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Clinical Investigations: ASTHMA |

Continuous vs Intermittent β-Agonists in the Treatment of Acute Adult Asthma*: A Systematic Review With Meta-analysis FREE TO VIEW

Gustavo J. Rodrigo, MD; Carlos Rodrigo, MD
Author and Funding Information

*From the Departamento de Emergencia, Hospital Central de las Fuerzas Armadas, Montevideo, Uruguay.

Correspondence to: Gustavo J. Rodrigo MD, Departamento de Emergencia, Hospital Central de las Fuerzas Armadas, Av 8 de Octubre 3020, Montevideo 11600, Uruguay; e-mail: gurodrig@adinet.com.uy.



Chest. 2002;122(1):160-165. doi:10.1378/chest.122.1.160
Text Size: A A A
Published online

Background: Since the late 1980s, there has been considerable clinical and academic interest in the use of continuous aerosolized bronchodilators for the treatment of patients with acute asthma. These studies have suggested that this therapy is safe, is at least as effective as intermittent nebulization, and may be superior to intermittent nebulization in patients with the most severely impaired pulmonary function.

Objectives: To determine whether continuous nebulization offered an advantage over intermittent nebulization for the treatment of adults with acute asthma in the emergency department (ED).

Design: Systematic review of randomized controlled trials of adults with acute asthma.

Outcomes: Change in pulmonary function tests as primary outcome, and admissions to the hospital and side effects as secondary outcomes.

Results: Six studies including 393 adults with acute asthma were selected. No significant differences were demonstrated between the two delivery methods in terms of pulmonary function measures obtained after 1 h of treatment (standardized mean difference [SMD], −0.15; 95% confidence interval [CI], −0.35 to 0.05) and after 2 to 3 h of treatment (SMD, −0.19; 95% CI, −0.39 to 0.01). No significant heterogeneity was demonstrated (p > 0.5). At the end of treatment, there was a significantly greater decrease in pulse rate when the continuous nebulizer was used (weighted mean difference [WMD], −6.82; 95% CI, −8.67 to −3.90 beats/min; χ2, 2.55; degrees of freedom [df], 4; p = 0.6). Additionally, the analysis showed a significant decrease of serum potassium concentration with the use of intermittent nebulization (WMD, 0.12; 95% CI, 0.24 to 0.01 mmol/L; χ2, 0.5; df, 2; p = 0.8). However, this finding was obtained on the analysis of only two trials. Finally, at the end of the study period, no significant differences were identified between patients treated with continuous or intermittent nebulization with respect to hospital admission (relative risk, 0.68; 95% CI, 0.33 to 1.38; χ2, 2.06; df, 1; p = 0.2).

Conclusions: Overall, this review supports the equivalence of continuous and intermittent albuterol nebulization in the treatment of acute adult asthma.

Figures in this Article

Inhaled β2-agonists are the drugs of choice with which to treat patients with acute severe asthma.12 In comparison to the systemic approach, inhalation is associated with a more rapid onset of action and fewer systemic side effects. However, there are controversies regarding the use of β2-agonists in the treatment of patients with acute asthma exacerbations. Thus, there is a consensus that frequent intermittent nebulizations (ie, every 20 min within the first hour) are appropriate, but continuous nebulization also has been proposed.,12 Thus, since the late 1980s, there has been considerable clinical and academic interest in the use of continuous aerosolized bronchodilators for the treatment of acute asthma.3 This method of therapy has potential advantages in terms of time, costs, and medication delivery. This feature may allow deeper penetration into the airways and greater reduction of bronchoconstriction. Furthermore, this may result in fewer side effects.

To date, a limited number of trials have examined this topic. Thus, therapy with continuous albuterol nebulization was considered to be better than intermittent therapy in children,48 but in adults the data are contradictory and do not allow definitive conclusions.910

We reviewed the literature to determine whether continuous nebulization offered an advantage over intermittent nebulization for the treatment of adults with acute asthma in the emergency department (ED). We also wanted to determine whether the intensity of the treatment and the severity of the exacerbation influenced the extent of the effect.

Literature Search and Selection of Studies

A computerized search was conducted to identify literature on the topic of continuous vs intermittent nebulization with β-agonists in adult patients with acute asthma. We searched for studies using the MEDLINE (1966 to 2001), EMBASE (1980 to 2001), and CINAHL (1982 to 2001) databases. The following MeSH terms were used in the search: Emerg* or acute or status and continuous or intermittent or multi-dose or dosage or nebuli* and β-agonist or bronchodilat*. Also, a search of the Cochrane Controlled Trials Register was completed using the above terms. Finally, we checked bibliographies of all trials and review articles that had been identified from the databases and medical journals to determine potentially relevant citations.

Criteria for considering trials included the following: (1) randomized controlled trials conducted in an ED setting; (2) studies with adult patients (ie, ≥ 18 years old) with acute asthma; (3) patients who had been randomized to receive either continuous or intermittent β-agonists early in the ED treatment (continuous β-agonist administration included frequent refilling of the nebulizer, use of a nebulizer and infusion pump, or use of a large-volume nebulizer with high-output extended aerosol respiratory therapy [HEART]); and (4) change in pulmonary function test results as the primary outcome (absolute or percent predicted of peak expiratory flow [PEF] and absolute or percent predicted FEV1), and side effects/adverse effects and admissions to the hospital as secondary outcomes. Assessments included up to 3 h of treatment. Because the peak bronchodilator effect after the administration of multiple doses of salbutamol/albuterol occurs within 1 to 2 h, and because approximately two thirds of adults with acute asthma require 90 min of treatment with inhaled β-agonists to have their conditions improve sufficiently to be discharged from the hospital, it is reasonable to expect significant improvement at this time.1112

The two authors independently examined (title and abstract) the output generated from the search. Any potentially relevant articles were obtained, irrespective of the original language of publication. From the full text of potentially relevant articles, the reviewers assessed each study independently, in terms of population, intervention, study design, and outcome, to determine whether the study met the inclusion criteria. Reviewers were masked to the authors’ names, the name of journal, and the date of publication. Agreement among reviewers was measured using κ statistics, and disagreement was settled by consensus. The methodological quality of each selected trial was assessed using the instrument of Jadad et al.13 This instrument evaluates the quality of randomization and blinding, and the reasons for withdrawal on a score of 0 (worst) to 5 (best).

Statistical Analysis

For continuous variables, a random-effects standardized mean difference (SMD) or a weighted mean difference (WMD) and the 95% confidence interval (CI) were calculated for each study. All similar studies were pooled using random-effects SMD or WMD and 95% CIs. The SMD, reported in SD units, was used when the change in the same pulmonary function test was reported in different units (ie, the weighted sum of each trials times the group mean difference divided by its pooled SD).1415 The WMD was reported for pulmonary function tests using the same unit of measure (ie, the weighted sum of each trial’s difference between the mean of the experimental and the control groups, reported on the same scale).16 The contribution of each trial to the pooled estimate was proportional to the inverse of the variance.17The homogeneity of the effect size was tested with the method of DerSimonian and Laird,18with p = 0.1 as the level of significance. Sensitivity analysis19 was performed to examine the effect on the results of the following: (1) the severity of airway obstruction (PEF or FEV1 < 50% of predicted vs ≥ 50% of predicted); (2) the methodological quality (Jadad et al,13 criteria, ≥ 3 vs < 3); and (3) β-agonist dose (high vs low). The meta-analysis was done with a computer software package (Review Manager, version 4.1 2000; Cochrane Collaboration and Update Software).

Sixty-two articles were identified in the initial search. Of these, reviewers found that 16 articles were potentially eligible. Most of the others were excluded because they were not clinical trials of patients with acute asthma. The reasons for subsequent exclusion were nonrandomized trials (n = 6),3,56,2022 only a continuous nebulization group included in the study (n = 1),23 only pediatric patients included in the study (n = 2),78 and non-acute asthma patients included in the study (n = 1).4 Finally, six articles were selected for inclusion in the meta-analysis.910,2427 Four studies were from the United States,9,24,2627 one was from Canada,25 and one was from Tunisia.12 The κ statistic for inter-rater agreement on the inclusion or exclusion of potential trials was 1.0. All studies reported pulmonary function measures, five reported heart rates, two reported admission rates, and two reported serum potassium concentrations. Table 1 shows the characteristics of the trials included in this review.

This review analyzed 393 adults who were included in six trials. One study27 compared nebulized albuterol at high doses (ie, 7.5 mg) or standard doses (ie, 2.5 mg) given either continuously for 1 h, or intermittently every hour for 2 h. The nebulized doses of albuterol in the first hour ranged between 2.5 and 16 mg. All studies reported the use of systemic corticosteroids, and four studies included acute severe asthma patients (pulmonary function, < 50% of predicted).

Effects on Pulmonary Function

In the six selected studies, a variety of pulmonary function tests was recorded over the ED stay. Four studies recorded FEV1 as percent predicted, three recorded absolute FEV1, and two recorded both percent predicted and absolute PEF. The results were pooled at 1 h and at 2 to 3 h after the start of treatment. No significant differences were demonstrated between the two delivery methods in terms of pulmonary function measurements at 1 h and at 2 to 3 h. More specifically, there were no significant group differences in the SMD observed after 1 h of treatment (SMD, −0.15; 95% CI, −0.35 to 0.05) and after 2 to 3 h of treatment (SMD, −0.19; 95% CI, −0.39 to 0.01) [Fig 1] . No significant heterogeneity was demonstrated (p > 0.5). In the same way, the four trials reporting responses to treatment as the change in the percentage predicted of FEV19,2527 did not show group differences after 1 h of treatment (WMD, −1.73; 95% CI, −8.51 to 5.05) and after 2 to 3 h of treatment (WMD, −2.18; 95% CI, −6.24 to 1.88).

Side Effects and Hospital Admissions

Five studies reported the final heart rate.910,2426 At the end of treatment, there was a significant, but small, decrease in pulse rate when continuous nebulization was administered (WMD, −6.82; 95% CI, −8.67 to −3.90 beats/min; χ2, 2.55; degrees of freedom [df], 4; p = 0.6) [Fig 2] . Additionally, only two studies,10,27 reported final serum potassium concentrations. The analysis showed a significant decrease with the use of intermittent nebulization (3.87 vs 3.76 mEq/L, respectively; WMD, 0.12; 95% CI, 0.24 to 0.01 mmol/L; χ2, 0.5; df, 2; p = 0.8). Finally, two studies,910 reported hospital admissions, and no significant differences were identified between patients treated with continuous or intermittent nebulization at the end of the study period (relative risk, 0.68; 95% CI, 0.33 to 1.38; χ2, 2.06; df, 1; p = 0.2). Side effects such as tremor and palpitations were reported so infrequently that they could not be considered in this review.

Sensitivity Analyses

The robustness of the findings to different assumptions was analyzed in sensitivity analyses. Three subgroup analyses were specified as follows a priori: severity of airway obstruction (pulmonary function, < 50% vs ≥ 50% of predicted); methodological quality (Jadad et al13 criteria, ≥ 3 vs < 3); and β-agonist dose (> 5 vs ≤ 5 mg/h of albuterol). Four studies included patients with severe asthma, and two included patients with moderate asthma. No benefit was observed after 1 h of treatment (severe asthma subgroup: SMD, −0.20; 95% CI, −0.43 to 0.04; χ2, 3.5; df, 4; p = 0.5; moderate asthma subgroup: SMD, −0.04; 95% CI, −0.40 to 0.33; χ2, 0.4; df, 1; p = 0.5) and after 2 to 3 h of treatment (severe asthma subgroup: SMD, −4.32; 95% CI, −8.78 to 0.14; χ2, 1.7; df, 4; p = 0.8; moderate asthma subgroup: SMD, 3.11; 95% CI, −10.2 to 16.4; χ2, 1.38; df, 1; p = 0.3). Similarly, the methodological quality of the studies did not alter the results. Thus, there were no differences in pulmonary function between high-quality and low-quality trials after 1 h of treatment (high-quality trials: SMD, −3.5; 95% CI, −11.6 to 4.6; χ2, 1.24; df, 1; p = 0.3; low-quality trials: SMD, −2.6; 95% CI, −7.3 to 1.9; χ2, 3.59; df, 4; p = 0.5) and after 2 to 3 h of treatment (high-quality trials: SMD, −3.4; 95% CI, −12.0 to 5.3; χ2, 0.89; df, 1; p = 0.4; low-quality trials: SMD, −3.6; 95% CI, −8.4 to 1.2; χ2, 3.26; df, 4; p = 0.5). Finally, there was no apparent impact on pulmonary function when high doses or low doses of albuterol were used after 1 h of treatment (high doses: SMD, 0.0; 95% CI, −0.34 to 0.33; χ2, 0.72; df, 2; p = 0.7; low doses: SMD, −0.14; 95% CI, −0.42 to 0.13; χ2, 3.38; df, 3; p = 0.3) and after 2 to 3 h of treatment (high doses: SMD, −2.92; 95% CI, −8.94 to 3.11; χ2, 0.86; df, 2; p = 0.6; low doses: SMD, −4.16; 95% CI, −10.1 to 1.81; χ2, 3.17; df, 3; p = 0.5).

The purpose of this systematic review is to determine whether continuous nebulization offered an advantage over intermittent nebulization for the treatment of adults with acute asthma in the ED. Our analysis failed to demonstrate statistically significant differences between groups in terms of pulmonary function or hospital admission rates. The intensity of treatment (ie, the albuterol dose) and the severity of the exacerbation did not influence the effect of either method of albuterol delivery. Thus, these findings argue against the routine use of continuous nebulization in the ED treatment of patients with acute asthma, and the decision to use one of these nebulization methods should be based on logistical and cost considerations. Overall, this review supports the equivalence of continuous and intermittent albuterol nebulization in the acute treatment of nonventilated adult asthma patients. Also, patients who received continuous nebulization showed lower pulse rates, and a decrease of serum potassium concentration compared to patients receiving the same β-agonist via intermittent nebulization. However, these findings are based on only five trials and two trials, respectively. Consequently, this information should be interpreted with caution. Since all studies reviewed excluded patients with life-threatening asthma (for example those patients who have been considered for ventilation), these results should not be extrapolated to this group.

This study met most of the methodological criteria that have been suggested for scientific reviews.2829 The patients had the typical features of patients with moderately to severely acute asthma when they presented for care to an ED. All of the included trials were randomized and placebo-controlled. Although sensitivity analysis and the tests of homogeneity suggest that the results of this meta-analysis were relatively robust, this review is limited by the number of selected trials and the quality of the data. Even if the number and size of the pooled studies were small, the exclusion of trials with lower reported methodological quality did not affect the conclusions. Clearly, the current conclusions may be seriously modified by the results of larger trials.30

In summary, for adults seen and assessed for acute exacerbations of asthma, this review found no significant differences between the two methods for treatment delivery. Consequently, the choice of delivery method should reflect practice situations and economic considerations. However, the findings are based on a reduced number of trials, and these conclusions may not apply to patients with life-threatening asthma.

Abbreviations: CI = confidence interval; df = degrees of freedom; ED = emergency department; HEART = high-output extended aerosol respiratory therapy; PEF = peak expiratory flow; SMD = standardized mean difference; WMD = weighted mean difference

Table Graphic Jump Location
Table 1. Summary of Included Trials
Figure Jump LinkFigure 1. Pooled SMD of the change in lung function 1 h and 2 to 3 h after the start of treatment. The width of the horizontal line represents the 95% CI around a point estimate (squares). The size of the point estimate represents the relative weight (% weight) of each trial in the pooled summary estimate (diamond). CN = continuous nebulization; IN = intermittent nebulization.Grahic Jump Location
Figure Jump LinkFigure 2. Pooled WMD of heart rate at the end of treatment. The width of horizontal line represents 95% the CI around a point estimate (squares). The size of the point estimate represents the relative weight (% weight) of each trial in the pooled summary estimate (diamond). See the legend of Figure 1 for abbreviations not used in the text.Grahic Jump Location
 National Asthma Education and Prevention Program. Expert panel report 2: guidelines for the diagnosis and management of asthma. April 1997; National Institutes of Health. Bethesda, MD: Publication No. 55–4051.
 
British Thoracic Society. British guidelines on asthma management: 1995 review and position statement.Thorax1997;52,S1-S24. [CrossRef]
 
Olshaker, J, Jerrand, D, Barish, RA, et al The efficacy and safety of a continuous albuterol protocol for the treatment of acute adult asthma attacks.Am J Emerg Med1993;11,131-133. [PubMed]
 
Moler, FW, Hurwitz, ME, Custer, JR Improvement in clinical asthma score and Paco2in children with severe asthma treated with continuously nebulized terbutaline.J Allergy Clin Immunol1988;81,1101-1109. [PubMed]
 
Portnoy, J, Aggaarwal, J Continuous terbutaline nebulization for the treatment of severe exacerbations of asthma in children.Ann Allergy1988;60,368-371. [PubMed]
 
Chipps, BE, Blackney, DA, Black, LE, et al Vortran high output extended aerosol respiratory therapy (HEART) for delivery of continuously nebulized terbutaline for the treatment of acute bronchospasm [abstract]. Pediatr Asthma Allergy Immunol. 1990;;471 ,.:477
 
Papo, M, Frank, J, Thompson, AE A prospective, randomized study of continuous versus intermittent nebulized albuterol for severe status asthmaticus in children.Crit Care Med1993;21,1479-1486. [PubMed]
 
Khine, H, Fuchs, SM, Saville, AL Continuous vs intermittent nebulized albuterol for emergency management of asthma.Acad Emerg Med1996;3,1019-1024. [PubMed]
 
Lin, RY, Sauter, D, Newman, T, et al Continuous versus intermittent albuterol nebulization in the treatment of acute asthma.Ann Emerg Med1993;22,1847-1853. [PubMed]
 
Besbes-Ouanes, L, Nouira, S, Elatrous, S, et al Continuous versus intermittent nebulization of salbutamol in acute severe asthma: a randomized, controlled trial.Ann Emerg Med2000;36,198-203. [PubMed]
 
Strauss, L, Hejal, R, Galan, G, et al Observations on the effects aerosolized albuterol in acute asthma.Am J Respir Crit Care Med1997;155,454-458. [PubMed]
 
Rodrigo, C, Rodrigo, G Therapeutic response patterns to high and cumulative doses of salbutamol in acute severe asthma.Chest1998;113,593-598. [PubMed]
 
Jadad, AR, Moore, RA, Carroll, D, et al Assessing the quality of reports of randomized clinical trials: is blinding necessary?.Control Clin Trials1996;17,1-12. [PubMed]
 
Glass, CV. Meta-analysis in social research. 1981; Sage. Beverly Hills, CA:.
 
Fleiss, JL The statistical basis of meta-analysis.Stat Methods Med Res1993;2,121-145. [PubMed]
 
Olkin, I Statistical and theoretical consideration in meta-analysis.Clin Epidemiol1995;48,133-146
 
Robins, J, Breslow, N, Greenland, S Estimators of the Mantel-Haenszel variance consistent in both sparse data and large-strata limiting models.Biometrics1986;42,311-323. [PubMed]
 
DerSimonian, R, Laird, N Meta-analysis in clinical trials.Control Clin Trials1986;7,177-188. [PubMed]
 
Egger, M, Davey Smith, G, Phillips, AN Meta-analysis: principles and procedures.BMJ1997;315,1533-1537. [PubMed]
 
Lin, RY, Smith, AJ, Hergenroeder, P High serum albuterol levels and tachycardia in adult asthmatics treated with high-dose continuously aerosolized albuterol.Chest1993;103,221-225. [PubMed]
 
Katz, RW, Kelly, HW, Crowley, MR, et al Safety of continuously nebulized albuterol for bronchospasm in infants and children.Pediatrics1993;92,666-669. [PubMed]
 
Baker, EK, Willsie, SK, Marinac, JS, et al Continuously nebulized albuterol in severe exacerbations of asthma in adults: a case controlled study.J Asthma1997;34,521-530. [PubMed]
 
Weber, EJ, Levitt, MA, Covington, JK, et al Effects of continuously nebulized ipratropium bromide plus albuterol on emergency department length of stay and hospital admission rates in patients with acute bronchospasm: a randomized, controlled trial.Chest1999;115,937-944. [PubMed]
 
Rudnitsky, GS, Eberlein, RS, Schoffstall, JM, et al Comparison of intermittent and continuously nebulized albuterol for treatment of asthma in an urban emergency department.Ann Emerg Med1993;22,1842-1846. [PubMed]
 
Colacone, A, Wolkove, N, Stern, E, et al Continuous nebulization of albuterol (salbutamol) in acute asthma.Chest1990;97,693-697. [PubMed]
 
Reisner, C, Kotch, A, Dworkin, G Continuous versus frequent intermittent nebulization of albuterol in acute asthma: a randomized, prospective study.Ann Allergy Asthma Immunol1995;75,41-47. [PubMed]
 
Shrestha, M, Bidadi, K, Gourlay, S, et al Continuous vs. intermittent albuterol at high and low doses, in the treatment of severe acute asthma in adults.Chest1996;110,42-47. [PubMed]
 
Oxman, AD, Guyatt, GH Guidelines for reading literature reviews.Can Med Assoc J1988;138,697-703
 
Oxman, AD, Cook, DJ, Guyatt, GH User’s guides to the medical literature: how to use an overview.JAMA1994;272,1367-1371. [PubMed]
 
Sim, I, Hlatky, MA Growing pains of meta-analysis.BMJ1996;313,702-703. [PubMed]
 

Figures

Figure Jump LinkFigure 1. Pooled SMD of the change in lung function 1 h and 2 to 3 h after the start of treatment. The width of the horizontal line represents the 95% CI around a point estimate (squares). The size of the point estimate represents the relative weight (% weight) of each trial in the pooled summary estimate (diamond). CN = continuous nebulization; IN = intermittent nebulization.Grahic Jump Location
Figure Jump LinkFigure 2. Pooled WMD of heart rate at the end of treatment. The width of horizontal line represents 95% the CI around a point estimate (squares). The size of the point estimate represents the relative weight (% weight) of each trial in the pooled summary estimate (diamond). See the legend of Figure 1 for abbreviations not used in the text.Grahic Jump Location

Tables

Table Graphic Jump Location
Table 1. Summary of Included Trials

References

 National Asthma Education and Prevention Program. Expert panel report 2: guidelines for the diagnosis and management of asthma. April 1997; National Institutes of Health. Bethesda, MD: Publication No. 55–4051.
 
British Thoracic Society. British guidelines on asthma management: 1995 review and position statement.Thorax1997;52,S1-S24. [CrossRef]
 
Olshaker, J, Jerrand, D, Barish, RA, et al The efficacy and safety of a continuous albuterol protocol for the treatment of acute adult asthma attacks.Am J Emerg Med1993;11,131-133. [PubMed]
 
Moler, FW, Hurwitz, ME, Custer, JR Improvement in clinical asthma score and Paco2in children with severe asthma treated with continuously nebulized terbutaline.J Allergy Clin Immunol1988;81,1101-1109. [PubMed]
 
Portnoy, J, Aggaarwal, J Continuous terbutaline nebulization for the treatment of severe exacerbations of asthma in children.Ann Allergy1988;60,368-371. [PubMed]
 
Chipps, BE, Blackney, DA, Black, LE, et al Vortran high output extended aerosol respiratory therapy (HEART) for delivery of continuously nebulized terbutaline for the treatment of acute bronchospasm [abstract]. Pediatr Asthma Allergy Immunol. 1990;;471 ,.:477
 
Papo, M, Frank, J, Thompson, AE A prospective, randomized study of continuous versus intermittent nebulized albuterol for severe status asthmaticus in children.Crit Care Med1993;21,1479-1486. [PubMed]
 
Khine, H, Fuchs, SM, Saville, AL Continuous vs intermittent nebulized albuterol for emergency management of asthma.Acad Emerg Med1996;3,1019-1024. [PubMed]
 
Lin, RY, Sauter, D, Newman, T, et al Continuous versus intermittent albuterol nebulization in the treatment of acute asthma.Ann Emerg Med1993;22,1847-1853. [PubMed]
 
Besbes-Ouanes, L, Nouira, S, Elatrous, S, et al Continuous versus intermittent nebulization of salbutamol in acute severe asthma: a randomized, controlled trial.Ann Emerg Med2000;36,198-203. [PubMed]
 
Strauss, L, Hejal, R, Galan, G, et al Observations on the effects aerosolized albuterol in acute asthma.Am J Respir Crit Care Med1997;155,454-458. [PubMed]
 
Rodrigo, C, Rodrigo, G Therapeutic response patterns to high and cumulative doses of salbutamol in acute severe asthma.Chest1998;113,593-598. [PubMed]
 
Jadad, AR, Moore, RA, Carroll, D, et al Assessing the quality of reports of randomized clinical trials: is blinding necessary?.Control Clin Trials1996;17,1-12. [PubMed]
 
Glass, CV. Meta-analysis in social research. 1981; Sage. Beverly Hills, CA:.
 
Fleiss, JL The statistical basis of meta-analysis.Stat Methods Med Res1993;2,121-145. [PubMed]
 
Olkin, I Statistical and theoretical consideration in meta-analysis.Clin Epidemiol1995;48,133-146
 
Robins, J, Breslow, N, Greenland, S Estimators of the Mantel-Haenszel variance consistent in both sparse data and large-strata limiting models.Biometrics1986;42,311-323. [PubMed]
 
DerSimonian, R, Laird, N Meta-analysis in clinical trials.Control Clin Trials1986;7,177-188. [PubMed]
 
Egger, M, Davey Smith, G, Phillips, AN Meta-analysis: principles and procedures.BMJ1997;315,1533-1537. [PubMed]
 
Lin, RY, Smith, AJ, Hergenroeder, P High serum albuterol levels and tachycardia in adult asthmatics treated with high-dose continuously aerosolized albuterol.Chest1993;103,221-225. [PubMed]
 
Katz, RW, Kelly, HW, Crowley, MR, et al Safety of continuously nebulized albuterol for bronchospasm in infants and children.Pediatrics1993;92,666-669. [PubMed]
 
Baker, EK, Willsie, SK, Marinac, JS, et al Continuously nebulized albuterol in severe exacerbations of asthma in adults: a case controlled study.J Asthma1997;34,521-530. [PubMed]
 
Weber, EJ, Levitt, MA, Covington, JK, et al Effects of continuously nebulized ipratropium bromide plus albuterol on emergency department length of stay and hospital admission rates in patients with acute bronchospasm: a randomized, controlled trial.Chest1999;115,937-944. [PubMed]
 
Rudnitsky, GS, Eberlein, RS, Schoffstall, JM, et al Comparison of intermittent and continuously nebulized albuterol for treatment of asthma in an urban emergency department.Ann Emerg Med1993;22,1842-1846. [PubMed]
 
Colacone, A, Wolkove, N, Stern, E, et al Continuous nebulization of albuterol (salbutamol) in acute asthma.Chest1990;97,693-697. [PubMed]
 
Reisner, C, Kotch, A, Dworkin, G Continuous versus frequent intermittent nebulization of albuterol in acute asthma: a randomized, prospective study.Ann Allergy Asthma Immunol1995;75,41-47. [PubMed]
 
Shrestha, M, Bidadi, K, Gourlay, S, et al Continuous vs. intermittent albuterol at high and low doses, in the treatment of severe acute asthma in adults.Chest1996;110,42-47. [PubMed]
 
Oxman, AD, Guyatt, GH Guidelines for reading literature reviews.Can Med Assoc J1988;138,697-703
 
Oxman, AD, Cook, DJ, Guyatt, GH User’s guides to the medical literature: how to use an overview.JAMA1994;272,1367-1371. [PubMed]
 
Sim, I, Hlatky, MA Growing pains of meta-analysis.BMJ1996;313,702-703. [PubMed]
 
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