Relationship between T-wave amplitude and oxygen pulse in guinea pigs in hyperbaric helium and hydrogen

Diving isknown to induce a change in the amplitude of the T wave(A_Tw) ofelectrocardiograms, but it is unknown whether this is linked to achange in cardiovascular performance. We analyzed A_Tw in guinea pigs at 10-60atm and 25-36°C, breathing 2%O₂ in either helium (heliox;n = 10) or hydrogen (hydrox;n = 9) for 1 h at each pressure. Coretemperature and electrocardiograms were detected by using implantedradiotelemeters. O₂ consumption rate was measured by using gas chromatography. In a previous study (S. R. Kayar and E. C. Parker. J. Appl.Physiol. 82: 988-997, 1997), we analyzed theO₂ pulse, i.e., theO₂ consumption rate per heartbeat, in the same animals. By multivariate regression analysis, weidentified variables that were significant toO₂ pulse: body surface area,chamber temperature, core temperature, and pressure. In this study,inclusion of A_Tw made asignificantly better model with fewer variables. After normalizing forchamber temperature and pressure, theO₂ pulse increased with increasing A_Tw in heliox(P = 0.001) but with decreasingA_Tw in hydrox(P < 0.001). ThusA_Tw is associated with thedifferences in O₂ pulse foranimals breathing heliox vs. hydrox.

     

Oxygen pulse in guinea pigs in hyperbaric helium and hydrogen

Kayar, Susan R., and Erich C. Parker. Oxygen pulse inguinea pigs in hyperbaric helium and hydrogen. J. Appl. Physiol. 82(3): 988-997, 1997.We analyzedO₂ pulse, the total volume of O₂ consumed per heart beat, inguinea pigs at pressures from 10 to 60 atmospheres. Animals were placedin a hyperbaric chamber and breathed 2%O₂ in either helium (heliox) orhydrogen (hydrox). Oxygen consumption rate(O₂) was measured by gaschromatographic analysis. Core temperature and heart rate were measuredby using surgically implanted radiotelemeters. TheO₂ was modulated over afourfold range by varying chamber temperature from 25 to 36°C. There was a direct correlation betweenO₂ and heartrate, which was significantly different for animals in heliox vs.hydrox (P = 0.003). By usingmultivariate regression analysis, we identified variables that weresignificant to O₂ pulse: bodysurface area, chamber temperature, core temperature, and pressure.After normalizing for all nonpressure variables, the residualO₂ pulse was found to decreasesignificantly (P = 0.02) with pressurefor animals in heliox but did not decrease significantly(P = 0.38) with pressure for animalsin hydrox over the range of pressures studied. This amounted to aroughly 25% lower O₂ pulse fornormothermic animals in 60 atmospheres heliox vs. hydrox. These resultssuggest that reduction of cardiovascular efficiency in a hyperbaricenvironment can be mitigated by the choice of breathing gas.

     

Oxygen transport during exercise in large mammals. I. Adaptive variation in oxygen demand

This study investigated mechanisms used by horses and steers to increase O2 uptake and delivery (VO2) from resting to maximal rates and identified the mechanisms that enable horses to achieve higher maximal rates of O2 consumption (VO2max) than steers. VO2 and circulatory variables were measured while Standardbred trotting horses and steers (450-kg body mass) stood quietly and ran on a treadmill at speeds up to those eliciting VO2max. As VO2 increased in both species, heart rate and circulating hemoglobin (Hb) concentration increased, thereby increasing O2 delivery by the circulation, while cardiac stroke volume remained unchanged. At VO2max arterial PCO2 increased from its resting value in horses but was unchanged in steers, and arterial PO2 decreased in both species. Although the horses hypoventilated and were hypoxemic at VO2max, no significant decrease in arterial Hb saturation occurred. VO2max of the horses was 2.6 times higher than that of the steers and was associated with a 100% larger cardiac output, 100% larger stroke volume, and 40% higher Hb concentration, whereas heart rates at VO2max were identical in the two species. The higher cardiac output of the horses at VO2max resulted from a 1.2-fold higher mean arterial pressure and 1.6-fold lower peripheral tissue resistance (associated with a larger skeletal muscle capillary bed). Both the magnitude of the difference in VO2max between horses and steers and the mechanisms used to achieve it are the same as observed in smaller pairs of mammalian species with large variation in aerobic capacity.     

On the likelihood of decompression sickness during H(2) biochemical decompression in pigs.

A probabilistic model was used to predict decompression sickness (DCS) outcome in pigs during exposures to hyperbaric H(2) to quantify the effects of H(2) biochemical decompression, a process in which metabolism of H(2) by intestinal microbes facilitates decompression. The data set included 109 exposures to 22-26 atm, ca. 88% H(2), 9% He, 2% O(2), 1% N(2), for 0.5-24 h. Single exponential kinetics described the tissue partial pressures (Ptis) of H(2) and He at time t: Ptis = integral (Pamb - Ptis). tau(-1) dt, where Pamb is ambient pressure and tau is a time constant. The probability of DCS [P(DCS)] was predicted from the risk function: P(DCS) = 1 - e(-r), where r = integral (Ptis(H(2)) + Ptis(He) - Thr - Pamb). Pamb(-1) dt, and Thr is a threshold parameter. Inclusion of a parameter (A) to estimate the effect of H(2) metabolism on P(DCS): Ptis(H(2)) = integral (Pamb - A - Ptis(H(2))). tau(-1) dt, significantly improved the prediction of P(DCS). Thus lower P(DCS) was predicted by microbial H(2) metabolism during H(2) biochemical decompression.     

O2 delivery at VO2max and oxidative capacity in muscles of standardbred horses.

The purpose of this study was to describe the relationships between 16 physiological, biochemical, and morphological variables presumed to relate to the oxidative capacity in quadriceps muscles or muscle parts in Standardbred horses. The variables included O2 delivery (blood flow) and mean capillary transit time (MTT) during treadmill locomotion at whole animal maximal O2 consumption (VO2max, 134 +/- 2 ml.min-1 x kg-1), capillary density and capillary-to-fiber ratio, myoglobin concentration, oxidative enzyme activities, glycolytic enzyme activities, fiber type populations, and fiber size. These components of muscle metabolic capacity were found to be interrelated to varying degrees using correlation matrix analysis, with lactate dehydrogenase activity showing the most significant correlations (n = 14) with other variables. Most of the "oxidative" variables occurred in the highest quantities in the deepest muscle of the group (vastus intermedius) and in the deepest parts of the other quadriceps muscles where the highest proportions of type I fibers were localized. The highest blood flow measured with microspheres in the muscle group during exercise was in vastus intermedius muscle (145 ml.min-1 x 100 g-1), and the lowest was in the superficial part of rectus femoris muscle (32 ml.min-1 x 100 g-1). Average muscle blood flow during exercise at whole animal VO2max was 116 ml.min-1 x 100 g-1. Because skeletal muscle comprised 43% of total body mass (453 +/- 34 kg), total muscle blood flow was estimated at 226 l/min, which was approximately 78% of total cardiac output (288 l/min).(ABSTRACT TRUNCATED AT 250 WORDS)     

Rat soleus muscle ultrastructure after hindlimb suspension

The aim of the present investigation was to determine, by quantitative electron microscopy, the effects of a 5-wk tail-suspension period on rat soleus muscle ultrastructure. A marked decline (-60%) in muscle mass occurred. The mean fiber cross-sectional area decreased to a greater extent (-75%) than the capillary-to-fiber ratio (-37%), leading to a higher capillary density (+148%) after hypokinesia. The total mitochondrial volume density remained unchanged, whereas the volume density of myofibrils was slightly but significantly reduced (-6%). A shift from subsarcolemmal to interfibrillar mitochondria occurred. Interfibrillar mitochondrial volume density was highest near the fiber border and decreased toward the fiber center. An increase in volume density of satellite cells suggested muscle regenerative events. Soleus atrophy with tail suspension greatly decreases the muscular volume but leaves the ultrastructural composition of muscle fibers relatively unaffected.     

Red blood cell rheological alterations in a rat model of ischemia-reperfusion injury

Red blood cell (RBC) deformability and aggregation characteristics were investigated in an experimental model of ischemia-reperfusion injury. Ischemia was produced in rat hind limb by occluding the femoral artery for 10 minutes, followed by reperfusion. Blood samples were obtained either following the ischemia or 15 minutes after reperfusion. RBC deformability measured by ektacytometry was found to be significantly impaired immediately after the end of ischemic period in the blood samples obtained from femoral vein of the ischemic limb, while there was no significant difference after 15 minutes of reperfusion. In contrast, RBC aggregability was found to be decreased only after the reperfusion period and this alteration was not only limited to the blood returning from the ischemic limb but was also observed in the samples obtained from non-ischemic, contralateral hind limb, indicating a systemic alteration. RBC electrophoresis studies suggested that the altered aggregability might be related to altered RBC surface properties including increased RBC surface charge density.     

Increasing activity of H(2)-metabolizing microbes lowers decompression sickness risk in pigs during H(2) dives.

The risk of decompression sickness (DCS) was modulated by varying the biochemical activity used to eliminate some of the hydrogen (H(2)) stored in the tissues of pigs (19.4 +/- 0.2 kg) during hyperbaric exposures to H(2). Treated pigs (n = 16) received intestinal injections of Methanobrevibacter smithii, a microbe that metabolizes H(2) to water and CH(4). Surgical controls (n = 10) received intestinal injections of saline, and an additional control group (n = 10) was untreated. Pigs were placed in a chamber and compressed to 24 atm abs (20.6-22.9 atm H(2)). After 3 h, the pigs were decompressed and observed for symptoms of DCS for 1 h. Pigs with M. smithii had a significantly lower (P < 0.05) incidence of DCS (44%; 7/16) than all controls (80%; 16/20). The DCS risk decreased with increasing activity of microbes injected (logistic regression, P < 0.05). Thus the supplemental tissue washout of the diluent gas by microbial metabolism was inversely correlated with DCS risk in a dose-dependent manner in this pig model.     

Modulation of decompression sickness risk in pigs with caffeine during H(2) biochemical decompression.

In H(2) biochemical decompression, H(2)-metabolizing intestinal microbes remove gas stored in tissues of animals breathing hyperbaric H(2), 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 O(2) 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 H(2), 0.3-0.5 atm O(2)) 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 H(2) biochemical decompression to humans.