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.