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Letters and Responses |
The article by Jones et al titled "Comparison of the Oxygen Cost of Breathing Exercises and Spontaneous Breathing in Patients With Stable Chronic Obstructive Pulmonary Disease" in the May 2003 issue adds valuable evidence to aid our decision on when to teach (or not to teach) breathing exercises to patients with chronic obstructive pulmonary disease (COPD). Their methods were clear and straightforward, and the patients were representative of the age (mean age=68.5 years) and severity of COPD in patients seen in many pulmonary rehabilitation programs (mean percentage of predicted forced expiratory volume in 1 second [FEV1]=39%). However, the article did raise some concerns.
First, much of the introduction discusses the topics of work of breathing (WOB) and ventilatory muscle efficiency and how these 2 factors may be affected in patients with COPD; however, WOB and ventilatory muscle efficiency are not adequately defined in relation to each other. The authors correctly define WOB, but not the rate of WOB, which is arguably more important; the rate of WOB is the product of WOB (for single breath) and respiratory rate (RR) (the rate of WOB=P·VT·RR, where P=inspiratory pressure and VT=tidal volume). Ventilatory muscle efficiency, or mechanical efficiency of breathing, was not defined, but is directly related to the rate of WOB by the following equation: ventilatory muscle efficiency=the rate of WOB/VO2·100%, where VO2 is oxygen consumption.1,2 Therefore, interventions such as pursed lip breathing (PLB) and diaphragmatic breathing (DB) that decrease RR also will decrease the rate of WOB, and if ventilatory muscle efficiency does not change, then VO2 will decrease, which is what Jones and colleagues' results indicate.
Second, given the wide range of body mass index (BMI) values (13.7–28.1 kg/m2; anorexic to overweight), VO2 values would be more conventionally and clearly expressed as mL/min/kg, rather than mL/min.3,4 Using data from 3 hypothetical patients with similar means and standard deviations for VO2 (mL/min) as well as mean BMI values (see Table), it is apparent that when body mass is accounted for in presenting VO2 results, the difference can be dramatic: subject 1's VO2 in mL/min is 25% lower than that of subject 3, but is 53% higher when expressed as mL/min/kg. Because VO2 results are not expressed relative to body mass, clear conclusions regarding the relevance of VO2 changes cannot be made. (Also, in Tab. 1 in the article by Jones et al, BMI is improperly defined as kg·m2, rather than kg/m2).
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We must keep in mind that these measurements were made at rest, whereas many physical therapy interventions occur with exercise. I have seen, in agreement with others,6,7 patients with COPD, new to pulmonary rehabilitation, who have not been taught PLB, but who do use it spontaneously; their pulse oximetry values and dyspnea almost universally improve.6 Therefore, I would argue that PLB, while not energetically more favorable at rest, is an important intervention for patients with COPD during exercise or during dyspneic episodes, when it may be more energetically favorable.
I congratulate the authors for helping us understand the effects of different breathing patterns on the oxygen cost of breathing in patients with COPD. Their results help clarify the circumstances under which we should or should not teach our patients to use PLB. I hope that future research will investigate the effect of various breathing patterns on VO2 in different positions and during exercise, as well as test interventions to alleviate dynamic hyperinflation.
Department of Physiology
School of Medicine
Medical College of Virginia Campus
Virginia Commonwealth University
Box 980551
Richmond, VA 23298-0551
Part-time Staff Physical Therapist
Department of Physical and Occupational Therapy
Duke University Hospital
Durham, NC
References
With respect to the first of his 2 main points, Mr Lowman attempted to reconstruct our data; however, his theoretical data do not approximate the actual data, which precludes much meaningful discussion. The mean value for oxygen consumption (VO2) in our study was 3.55 mL O2/kg/min rather than the 2.9 as calculated theoretically by Mr Lowman. As a point of clarification, the value of 3.5 mL O2/kg/min (resting oxygen consumption in individuals with no cardiovascular/cardiopulmonary impairment) is comparable to other physiologic measurements in that it is associated with some inherent variability and potential systematic measurement differences even with adherence to strict calibration procedures. In our study, therefore, the pretesting conditions were strictly standardized across subjects, and the measurements were obtained using the same equipment, calibrated in the same way, by the same individuals, in the same place, under the same ambient conditions (see the "Method" section of our article).
Mr Lowman's second main point queried the clinical significance of our findings, given that we studied subjects in a supine position and that the body mass index of our subjects ranged from relatively overweight to underweight. First, we refer him to the "Method" section of the article in which we explain that the subjects were studied in a supine position purposefully so that we could use the least intrusive VO2 measurement method. This was important because, to examine oxygen cost without introducing ventilatory measurement artifact in this patient population, we needed to use a canopy that requires the patient to lie quietly in a supine position (and not in a sitting position, let alone exercising) so the canopy could be positioned over the head, secured for possible air leaks, and safely supported by a plinth.
Alternative methods alter breathing. For example, a mouthpiece and noseclip alter dead space and breathing pattern and may not be tolerated by patients with impaired pulmonary function. In addition, a face mask needs to fit snugly. This can also alter normal breathing and can contribute to claustrophobia in patients who are prone to dyspnea and distort the breathing pattern further. Both alternatives are likely to alter energy demands.
We anticipate that other investigators will help extend our work and that of others, with the development and use of artifact-free technology, in the study of breathing patterns adopted spontaneously by patients experiencing dyspnea and in the study of breathing energetics. With respect to body mass, the patients we studied represented a range of body masses, which was desirable experimentally to enhance the generalizability of the results to the population of patients with chronic obstructive pulmonary disease (COPD) in the real world. To control the contribution of intersubject variation, we chose to use a robust, within-subject experimental design with a judicious sample size (see the "Data Analysis" section of our article) to avoid the between-group differences that may have occurred had we used a between-subject design, where age, body weight, and inherent variability of resting VO2 could have introduced marked systematic sampling error.
We believe Mr Lowman's observation of and purported concurrence with the literature2,3 regarding improvement in status with breathing exercises in "patients with COPD...who have not been taught PLB [pursed lip breathing], but who do use it spontaneously" is somewhat overstated. Although we cannot comment on Mr Lowman's personal experience, we are familiar with the work of Tiep and colleagues2 and Mueller and colleagues,3 whom he cites. Tiep and coworkers stated, "None of the subjects had experienced pursed lip breathing training within a year of the study, and they were neither naturally nor by training pursed-lips breathers."2
With respect to their subject selection, Mueller and colleagues stated that their subjects "had to have received at some time in the past adequate instruction in PLB."3 That their subjects used PLB spontaneously cannot be presumed, because this was not stated explicitly. Whether patients spontaneously adopt PLB, and under what conditions and to what degree, should neither be overstated nor understated, because this is the crux of the matter in understanding the normal determinants of breathing pattern in patients with chronic lung conditions.
We do not dispute the literature or Mr Lowman's observation that the status of some patients improves when they adopt PLB. In fact, we would proffer that this observation is what gave physical therapists the idea of using PLB as an intervention in the first place. However, we would argue that physical therapists have taken the observation of a naturally occurring phenomenon in some patients and applied it therapeutically, believing that the abnormal breathing pattern in patients with COPD is the cause of the ventilatory distress in their patients and that changing this pattern is the solution. This notion is dispelled by the literature reported in our article supporting variable effects of breathing exercises, including detrimental effects. Therefore, we would argue that to prescribe the most efficacious intervention, we need to understand when and why some patients adopt this breathing pattern spontaneously and to explain this phenomenon based on an analysis of respiratory mechanics and energetics and subjective perception of dyspnea. By understanding the underlying mechanism, we can direct interventions specifically to the underlying cause of ventilatory distress.
It was a pleasure to be able to share our work and engage in intellectual discussion regarding this important issue of breathing exercises, which have been a mainstay of time-honored—rather than evidence-based—conventional chest physical therapist practice. To conclude, we would like to emphasize the original focus of our study and its clinical relevance. Our data lend support for viewing mechanical efficiency and energetic efficiency of respiration as clinically distinct. Given that imposed breathing patterns associated with reduced oxygen cost are not adopted routinely or sustained when taught to patients with COPD who are prone to ventilatory distress, such intervention is counterintuitive considering the established indications for breathing exercises. This topic, therefore, is of considerable clinical interest and importance. Viewing abnormal breathing patterns in patients with COPD as the effect rather than the cause of their problem is a novel approach. This shift in orientation should help elucidate physical therapy interventions that can be targeted to the cause rather than the effect, with the promise of superior outcomes.
Detailed studies of spontaneous breathing patterns in patients with COPD at rest and during varying intensities of exercise are warranted to elucidate the mechanical impairments of ventilation and their energy consequences. These studies then could provide a cogent rationale, if any, for breathing exercises or provide support for evidence-based alternatives. If sufficient evidence for breathing exercises such as PLB is demonstrated, we will need to prescribe this intervention in a targeted manner, that is, for particular patients and for particular conditions. In light of contemporary evidence related to a primary focus on oxygen transport overall in patients with COPD rather than a narrow focus on airways and lungs, the relevant question is: "What effect do breathing exercises have on lung function and dyspnea over and above that resulting from mobilization/exercise and body positioning?"
Last, we concur that the development of clinical tools that measure the power as well as the work of breathing would be a major advance in physical therapy.
Department of Rehabilitation Science
The Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong
School of Rehabilitation Sciences
T-325, 2211 Wesbrook Mall
Vancouver, British Columbia, Canada
V6T 2B5
Caritas Medical Centre, Kowloon, Hong Kong
References
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