Severe Chronic Airways Obstruction Research Articles

Long-term oxygen therapy in COPD: what is the evidence?

Thomas L Petty was the founder of modern long-term oxygen therapy (LTOT).1 In 1967, Petty's group published the seminal study of six patients with severe chronic airway obstruction, in whom LTOT improved clinical status, exercise capacity, secondary erythrocythemia, and pulmonary hypertension.2 Notably, all patients had severe hypoxaemia (partial pressure of oxygen [PaO2] 41·5–46·5 mm Hg) and four patients were severely hypercapnic (partial pressure of carbon dioxide [PaCO2] 55·0–61·5 mm Hg). Thereafter, two randomised controlled trials that were initiated in the 1970s established a clear survival benefit related to LTOT in patients with chronic obstructive pulmonary disease (COPD) with severe resting hypoxaemia: the Nocturnal Oxygen Therapy Trial (NOTT)3 and the British Medical Research Council (BMRC) trial.

Repeatability of exhaled nitric oxide measurements in patients with COPD

The assessment of the presence of eosinophilic airway inflammation may help in predicting the steroid response in subjects with respiratory symptoms. Unlike patients with asthma, only a subset of patients with chronic obstructive pulmonary disease (COPD) benefits from steroid treatment. Fractional exhaled nitric oxide (FENO) is a useful surrogate marker for eosinophilic airway inflammation, but data on the repeatability of FENO measurements in COPD needed for the assessment of significant change are insufficient. The aim of this study was to assess the short-term repeatability of FENO measurement in subjects with moderate to very severe chronic airway obstruction compared to that in healthy subjects. We studied 20 patients with stable COPD and 20 healthy subjects, and determined FENO (flow rate 50 ml s−1) three times: at baseline, 10 min and 24 h after baseline. Spirometry was performed on the first study day after the FENO measurements. The median FENO concentration in patients with COPD was 15·6 ppb, and in healthy subjects, 15·2 ppb. The coefficient of variation (CoV) for 24-h measurements was 12·4% in COPD patients, and 15·9% in healthy subjects. Among COPD patients with global initiative for chronic obstructive lung disease stage 2 disease, the CoV was 13·7%, and among those with stage 3–4 disease, 10·5%. The findings indicate that the short-term repeatability of FENO measurement in patients with moderate to very severe COPD is equally good as in healthy subjects. A change in FENO exceeding 24% is likely to reflect a minimum measurable change in COPD.

Wirkung von Almitrin und Sauerstoff auf die arteriellen Blutgase bei chronischer respiratorischer Insuffizienz

In patients with respiratory failure, almitrine causes an increase in arterial O2 partial pressure (paO2) by stimulating peripheral chemoreceptors. Since such patients often require long-term O2 therapy inhibiting peripheral chemoreceptor activity, the effect on arterial blood gases by almitrine (100 mg orally) and increasing amounts of nasally supplied O2 was studied. In ten patients with severe chronic airways obstruction and pulmonary emphysema, almitrine produced a mean increase of paO2 by 5.2 mm Hg both while breathing normal air and during nasal administration of O2 (0.5-2.0 l/min), whereas paCO2 levels remained constant. In 4 patients with pulmonary fibrosis almitrine did not change the paO2. These results show that patients with chronic airways obstruction and pulmonary emphysema respond to a single dose of 100 mg almitrine by an increase of paO2 equivalent to that achieved by administering 1.0 l/min of O2 via the nose.

Cardioselective beta-blockers for chronic obstructive pulmonary disease.

Beta-blocker therapy has a proven mortality benefit in patients with hypertension, heart failure and coronary artery disease, as well as during the perioperative period. These drugs have traditionally been considered contraindicated in patients with chronic obstructive pulmonary disease (COPD). To assess the effect of cardioselective beta-blockers on respiratory function of patients with COPD. A comprehensive search of the Cochrane Airways Group Specialised Register (derived from systematic searches of CENTRAL, MEDLINE, EMBASE and CINAHL) was carried out to identify randomised blinded controlled trials from 1966 to May 2005. We did not exclude trials on the basis of language. Randomised, blinded, controlled trials of single dose or longer duration that studied the effects of cardioselective beta-blockers on the forced expiratory volume in 1 second (FEV1) or symptoms in patients with COPD. Two independent reviewers extracted data from the selected articles, reconciling differences by consensus. Two interventions studied were the administration of beta-blocker, given either as a single dose or for longer duration, and the use of beta2-agonist given after the study drug. Eleven studies of single-dose treatment and 9 of treatment for longer durations, ranging from 2 days to 12 weeks, met selection criteria. Cardioselective beta-blockers, given as a single dose or for longer duration, produced no change in FEV1 or respiratory symptoms compared to placebo, and did not affect the FEV1 treatment response to beta2-agonists. A subgroup analysis revealed no change in results for those participants with severe chronic airways obstruction or for those with a reversible obstructive component. Cardioselective beta-blockers, given to patients with COPD in the identified studies did not produce adverse respiratory effects. Given their demonstrated benefit in conditions such as heart failure, coronary artery disease and hypertension, cardioselective beta-blockers should not be routinely withheld from patients with COPD.

Effect of formoterol/budesonide combination on arterial blood gases in patients with acute exacerbation of COPD

Patients with severe chronic airway obstruction might suffer dangerous hypoxemia after administration of a beta-agonist despite bronchodilation. We first compared the acute effects on gas exchange of two doses of formoterol Turbuhaler (9 and 18 microg) in 10 patients with acute exacerbation of COPD. Afterwards, we compared the acute effects of formoterol Turbuhaler 9 microug with those of formoterol/budesonide combination in a single inhaler (Turbuhaler) 9/320 microg in 10 other patients with acute exacerbation of COPD. Finally, we compared the changes in PaO(2) induced by formoterol Turbuhaler 9 microg or formoterol/budesonide combination in a single inhaler (Turbuhaler) 9/320 microg with those in FEV(1) in 10 other patients with acute exacerbation of COPD. Each agent was given on separate days, and the patients' arterial blood gases were measured at baseline and at intervals of 120 min. Small but statistically significant declines in PaO(2) were found after administration of both formoterol 9 and 18 microg. In the second group of patients, formoterol 9 microg alone again induced a significant decrease in PaO(2). However, the simultaneous administration of budesonide 320 microg significantly reduced the acute effect of formoterol on PaO(2). In a third group of 10 patients we confirmed a small but significant decrease in PaO(2) after formoterol alone and the reduction of this effect when budesonide was administered simultaneously. Moreover, we also documented that addition of budesonide amplified the fast onset of action of formoterol. These results suggest that when treating patients suffering from acute exacerbation of COPD with formoterol, it is prudent to check their arterial blood gases. In any case, combined administration of formoterol and budesonide reduces the potential for acute effects of formoterol on blood-gas tensions.

Energetic cost of breathing, body composition, and pulmonary function in horses with recurrent airway obstruction.

This study was conducted to determine whether horses with naturally occurring, severe chronic recurrent airway obstruction (RAO) 1). have a greater resting energy expenditure (REE) than control horses, 2). suffer body mass depletion, and 3). have significantly decreased REE after bronchodilation and, therefore, also 4). whether increased work of breathing contributes to the cachexia seen in some horses with RAO. Six RAO horses and six control horses underwent indirect calorimetric measures of REE and pulmonary function testing using the esophageal balloon-pneumotachograph method before and after treatment with ipratropium bromide, a parasympatholytic bronchodilator agent, at 4-h intervals for a 24-h period. Body condition scoring was performed, and an estimate of fat mass was determined via B-mode ultrasonography. O(2) and CO(2) fractions, respiratory airflow, respiratory rate, and pleural pressure changes were recorded, and O(2) consumption, CO(2) production, REE, pulmonary resistance, dynamic elastance, and tidal volume were calculated. In addition, we performed lung function testing and calorimetry both before and after sedation in two control horses. RAO horses had significantly lower body condition scores (2.8 +/- 1.0 vs. 6.4 +/- 1.2) and significantly greater O(2) consumption than controls (4.93 +/- 1.30 vs. 2.93 +/- 0.70 ml.kg(-1).min(-1)). After bronchodilation, there was no significant difference in O(2) consumption between RAO horses and controls, although there remained evidence of residual airway obstruction. There was a strong correlation between O(2) consumption and indexes of airway obstruction. Xylazine sedation was not associated with changes in pulmonary function but did result in markedly decreased REE in controls.

Nutritional Status and Muscle Strength in Patients with Emphysema and Severe α1-Antitrypsin Deficiency

The association between emphysema and weight loss is well known. Severe alpha(1)-antitrypsin deficiency is an important risk factor for the development of emphysema. To study nutritional status and muscle strength in patients with severe alpha(1)-antitrypsin deficiency and emphysema. Fifteen alpha(1)-antitrypsin-deficient patients with emphysema (7 men) and 18 healthy control subjects (9 men) were included in the study. Total body protein (TBP) was measured by in vivo neutron activation analysis of nitrogen. Lean body mass (LBM) was estimated from measurement of total body potassium. In analogy with body mass index (BMI), TBP index and LBM index were calculated as TBP/height squared and LBM/height squared, respectively. Respiratory muscle strength was studied by maximal inspiratory pressure (PImax) and maximal expiratory pressure (PEmax), and skeletal muscle strength by handgrip test. Plasma albumin, transthyretin, and retinol-binding protein concentrations were analyzed as biochemical markers of nutritional status. In the alpha(1)-antitrypsin-deficient individuals, lung function test results were consistent with severe chronic airway obstruction, whereas the healthy control subjects had normal lung function. No significant differences were found in age, body weight, or BMI between the groups. TBP (p < 0.05), TBP index (p < 0.001), LBM index (p < 0.05), and plasma concentration of transthyretin (p < 0.01) were significantly lower in the patients than in the control subjects. There was a significant correlation between TBP and LBM (p < 0.001), and between TBP and body weight (p < 0.001). In the male subgroup, PImax (p < 0.05) and PEmax (p < 0.05) were significantly lower in the patients than in the control subjects. In the female subgroup, handgrip strength was significantly lower in the patients than in the control subjects (p < 0.05). Body weight was significantly correlated with handgrip test (p < 0.05) in the male patients. In the female patients, body weight was significantly correlated with PImax (p < 0.05), LBM with PEmax (p < 0.05), and LBM with handgrip test (p < 0.01). Reduced TBP and plasma transthyretin concentration in alpha(1)-antitrypsin-deficient patients with emphysema may indicate early signs of malnutrition.

Cardioselective beta-blockers for chronic obstructive pulmonary disease.

Beta-blocker therapy has a proven mortality benefit in patients with hypertension, heart failure and coronary artery disease, as well as during the perioperative period. These drugs have traditionally been considered contraindicated in patients with chronic obstructive pulmonary disease (COPD). To assess the effect of cardioselective beta-blockers on respiratory function of patients with COPD. A comprehensive search of EMBASE, MEDLINE and CINAHL was performed using the Cochrane Airways Group registry to identify randomised blinded controlled trials from 1966 to May 2001. The search was completed using the terms: asthma*, bronchial hyperreactivity*, respiratory sounds*, wheez*, obstructive lung disease* or obstructive airway disease*, and adrenergic antagonist*, sympatholytic* or adrenergic receptor block*. We did not exclude trials on the basis of language. Randomised, blinded, controlled trials of single dose or longer duration that studied the effects of cardioselective beta-blockers on the forced expiratory volume in 1 second (FEV1) or symptoms in patients with COPD. Two independent reviewers extracted data from the selected articles, reconciling differences by consensus. Two interventions studied were the administration of beta-blocker, given either as a single dose or for longer duration, and the use of beta2-agonist given after the study drug. Eleven studies of single-dose treatment and 8 of treatment for longer duration, ranging from 2 days to 12 weeks, met selection criteria. Cardioselective beta-blockers produced no statistically significant change in FEV1 or respiratory symptoms compared to placebo, given as a single dose (Weighted Mean Difference -2.05% [95% Confidence interval, -6.05 to 1.96%]) or for longer duration (WMD -2.55% [95% CI, -5.94 to 0.84]), and did not significantly affect the FEV1 treatment response to beta2-agonists. Exacerbations and hospitalizations were recorded in all trials, but none occurred during the periods of study, in either group. A subgroup analysis revealed no significant change in results for those participants with severe chronic airways obstruction or for those with a reversible obstructive component. The available evidence suggests that cardioselective beta-blockers, given to patients with COPD do not produce a significant short-term reduction in airway function or in the incidence of COPD exacerbations. However, the trials were small and of short duration. Given their demonstrated benefit in conditions such as heart failure, coronary artery disease and hypertension, cardioselective beta-blockers should be considered for patients with COPD, but administered with careful monitoring since data concerning long term administration and their effects during exacerbations are not available.

Interrupter resistance in preschool children: measurement characteristics and reference values.

There is a need for quick, reliable, and noninvasive lung function tests to assess airway obstruction in preschool children both for pediatric pulmonary care as well as for research purposes. We studied feasibility, reproducibility, and validity of measurements of the respiratory system using the interrupter technique (interrupter resistance [Rint]) and obtained reference values in children from a general population, 2 to 7 yr of age. Accuracy was studied by comparisons of Rint with plethysmographic airway resistance (Raw) in 20 patients (7 to 14 yr) with mild to severe chronic airways obstruction and was satisfactory in patients with FEV(1) > 60% predicted. The technique proved sensitive enough to detect changes in airway caliber within a small group of 12 children who developed mild respiratory tract infections. Among children from a general population, subgroups with mild respiratory symptoms or mild respiratory disease had higher mean Rint values. Airway obstruction was better detected using expiratory rather than inspiratory interruptions, both programmed at peak tidal ventilatory flow. Reproducibility within subjects was satisfactory (intraclass correlation 0.82 and 0.79). The same applied to interobserver agreement (intraclass correlation 0.98). The interrupter technique proves to be a reliable and practical test of airway function, suitable for clinical and epidemiologic studies in preschool children.

Difference between functional residual capacity and elastic equilibrium volume in patients with chronic obstructive pulmonary disease.

A study was performed to determine the elastic equilibrium volume (Vr) of the respiratory system in patients with chronic obstructive pulmonary disease (COPD). Voluntary relaxed expiration from total lung capacity (TLC) was studied in three groups of subjects: seven patients with severe chronic airways obstruction (COPD), 10 normal subjects, and 15 subjects with restrictive disease. In the normal subjects and the patients with restrictive disease voluntary relaxed expiration from TLC stopped close to end tidal volume (FRC) and the volume expired in this manoeuvre was less than that expired in a slow vital capacity manoeuvre (SVC). In the patients with COPD the voluntary relaxed expiration continued beyond the end tidal volume (FRC) and the volume expired was not different from the SVC. Oesophageal (pleural) pressures and surface diaphragmatic EMG recordings in the patients with COPD supported the premise that relaxation was achieved. In patients with COPD, end tidal volume (FRC) is higher than the elastic equilibrium volume, Vr, of the respiratory system. This is in contrast to patients with restrictive disease and normal subjects in whom end tidal volume (FRC) is close to Vr. This study shows that, in patients with severe chronic obstructive pulmonary disease, Vr is at least as small as residual volume (RV).

Salbutamol powder inhaled from the Diskhaler compared to salbutamol as nebulizer solution in severe chronic airways obstruction

The bronchodilatory effect of four doses of salbutamol powder (1·6 mg) from a multi-dose dry powder inhaler, the Diskhaler, was compared to the effect of 2·5 ml salbutamol nebulizer solution (1 mg ml −1) from a jet nebulizer, Pari Inhalierboy, in a randomized, double-blind, double-dummy, cross-over study performed on 2 consecutive days. Thirty-two patients with severe chronic obstructive pulmonary disease (COPD), a mean FEV 1=29% of predicted value, and at least a 15% increase in FEV 1 after inhaling 5 mg nebulized terbutaline were included. Twenty-eight patients were evaluated: 17 women and 11 men with a mean age of 67 years (range 53–82 years). The mean increases in FEV 1 were greater after inhalation via the Diskhaler, although there was no difference in the patients' subjective assessment of the treatments. The powder inhaler was also effective in patients with the lowest baseline FEV 1 and the lowest inspiratory peak flow through the inhaler. The study demonstrates that dry powder inhalation of salbutamol via a Diskhaler is at least as effective as inhalation of salbutamol via a jet nebulizer in providing bronchodilation in patients with severe COPD.

Can long-term oxygen therapy improve exercise capacity and prognosis?

The lack of studies as to whether the correction of desaturations during exercise can play a role in improving survival still leaves many problems usually met in the common practice open. (1) Why prescribe long-term oxygen therapy (LTOT) on exercise? Up to now, supplemental oxygen during exercise seems more an approach to the 'dyspnea symptom' than a pivotal component of a comprehensive strategy for long-term management of severe chronic airway obstruction. (2) Who needs LTOT on exercise? It seems reasonable to correct desaturations if this leads to a substantial improvement in exercise tolerance. As to the method of clinical assessment, pulse oximetry can be used for measuring desaturation between rest and exercise, although absolute values are not reliable. (3) How to prescribe LTOT on exercise? In practice, the O2 flow able to prevent desaturation on exercise, restoring an SaO2 greater than 90%, is the usual prescription criterion after an appropriate testing able to demonstrate a significant increase in exercise tolerance and conducted comparing the results breathing air with those on O2, the patient being unaware of the inhaled mixture. (4) How to administrate LTOT on exercise? A portable source is usually employed to allow the greatest possible independency. A reservoir nasal cannula can halve the oxygen wastage and is less expensive than a pulse demand valve. In patients needing 24-hour oxygen therapy the transtracheal catheter is being used more and more at present.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of Asthma among Elderly Adults in a Sample of the General Population

This article describes the characteristics and course of asthma among subjects who were older than 65 years at the time of enrollment in a longitudinal study of a general population sample. It was present in 3.8 percent of men and 7.1 percent of women. An additional 4.1 percent of men reported having "asthma," but they also had seen a physician for "emphysema" and had smoked significantly; their "asthma" diagnosis is regarded as highly questionable. They did not show the elevated rate of allergy skin test reactivity of high serum IgE levels that were characteristic of other asthmatics. Many of the elderly asthmatics (mean age, 72 years) had severe disease with marked ventilatory impairment. There was a close relationship between the severity of wheezing complaints and impairment of the FEV1. Of the 46 patients, 48 percent reported an onset before age 40 years. There was no relationship between severity and age of onset or duration of disease. A second diagnosis of "chronic bronchitis" was reported by 46 percent of the asthmatics, but this did not delineate a distinctive group with late-onset, smoking-related disease. Death rates in the asthmatics tended to be higher than in nonasthmatics (odds ratio, 1.9; CI, 0.998 to 3.70, after stratifying by sex). Over a mean follow-up of 7.44 years, most symptoms as well as the FEV1 remained relatively stable. Chronic productive cough did tend to remit (p less than 0.01), but this was noted in the nonasthmatics as well. We estimate that no more than 19 percent of the asthmatics went into complete remission during follow-up. Most of these had mild initial symptoms; there were no remissions in subjects with severe disease at the time of entry. We concluded that asthma in the elderly is not a rare disease and may be associated with severe symptoms and chronic airways obstruction. If severe, it rarely goes into complete remission but tends to remain a severe, disabling disorder.

Domiciliary nebulized terbutaline in severe chronic airways obstruction

Forty-eight patients with severe chronic airways obstruction were given 5 mg terbutaline or placebo from a nebulizer twice daily for 2 + 2 weeks. Twenty three patients preferred terbutaline, 9 placebo and 16 had no preference. The baseline lung function and the 6 minute walking distance were not increased after the terbutaline period. The patients who preferred terbutaline indicated less dyspnoea after the terbutaline period as compared to the placebo period, but did not differ with regard to lung function or walking distance after the terbutaline treatment. The physiology behind the subjective relief from the terbutaline inhalations remains unexplained.

Terbutaline as powder inhalation from Bricanyl Turbuhaler compared to terbutaline as nebulizer solution in severe chronic airways obstruction

Twenty two adults with severe chronic airways obstruction--FEV1 from 14 to 57% of predicted--participated on two consecutive days in an open cross over comparison of inhalation of 2 mg terbutaline powder by Bricanyl Turbuhaler and 5 mg terbutaline by a conventional jet nebulizer (Pari Inhalierboy). All participants were domiciliary users of nebulized terbutaline previous to inclusion in this study. The mean (SD) maximal increase in FEV1 within the 60 minutes following inhalation by Turbuhaler and by Pari Inhalierboy was 27% (15%) and 28% (15%). The mean (SD) maximal increase in FVC was 30% (20%) and 29% (16%). The differences were not statistically significant. All participants were able to produce inspiratory flows high enough to use the Turbuhaler. The study suggests that it will be possible to replace the use of nebulizers with powder inhalations, as far as the bronchodilating effects are concerned.

High dose salbutamol in chronic airflow obstruction: comparison of nebulizer with Rotacaps

Patients with severe chronic airways obstruction often respond poorly to inhaled salbutamol in conventional dosage from a pressurized aerosol. We have investigated the response to high dose salbutamol in 18 patients with chronic airways obstruction (mean age 64.4 years, mean FEV1 40.2% predicted, less than 15% response to 400 micrograms salbutamol). Patients were given 1 mg, 2 mg and 5 mg of salbutamol by either Rotacaps or nebulizer in random order on separate days. Response was measured over 8 h as a change in airway calibre (FEV1, FVC and PEF). The frequency of tremor, palpitations and arrhythmias was noted. Area under the curve for change from baseline values for FEV1, FVC and PEF showed that larger doses of salbutamol (nebulized or Rotacaps) result in larger areas from all three variables. Analysis of peak response, for FEV1, FVC and PEF showed no significant difference for the six treatments used except that the peak response for 2 mg Rotacaps for FEV1 was significantly larger than that for 1 mg Rotacaps (P less than 0.05). The results of this study show that in severe chronic airflow obstruction 1, 2 and 5 mg of salbutamol inhaled via a rotahaler device are just as effective as 1, 2 and 5 mg salbutamol inhaled from a nebulizer in producing bronchodilation. High dose salbutamol is well tolerated and side effects are minimal. Rotacaps have the advantage of being more portable than a wet nebulizer.

Effect of chronic airways obstruction on measurement of the single-breath carbon-monoxide-diffusing capacity.

We have examined the effect of chronic airways obstruction on the measurement of the single-breath carbon-monoxide-diffusing capacity (DLCLSB). We reviewed the results of 136 consecutive pulmonary function tests (comprising standard spirometry, helium dilution lung volumes and DLCOSB) obtained in patients who had an FEV1/FVC less than 70%. We calculated DLCOSB using two different values for alveolar volume (VA). In the first method (HeDL), VA was measured by single-breath dilution of helium during the test. In the second method (RbDL), VA was measured as the sum of the inspiratory vital capacity, performed during the test, and the residual volume, determined separately by helium rebreathing. The mean HeDL/RbDL, reflecting disparity between computations of DLCOSB in individual subjects was 0.85 +/- 0.13 in patients with moderate obstruction (40 less than or equal to FEV1/FVC% less than 60) and was 0.80 +/- 0.14 in those with severe obstruction (FEV1/FVC% less than 40). The mean HeDL/RbDL was lowest (0.73 +/- 0.12) in those with severe elevation of RV/TLC (RV/TLC% greater than 60). HeDL/RbDL correlated best with RV/TLC (r = -0.71, p less than 0.001). Unexplained variance in HeDL/RbDL was not significantly reduced by including the relationship between HeDL/RbDL and pulmonary function indices commonly used to measure airways resistance. These data suggest (1) the difference between HeDL and RbDL in patients with moderate and severe chronic airways obstruction is greater than previously reported; (2) the disparity between HeDL and RbDL stems from slow space ventilation rather than from increased resistance to air flow, and (3) HeDL underestimates gas transfer in poorly ventilated lung compartments.

Changes in Blood Gas Levels following Nebuhaler and Nebuliser Administration of Terbutaline in Severe Chronic Airways Obstruction

Changes in Blood Gas Levels following Nebuhaler and Nebuliser Administration of Terbutaline in Severe Chronic Airways Obstruction

Effect of Inspiratory Flow Rate on Gas Exchange during Mechanical Ventilation

We studied the effect of changes in inspiratory flow rate () on gas exchange and respiratory system mechanics during mechanical ventilation. Seven patients with respiratory failure caused by severe chronic airway obstruction (CAO group) and 6 patients with other causes of respiratory failure (ORF group) were studied at 3 settings, 40 L/min, 70 L/min, and 100 L/min. In the CAO group, as increased from 40 L/min to 100 L/min, arterial oxygen tension increased 18%, venous admixture decreased 23.2%, dead space to tidal volume ratio decreased 23.7%, and tidal compliance of the respiratory system increased 31.2%. In the ORF group, as increased, there was little change in most parameters measured. The changes in the CAO group may be explained by redistribution of ventilation to low ventilation-perfusion ratio units, which empty more completely during the longer expiratory times allowed at high . Using a simple two-compartment lung model we determined that an increase in expiratory time from 4.2 to 6.1 s (the incr...

Effect of inspiratory flow rate on gas exchange during mechanical ventilation.

We studied the effect of changes in inspiratory flow rate (VI) on gas exchange and respiratory system mechanics during mechanical ventilation. Seven patients with respiratory failure caused by severe chronic airway obstruction (CAO group) and 6 patients with other causes of respiratory failure (ORF group) were studied at 3 VI settings, 40 L/min, 70 L/min, and 100 L/min. In the CAO group, as VI increased from 40 L/min to 100 L/min, arterial oxygen tension increased 18%, venous admixture decreased 23.2%, dead space to tidal volume ratio decreased 23.7%, and tidal compliance of the respiratory system increased 31.2%. In the ORF group, as VI increased, there was little change in most parameters measured. The changes in the CAO group may be explained by redistribution of ventilation to low ventilation-perfusion ratio units, which empty more completely during the longer expiratory times allowed at high VI. Using a simple two-compartment lung model we determined that an increase in expiratory time from 4.2 to 6.1 s (the increase observed in the CAO group as VI increased from 40 L/min to 100 L/min) will result in redistribution of ventilation to long expiratory time constant respiratory units and produce changes in lung mechanics and gas exchange similar to those observed experimentally. We concluded that mechanical ventilation at high VI in patients with CAO is associated with improvement in gas exchange and more even distribution of ventilation, which appear to be due to the increased time available for alveolar emptying.