Exercise VE and physical performance at altitude are not affected by menstrual cycle phase.

We hypothesized that progesterone-mediated ventilatory stimulation during the midluteal phase of the menstrual cycle would increase exercise minute ventilation (VE; l/min) at sea level (SL) and with acute altitude (AA) exposure but would only increase arterial O2 saturation (SaO2, %) with AA exposure. We further hypothesized that an increased exercise SaO2 with AA exposure would enhance O2 transport and improve both peak O2 uptake (VO2 peak; ml x kg-1 x min-1) and submaximal exercise time to exhaustion (Exh; min) in the midluteal phase. Eight female lowlanders [33 +/- 3 (mean +/- SD) yr, 58 +/- 6 kg] completed a VO2 peak and Exh test at 70% of their altitude-specific VO2 peak at SL and with AA exposure to 4,300 m in a hypobaric chamber (446 mmHg) in their early follicular and midluteal phases. Progesterone levels increased (P < 0.05) approximately 20-fold from the early follicular to midluteal phase at SL and AA. Peak VE (101 +/- 17) and submaximal VE (55 +/- 9) were not affected by cycle phase or altitude. Submaximal SaO2 did not differ between cycle phases at SL, but it was 3% higher during the midluteal phase with AA exposure. Neither VO2 peak nor Exh time was affected by cycle phase at SL or AA. We conclude that, despite significantly increased progesterone levels in the midluteal phase, exercise VE is not increased at SL or AA. Moreover, neither maximal nor submaximal exercise performance is affected by menstrual cycle phase at SL or AA.

Ventilatory capacities at sea level and high altitude.

Because air is less dense at high altitude (HA), airway resistance is reduced and maximum inspiratory and expiratory flows are greater than at sea level (SL). Despite the reduction in airway resistance, ventilatory muscle endurance may be decreased by hypobaric hypoxia and, thus, may be a factor in limiting exercise at HA. To explore the effects of HA on ventilatory capacities and their relation to ventilatory demands of exercise, we measured 15-s maximum voluntary ventilation (MVV), 15-min maximum sustainable ventilation (MSV), and maximum airway pressures (Plmax and PEmax) in 18 healthy young men at SL and HA (Pikes Peak, 4300 m, or hypobaric chamber, PB approximately 460 mmHg). In eight of these subjects ventilatory capacities were compared with exercise ventilations. We also measured the effects of 36% O2 on the MSV in 12 of the subjects exposed to simulated altitude. Similar results were obtained at either simulated or actual HA. We found that MVV increased (p < 0.001) by 20% and the MSV (p < 0.001) by 15% at HA. Administration of 36% O2 at HA increased MSV further by 5% with no effect on MVV. No effect of HA on maximum inspiratory and expiratory pressures was found. We confirmed previous findings of modest increases in forced 1-s expired volume (FEV1) and slight decreases in forced vital capacity (FVC) at HA. At both SL and HA, the MSV exceeded the ventilatory demands of submaximal cycle exercise that could be sustained for about 30 min. During progressive cycle exercise to exhaustion, however, peak VE was not different from MVV, either at SL or HA. We conclude that the small, but significant, increase in MSV with 36% O2 administration at HA suggests that hypoxia decreases ventilatory endurance for flow loads as determined by the MSV. Thus, the possibility that ventilatory limits have a role in cessation of exercise at high altitude cannot be ruled out.

Blood oxygen saturation measured in the presence of a mixture of cyclopropane, sulfur hexafluoride, and ethane.

Some anesthetic gases interfere with the determination of blood O2 values. We evaluated, for its potential for such interference, a gas mixture containing (v/v) approximately 70% ethane, 20% sulfur hexafluoride, and 10% cyclopropane, as is currently used in trace amounts to determine ventilation-perfusion (v/Q) ratios. Normal human blood samples were first tonometered with control gas mixtures containing (v/v): (1) 20.9% O2, 0.04% CO2; (2) 16.0% O2, 4.0% CO2; and (3) 9.9% O2, 7.8% CO2. A large quantity (20.9%) of the v/Q mixture was blended experimentally into the control mixture and the tonometry repeated. The entire experiment was then repeated substituting pure N2 for the v/Q mixture as a dilution control. O2 values were determined by three methods: (1) a polarographic electrode, ABL-300 (ABL); (2) a spectrophotometric method, Co-Oximeter (COOX); and (3) a galvanic cell, Lex-O2-Con (LEX). The v/Q gas mixture lowered significantly all measured LEX values by 2.5-3.6 saturation percent (sat%), but showed no effect, dilution or otherwise, on the O2 values determined by the COOX and ABL methods. The N2 dilution lowered the LEX values by an average of only 0.9 sat%; the ABL and the COOX were approximately 0.6 sat% lower. We therefore suggest that, if any of these O2 measurement methods are used in the presence of the v/Q mixture, baseline O2 values should be determined both before and after injection of the mixture into the bloodstream, prior to performing other experimental manipulations. The difference between the two values, if any, can then be used to interpret subsequent results.

Effects of chemical defense clothing and individual equipment on ventilatory function and subjective reactions.

BACKGROUND: We hypothesized that the multiple layers of protective clothing (PC) of a Chemical Defense Uniform (CDU) with body armor (BA), and load bearing equipment (LBE) impose an external impedance on breathing mechanics in addition to that caused by the chemical-biological protective mask (CB mask). METHODS: We measured the effects of wearing the CDU with BA + LBE on the mechanics and pattern of breathing and subjective reactions in 15 male soldiers during rest and sustained exercise (600 W). Heat strain on each soldier was made minimal and equivalent between the battle dress uniform (BDU) and CDU configurations. RESULTS: Results from wearing the CDU were contrasted with data from wearing a physical training uniform (PTU), or BDU. At rest, the CDU decreased maximal voluntary ventilation (MVV) by 25% (p < 0.05) compared with the PTU. The CB mask alone reduced MVV 20% (p < 0.05), and the PC + BA + LBE components of the CDU contributed an additional 5% decrement (p = 0.059). The CB mask decreased maximal respiratory flows (p < 0.05), whereas the PC + BA + LBE components decreased lung volumes (p < 0.05), and potentiated the decrement of respiratory flows. Total respiratory system elastance (Ers) was increased by 16% (p < 0.05) in the CDU. During exercise, VE was comparable for both the CDU and BDU, but VT was smaller (p < 0.01) and fR greater (p < 0.05) in the CDU; a breathing pattern consistent with the increased Ers. Perceptions of anxiety, not getting enough air, not breathing the way one usually does, and not being relaxed were greater (p < 0.05) during exercise in the CDU than in the BDU. CONCLUSIONS: This study shows that some of the respiratory impairment associated with the wearing of CDU is created by the PC, BA and LBE which present a unique external constraint on the chest wall. These impairments to ventilation may provoke more aversive effects than the larger resistive loads imposed by the CB mask.

Effects of chronic hypoxia on the pharmacokinetics of caffeine and cardio-green in micro swine.

METHODS: The pharmacokinetics of caffeine and cardio-green (ICG) were examined in four micro swine at sea level (SEA) and following 21 d continuous exposure to 4600 m (ALT) in a hypobaric chamber. Caffeine (84.7 mg) and ICG (10 mg) were administered as separate intravenous boluses and sequential blood samples collected. RESULTS: Caffeine clearance significantly (p < 0.05) increased in ALT (96.8 +/- 20.0 ml.min-1) as compared to SEA (53.6 +/- 24.8 ml.min-1), demonstrating that liver function increased in ALT. There was no significant change in the ratio of primary metabolites to caffeine, suggesting that the increase in clearance was not due to a change in the rate of caffeine metabolism. ICG clearance significantly increased in ALT (179.8 +/- 57.4 ml.min-1) as compared to SEA (84.4 +/- 28.9 ml.min-1) indicating that hepatic blood flow (HBF) increased. CONCLUSION: These results demonstrate that chronic exposure to 4600 m increases the clearance of caffeine and ICG in the micro swine model and suggests that the increase in caffeine clearance is related to HBF.

Effect of caffeine on submaximal exercise performance at altitude.

The purpose of this study was to determine if caffeine (CAF) could enhance exercise performance at high altitude (HA). Eight males (17 to 24 years) performed two submaximal endurance tests to exhaustion (ETX) while cycling at approximately 80% of their altitude-specific maximal aerobic power during each of three phases: 1) sea level (SL); 2) after 1 h (acute) at 4,300 m; and 3) after 2 weeks (chronic) at 4,300 m. Subjects received either CAF (4 mg.kg-1) or a placebo drink 1 h prior to each ETX bout at each phase in a double-blind crossover design. ETX was little affected during CAF treatment at SL (26.33 to 27.51 min, p = 0.21) but was increased by 54% (22.77 to 35.10 min, p = 0.004) during acute HA exposure and tended to improve (24%, 30.52 to 38.63 min, p = 0.084) during chronic HA exposure. The change in ETX during acute ALT could not be accounted for by differences in substrate metabolism, Q, diet, or RPE, but may have been due to either a CAF-induced increase in tidal volume or to a lessening of an ALT-induced impairment in muscular force production during submaximal exercise.

Hypobaric hypoxia (380 Torr) decreases intracellular and total body water in goats.

The effects of prolonged hypoxia on body water distribution was studied in four unanesthetized adult goats (Capra lircus) at sea level and after 16 days in a hypobaric chamber [(380 Torr, 5,500 m, 24 +/- 1 degrees C); arterial PO2 = 27 +/- 2 (SE) Torr]. Total body water (TBW), extracellular fluid volume (ECF), and plasma volume (PV) were determined with 3H2O, [14C]inulin, and indocyanine green dye, respectively. Blood volume (BV) [BV = 100PV/(100 - hematocrit)], erythrocyte volume (RCV) (RCV = BV - PV), and intracellular fluid (ICF) (ICF = TBW - ECF) and interstitial fluid (ISF) (ISF = ECF - PV) volumes were calculated. Hypoxia resulted in increased pulmonary ventilation and arterial pH and decreased arterial PCO2 and PO2 (P less than 0.05). In addition, body mass (-7.1%), TBW (-9.1%), and ICF volume (-14.4%) all decreased, whereas ECF (+11.7%) and ISF (+27.7%) volumes increased (P less than 0.05). The decrease in TBW accounted for 89% of the loss of body mass. Although PV decreased significantly (-15.3%), BV was unchanged because of an offsetting increase in RCV (+39.5%; P less than 0.05). We conclude that, in adult goats, prolonged hypobaric hypoxia results in decreases in TBW volume, ICF volume, and PV, with concomitant increases in ECF and ISF volumes.

A latex mask for collection of expired gases from goats.

A latex respiratory mask was developed to collect expired gas from goats in studies of ventilatory adaptation to high altitude. The snouts of 15 animals were dimensioned by taking girth measurements. The procedure requires a rough positive mold of wood which is repeatedly submerged into latex, then slowly rotated upon removal until skim-like layers form a mask. A supporting harness is attached to the mask by use of three flat straps connected by Dot fasteners. A period of several hours per day for up to 2 weeks may be necessary to allow the animal to become accustomed to wearing the mask.

The use of tympanometry to detect aerotitis media in hypobaric chamber operations.

Diagnosis and quantification of aerotitis media were performed using a modified commercially-available tympanometer under hypobaric conditions. Subjects were 22 males and 9 females, 22-43 years of age, who were tested in each ear with the tympanometer prior to and after exposure, sequentially at the barometric pressure plateaus of 706, 656, 609, 586, 564, and 522 mm Hg, and following an induced ear block during a 1-min descent from 522 to 586 mm Hg. Each subject was examined once either alone or in pairs during a 90-min exposure. Aerotitis media was detected using tympanometry at simulated altitude as evidenced by the difference between measurements made during induced ear blocks and those made prior to inducement, as well as following relief of the pressure differential with the Valsalva maneuver. There were no significant differences between pre- and post-induced aerotitis media values at 586 mm Hg, or between pre- and post-hypobaria. Our study suggests that tympanometry can be a valuable tool in managing aerotitis media in the aeromedical environment.

Operation Everest II: comparison of four instruments for measuring blood O2 saturation.

The bias and precision of four different methods for determining O2 saturation (SO2) were evaluated during a study of hypobaric hypoxia conducted with seven male subjects exposed progressively over a 40-day period to simulated altitudes from sea level (760 Torr) to 8,840 m (240 Torr). SO2 of arterial and mixed venous blood samples were measured with the Instrumentation Laboratory 282 CO-oximeter (CO-OX), the Radiometer ABL-300 (ABL), and the Lex-O2-Con-K (LEX). Noninvasive measurements of arterial SO2 were made with a Hewlett-Packard 47201A ear oximeter (EAR-OX). The CO-OX method was used as a secondary standard for comparison with the other methods because it has been validated against the classical Van Slyke method over a wide physiological range (Maas et al., Clin. Chim. Acta 29: 303-309, 1970). The LEX results most closely approximated but consistently underestimated those of the CO-OX: LEX = 0.93 CO-OX -0.86, standard error of the estimate (SEE) = 5.17, r = 0.98, n = 670. The ABL method appeared to combine two linear trends: for SO2 greater than 75%, ABL = 0.84 CO-OX +14.4, SEE = 1.77, r = 0.97, n = 369; less than 75%, ABL = 0.98 CO-OX +5.9, SEE = 4.44, r = 0.97, n = 486. The EAR-OX results were found to approximate those of the CO-OX at SO2 values only greater than 65%: EAR-OX = 1.07 CO-OX -6.12, SEE = 7.71, r = 0.98, n = 326.(ABSTRACT TRUNCATED AT 250 WORDS)

A reusable adapter for collection of cerebrospinal fluid in chronically cannulated goats.

A lightweight, adjustable adapter has been designed for chronic cannulation of goats (Capra hircus) which provides an accurate, safe means of sampling cerebrospinal fluid (CSF). This cisternal cannula has been used for continuous perfusion of synthetic CSF into the fourth ventricle in unanesthetized goats. This method also has been used for examining changes in ionic composition of the CSF and cerebral interstitial fluid (ISF) during physiologic adaptations to high altitude (2-5).

Acute mountain sickness at 4500 m is not altered by repeated eight-hour exposures to 3200-3550 m normobaric hypoxic equivalent.

A lightweight device, designed to supply inspired air at 12.8% O2 concentration (PO2 equivalent to 3960 m altitude) by recirculating a portion of each expired breath after CO2 removal, was tested at sea-level for its ability to induce altitude acclimation. Twelve young men (experimental group) breathed from the device for 7.5-8 h each day for 10 successive days. On the morning of day 1, inspired O2 concentrations averaged 12.8%, as intended, but increased by noontime and remained elevated thereafter. This raised the average hypoxic stimulus to 13.8 +/- 0.9% (PO2 equivalent to 3370 +/- 517 m altitude) for the entire 10-d period. Ten other young men (control group) breathed normoxic air from a placebo device of identical appearance on the same schedule. On day 10, both groups were exposed for the next 2 d to 4500 m altitude in a hypobaric chamber to assess the effect of the treatment on acute mountain sickness (AMS). After the sea level treatment, the experimental group showed no significant differences from control in resting ventilatory rate, respiratory frequency or end tidal PO2, but end-tidal PCO2 was lower; there was no indication of hemoconcentration. At altitude, both groups showed the expected decreases in end-tidal PO2 and PCO2, and increases in hemoglobin concentration and hematocrit indicative of hemoconcentration, with no differences between them. Neither incidence nor severity of AMS differed significantly between groups, but the experimental group had a lower incidence rate than historical controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Altitude acclimatization attenuates plasma ammonia accumulation during submaximal exercise.

This study examined the effects of acclimatization to 4,300 m altitude on changes in plasma ammonia concentrations with 30 min of submaximal [75% maximal O2 uptake (VO2max)] cycle exercise. Human test subjects were divided into a sedentary (n = 6) and active group (n = 5). Maximal uptake (VO2max) was determined at sea level and at high altitude (HA; 4,300 m) after acute (t less than 24 h) and chronic (t = 13 days) exposure. The VO2max of both groups decreased 32% with acute HA when compared with sea level. In the sedentary group, VO2max decreased an additional 16% after 13 days of continuous residence at 4,300 m, whereas VO2max in the active group showed no further change. In both sedentary and active subjects, plasma ammonia concentrations were increased (P less than 0.05) over resting levels immediately after submaximal exercise at sea level as well as during acute HA exposure. With chronic HA exposure, the active group showed no increase in plasma ammonia immediately after submaximal exercise, whereas the postexercise ammonia in the sedentary group was elevated but to a lesser extent than at sea level or with acute HA exposure. Thus postexercise plasma ammonia concentration was decreased with altitude acclimatization when compared with ammonia concentrations following exercise performed at the same relative intensity at sea level or acute HA. This decrease in ammonia accumulation may contribute to enhanced endurance performance and altered substrate utilization with exercise following acclimatization to altitude.

Naloxone does not affect ventilatory responses to hypoxia and hypercapnia in rats.

Ventilatory responses (tidal volume, respiratory frequency, and minute ventilation) to steady-state hypoxia and steady-state hypercapnia were measured plethysmographically in awake unrestrained adult rats, before and after subcutaneous injection of placebo (saline) naloxone in doses up to 5.0 mg/kg. Naloxone did not alter the ventilatory responses to hypoxia or hypercapnia.