| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 Environmental Physiology Department, Naval Medical Research Center, Silver Spring, Maryland 20910-7500; and 2 Department of Microbiology, University of Georgia, Athens, Georgia 30602
 |
ABSTRACT |
|---|
 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
In H2 biochemical decompression, H2-metabolizing intestinal microbes remove gas stored in tissues of animals breathing hyperbaric H2, thereby reducing decompression sickness (DCS) risk. We hypothesized that increasing intestinal perfusion in pigs would increase the activity of intestinal Methanobrevibacter smithii, lowering DCS incidence further. Pigs (Sus scrofa, 17-23 kg, n = 20) that ingested caffeine (5 mg/kg) increased O2 consumption rate in 1 atm air by ~20% for at least 3 h. Pigs were given caffeine alone or caffeine plus injections of M. smithii. Animals were compressed to 24 atm (20.5-23.1 atm H2, 0.3-0.5 atm O2) for 3 h, then decompressed and observed for signs of DCS. In previous studies, DCS incidence in animals without caffeine treatment was significantly (P < 0.05) lower with M. smithii injections (7/16) than in controls (9/10). However, contrary to our hypothesis, DCS incidence was marginally higher (P = 0.057) in animals that received caffeine and M. smithii (9/10) than in animals that received caffeine but no M. smithii (4/10). More information on gas kinetics is needed before extending H2 biochemical decompression to humans.
cardiac output; hydrogen diving; hyperbaria; intestine; perfusion
| |
INTRODUCTION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
HYDROGEN, BY VIRTUE OF BEING the smallest gas molecule and possessing unusual narcotic properties under pressure, is a suitable diluent to O2 in a breathing gas mixture for deep dives by humans to 100-600 m (10-60 atm) (1). By conventional methods, safe decompression of human divers after an excursion to 600 m of more than 24 h duration requires on the order of 2 wk. H2 biochemical decompression is a novel process for reducing the risk of decompression sickness (DCS) and shortening decompression time from deep H2 dives. This process is based on the active removal of a critical fraction of the H2 dissolved in the tissues of divers by means of intestinal microbes that metabolize H2 to CH4 (6, 10-12). The present study examines a possible approach to increasing the rate of delivery of H2 to the intestinal microbes, thereby potentially increasing the benefits of H2 biochemical decompression.
It has been shown that after some compression and decompression
sequences, pigs with a higher activity of H2 metabolism by their native intestinal flora had a lower DCS incidence compared with
pigs with a lower activity (10). By injecting additional methanogenic microbes into the intestines, the CH4 release
rate (
CH4) from pigs increased
significantly, and the DCS incidence was lower compared with control
animals (6, 11). It appeared that injections of increasing
methanogenic activity increased the
CH4 up to a certain point, beyond
which greater injected activity did not usually elicit further
increases in CH4. A similar result
was observed in an earlier study in rats (12). It has been
suggested that H2 metabolic activity within the intestine is limited by the rate of supply of H2 via vascular
perfusion (12). If this hypothesis is correct, then
increasing the blood flow to the intestines should increase the
CH4 and reduce the DCS risk even
further for a given activity of injected methanogens.
We sought a pharmacological means of increasing vascular perfusion
strictly to the intestine but could not identify a drug with this
selectivity. As a second choice, we tested our hypothesis by oral
administration of caffeine to pigs. Caffeine has been shown to
stimulate resting heart rate, cardiac output, or metabolic rate in a
number of studies (5, 9, 13, 17) but not all studies
(7, 16). After caffeine administration, animals were subjected to the same sequence of pressurization and depressurization in hyperbaric H2 as used previously to test for
CH4 and DCS incidence (11).
| |
MATERIALS AND METHODS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
Animals and treadmill training.
Male Yorkshire pigs [Sus scrofa; n = 20;
body mass range = 16.9-23.4 kg; mean body mass (±SD) = 19.5 ± 1.9 kg; Table 1] were used
for all experiments. The animals were randomly assigned to one of two
groups: caffeine alone (CA+INJ
, n = 10, 20.0 ± 2.0 kg) or animals given caffeine and also injected with
H2-metabolizing microbes (CA+INJ+, n = 10, 18.9 ± 1.7 kg; 1,222 ± 439 µmol CH4/min injected activity) before hyperbaric exposure. There was no difference in mass between the two groups of animals (P > 0.2, two-tailed Student's t-test; Table 1). The animals were
housed in an accredited animal care facility. They were fed once daily
in the morning with laboratory animal chow (Harlan Teklad, Madison, WI;
2% by body weight) and had an unlimited supply of water. Animals were used singly in experiments. On the day before each experiment, the
animal was fed a second time in the afternoon (Table
2). On the day of the experiment, no food
was made available to the animal until entry into the chamber. All
experimental procedures were approved by an Animal Care and Use
Committee, and the experiments reported were conducted according to the
principles presented in the "Guide for the Care and Use of Laboratory
Animals," Institute of Laboratory Animal Resources, National Research
Council, 1996.
|
|
Culturing of the methanogen and activity assay.
A sample culture of Methanobrevibacter smithii (strain PS)
was obtained from Dr. Terry Miller (Wadsworth Center for Laboratories and Research, Albany, NY) and was grown in an atmosphere of
H2/CO2 (80:20 vol/vol; 3 atm) at 37°C at the
University of Georgia. Stock cultures were maintained in a modified
medium 1 (3, 11). Growth of M. smithii took
place in a 14-liter fermentor using 11 liters of the modified medium 1. The fermentor was sparged with H2/CO2. At least
1 h before inoculation, the temperature was set to 37°C, and 10 ml of Na2S · 9 H2O (20% wt/vol) were
added. The inoculum size was ~1% vol/vol of the total medium volume. Details of growth phase conditions appear elsewhere (11).
Before harvesting, the rate of methane production ranged from 7 to 16 µmol CH4 · min
1 · ml
culture1, and cell absorbance ranged from 2.0 to 3.1 OD600.
Caffeine administration. To treat animals with caffeine, the pigs were fed a 100-mg tablet of caffeine (5 mg/kg po; CVS Pharmacy, Woonsocket, RI; Table 2). This tablet was apparently sufficiently palatable to animals, when offered with a few food pellets, so that special effort to induce the pigs to swallow a tablet was seldom needed.
Surgery for injection of M. smithii. The surgical procedure has been described in detail elsewhere (Ref. 11; Table 2). Animals were prepared for surgery by preanesthetizing them with injections of ketamine HCl (20 mg/kg im; Fort Dodge Laboratories, Fort Dodge, IA) and xylazine (Rompun, 2 mg/kg im; Bayer, Shawnee Mission, KS). Animals were then kept at a surgical plane of anesthesia with inhaled isoflurane (Abbott Laboratories, Chicago, IL) and O2. With the use of an aseptic technique, a midline incision of ~10 cm was made in the abdomen, and the cecum and spiral colon were exteriorized. Injections of M. smithii culture were made into the cecum and spiral colon, with total injectate volumes ranging from 22 to 121 ml and total activities injected ranging from 500 to 1,800 µmol CH4/min (Table 1). All needle puncture sites were sealed with a drop of surgical cement (Vetbond, 3M, St. Paul, MN). The intestines were moistened externally with saline and returned to the abdomen, and the incision was closed with sutures. As the animal recovered from the anesthesia, yohimbine (2 mg iv; Lloyd Laboratories, Shenandoah, IA) was injected into an ear vein to act as an antagonist to the xylazine. Animals appeared to be fully recovered from the anesthesia in 1-2 h. They were then placed in the compression chamber to commence the experiment. Food and water were freely available in the chamber.
O2 consumption and heart rate measurements in 1 atm air. The animal was placed in a clear plastic box with internal dimensions of 90 cm × 60 cm × 55 cm (~300 liters). A vacuum pump (wet/dry industrial shop-vac model 700M, Shop-Vac, Williamsport, PA) was attached to the box to create a flow of air through the box of 14 l/min. A second vacuum pump extracted an additional 0.8 l/min from the box. The excurrent gas from the second pump was used to supply samples for analysis of O2 (model 755A, Beckman Industrial, Fullerton, CA) and CO2 (medical gas analyzer, Beckman Industrial). The gas analyzers were calibrated by using purchased gas mixtures of known composition (12.3% O2, 4.97% CO2, balance N2; 21.8% O2, 0% CO2, balance N2; or pure N2 Air Products and Chemicals, Allentown, PA) before and after each experiment. The temperature inside the box was measured by using a thermistor (no. 402 Yellow Springs Instruments, Yellow Springs, OH). A canister with anhydrous CaSO4 (WA Hammond Drierite, Xenia, OH) was placed downstream of the box to remove water vapor. Consequently, all gases were assumed to be dry when correcting the flow rate from the box. During an experiment, gas analysis data were automatically logged each minute, corrected to STPD, and saved to a file with a routine in Lab VIEW 5.0 (National Instruments, Austin, TX).
The animal was kept in the box for 60 min (n = 14 pigs) or 210 min (n = 5 pigs) to measure its O2 consumption rate (O2). The
measurement of O2 for several hours
was used to test for systematic temporal change of
O2 during extended confinement. Next, the animal was fed a tablet of caffeine, and the
O2 was measured again for either 60 min (n = 10 pigs) or 210 min (n = 5 pigs).
O2 was computed from the readings of
the last 30 or 180 min of confinement by using the Z transformation
(4) to correct the gas analysis data to equilibrium
conditions. Data from the initial 30 min in the box were not used to
allow the animal to acclimate to confinement and for the caffeine to
take effect (17).
An oximeter (Vet/Ox 4404, Heska, Waukesha, WI) was attached to the tail
in a subset of animals (n = 6 pigs). The oximeter allowed measurement of the heart rate and blood O2
saturation for 1 h before and after caffeine administration.
CH4 in 1 atm air.
CH4 was measured in animals at 1 atm
for 30 min, before and after oral administration of 100 mg of caffeine.
After caffeine administration, the animal was allowed to walk around
freely for 30 min before measurement of
CH4 commenced. This period of time allowed the caffeine to take effect (17).
O2 measurement.
Gas samples (~500 µl) were taken with a gas-tight syringe from a
sample hole covered with a rubber membrane. A 100-µl sample of the
gas was injected into a gas chromatograph (HP 5890 series II,
Hewlett-Packard, Wilmington, DE) and analyzed for [CH4]
(ppm). [CH4] was converted to micromoles of
CH4 by correcting for the volume of the box, assuming that
the animal displaced a volume (liters) equal to its body mass
(kilograms), and converting the values to STPD. The
CH4 (µmol CH4/min) was
calculated as the change in [CH4] over time. To avoid
build-up of CO2 in the box, the experiment was limited to
30 min, after which the lid of the box was taken off, and the air in
the box was exchanged by using a fan. Each experiment was made in
duplicate and the CH4 was taken as
the average of the two independent measurements.
CH4 was not calculated in 1 atm air
for animals that had received injections of M. smithii. This
was due to the need for haste in placing injected animals in the
chamber and commencing the hyperbaric exposure while the M. smithii cultures were likely to be retained within the intestines.
Dive protocol. Immediately before the hyperbaric experiment, each animal was given caffeine and placed in a dry hyperbaric chamber (5,665-liter internal volume, WSF Industries, Buffalo, NY). Subsequently, the animal was compressed as described in detail elsewhere (11). The chronological events of the compression and decompression sequence are summarized in Table 2.
In brief, one pig was placed in the compression chamber for each experiment. A stream of gas flowed continuously from the chamber to a gas chromatograph (model 5890A series II, Hewlett-Packard) that was calibrated before and after each experiment. Automated analysis of O2, N2, He, H2, and CH4 occurred every 12 min throughout the experiment. The hyperbaric chamber was initially pressurized to 11 atm (absolute pressure) with He, with concomitant addition of O2 to keep the chamber atmosphere normoxic to slightly hyperoxic (PO2 = 0.2-0.4 atm). After 11 atm was reached, the chamber was flushed with H2 and small volumes of O2 for ~30 min. The initial compression with He followed by replacement with H2 was performed to maintain a noncombustible and breathable mixture of H2 and O2 within the chamber (11). After the flush, the chamber was further pressurized to 24 atm with H2 and O2 at a rate of 0.45 atm/min. When 24 atm was reached, the pressure was maintained constant (±0.3 atm) for 3 h by continuous addition of H2 and O2 to make up for the gas exhausted to the gas chromatograph. Final H2 concentration in the chamber was 85-96% (Table 1). Reported values (Table 1) for H2 concentration are the means of the final three gas chromatograph readings at 24 atm. There was no difference in H2 concentration between the CA+INJ and CA+INJ+ groups (P > 0.1, two-tailed Student's
t-test; Table 1).
Throughout most of the time at 24 atm, the animal was free to rest or
move about its space within the chamber at will. The chamber
temperature at 24 atm was maintained at 32°C, a seemingly comfortable
level for the animals, as judged by absence of shivering with blanched
skin or rapid breathing with flushed skin. After 2.5 h at 24 atm,
the animal was made to walk on the treadmill within the chamber for 5 min to observe its gait before decompression.
After 3 h (±30 s) at 24 atm, the chamber was depressurized at
0.90 atm/min to 11 atm. The animal was observed for severe symptoms of
DCS for 1 h at 11 atm as it walked intermittently on the chamber treadmill. These symptoms were primarily neurological and included falling, difficulty standing or righting after falling, and seizures. Some animals had labored breathing, which may have indicated
cardiopulmonary DCS in addition to neurological DCS. Many animals were
also observed to have signs of skin DCS (conspicuous lavender to dark
purple mottling of the skin, with or without itching), but these signs alone did not warrant a diagnosis of severe DCS. Mild, transient behavioral changes (agitation or lethargy) were also not considered sufficient for a diagnosis of severe DCS. Once the diagnosis was made
or the hour had passed without evidence of DCS, the animal was quickly
killed by asphyxiation with He. The chamber was returned to 1 atm after
the animal was dead.
Corrected CH4.
The change in the chamber [CH4] during the time at
constant pressure was corrected by using the Z transformation
(4) to estimate the CH4
under equilibrium conditions, as described earlier (11).
Values are reported as means ± SD of five pairs of
chromatographic readings at 24 atm, with a time change of 120 min
(Table 1). The mean flow rate through the chamber was 115 l/min.
| |
RESULTS |
|---|
|
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
There was no systematic temporal change in
O2 (P > 0.4;
repeated-measures single-factor ANOVA) over a 1-3.5 h period
before caffeine administration in 1 atm air. Thus baseline
O2 was represented by a single value
in air (CAAir; Fig. 1). During the
first hour after caffeine administration (CA+Air),
O2 was 24% higher than baseline
(Fig. 1). O2 remained significantly
higher after caffeine administration for at least 180 min
(P < 0.05, two-tailed paired t-test; Fig.
1). Heart rate was significantly higher by 7-40 beats/min after
caffeine administration in three animals, whereas it was unchanged in
three animals (Table 3).
|
|
Mean CH4 for animals in 1 atm air
was 14.2 ± 13.2 µmol CH4/min before caffeine
administration and 13.8 ± 11.1 µmol CH4/min after
caffeine administration. These values are not different from each other
(P > 0.50, two-tailed Student's t-test).
During the hyperbaric experiments, DCS incidence in CA+INJ+ animals
(9/10) was marginally higher than in CA+INJ animals (4/10; P = 0.057, Fisher exact test; Table 1, Fig.
2).
CH4 in animals in the CA+INJ+ group
was significantly higher (P < 0.05, two-tailed Student's t-test) by >50% compared with animals in the
CA+INJ group (Table 1, Fig. 3).
|
|
| |
DISCUSSION |
|---|
|
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
H2 biochemical decompression has been demonstrated to
reduce DCS risk in two different animal models after a number of
different exposures to hyperbaric H2 (6,
10-12). Mathematical modeling has supported our concept
that H2 biochemical decompression reduces DCS risk by
lowering tissue H2 content via microbial H2
metabolism, with significant correlations between microbial activity
injected, CH4 from animals, and DCS
outcome (6). Even the native intestinal flora in pigs can
significantly reduce DCS risk if the methanogenic activity is
sufficiently high (10).
The caffeine treatments of the present study led to a result even more disappointing than a failure to support our hypothesis of lowering DCS risk further for a given activity of methanogens injected. The caffeine treatments were actually associated with an increased risk of DCS for animals receiving injections of methanogens (Fig. 2). This unexpected result deserves careful consideration since we had been hoping to offer biochemical decompression, first for H2 diving and eventually for N2 diving, to human divers in the foreseeable future.
Caffeine, a methylxanthine, exerts a number of physiological
effects (5, 7-9, 16, 17), including the
stimulation of gastric acid and digestive enzyme secretion
(9). Caffeine does not selectively increase intestinal
perfusion but can increase cardiac output and, therefore, perfusion to
many vascular beds (9).
O2 values measured before caffeine
administration for the pigs in this study are within the normal range
for these animals, when normalized for body mass and age effects
(15, 18). The higher O2
in air (which is the only option we had for measuring O2, given the technical difficulties of
working with large volumes of hyperbaric H2) after caffeine
treatment (Fig. 1) supports our assumption that the caffeine was
increasing cardiac output in these animals. This elevation in
O2 lasted at least for a time span
corresponding to the length of the chosen hyperbaric exposure. The
variable effect on heart rate of individual animals (Table 3) may
reasonably reflect changes in stroke volume as well as heart rate after
caffeine administration.
Consequently, the treatment of these animals with caffeine is likely to offer mixed effects bearing on DCS outcome. Increasing the cardiac output may increase the rate of H2 uptake throughout the body during the hyperbaric exposure for the period of hours needed before attaining H2 saturation (Fahlman, unpublished observation). However, if some fraction of the higher cardiac output increases the perfusion of the intestines, this should potentially increase the rate of H2 elimination across the intestine by the metabolism of the methanogens. The net effect on subsequent DCS risk is likely to be determined by whether the increased H2 uptake or the increased H2 elimination is greater.
Under normal atmospheric conditions, intestinal CH4
production in mammals is almost exclusively attributable to metabolism of H2 generated by other intestinal microbes
(14). The lack of change in
CH4 after caffeine administration in
1 atm air was expected because increased cardiac output, and
potentially increased perfusion to the intestines, would not alter the
supply of H2 within the intestines of air-breathing animals.
To help understand CH4 and DCS
incidence in these experiments, we compared the data from this study
with those of an earlier study that did not include caffeine treatment
(11). In that study, animals were exposed to the same
compression and decompression sequence in H2 and the same
range of H2 concentrations as in the present study; some
animals had only their own native intestinal flora (CAINJ;
n = 10) and others received intestinal injections of
M. smithii (CAINJ+; n = 16).
Before making any pairwise statistical comparisons between the animal
groups in the two studies, we first performed statistical procedures on
the data from the four groups together. Body masses did not differ
among the four groups (ANOVA, P > 0.10); the null
hypothesis that CH4 was the same in
all four groups was rejected (ANOVA, P < 0.001); and
the null hypothesis that the DCS outcome for the two groups with
caffeine was the same as the outcome for the two groups without
caffeine was rejected (
2 test, P < 0.01).
Mean CH4 in hyperbaric
H2 was nearly twofold higher (P < 0.01, two-tailed Student's t-test) in CA+INJ animals compared with CAINJ animals (Fig. 3). This supports the hypothesis that caffeine ingestion increased intestinal perfusion and that intestinal methanogenesis (using only a native population of microbes) is to some
extent perfusion limited when there is an ample source of
H2 supplied from the blood.
Pigs from the CA+INJ group had a marginally lower incidence of DCS
(4/10) than those from the CAINJ group (9/10; P = 0.057, Fisher exact test; Fig. 2). This result is also as expected if the caffeine treatment increased the rate of supply of blood-borne H2 to the microbes, thereby increasing the H2
elimination process. Any changes in H2 uptake in the
CA+INJ group would be undetectable in these experiments because we
had no assay for H2 uptake.
In designing this study, we predicted that the CA+INJ+ group would have
the highest CH4 and lowest DCS risk of
any animal group. Instead, the CA+INJ+ animals released CH4
at a mean rate that was similar to that of the CAINJ+ pigs
(P > 0.40, two-tailed Student's t-test;
Fig. 3). When we compared methanogenic activity injected into animals
to CH4 from animals in the CAINJ+
and CA+INJ+ groups, no significant differences were found (Fig.
4). The CA+INJ+ animals had a higher
incidence of DCS (9/10) than the CAINJ+ animals (7/16;
P < 0.05, Fisher exact test; Fig. 2). One explanation
for this observation, which we explore below, is that a higher cardiac
output after caffeine administration and surgery may have had a greater
effect on H2 uptake than on H2 elimination,
thereby increasing DCS risk.
|
In the prior study (11), we included a group of pigs that
underwent the same surgical procedure as the methanogen-treated animals
except that their intestinal injections were of deoxygenated saline.
The pigs in this surgical control group had a similarly high incidence
of DCS (7/10) and a CH4 (34 ± 16 µmol CH4/min) that was nearly identical to that of the
controls without surgery (CAINJ; Figs. 2 and 3). Thus we did not
expect to need a surgical control group for the animals with caffeine
treatment. We now suspect that such a caffeine plus surgical control
group may have revealed that the caffeine treatment coupled with
surgery had the effect of increasing cardiac output without allowing an
increase in intestinal perfusion for pharmacological reasons we
presently do not understand. This combination of caffeine and surgery
may have augmented H2 uptake but kept H2
elimination from rising, leading to an elevated risk of DCS in this
hypothetical caffeine plus surgical control group.
The issue of cardiac output in relation to intestinal perfusion is a very realistic concern for bringing biochemical decompression from laboratory animal studies to human field use. Exercise is known to increase cardiac output, with selectively elevated perfusion of the muscles in demand, and decreased perfusion of less critical tissues such as intestine (2). Thus future laboratory studies of biochemical decompression should be careful to include exercise and its effects on blood flow distribution, along with analyses of gas uptake and elimination rates with and without intestinal microbes.
We conclude that that the administration of caffeine to pigs increased
their cardiac output as suggested by their increased O2. Caffeine administration also
increased CH4 during hyperbaric H2 exposure in animals with a native intestinal flora,
which led to a decreased DCS incidence. The anomalous effect of
increased DCS incidence in CA+INJ+ can only be speculatively attributed to an increased H2 uptake without a matching H2
elimination by intestinal methanogens. Thus there is much that we do
not understand about gas fluxes during biochemical decompression, and
this missing information will be critical to offering this process to
facilitate diving in humans.
| |
ACKNOWLEDGEMENTS |
|---|
We are grateful as ever to the Phase III support team, Richard Ayres, Jerry Morris, Roland Ramsey, and Chief Anthony Ruopoli for devotion to doing things well. We also thank Diana Temple for editorial assistance and critical reading of this manuscript.
| |
FOOTNOTES |
|---|
This work was funded by the Naval Medical Research and Development Command Work Unit no. 61153N MR04101.00D-1103. The opinions and assertions contained herein are the private ones of the authors and are not to be construed as official or reflecting the views of the Navy Department and the naval service at large.
Present address of A. Fahlman: School of Biosciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
Address for reprint requests and other correspondence: S. R. Kayar, National Center for Research Resources, National Institutes of Health, 6705 Rockledge Dr., Suite 6030, Bethesda, MD 20892-7965 (E-mail: kayars{at}ncrr.nih.gov).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
July 19, 2002;10.1152/japplphysiol.00349.2002
Received 18 April 2002; accepted in final form 2 July 2002.
| |
REFERENCES |
|---|
|
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
|
|---|
1.
Abraini, JH,
Gardette-Chauffour MC,
Martinez E,
Rostain JC,
and
Lemaire C.
Psychophysiological reactions in humans during an open sea dive to 500 m with a hydrogen-helium-oxygen mixture.
J Appl Physiol
76:
1113-1118,
1994
2.
Armstrong, RB,
Delp MD,
Goljan EF,
and
Laughlin MH.
Distribution of blood flow in muscles of miniature swine during exercise.
J Appl Physiol
62:
1285-1298,
1987
3.
Balch, WE,
Fox GE,
Magrum LJ,
Woese CR,
and
Wolfe RS.
Methanogens: reevaluation of a unique biological group.
Microbiol Rev
43:
260-296,
1979
4.
Bartholomew, GA,
Vleck D,
and
Vleck CM.
Instantaneous measurements of oxygen consumption during pre-flight warm-up and post-flight cooling in sphingid and saturniid moths.
J Exp Biol
90:
17-32,
1981
5.
Bauer, J,
Maier K,
Linderkamp O,
and
Hentschel R.
Effect of caffeine on oxygen consumption and metabolic rate in very low birth weight infants with idiopathic apnea.
Pediatrics
107:
660-663,
2001
6.
Fahlman, A,
Tikuisis P,
Himm JF,
Weathersby PK,
and
Kayar SR.
On the likelihood of decompression sickness during H2 biochemical decompression in pigs.
J Appl Physiol
91:
2720-2729,
2001
7.
Graham, TE,
Sathasivam P,
and
MacNaughton KW.
Influence of cold, exercise, and caffeine on catecholamines and metabolism in men.
J Appl Physiol
70:
2052-2058,
1991
8.
Greer, F,
McLean C,
and
Graham TE.
Caffeine, performance, and metabolism during repeated Wingate exercise tests.
J Appl Physiol
85:
1502-1508,
1998
9.
Katzung, BG.
Basic & Clinical Pharmacology. Norwalk, CT: Appleton & Lange, 1992, p. 284.
10.
Kayar, SR,
and
Fahlman A.
Decompression sickness risk reduced by native intestinal flora in pigs after H2 dives.
Undersea Hyperb Med
28:
89-97,
2001[ISI][Medline].
11.
Kayar, SR,
Fahlman A,
Lin WC,
and
Whitman WB.
Increasing activity of H2-metabolizing microbes lowers decompression sickness risk in pigs during H2 dives.
J Appl Physiol
91:
2713-2719,
2001
12.
Kayar, SR,
Miller TL,
Wolin MJ,
Aukhert EO,
Axley MJ,
and
Kiesow LA.
Decompression sickness risk in rats by microbial removal of dissolved gas.
Am J Physiol Regul Integr Comp Physiol
275:
R677-R682,
1998
13.
LeBlanc, J,
Jobin M,
Cote J,
Samson P,
and
Labrie A.
Enhanced metabolic response to caffeine in exercise-trained human subjects.
J Appl Physiol
59:
832-837,
1985
14.
Miller, TL.
Biogenic sources of methane.
In: Microbial Production and Consumption of Greenhouse Gases: Methane, Nitrogen Oxides, and Halomethanes, edited by Rogers JE,
and Whitman WB.. Washington DC: Am. Soc. for Microbiology, 1991, p. 175-187.
15.
Prosser, CL.
Comparative Animal Physiology. Philadelphia, PA: Saunders, 1973.
16.
Roy, BD,
Bosman MJ,
and
Tarnopolsky MA.
An acute oral dose of caffeine does not alter glucose kinetics during prolonged dynamic exercise in trained endurance athletes.
Eur J Appl Physiol
85:
280-286,
2001[ISI][Medline].
17.
Sewester, CS.
Drug Facts and Comparisons. St. Louis, MO: Wolters Kluwer, 1995, p. 1166-1168.
18.
Wieser, W.
A distinction must be made between the ontogeny and the phylogeny of metabolism in order to understand the mass exponent of energy metabolism.
Respir Physiol
55:
1-9,
1984[ISI][Medline].
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
P = 0.10 (paired t-test).
Marginally different from the CA+INJ
