strength. For the trimix group visuomotor coordination was impaired. In the cognitive performance tests the heliox group was most impaired at 250 msw, whereas the trimix group was most impaired at 300 msw. Before the gas change to heliox the trimix group showed impaired performance in some tests. After the gas change, performance returned to predive levels, except for perceptual speed. There was an increase in tremor immediately after the gas change. CONCLUSION: There was a marked HPNS effect during compression on heliox. This effect was not observed in the trimix group, but this group was mildly intoxicated on reaching saturation depth. The tests indicated slight narcotic effects also after 26 h, but this disappeared after the change in heliox.

Adult↗
PubMed1982

Effect of compression rate on use of trimix to ameliorate HPNS in man to 686 m (2250 ft).

Previous man dives, in a series designed to evaluate the physiological effects of helium, nitrogen, and oxygen (trimix), investigated the comparative effects of 5% vs. 10% nitrogen with fast compression (12 h 20 min) to 460 m (1509 ft) and subsequent compression over 2.5 d to 650 m (2132 ft). In 1981 three divers were compressed twice as slowly as for these earlier dives to 650 m over a period of 6 d 8 h using 10% N2 in heliox. An extensive series of studies were made over 4 d 15 h at 650 m before further compression to 686 m (2250 ft) for a stay of 24 h with extensive tests of psychological and neurophysiological performance, pulmonary function, reflex, Doppler, and other studies. High pressure nervous syndrome (HPNS) tremors and EEG theta activity increases were effectively controlled with no nausea, vomiting, or somnolence (microsleep). Some euphoria was present. At 686 m there was a 20% to 30% impairment of concentration and attention; otherwise the physical condition of the divers was fine and they completed all tasks without difficulty. Slow compression prevented the initial large performance decrement of 40% to 50% on Day 1 as found in previous fast-compression dives. Otherwise the performance tests showed much the same decrement of 15% to 20% seen in earlier dives; deeper than 570 m (1870 ft), however, the addition test was worse, with a decrement of 35%. The results are discussed with respect to the previous two dives with faster compression, and the possible nonlinearity of nitrogen antagonism of HPNS is considered.

Adult↗
PubMed1981

Enhancement of cortical evoked potentials by high atmospheric pressures of helium.

Adult guinea pigs with electrodes chronically implanted in the optic chiasm (OC(, lateral geniculate nucleus (LGN), and visual cortex (CX) were compressed at 1 bar/min to 120 bars pressure in helium-oxygen. Body temperature was controlled to within +/- 1 degree C, CO2 was removed by a soda-lime absorbent. Electrical stimuli (50 microA, 0.05 msec) were delivered to the OC at 9.6/sec at 10 bar intervals, with pressure held constant. Pressure caused only negligible changes inthe amplitudes of pre- and postsynaptic components of potentials in the LGN. Amplitudes of evoked potentials in the CX increased approximately linearly with pressure, reaching values of up to 300% above normobaric. Latencies did not change independently of temperature. Excitability of nerve fibers changed very little, but intracortical synaptic transmission was substantially enhanced. Differences in the effects of elevated helium pressure at the LGN and CX may be attributed to differences in organization of local circuits.

Animals↗
PubMed1981

Hydrostatic pressure reduces synaptic efficiency by inhibiting transmitter release.

The amplitude of post-synaptic potentials at an identified synapse in the Aplysia central nervous system can be modulated by the application of hydrostatic pressure. Postsynaptically controlled functions, including responses to acetylcholine and the time course of decay of the synaptic responses, remain unaffected by compression. In contrast, frequency facilitation and posttetanic potentiation, which result from presynaptic processes, are altered by pressure in a manner similar to that of agents that block transmitter release. It is concluded that pressure reduces synaptic efficiency by interfering with certain presynaptic mechanisms associated with transmitter release.

Acetylcholine↗
PubMed1981

Control of HPNS in humans during rapid compression with trimix to 650 m (2131 ft).

In 1978 a series of deep trimix (He-N2-O2) dives was initiated to establish the relationship between a given nitrogen percentage and the rate of compression required to prevent the high pressure nervous syndrome (HPNS) at 460 m (1509 ft) and to determine the effects of inspired gas density, hydrostatic pressure, and narcosis o various circulatory and respiratory parameter, including the presence of dyspnea. In 1979, three human subjects were compressed to 460 m in 12 h 20 min with 5% N2 in He-O2. This resulted in nausea, vomiting, fatigue, tremors, and other signs and symptoms of HPNS that were especially prominent on arrival at that depth but had much improved by Day 2. In March 1980 the same profile was repeated but with 10% N2 in He-O2. The divers arrived at 460 m with virtually no symptoms of HPNS, but the psychometric performance, as for Atlantis I, still was decreased by some 40% on Day 1 and recovered to some 15% by Day 2. After 5-6 days at 460 m further extension of the dive to 650 m (2132 ft) with a 7.7% N2 mixture for 24 h showed similar control of symptoms of HPNS, although inspiratory resting dyspnea was present in one subject. The results are discussed in relation to the interactions of nitrogen percentage and rate of compression.

Air↗
PubMed1980

High pressure, diazepam and pH influences on ketamine anaesthesia in the rat.

Ketamine infusion anaesthesia was studied in rats breathing oxygen/helium under hyperbaric conditions from 1 to 90 ATA. No pressure reversal of the analgesic effect was found. Premedication with diazepam failed to improve the analgesic action of ketamine, but influenced the respiratory pattern. Blood-gas analyses showed a marked relationship between arterial pH and the ketamine dose requirement for stable anaesthesia.

Anesthesia↗
PubMed1980

Diazepam under hyperbaric conditions in rats.

The effect of diazepam, expressed as loss of the righting reflex, in 60 adult rats compressed with oxygen/helium to 90 ATA was compared to 18 diazepam-treated rats at 1 ATA. A marked pressure reversal of the loss of righting reflex was found. Convulsions provoked by high helium pressure were prevented by diazepam and the symptoms of High Pressure Nervous Syndrome (HPNS) were reduced.

Anesthetics↗
PubMed1980

A versatile high-pressure chamber for electrophysiological measurements.

A modular high-pressure chamber is described. This chamber will allow stable microelectrode recordings to be made for a variety of intracellular preparations at pressure of 200-300 atmospheres absolute. Its features include internal temperature control, easy visibility, continuous perfusion, electrical penetrations, and manipulation of several internal controls while the system is at pressure. The small size and high versatility of the Wilson chamber make it a convenient and inexpensive research unit for experiments in the moderately high-pressure ranges that affect biological preparations.

Atmosphere Exposure Chambers↗
PubMed1980

Lithium effects: protection against nitrogen narcosis, potentiation of HPNS.

The effect of either 10, 8, or 6 meq/kg of intraperitoneal lithium chloride or sodium chloride on the loss of righting response (RR50) produced by 18.2 ATA N2-ATA O2 was examined in rats. Results were compared to the effects of 10, 8, 6, or 4 meq/kg of intraperitoneal lithium chloride given 24 h before determination of the convulsion pressure (PC) in 40 rats compressed with He-O2 at 160 atm/h at 37 +/- 0.5 degrees C. Lithium (10 meq/kg) prevented the nitrogen-narcosis-induced loss of righting response but significantly potentiated the pressure (depth) at which convulsions and tremors occurred.

Animals↗
PubMed1979

Can regular solution theory be applied to lipid bilayer membranes?

Direct measurement of the partition coefficient of n-hexane into phosphatidylcholine and phosphatidylcholine-cholesterol bilayers showed that (a) isotropic liquids are not good models for lipid bilayers and (b), Regular Solution Theory cannot, in general, be applied to lipid bilayer membranes at temperatures above their phase transition. Theoretical and experimental evidence given.

Cholesterol↗
PubMed1979

Spinal cord seizures elicited by high pressures of helium.

Rats with complete spinal transections were compressed in helium-oxygen to 120 bars. Tremors and increased EMG activity in limbs rostral as well as caudal to the lesions were observed beginning at 30 bars. Spinal seizures occurred at 95 bars, similar to cortical seizure thresholds of intact rats. Denervated limbs remained flaccid throughout the dives. No rostro-caudal progression of symptoms was evident in normal animals, but fluctuation of symptoms with increasing pressure was frequently observed. These findings are consistent with the hypothesis that the effects of pressure on aggregates of neurons exceed those on isolated components.

Animals↗
PubMed1979

Effect of interaction of volatile anesthetics and high hydrostatic pressure on central neurons.

Autoactive neurons in the central nervous system of Helix and Aplysia were studied after exposure to several volatile anesthetics and under compression by mineral oil. Voltage clamp studies reveal that halothane will eliminate the slow inward current that underlies oscillatory activity in burster neurons, while high pressure shifts the negative resistance region of the current without causing its elimination. Simultaneous application of the anesthetic and pressure results in the loss of inward current over a time course similar to that of halothane application alone. It is concluded that in this system, pressure and anesthetics are not acting antagonistically at the site in the membrane that controls slow wave conductances.

Anesthetics↗