Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

Hyperthermia induces injury to the intestinal mucosa in the mouse: evidence for an oxidative stress mechanism

Loss of the intestinal barrier is critical to the clinical course of heat illness, but the underlying mechanisms are still poorly understood. We tested the hypothesis that conditions characteristic of mild heatstroke in mice are associated with injury to the epithelial lining of the intestinal tract and comprise a critical component of barrier dysfunction. Anesthetized mice were gavaged with 4 kDa FITC-dextran (FD-4) and exposed to increasing core temperatures, briefly reaching 42.4°C, followed by 30 min recovery. Arterial samples were collected to measure FD-4 concentration in plasma (in vivo gastrointestinal permeability). The small intestines were then removed to measure histological evidence of injury. Hyperthermia resulted in a ≈2.5-fold elevation in plasma FD-4 and was always associated with significant histological evidence of injury to the epithelial lining compared with matched controls, particularly in the duodenum. When isolated intestinal segments from control animals were exposed to ≥41.5°C, marked increases in permeability were observed within 60 min. These changes were associated with release of lactate dehydrogenase, evidence of protein oxidation via carbonyl formation and histological damage. Coincubation with N-acetylcysteine protected in vitro permeability during hyperthermia and reduced histological damage and protein oxidation. Chelation of intracellular Ca(2+) to block tight junction opening during 41.5°C exposure failed to reduce the permeability of in vitro segments. The results demonstrate that hyperthermia exposure in mouse intestine, at temperatures at or below those necessary to induce mild heatstroke, cause rapid and substantial injury to the intestinal lining that may be attributed, in part, to oxidative stress.

Superoxide scavengers augment contractile but not energetic responses to hypoxia in rat diaphragm

Acute exposure to severe hypoxia depresses contractile function and induces adaptations in skeletal muscle that are only partially understood. Previous studies have demonstrated that antioxidants (AOXs) given during hypoxia partially protect contractile function, but this has not been a universal finding. This study confirms that specific AOXs, known to act primarily as superoxide scavengers, protect contractile function in severe hypoxia. Furthermore, the hypothesis is tested that the mechanism of protection involves preservation of high-energy phosphates (ATP, creatine phosphate) and reductions of P(i). Rat diaphragm muscle strips were treated with AOXs and subjected to 30 min of hypoxia. Contractile function was examined by using twitch and tetanic stimulations and the degree of elevation in passive force occurring during hypoxia (contracture). High-energy phosphates were measured at the end of 30-min hypoxia exposure. Treatment with the superoxide scavengers 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron, 10 mM) or Mn(III)tetrakis(1-methyl-4-pyridyl) porphyrin pentachloride (50 microM) suppressed contracture during hypoxia and protected maximum tetanic force. N-acetylcysteine (10 or 18 mM) had no influence on tetanic force production. Contracture during hypoxia without AOXs was also shown to be dependent on the extracellular Ca(2+) concentration. Although hypoxia resulted in only small reductions in ATP concentration, creatine phosphate concentration was decreased to approximately 10% of control. There were no consistent influences of the AOX treatments on high-energy phosphates during hypoxia. The results demonstrate that superoxide scavengers can protect contractile function and reduce contracture in hypoxia through a mechanism that does not involve preservation of high-energy phosphates.

Invited review: Adaptive responses of skeletal muscle to intermittent hypoxia: the known and the unknown

Intermittent hypoxia (IH) describes conditions of repeated, transient reductions in O2 that may trigger unique adaptations. Rest periods during IH may avoid potentially detrimental effects of long-term O2 deprivation. For skeletal muscle, IH can occur in conditions of obstructive sleep apnea, transient altitude exposures (with or without exercise), intermittent claudication, cardiopulmonary resuscitation, neonatal blood flow obstruction, and diving responses of marine animals. Although it is likely that adaptations in these conditions vary, some patterns emerge. Low levels of hypoxia shift metabolic enzyme activity toward greater aerobic poise; extreme hypoxia shifts metabolism toward greater anaerobic potential. Some conditions of IH may also inhibit lactate release during exercise. Many related cellular phenomena could be involved in the response, including activation of specific O2 sensors, reactive oxygen and nitrogen species, preconditioning, hypoxia-induced transcription factors, regulation of ion channels, and influences of paracrine/hormonal stimuli. The net effect of a variety of adaptive programs to IH may be to preserve contractile function and cell integrity in hypoxia or anoxia, a response that does not always translate into improvements in exercise performance.

Selected Contribution: Improved anoxic tolerance in rat diaphragm following intermittent hypoxia

Intermittent hypoxia (IH), associated with obstructive sleep apnea, initiates adaptive physiological responses in a variety of organs. Little is known about its influence on diaphragm. IH was simulated by exposing rats to alternating 15-s cycles of 5% O2 and 21% O2 for 5 min, 9 sets/h, 8 h/day, for 10 days. Controls did not experience IH. Diaphragms were excised 20-36 h after IH. Diaphragm bundles were studied in vitro or analyzed for myosin heavy chain isoform composition. No differences in maximum tetanic stress were observed between groups. However, peak twitch stress (P < 0.005), twitch half-relaxation time (P < 0.02), and tetanic stress at 20 or 30 Hz (P < 0.05) were elevated in IH. No differences in expression of myosin heavy chain isoforms or susceptibility to fatigue were seen. Contractile function after 30 min of anoxia (95% N2-5% CO2) was markedly preserved at all stimulation frequencies during IH and at low frequencies after 15 min of reoxygenation. Anoxia-induced increases in passive muscle force were eliminated in the IH animals (P < 0.01). These results demonstrate that IH induces adaptive responses in the diaphragm that preserve its function in anoxia.

NMR spin trapping: detection of free radical reactions with a new fluorinated DMPO analog

Electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin trapping were used for detection of free radical reactions utilizing a new fluorinated analog of DMPO, 4-hydroxy-5,5-dimethyl-2-trifluoromethylpyrroline-1-oxide (FDMPO). The parent FDMPO spin trap exhibits a single 19F-NMR resonance at -66.0 ppm. The signal to noise ratio improved 10.4-fold compared to 31P-NMR sensitivity of the phosphorus-containing spin trap, DEPMPO. The spin adducts of FDMPO with .OH, .CH3, and .CH2OH were characterized. Competitive spin trapping of FDMPO with DMPO showed that both have similar rates of addition of .OH and C-centered radicals. The corresponding paramagnetic spin adducts of FDMPO were extremely stable to degradation. In the presence of ascorbate, reaction products from C-centered radicals resulted in the appearance of two additional 19F-NMR signals at -78.6 and -80 ppm for FDMPO/ .CH(3) and at -74.6 and -76.75 ppm for FDMPO/ .CH(2)OH. In each case, these peaks were assigned to the two stereoisomers of their respective, reduced hydroxylamines. The identification of the hydroxylamines for FDMPO/ .CH3 was confirmed by EPR and 19F-NMR spectra of independently synthesized samples. In summary, spin adducts of FDMPO were highly stable for ESR. For NMR spin trapping, FDMPO showed improved signal to noise and similar spin trapping efficiency compared to DEPMPO.

Unique in vivo applications of spin traps

The ultimate goal of in vivo electron spin resonance (ESR) spin trapping is to provide a window to the characterization and quantification of free radicals with time within living organisms. However, the practical application of in vivo ESR to systems involving reactive oxygen radicals has proven challenging. Some of these limitations relate to instrument sensitivity and particularly to the relative stability of these radicals and their nitrone adducts, as well as toxicity limitations with dosing. Our aim here is to review the strengths and weaknesses of both traditional and in vivo ESR spin trapping and to describe new approaches that couple the strengths of spin trapping with methodologies that promise to overcome some of the problems, in particular that of radical adduct decomposition. The new, complementary techniques include: (i) NMR spin trapping, which monitors new NMR lines resulting from diamagnetic products of radical spin adduct degradation and reduction, (ii) detection of *NO by ESR with dithiocarbamate: Fe(II) "spin trap-like" complexes, (iii) MRI spin trapping, which images the dithiocarbamate: Fe(II)-NO complexes by proton relaxation contrast enhancement, and (iv) the use of ESR to follow the reactions of sulfhydryl groups with dithiol biradical spin labels to form "thiol spin label adducts," for monitoring intracellular redox states of glutathione and other thiols. Although some of these approaches are in their infancy, they show promise of adding to the arsenal of techniques to measure and possibly "image" oxidative stress in living organisms in real time.

Intra- and extracellular measurement of reactive oxygen species produced during heat stress in diaphragm muscle

Skeletal muscles are exposed to increased temperatures during intense exercise, particularly in high environmental temperatures. We hypothesized that heat may directly stimulate the reactive oxygen species (ROS) formation in diaphragm (one kind of skeletal muscle) and thus potentially play a role in contractile and metabolic activity. Laser scan confocal microscopy was used to study the conversion of hydroethidine (a probe for intracellular ROS) to ethidium (ET) in mouse diaphragm. During a 30-min period, heat (42 degrees C) increased ET fluorescence by 24 +/- 4%, whereas in control (37 degrees C), fluorescence decreased by 8 +/- 1% compared with baseline (P < 0.001). The superoxide scavenger Tiron (10 mM) abolished the rise in intracellular fluorescence, whereas extracellular superoxide dismutase (SOD; 5,000 U/ml) had no significant effect. Reduction of oxidized cytochrome c was used to detect extracellular ROS in rat diaphragm. After 45 min, 53 +/- 7 nmol cytochrome c. g dry wt(-1). ml(-1) were reduced in heat compared with 22 +/- 13 nmol. g(-1). ml(-1) in controls (P < 0.001). SOD decreased cytochrome c reduction in heat to control levels. The results suggest that heat stress stimulates intracellular and extracellular superoxide production, which may contribute to the physiological responses to severe exercise or the pathology of heat shock.

Surfactant treatment impairs gas exchange in a canine model of acute lung injury

The effectiveness of surfactant (SURF) treatment in acute lung injury in the adult is controversial. In this study, we tested the effectiveness of early surfactant treatment in a commonly used animal model of acute lung injury, phorbol-myristate acetate (PMA), to see if it would attenuate the progression of lung injury. We measured the effect on lung compliance and whether positive end-expiratory pressure (PEEP) (10 cm H2O) during SURF administration had a synergistic effect.

METHODS

Four groups of anesthetized dogs were studied: a) normals; b) PMA injury only; c) PMA injury + SURF; and d) PMA + SURF + PEEP. Lung injury was induced with 25-30 microg/kg of PMA. Responses were measured over 7 hrs. Surfactant was administered in the form of Survanta, 4 x 25 mg/kg doses via tracheal instillation 2.5 hrs after PMA. For the group receiving PEEP, 10 cm H2O PEEP was begun 1.5 hrs after PMA, 1 hr before SURF. Postmortem, the left lung was excised and inflated three times to total lung capacity (volume at 30 cm H2O) and expiratory compliance was measured with 25-100 mL volume increments. The trachea was then clamped and trapped volume was determined by water displacement.

RESULTS

PMA-induced lung injury significantly reduced expiratory compliance and total lung capacity (p < .05 from normal). Wet/dry lung weights did not differ between groups. SURF without PEEP further decreased lung compliance as compared with PMA only.

CONCLUSIONS

SURF administration after PMA injury causes marked reductions in lung compliance when no PEEP is coadministered. However, the loss of static expiratory lung compliance appears partly ameliorated by application of PEEP + SURF. Given that tracheal instillation of SURF is known to acutely elevate lung impedance in the first few hours after administration, coadministration of PEEP appears to be critically important in counteracting these early effects of surfactant instillation on the lung.

Increased susceptibility to pulmonary emphysema among HIV-seropositive smokers

BACKGROUND

Previous uncontrolled reports have suggested that HIV-seropositive persons develop an accelerated form of emphysema.

OBJECTIVE

To characterize the risk for emphysema in a stable HIV-seropositive outpatient population.

DESIGN

Controlled, cross-sectional analysis.

SETTING

Midwestern urban community.

PARTICIPANTS

HIV-seropositive persons (n = 114) without AIDS-related pulmonary complications and HIV-seronegative controls (n = 44), matched for age and smoking history.

MEASUREMENTS

Measurement of pulmonary function, bronchoalveolar lavage, and high-resolution computed tomography of the chest.

RESULTS

The incidence of emphysema was 15% (17 of 114) in the HIV-seropositive group compared with 2% (1 of 44) in the HIV-seronegative group (P = 0.025). The incidence of emphysema in participants with a smoking history of 12 pack-years or greater was 37% (14 of 38 persons) in the HIV-seropositive group compared with 0% (0 of 14 persons) in the HIV-seronegative group (P = 0.011). The percentage of cytotoxic lymphocytes in lavage fluid was much higher in HIV-seropositive smokers with emphysema.

CONCLUSIONS

Infection with HIV accelerates the onset of smoking-induced emphysema. The results of this study support the emerging concept that cytotoxic lymphocytes may have an important role in emphysema pathogenesis.

Oxidants and skeletal muscle function: physiologic and pathophysiologic implications

Previous studies have demonstrated that skeletal muscles generate considerable reactive oxygen during intense muscle contraction. However, the significance of this phenomenon and whether it represents normal physiology or pathology are poorly understood. Treatment with exogenous antioxidants suggests that normal redox tone during contraction is influencing ongoing contractile function, both at rest and during intense exercise. This could represent the influence of redox-sensitive proteins responsible for excitation-contraction coupling or redox-sensitive metabolic enzymes. Some conditions associated with intense exercise, such as local tissue hypoxia or elevated tissue temperatures, could also contribute to reactive oxygen production. Evidence that muscle conditioning results in upregulation of antioxidant defenses also suggests a close relationship between reactive oxygen and contractile activity. Therefore, there appears to be a significant role for reactive oxygen in normal muscle physiology. However, a number of conditions may lead to an imbalance of oxidant production and antioxidant defense, and these, presumably, do create conditions of oxidant stress. Ischemia-reperfusion, severe hypoxia, severe heat stress, septic shock, and stretch-induced injury may all lead to oxidant-mediated injury to myocytes, resulting in mechanical dysfunction.

NMR spin trapping: detection of free radical reactions using a phosphorus-containing nitrone spin trap

This study employs (31)P-nuclear magnetic resonance (NMR) to probe for changes in molecular structure arising from reactions between free radicals and a phosphorus-containing nitrone spin trap, 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). A number of biologically relevant free radical reactions were detected: a) reactions of DEPMPO with ( small middle dot)OH resulted in a new (31)P-NMR resonance at 27.05 ppm (shifted from the parent compound at 23.67 ppm); evidence suggests that this species is a diamagnetic hydroxy-pyrrolidone reduction product; b) (31)P-NMR spectra of DEPMPO/( small middle dot)CH(3) reactions resulted in peaks at 24.54, 30.83, and 32.31 ppm, while DEPMPO/( small middle dot)CH(2)OH produced peaks at 24.05, 30.80 and 32.52 ppm; in the presence of excess ascorbate, only resonances between 30 and 32 ppm were evident, which we have tentatively assigned to the hydroxylamine isomers of their respective adducts; and c) reaction of DEPMPO with O(2)( small middle dot-), produced by xanthine/xanthine oxidase or stimulated neutrophils, resulted in a single line, indistinguishable from DEPMPO/( small middle dot)OH reaction products. We conclude that NMR spin trapping is a useful approach for detecting free radical reaction pathways. It may have future applications for human free radical biology and imaging. Magn Reson Med 42:228-234, 1999.

The pathophysiology of pulmonary diffusion impairment in human immunodeficiency virus infection

Numerous reports have demonstrated that prior to the development of acquired immunodeficiency syndrome (AIDS)-related pulmonary complications, human immunodeficiency virus-positive (HIV+) individuals commonly develop unexplained reductions in pulmonary diffusing capacity (DLCO). The potential relevance of this observation is underscored by recent data demonstrating that reductions in DLCO independently predict the subsequent development of opportunistic pneumonia. To delineate the alterations in gas exchange associated with HIV, we investigated a group of HIV+ subjects with unexplained reductions in DLCO, using high-resolution computed tomography (HRCT) of the chest and a separation of diffusing capacity into its membrane (Dm) and capillary blood volume (Vc) components. We compared this abnormal group with HIV+ subjects with more normal gas exchange and also with a group of HIV- volunteers matched for age and smoking history. Compared with other groups, the HIV+ group with diffusion impairment demonstrated prominent reductions in Vc, despite a well-preserved total lung capacity (TLC). HRCT demonstrated virtually no evidence of interstitial fibrosis in any HIV+ subject, but evidence of early emphysema that significantly correlated with DLCO. Our results suggest that the previously reported impairment in pulmonary gas exchange in the HIV+ population involves loss of Vc and likely represents the development of early emphysema.

Focal air trapping in patients with HIV infection: CT evaluation and correlation with pulmonary function test results

OBJECTIVE

HIV-positive individuals commonly have symptoms of airway disease. We evaluated thin-section CT scans of HIV-infected individuals during inspiration and expiration for evidence of focal air trapping. We also correlated imaging findings with pulmonary function test results.

SUBJECTS AND METHODS

Fifty-nine subjects, 48 of whom were HIV-positive and 11 of whom were HIV-negative, underwent thin-section CT of the thorax during inspiration and expiration. All subjects also underwent pulmonary function tests. Two radiologists, who were unaware of the subjects' HIV status and smoking history and of the results of pulmonary function tests, evaluated the CT scans for the presence and severity of focal air trapping.

RESULTS

Expiratory CT revealed focal air trapping in 33 subjects: 30 were HIV-positive and three were HIV-negative (p = .0338). The mean values of forced expiratory volume in 1 sec (FEV1), forced mid expiratory flow, and diffusion capacity (DL(CO)) were significantly lower for subjects with focal air trapping (mean = 88.85, 84.52, and 80.80, respectively) than for those with normal findings on CT (mean = 100.84, 99.24, and 95.82, respectively; p = .001, p = .021, and p = .003, respectively). We found no significant differences in smoking history between HIV-positive and HIV-negative subjects. Severe air trapping on expiratory CT scans was seen in three subjects: All three had HIV infection, low CD4 counts, and abnormally decreased FEV1 and DL(CO) values.

CONCLUSION

Focal air trapping was a common finding on thoracic CT scans obtained during expiration in HIV-positive subjects. In addition, focal air trapping was associated with significantly lower FEV1, forced mid expiratory flow, and DL(CO) values than those found for subjects in whom CT revealed no focal air trapping. These results suggest that small airways disease may accompany a decline in pulmonary function in HIV-positive individuals.

Cigarette smoking in HIV infection induces a suppressive inflammatory environment in the lung

Lung lymphocyte numbers are frequently increased in human immunodeficiency virus (HIV)-infected individuals in the absence of lung infection, and may play a critical role in viral surveillance and protection against new infections. In this context, cigarette smoking by HIV-infected individuals has been associated with a relative increase in the peripheral blood CD4(+) T-lymphocyte count as compared with that of nonsmokers. Because lung defense is local, the aim of the present study was to determine whether cigarette smoking had a significant impact on local lung defenses in HIV-infected individuals. The numbers and subtypes of bronchoalveolar lymphocytes and the ability of lung lavage cells to produce proinflammatory cytokines were compared in 58 smokers and 34 nonsmokers. In contrast to a trend toward an increase in peripheral blood CD4(+) cell counts among nonsmokers, smokers had significant depressions in both the percentage and absolute numbers of CD4(+) and CD8(+) cells in their bronchoalveolar lavage fluid (BALF). A decrease in CD4(+)/CD8(+) cell ratios was also seen with smoking. In addition, production of both interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) was suppressed with cigarette smoking. These observations show that cigarette smoking is associated with suppression in localized lung defenses, and suggest that smoking cessation may have a positive impact on lung defenses in HIV-infected smokers.

Antioxidants protect rat diaphragmatic muscle function under hypoxic conditions

In hypoxia, mitochondrial respiration is decreased, thereby leading to a buildup of reducing equivalents that cannot be transferred to O2 at the cytochrome oxidase. This condition, called reductive stress, can paradoxically lead to enhanced formation of reactive O2 species, or a decrease in the ability of the cell to defend against an oxidative stress. We hypothesized that antioxidants would protect tissues under conditions of hypoxia. Rat diaphragm strips were incubated in tissue baths containing one of four antioxidants: N-acetyl-L-cysteine, dimethyl sulfoxide, superoxide dismutase, or Tiron. The strips were directly stimulated in an electrical field. Force-frequency relationships were studied under baseline oxygenation (95% O2-5% CO2), after 30 min of hypoxia (95% N2-5% CO2), and 30 min after reoxygenation. In all tissues, antioxidants markedly attenuated the loss of contractile function during hypoxia (P < 0.01) and also significantly improved recovery on reoxygenation (P < 0.05). We conclude that both intracellular and extracellular antioxidants improve skeletal muscle contractile function in hypoxia and facilitate recovery during reoxygenation in an in vitro system. The strong influence of antioxidants during hypoxic exposure suggests that they can be as effective in protecting cell function in a reducing environment as they have been in oxidizing environments.

Dithiothreitol improves recovery from in vitro diaphragm fatigue

There is increasing evidence that reactive oxygen species are produced during strenuous skeletal muscle work and that they contribute to the development of muscle fatigue. Although the precise cellular mechanisms underlying such a phenomenon remain obscure, it has been hypothesized that endogenously produced reactive oxygen species may down-regulate force production during fatigue by oxidizing critical sulfhydryl groups on important contractile proteins. To test this hypothesis, we fatigued rat diaphragm strips in vitro for 4 min at 20 Hz stimulation and a duty cycle of 0.33. Following fatigue, the tissue baths were drained and randomly replaced with either physiologic saline or physiologic saline containing the disulfide reducing agent, dithiothreitol (DTT) at varying doses (0.1-5.0 mM). Force-frequency characteristics were then measured over a 90-min recovery period. At the 0.5 and 1.0 mM doses, DTT treatment was associated with significantly greater force production in the recovery period. DTT's effects were observed at most frequencies tested, but appeared more prominent at the higher frequencies. The beneficial effects of DTT were not evident at the 0.1 or 5.0 mM doses and appeared to be specific for fatigued muscle. These recovery-enhancing effects of a potent disulfide reducing agent suggest that important contractile proteins may be oxidized during fatigue; such changes may be readily reversible.

Biological reactions of peroxynitrite: evidence for an alternative pathway of salicylate hydroxylation

Salicylate hydroxylation has often been used as an assay of hydroxyl radical production in vivo. We have examined here if hydroxylation of salicylate might also occur by its reaction with peroxynitrite. To test this hypothesis, we exposed salicylate to various concentrations of peroxynitrite, in vitro. We observed the hydroxylation of salicylate at 37 degrees C by peroxynitrite at pH 6, 7 and 7.5, where the primary products had similar retention times on HPLC to 2,3- and 2,5-dihydroxybenzoic acid. The product yields were pH dependent with maximal amounts formed at pH 6. Furthermore, the relative concentration of 2,3- to 2,5-dihydroxybenzoic acid increased with decreasing pH. Nitration of salicylate was also observed and both nitration and hydroxylation reaction products were confirmed independently by mass spectrometry. The spin trap N-t-butyl-alpha-phenylnitrone (PBN), with or without dimethyl sulfoxide (DMSO), was incapable of trapping the peroxynitrite decomposition intermediates. Moreover, free radical adducts of the type PBN/.CH3 and PBN/.OH were susceptible to destruction by peroxynitrite (pH 7, 0.1 M phosphate buffer). These results suggest direct peroxynitrite hydroxylation of salicylate and that the presence of hydroxyl radicals is not a prerequisite for hydroxylation reactions.

High-dose furosemide alters gas exchange in a model of acute lung injury

PURPOSE

Furosemide is often used to reduce edema in patients with acute respiratory distress syndrome (ARDS). It was hypothesized that furosemide would reduce lung water and improve gas exchange in a phorbol-myristate acetate (PMA) model of acute lung injury.

METHODS

Two groups of mongrel dogs received PMA (25 to 30 micrograms/kg) and continuous saline at 10 mL/kg/h; one group received PMA plus two 1-mg/kg doses of furosemide at 1 and 2 hours after PMA. Arterial blood gases on F1O2 = 1.0 and double-dilution lung water were measured at intervals over 7 hours.

RESULTS

In dogs receiving PMA+furosemide, AaDO2 and shunt fraction increased compared with dogs receiving PMA only (AaDO2, P = .014; shunt, P = .017). There were no significant differences between the groups in lung water (P = .34) during the experiment or in wet/dry weight postmortem. Urine flow was markedly reduced in both groups; the kidneys appeared unresponsive to the diuretic effects of furosemide. Significant elevations in hematocrit and pulmonary vascular resistance were seen in furosemide-treated compared with PMA-only dogs.

CONCLUSIONS

In this model of ARDS, which results in the absence of effective kidney function and multiple organ failure, furosemide compromises alveolar-capillary gas exchange and fails to influence the time course of lung water accumulation. The results suggest that the nondiuretic affects of furosemide cannot explain its purported clinical utility in ARDS.

N-tert-butyl-alpha-phenylnitrone: a free radical trap with unanticipated effects on diaphragm function

The spin trap N-tert-butyl-alpha-phenylnitrone (PBN) has a high avidity for free radical species and hence functions as an antioxidant in many biological systems. As such, we hypothesized that PBN would have powerful antioxidant effects on muscle function. We examined the effects of PBN on directly stimulated in vitro (37 degrees C) rat diaphragm. First, a dose-response curve for the effects of PBN on force frequency (n = 8) was established by comparing PBN-treated muscle strips (0.01-10 mM) with time- and stimulus-matched control strips. Second, the effect of 1.0 mM PBN on muscle endurance (n = 8) was established. Our findings were as follows. 1) Compared with baseline, peak twitch and low-frequency muscle tensions increased in a dose-dependent fashion, with peak effects at 1.0 mM PBN. 2) Muscle function at all stimulation frequencies was depressed at doses above 1.0 mM PBN. 3) Complete inhibition at 10 mM PBN was reversed with caffeine administration or washout. 4) During early fatigue, 1.0 mM PBN facilitated force. However, endurance time decreased in the PBN-treated group. We conclude that PBN has direct reversible dose-dependent effects on diaphragm function. However, facilitation of low-frequency forces and the lack of fatigue-attenuating properties suggest that PBN has atypical antioxidant effects on muscle function.

Clinical assessment of the respiratory muscles

This review examines approaches to evaluation of the respiratory muscles and describes new techniques that may be more quantitative, less effort dependent, and less invasive than conventional methods. To evaluate strength of the respiratory muscles, maximum inspiratory and expiratory pressures remain useful measures. Potential methodologic errors, however, necessitate careful technique. Evaluation of the twitch response to direct phrenic nerve stimulation may ultimately prove more quantitative and less effort dependent than measurements of maximum pressure. Many techniques are also available to measure endurance of respiratory muscles, but most are less than satisfactory outside the research environment because of poor reproducibility and other procedural difficulties. The maximum incremental resistive loading test, however, has proven to be practical and well tolerated. There is little substitute for careful clinical observation of respiratory muscle coordination and movement, particularly in the patient with suspected respiratory muscle weakness or chest wall distortion. In conclusion, though the respiratory muscles are difficult to evaluate, techniques are available that can be quite helpful for assessment, particularly in response to interventions such as rehabilitation.

Effects of N-acetylcysteine on in vitro diaphragm function are temperature dependent

Recent evidence has shown that systemic administration of N-acetylcysteine (NAC), a compound structurally similar to the intracellular antioxidant glutathione, inhibits skeletal muscle fatigue. To further elucidate the actions of NAC, we studied its effects on in vitro rat diaphragm contractile function. Rat diaphragm strips were incubated in tissue baths containing physiological salt solution (n = 29) or physiological salt solution containing 4 mg/ml of NAC (n = 29). Strips were stimulated by either indirect or direct means. After determination of baseline contractile characteristics, strips were fatigued for 4 min at 20 Hz (1 train/s, 0.33 ms train duration). Force-frequency relationships were then studied over a 60-min recovery period. We found that 1) NAC had significant effects on the baseline force-frequency relationship; treated strips had increased peak tension but diminished twitch tension and accelerated twitch kinetics; 2) NAC had significant fatigue-sparing effects that were magnified at 37 degrees C; and 3) NAC treatment did not improve postfatigue recovery. The effects of NAC were generally independent of the stimulation method. We conclude that NAC has direct temperature-dependent effects on diaphragm function. These effects are consistent with the properties of NAC as an antioxidant and suggest important but complex effects of oxidant stress on skeletal muscle.

Detection of free radicals in blood by electron spin resonance in a model of respiratory failure in the rat

Previous work has suggested that a free radical mechanism is involved in some types of muscle fatigue and that there can be free radicals released extracellularly. Because muscle fatigue may be an important factor in respiratory failure, the authors tested the hypothesis that increased concentrations of free radicals could be detected in the blood of animals undergoing severe resistive loading to respiratory failure. An ex vivo spin trapping technique with alpha-phenyl-N-tert-butylnitrone (PBN) was used to investigate the possible formation of free radicals in systemic blood samples by electron spin resonance (ESR) spectrometry. After 2.5-3 h of severe inspiratory resistive loading with 70% supplemental inspired oxygen, free radical levels in the form of PBN-adducts were found to rise significantly over the control group breathing room air and the control group breathing 70% oxygen (p < 0.05, N = 8). There were no significant differences between control groups breathing room air and control groups breathing 70% oxygen. This study presents direct evidence that free radicals are produced ex vivo and that they can be detected in the systemic circulation due to excessive resistive loading of the respiratory muscles.

Detection of free radicals by electron spin resonance in rat diaphragm after resistive loading

Indirect evidence supports free radical production in the diaphragm under excessive mechanical loads in both in vitro and in situ preparations. We hypothesized that free radicals are produced in the diaphragm with loads in vivo at a sufficient concentration to be detected by electron spin resonance (ESR) spectroscopy. Anesthetized rats underwent severe inspiratory resistive loading for 2.5-3 h with maintenance of blood oxygenation and arterial blood pressure by breathing 70% oxygen. The ESR spectra of four samples (freeze-clamped at liquid nitrogen temperature) from each experimental animal were compared with the spectra from a control animal breathing air and a control animal breathing 70% oxygen. We observed 1) an approximately 30% increase in intensity of free radical signal in experimental animals (n = 10) compared with control animals breathing oxygen (n = 10; P < 0.01) and control animals breathing air (n = 10; P < 0.05), 2) that oxygen alone had no effect on the ESR spectrum, and 3) the intensity of the ESR signal decreased approximately 25% in the experimental group when samples were taken 10 min postmortem, whereas no difference in signal was observed for control animals. We conclude that the diaphragm shows an increased production of free radicals associated with respiratory failure induced by resistive breathing.

High- vs low-intensity inspiratory muscle interval training in patients with COPD

This study determined the effect of a high vs low resistive inspiratory muscle interval training protocol on inspiratory muscle strength (PImax), incremental inspiratory threshold loading (Pitl), inspiratory muscle endurance (IE), and 12-minute distance test (12 MD) in severely impaired patients with COPD. We used a double-blind, two-group, repeated-measure design. Group 1 (n = 12) received supervised high resistive loading at approximately 52 percent PImax and group 2 (n = 8) received supervised low resistive loading at approximately 22 percent PImax. All subjects trained three times weekly (progressing from 5 min per session in week 1 to 18 min per session in week 12) for 12 weeks. After three practice sessions, measures of PImax, Pitl, IE, and 12 MD were taken at baseline, at 4-week intervals, and within 72 h of completing the protocol. Group 1 showed significant improvement in all four dependent variables while group 2 improved in Pitl, IE, and 12 MD. The results suggest there is no significant difference between high and low resistive interval training in more severely impaired patients with COPD.

Loss of diaphragm glutathione is associated with respiratory failure induced by resistive breathing

It has been suggested that oxidant stress may contribute to dysfunction of respiratory muscles undergoing severe work loads. We examined changes in glutathione content and redox status in the diaphragm and intercostal muscles of anesthetized Sprague-Dawley rats exposed to prolonged inspiratory resistive loading while breathing 70% O2. These results were compared with those from control groups breathing air or 70% O2. Changes in liver glutathione were also examined. Freeze-clamping and an enzymatic recycling assay were used. Results show that 1) in controls, glutathione content was higher in the diaphragm than in the intercostals, 2) severe hypercapnic acidosis without hypoxemia was present with loading, 3) total diaphragm glutathione decreased approximately 35% with no increase in glutathione oxidation with resistive breathing, whereas intercostal and liver glutathione remained unchanged, and 4) the drop in diaphragm glutathione correlated significantly with the drop in minute ventilation and the increase in arterial PCO2, whereas it was not directly related to intensity of respiratory muscle activity. In conclusion, although diaphragm susceptibility to oxidant stress may be increased with resistive breathing, it is unlikely that the modest decrease in total glutathione contributed significantly to respiratory failure in this model.

Improved pulmonary function and exercise tolerance with inspiratory muscle conditioning in children with cystic fibrosis

This study documented the effect of inspiratory muscle conditioning in children with cystic fibrosis. Subjects, ages 7 to 14 years, were divided into two groups. The experimental group (n = 10) trained at a high pressure load (> or = 29 cm H2O) and the control group (n = 10) trained at a minimal pressure load (< or = 15 cm H2O), using a threshold loading device. Subjects trained 30 min a day for 10 weeks. Pulmonary function, inspiratory muscle strength, and exercise tolerance were measured at the beginning and end of the training period. Pulmonary function was measured by body plethysmography. Inspiratory muscle strength was determined by standard measures of maximal inspiratory pressure against an occluded airway. Exercise tolerance was measured by the length of time subjects could walk on a treadmill. Findings indicated that the experimental group showed significant increases in inspiratory muscle strength, vital capacity, total lung capacity, and exercise tolerance in comparison to the control group.

Hydroxylation of salicylate by the in vitro diaphragm: evidence for hydroxyl radical production during fatigue

There is increasing evidence that oxygen-derived free radicals produced during strenuous work by the diaphragm may contribute to diaphragm fatigue and/or injury. However, the precise identity of these oxygen radicals remains unknown, inasmuch as oxygen free radicals are extremely short lived and their detection in biologic systems is quite difficult. There is recent evidence that the salicylate-trapping method may be a useful means of monitoring tissue production of hydroxyl radical (.OH). This method is predicated on the fact that salicylate's phenolic ring can be attacked by .OH at the 3 or 5 position to yield 2,3- or 2,5-dihydroxybenzoic acid (DHB). These metabolites are stable and can be identified by high-performance liquid chromatography (HPLC) coupled with electrochemical or ultraviolet detection. To test the hypothesis that hydroxylated salicylates are produced during diaphragm fatigue, we exposed in vitro rat diaphragm strips to a physiological saline solution containing 2.0 mM sodium salicylate for approximately 15 min. The solution was then removed, and the strips were fatigued (20 Hz, 200-ms train duration, 1 train/s) via phrenic nerve stimulation for 30 s-10 min. The diaphragm strips were subsequently homogenized, and the homogenate was analyzed by HPLC coupled with ultraviolet detection. Levels of 2,3-DHB were significantly higher in fatigued than in control nonfatigued strips. There was also a significant correlation between the amount of 2,3-DHB in the fatigued muscle and the accumulated tension-time product developed during fatigue. 2,5-DHB was not consistently identified in control or experimental strips.(ABSTRACT TRUNCATED AT 250 WORDS)

Tumor necrosis factor and endotoxin do not directly affect in vitro diaphragm function

Ventilatory pump failure can occur in the setting of severe infection. Recent in vivo studies have shown a significant decrease in diaphragm force production in rats with pneumococcal sepsis and sepsis secondary to Escherichia coli endotoxin. We hypothesized that diaphragm impairment during sepsis may be mediated by a direct effect of tumor necrosis factor-alpha (TNF) or endotoxin. To test this hypothesis we studied the mechanical characteristics of isolated rat diaphragm strips in tissue baths containing rTNF-alpha or endotoxin and compared the results with control strips. The strips were stimulated to contract isometrically in the tissue baths that were aerated with 95% O2-5% CO2. Baseline force-frequency determinations were made at 60 min. Following this, the strips were fatigued over a 4-min period (20 Hz, 0.33-s trains, 1 train/s) and force-frequency relationships determined 30 s, 10 min, and 60 min post-fatigue. There were no significant differences found between control and experimental strips in any aspect of contractile function tested, including force-frequency characteristics, fatiguability, and recovery from fatigue. Using an isolated cell line assay (L929), we found evidence of attenuated cytotoxicity of TNF at 26 degrees C compared with 37 degrees C. Therefore, we repeated the experiments studying the effects of TNF on in vitro muscle at 37 degrees C. We once again found no effect of TNF on contractile function. We conclude that the impairment of diaphragm function during sepsis is not mediated by a direct effect of TNF or endotoxin.

Marked pulmonary function abnormalities in a case of HIV-associated pulmonary hypertension

Recent reports have suggested an association between primary pulmonary hypertension and human immunodeficiency virus (HIV) infection. This appears to be an accelerated syndrome, associated with a relatively brief duration of symptoms, yet prominent right ventricular failure and severe pulmonary hypertension on presentation. We present a case of a primary pulmonary hypertension in a 35-year-old HIV-seropositive hemophiliac. His accelerated clinical course is consistent with previously reported cases of HIV-related pulmonary hypertension. However, this patient's pulmonary function tests revealed marked hyperinflation, a decreased diffusing capacity, and no airflow obstruction. To our knowledge, this very usual constellation of pulmonary function changes has not been described previously in this syndrome.

Preservation of sustainable inspiratory muscle pressure at increased end-expiratory lung volume

Previous studies in isolated muscles have shown that decreases in muscle length reduce the loss of force resulting from fatigue in response to repeated maximal stimulations. However, increases in end-expiratory lung volume (EEV), which presumably decrease the length of the inspiratory muscles, appear to make the inspiratory muscles more susceptible to fatigue. To address this paradox, we studied the influence of changes in EEV on inspiratory muscle fatigue resulting from repeated maximal voluntary inspirations for 15 min in normal humans. Tidal volume and breath timing were constant between runs. Fatigue runs were compared with atmospheric, positive or negative pressures applied to the mouth, sufficient to change EEV by approximately +30% or -20% of inspiratory capacity. Although the maximal initial pressure-time product for the inspiratory muscles (PTmus) was reduced by increased EEV, sustainable PTmus was not significantly affected. In contrast, both initial and sustainable pressure-time products for the diaphragm were reduced at elevated EEV. The rate at which the fatigue process developed was also reduced at increased EEV. There were no significant effects of decreased EEV on any measured pressures. We conclude that when EEV is elevated, within a moderate range, sustainable inspiratory muscle pressure is preserved. However, the contribution of the diaphragm to inspiratory pressure development during fatigue may be uniquely compromised by increased EEV.

Kinetics of CO uptake and diffusing capacity in transition from rest to steady-state exercise

In the transition from rest to steady-state exercise, O2 uptake from the lungs (VO2) depends on the product of pulmonary blood flow and pulmonary arteriovenous O2 content difference. The kinetics of pulmonary blood flow are believed to be somewhat faster than changes in pulmonary arteriovenous O2 content difference. We hypothesized that during CO breathing, the kinetics of CO uptake (VCO) and diffusing capacity for CO (DLCO) should be faster than VO2 because changes in pulmonary arteriovenous CO content difference should be relatively small. Six subjects went abruptly from rest to constant exercise (inspired CO fraction = 0.0005) at 40, 60, and 80% of their peak VO2, measured with an incremental test (VO2peak). At all exercise levels, DLCO and VCO rose faster than VO2 (P less than 0.001), and DLCO rose faster than VCO (P less than 0.001). For example, at 40% VO2peak, the time constant (tau) for DLCO in phase 2 was 19 +/- 5 (SD), 24 +/- 5 s for VCO, and 33 +/- 5 s for VO2. Both VCO and DLCO increased with exercise intensity but to a lesser degree than VO2 at all exercise intensities (P less than 0.001). In addition, no significant rise in DLCO was observed between 60 and 80% VO2peak. We conclude that the kinetics of VCO and DLCO are faster than VO2, suggesting that VCO and DLCO kinetics reflect, to a greater extent, changes in pulmonary blood flow and thus recruitment of alveolar-capillary surface area. However, other factors, such as the time course of ventilation, may also be involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Emphysema-like pulmonary disease associated with human immunodeficiency virus infection

OBJECTIVE

To describe a possible association between prolonged infection with human immunodeficiency virus (HIV) and a pathophysiologic process suggestive of pulmonary emphysema.

DESIGN

Case series.

SETTING

The Ohio State University Hospital, Columbus, Ohio.

MEASUREMENTS AND MAIN RESULTS

We describe four HIV-seropositive individuals ranging in age from 32 to 55 years who presented with dyspnea. Radiographic examination of the chest showed no infiltrates. All patients were presumed to have had prolonged HIV infection (mean CD4 count, 99.8 +/- 43 cells/mm3), but none had a previous history of pneumonia or opportunistic infections. Comprehensive examination of bronchoalveolar lavage fluid showed no pathogens or other complications of HIV infection. All patients had markedly abnormal pulmonary function tests that were suggestive of emphysema with air-trapping, hyperinflation, and a markedly decreased diffusing capacity. However, only minimal evidence of airflow obstruction was noted. Three patients subsequently had high-resolution computed tomographic scans of the chest that revealed emphysema-like bullous changes. Known causes of emphysema were not present in these patients.

CONCLUSIONS

Our findings support an association between prolonged HIV infection and an emphysema-like process. This syndrome may occur in the absence of previous pulmonary infections or apparent pulmonary complications and is characterized by unusual pulmonary function test abnormalities.

Muscle shortening increases sensitivity of fatigue to severe hypoxia in canine diaphragm

The effects of inspired O2 on diaphragm tension development during fatigue were assessed using isovelocity (n = 6) and isometric (n = 6) muscle contractions performed during a series of exposures to moderate hypoxia [fraction of inspired O2 (FIO2) = 0.13], hyperoxia (FIO2 = 1), and severe hypoxia (FIO2 = 0.09). Muscle strips were created in situ from the canine diaphragm, attached to a linear ergometer, and electrically stimulated (30 Hz) to contract (contraction = 1.5 s/relaxation = 2 s) from optimal muscle length (Lo = 8.9 cm). Isovelocity contractions shortened to 0.70 Lo, resulting in a mean power output of 210 mW/cm2. Fatigue trials of 35 min duration were performed while inspired O2 was sequentially changed between the experimental mixtures and normoxia (FIO2 = 0.21) for 5-min periods. In this series, severe hypoxia consistently decreased isovelocity tension development by an average of 0.1 kg/cm2 (P less than 0.05), which was followed by a recovery of tension (P less than 0.05) on return to normoxia. These responses were not consistently observed in isometric trials. Neither isovelocity nor isometric tension development was influenced by moderate hypoxia or hyperoxia. These results demonstrate that the in situ diaphragm is relatively insensitive to rapid changes in O2 supply over a broad range and that the tension development of the shortening diaphragm appears to be more susceptible to severe hypoxia during fatigue. This may reflect a difference in either the metabolic or blood flow characteristics of shortening contractions of the diaphragm.

Sustainable inspiratory pressures over varying flows, volumes, and duty cycles

This study identifies the influence of flow (0.5-2.0 l/s), duty cycle (0.29-0.57), and tidal volume (1.08-2.16 liters) on sustainable inspiratory muscle pressure (Pmus) and transdiaphragmatic pressure (Pdi) development. Six normal humans performed endurance tests using an isoflow method, which allowed for measurements of maximum dynamic Pmus and Pdi, with controlled lung inflation. The subjects repeated maximum dynamic voluntary inspirations for 10 min. Pressures dropped exponentially from initial measurements at rest (Pmusi or Pdi) to sustainable values (Pmus or Pdis). As flow and tidal volume increased, maximum initial and sustainable pressures decreased significantly. However, at a constant duty cycle, the sustainable dynamic pressures remained predictable fractions of initial dynamic pressures (i.e., Pmuss/Pmusi or Pdis/Pdii), regardless of changes in flow and tidal volume. In contrast, as duty cycle increased, the sustainable fractions significantly decreased for both Pdi and Pmus. For example, at a duty cycle of 0.29, Pmuss/Pmusi was approximately 0.71, and at a duty cycle of 0.57, Pmuss/Pmusi was approximately 0.62. Calculated sustainable pressure-time indexes varied significantly between 0.16 to 0.32 for Pmus and 0.11 to 0.22 for Pdi over the breathing patterns studied. We conclude that 1) the maximum dynamic pressure that can be sustained at a given duty cycle is a predictable fraction of the maximum dynamic pressure that can be generated at rest when measured under the same conditions of inspiration and 2) the sustainable fraction of initial dynamic pressure significantly decreases with increasing duty cycle.

Increased fatigue of isovelocity vs. isometric contractions of canine diaphragm

A comparison of fatigue as a loss of force with repeated contractions over time was performed in canine respiratory muscle by isometric (nonshortening) and isovelocity (shortening) contractions. In situ diaphragm muscle strips were attached to a linear ergometer and electrically stimulated (30 or 40 Hz) via the left phrenic nerve to produce either isometric (n = 12) or isovelocity (n = 12) contractions (1.5 s) from optimal muscle length (Lo = 8.8 cm). Similar velocities of shortening between isovelocity experiments [0.19 +/- 0.02 (SD) Lo/S] were produced by maximizing the mean power output (Wmax = 210 +/- 27 mW/cm2) that could be developed over 1.5 s when displacement was approximately 0.30 Lo. Initial peak isometric tension was 1.98 kg/cm2, whereas initial peak isovelocity tension was 1.84 kg/mc2 (P less than 0.01) or 93% of initial isometric tension. Fatigue trials of 5 min were conducted on muscles contracting at a constant duty cycle (0.43). At the end of the trials, peak isovelocity tension had fallen to 50% of initial isometric tension (P less than 0.01), whereas peak isometric tension had only fallen by 27%. These results indicate that muscle shortening during force production has a significant influence on diaphragm muscle fatigue. We conclude that the effects of shortening on fatigue must be considered in models of respiratory muscle function, because these muscles typically shorten during breathing.

Hyperoxia and moderate hypoxia fail to affect inspiratory muscle fatigue in humans

Normal human subjects (n = 7) breathing 21% O2 (normoxia), 13% O2 (hypoxia), or 100% O2 (hyperoxia) performed repeated maximal inspiratory maneuvers (inspiratory duration = 1.5 s, total breath duration = 3.5 s) on an "isoflow" system, which delivered a constant mouth flow (1.25 or 1 l/s) while maintaining normocapnia (5.5% end-tidal CO2). Respective mean arterial O2 saturation values (ear lobe oximetry) were 98 +/- 1, 91 +/- 4 (P less than or equal to 0.01), and 99 +/- 1% (NS). Maximal mouth pressure (Pm) was measured during inspirations at rest and during a 10-min fatigue trial, and the Pm measurements obtained during the fatigue trials were fit to an exponential equation. The parameters of the equation included the time constant (tau), which describes the rate of decay of Pm from the initial pressure (Pi) to the asymptote, or "sustainable" pressure (Ps). The mean fraction of Pm remaining at the end of the fatigue trials (Ps/Pi) was 63 +/- 5%. No significant differences in Pi, Ps, or tau were observed between O2 treatments. This suggests that fatigue of the inspiratory muscles in normal humans occurs by a mechanism that is insensitive to changes in blood O2 content that occur during inspiration of O2 in the range of 13-100%.

Accelerated decay of inspiratory pressure during hypercapnic endurance trials in humans

Seven normal human subjects inspired a CO2-O2 mixture from a constant-flow generator while performing maximal inspiratory maneuvers from functional residual capacity. End-tidal CO2 (ETCO2) was maintained at either 5.5 (normocapnia), 3.5 (hypocapnia), or 7% (hypercapnia) on separate testing days. Subjects attained maximal mouth pressure (Pm) while breathing at either 1.25 or 1 l/s, utilizing a fixed breathing pattern (duty cycle 0.43) with an inspiratory time of 1.5 s. Maximal Pm was measured at rest and then during a 10-min endurance trial in which subjects repeated maximal voluntary inspirations with constant flow and breathing pattern. The endurance Pm data were fit to nonlinear exponential regression. The results indicated that 1) maximal Pm at rest was unaffected by changing ETCO2; 2) the rate of Pm decay over time was accelerated by hypercapnia, whereas hypocapnia showed no consistent effects; and 3) "sustainable" Pm, attained toward the end of the endurance trial, was not decreased; therefore sustainable force output was preserved in response to changing ETCO2.

Fatigue of the inspiratory muscle pump in humans: an isoflow approach

A new method is described for measurement of inspiratory muscle endurance in humans that is based on isokinetic principles of muscle testing (i.e., measurement of maximum force during a constant velocity of shortening). Subjects inspired maximally while their lungs were inflated at a constant rate during each breath for 10 min. Inspiratory and expiratory time, flow rate, tidal volume, and end-tidal CO2 were maintained constant. In each subject, maximum inspiratory mouth pressure exponentially decayed over the first few minutes to an apparent sustainable value. Repeated tests in experienced subjects showed high reproducibility of sustainable pressure measurements. To determine the effects of flow, endurance tests were repeated in four subjects at flows of 0.75, 1.0, and 1.25 l/s, with a constant duty cycle. As flow increased, the maximum pressures that could be attained at rest and the maximum sustainable pressures decreased. At each flow, the sustainable pressure remained a constant fraction of the maximum pressure attainable at rest. We interpret the decay in mouth pressure during isoflow endurance tests to directly reflect the loss of net inspiratory muscle force available by maximum voluntary activation of the inspiratory pump.

Effects of swim training on lung volumes and inspiratory muscle conditioning

Lung volumes and inspiratory muscle (IM) function tests were measured in 16 competitive female swimmers (age 19 +/- 1 yr) before and after 12 wk of swim training. Eight underwent additional IM training; the remaining eight were controls. Vital capacity (VC) increased 0.25 +/- 0.25 liters (P less than 0.01), functional residual capacity (FRC) increased 0.39 +/- 0.29 liters (P less than 0.001), and total lung capacity (TLC) increased 0.35 +/- 0.47 (P less than 0.025) in swimmers, irrespective of IM training. Residual volume (RV) did not change. Maximum inspiratory mouth pressure (PImax) measured at FRC changed -43 +/- 18 cmH2O (P less than 0.005) in swimmers undergoing IM conditioning and -29 +/- 25 (P less than 0.05) in controls. The time that 65% of prestudy PImax could be endured increased in IM trainers (P less than 0.001) and controls (P less than 0.05). All results were compared with similar IM training in normal females (age 21.1 +/- 0.8 yr) in which significant increases in PImax and endurance were observed in IM trainers only with no changes in VC, FRC, or TLC (Clanton et al., Chest 87: 62-66, 1985). We conclude that 1) swim training in mature females increases VC, TLC, and FRC with no effect on RV, and 2) swim training increases IM strength and endurance measured near FRC.

A simple dosimeter for bronchial provocation testing using a solid-state electronic timing module

In conducting inhalational challenge tests of airway responsiveness to methacholine, we prefer to deliver the challenge aerosol intermittently, using a dosimeter that turns the nebulizer on for about 0.5 to 0.6 second to deliver each "puff." To avoid the considerable expense of commercially available dosimeters, we constructed our own electronically controlled device, which, in conjunction with a DeVilbiss Model 646 nebulizer, is easily calibrated and permits reproducible and precise timing of challenge puffs. The device was constructed of readily available components and cost us only $100. An experienced technician can use it to trigger on the nebulizer within 0.5 second of the start of inspiration, and in more than one year's experience we have found the system to be completely satisfactory and reliable.

Effects of breathing pattern on inspiratory muscle endurance in humans

Endurance of the inspiratory muscles was measured in normal volunteers using a threshold resistance that produced a relatively constant mouth-pressure load, independent of inspiratory flow rate (VTI). Breathing pattern was controlled by visual feedback from an oscilloscope. Endurance was measured as the length of time (Tlim) a target VTI could be maintained with maximum effort. Effects of changes in breathing pattern on Tlim were compared with control measurements made the same day. Increases in VTI or in duty cycle (inspiratory time/total period) shortened Tlim, whereas decreases lengthened Tlim. However, effects of changes in VTI were less than equivalent changes in tidal volume produced by alterations in duty cycle. Furthermore, when two breathing pattern changes were altered simultaneously to keep the rate of external inspiratory work (Winsp) constant, significant effects due to changes in duty cycle were still observed. In conclusion, 1) both VTI and duty cycle have significant effects on measurements of inspiratory muscle endurance and 2) the effects of VTI are less than the effects of duty cycle for the same Winsp.

Effect of sinusoidal forcing of ventilatory volume on avian breathing frequency

Awake chickens were unidirectionally ventilated at 3.6 l . min-1 with 3.2-4.8% CO2 in air. The air sacs on each side were made confluent and implanted with exit tubes connected to the following three devices: 1) a system of constant-flow generators which remove air at exactly the same rate that it entered the trachea, allowing no port for spontaneous volume changes; 2) a sinusoidal pump to force volume changes in the chicken; and 3) a pressure transducer to record air sac pressure, which reflected the sum of two pressure components, the passive pressure changes created by the pump and the active pressure changes due to breathing efforts. Over a range of pump frequencies, the amplitude of measured air sac pressure changes varied inversely with frequency. Above and below this range, pressure showed a beat pattern, indicating a difference in the frequencies of the two pressure components. Within the range lacking a beat pattern, breathing movements and the pump stroke had the same frequency. This range was greater at increased stroke volume. Breathing efforts worked with the pump at the high end of the range and against the pump at the low end. These findings show further evidence of the presence of a response to volume forcing and fit a previously described volume threshold model.

Inspiratory muscle conditioning using a threshold loading device

We demonstrate the effectiveness of a new conditioning technique for increasing the strength and endurance of the inspiratory muscles. The technique employs a threshold loading device which allows for maximization of exercise intensity with a minimum of exercise duration. After ten weeks, with approximately 25 minutes of exercise time per week, four test subjects showed an average increase in maximum inspiratory pressure (PImax) of 50 (+/- 9 SD) cm H2O (p less than 0.02), whereas four control subjects undergoing submaximal inspiratory muscle exercise showed no significant change. The time the test subjects could endure 65 percent of their prestudy PImax increased from an average of 3.58 +/- 1.65 SD min to over 10 min in all four subjects. No significant change was seen in control subjects. Further testing showed the test subjects could endure 100 percent of their prestudy PImax after conditioning for an average duration 5.15 +/- 1.65 min. This technique should be useful for conditioning the inspiratory muscles in subjects with pulmonary disease.

Ventilatory pressure loading at constant pulmonary FCO2 in Gallus domesticus

Seven White Leghorn roosters were unidirectionally ventilated at constant flows and CO2 concentrations. The birds were awake and stood or crouched in a plethysmograph. A servo system clamped the pressure in the air sacs at constant values from -10 to +10 cm H2O in 2 cm H2O increments. Therefore, the animals could inflate or deflate the air sacs with breathing movements without affecting intrapulmonary pressures. Decreasing air sac pressure less than atmospheric caused inspiratory duration (TI), expiratory duration (TE), total period (TTOT) and tidal volume (VR) to decrease, and the ratio, TI/TE to increase. Increasing air sac pressures to 6 cm H2O above atmospheric caused, TE to increase, TI and TI/TE to decrease and VT and TTOT to change very little. After bilateral vagotomy air sac pressure changes caused little or no changes in TI, TE, TTOT or TI/TE, but produced percentage changes in VT similar to before vagotomy. Comparison of end expiratory volumes with apneic volumes (produced by lowering CO2 in the insulfating gas) over the range of air sac pressures clamped shows: (1) chickens actively exhale at pressures as low as -10 cm H2O, and (2) the change of mean air sac volume due to imposed pressure is less during breathing than during apnea. These findings, we believe, are due to a reflex initiated by mechanoreceptors with projections in the vagus nerves.

Effects of hypercapnia on Breuer-Hering threshold for inspiratory termination

The effects of CO2 concentration on the timing of inspiratory duration (TI) and expiratory duration (TE) and the responses to lung inflation were studied in decerebrate paralyzed cats. With lung volume held at functional residual capacity during the breath cycle, hypercapnia (fractional concentration of inspired CO2 = 0.04) caused variable changes in TI and significant increases in TE. To obtain the Breuer-Hering threshold relationship [tidal volume (VT) vs. TI] and the timing relationship between TE and the preceding TI (TE vs. TI), ramp inflations of various sizes were used to terminate inspiration at different times in the breath cycle. Hypercapnia caused the VT vs. TI curves to shift in an upward direction so that at higher lung volumes TI was lengthened. Also, the slope of the TE vs. TI relationship was increased. The results suggest that hypercapnia diminished the sensitivity of the Breuer-Hering reflex to the lung volume, thus allowing volume to increase with little effect on TI. In addition, TE appears to become more sensitive to changes in the preceding TI. A model is presented which provides a possible neural mechanism for these responses.

Ventilatory phase duration in the chicken: role of mechanical and CO2 feedback

Awake upright White Leghorn roosters (Gallus domesticus) were unidirectionally ventilated. Electromyographic activity from inspiratory and expiratory muscles was recorded to demarcate inspiration and expiration. During inspiration, the rate of inflation of the air sac system was varied while the CO2 concentration of the gas passing through the lungs was maintained constant. Inspiratory duration was inversely related to the rate of inflation, producing an inspiratory volume-time threshold (VT) curve with a negative slope. When the CO2 concentration was increased in the lungs, the inspiratory VT curve shifted to the right with a concurrent increase in slope. If the rate of deflation was varied during expiration, it was found that expiratory duration was inversely related to the rate of deflation, producing an expiratory VT curve with a positive slope. Increasing the CO2 concentration shifted the curve to the left with an increased slope. These results indicate that inspiratory and expiratory phase durations are a function of both mechanical and chemical feedback.