Canadian Neighbor Pharmacy: Discussion of Resistive Breathing Training in Patients with Chronic Obstructive Pulmonary Disease
We have shown that resistive breathing training did not improve the ability to breathe through higher resistances. Furthermore, we found no improvement in ventilatory muscular endurance as measured by the maximal sustained ventilatory capacity and no change in maximal respiratory pressures; however, we did find that an altered pattern of breathing with a longer Ti but lower Pm, breathing frequency, and external resistive work was associated with an improved ability to inspire through smaller orifices.
The majority of our patients failed to improve resistive breathing performance after training. This is in contrast to most previous investigators, who showed that overall resistive breathing training improves the ability to breathe through inspiratory resistances in patients with COPD. In the previous studies of resistive training, the duration and frequency of the training was similar to our study, varying between four and six weeks with two to three 15-minute sessions per day, respectively; however, it should be noted that in one study a control group which used a sham treatment also improved their resistive breathing performance, although in two other studies the control group showed no change. Several factors may play a role in explaining the differing results, and these include (1) alteration in breathing strategy, (2) an inadequate training stimulus, and (3) an inadequate recovery from fatigue between tests.
Alteration in Breathing Strategy
The ability to perform resistive breathing is strongly related to breathing strategy. This has been expressed by Bellemare and Grassino as the tension time index, which is the product of the transdiaphragmatic pressure (Pdi)/Pdi max ratio and the duty cycle (inspiratory time/total breathing time [Ti/Ttot]). When this ratio is less than 0.15, both normal subjects and patients were able to perform resistive breathing without the development of fatigue. For tension time indices greater than 0.15, fatigue developed, and there was an inverse relationship between the level of the tension time index and the endurance time. More recently, Collett and co-workers have shown that there is a linear relationship between the oxygen cost of breathing (Vo2 resp) and the work rate across external resistances and that the latter is a function of the tension time index and the mean inspiratory flow rate. The importance of this work is that it shows that the tension time index only describes the Vo2 resp when inspiratory flow rates are constrained. For increasing inspiratory flow rates, there is an increase in the Vo2 resp and the work rate even when the tension time index is constant. More recently, endurance of the inspiratory muscles has been shown to vary inversely with the inspiratory flow rate even for the same tension time indices. The work of Jones et al confirmed the relationship between pressure time indices and the increase in the oxygen cost of external work. Furthermore, these investigations showed that the rating of perceived effort (RPE), as measured by a Borg scale during resistive breathing, was strongly related to the Pm, Ti, and breathing frequency (fb). This relationship was similar during both a freely adopted and constrained breathing pattern and was described by the equation, RPE = Pm134 X Ti X fb. COPD may be treated by inhalation of different preparations which may be ordered via Canadian Neighbor Pharmacy.
We did not specifically measure perceived effort in our study, but we noted that an improved endurance at the orifice one smaller than the critical orifice was associated with the breathing pattern which would reduce the RPE as defined by the previous relationship. Because the RPE is affected principally by the Pm, the fall in Pm would outweigh the increase in TI observed when patients were coached (Fig 6). The five patients who increased their resistive breathing time showed a decrease in the calculated RPE, while the two patients who failed to increase their endurance also failed to reduce the calculated RPE (Fig 7); however, our study was not designed to investigate the relative importance of RPE, Vo2 resp, and external resistive work in determining resistive breathing performance, and any of these variables singly or in combination could have played a role in improving resistive breathing performance with coaching.
Despite the well-known relationship between resistive breathing endurance and breathing strategy, there are very few data on these variables in the previous studies in COPD. The Pm and pattern of breathing was not monitored during training, and only in a few was the Pm recorded during the testing of resistive breathing, although it is not clear if the signal was displayed to the patients. One group of investigators who noted that their patients tended to take longer, slower breaths actually suggested that the alteration in breathing pattern may have been responsible for the improvement but did not make measurements of the various patterns. In another study the investigators encouraged their patients to use a pattern of long slow breaths, but again no measurements of strategy were made. It was only in the study of Clanton and coworkers that breathing strategy was recorded during both testing and training, but this study dealt with young normal women. Without this information, it is not possible to judge if the previously described improvements in resistive breathing in patients with COPD were due to real increases in ventilatory muscular function or were secondary to changes in breathing pattern.
Inadequate Training Stimulus
There is a vast body of information which deals with the appropriate intensity, duration, and frequency of training necessary to induce the classic training responses for whole-body exercise. This information is as yet not available for ventilatory muscle training, and in fact the recommended methods of training differ greatly in their strategies. In the resistive form, breathing frequency is generally within the normal range, whereas the respiratory pressures are increased by breathing through the resistors.
Whereas most authors have used two to three 15-minute sessions per day for periods varying from four to six weeks, this does not appear to be essential to elicit a training response. In the study by Clanton et al in normal subjects, the total training time was only IVi minutes daily (25 percent of the total time performed by patients with COPD); however, the intensity was considerably higher with the subjects aiming for maximal mouth pressure with each inspiration, and the duty cycle was controlled. In this study, there was improvement in both strength, as measured by the MIP, and endurance, as measured by the ability to breathe while following pressure and flow target. If training for such short duration is efficacious in patients with COPD, this would facilitate the use of resistive training, but the appropriate proportions of intensity and duration remain to be determined. Work from studies in animals suggests that alteration of these factors determines the pattern of oxidative enzyme enhancement. High-intensity short-duration work predominantly affects the high-glycolytic low-oxidative fibers, whereas low-intensity longer-duration exercise predominantly affects high-oxidative low-glycolytic muscle fibers. Furthermore, in humans, it has been shown that a program of combined strength and endurance training will have the same effect as endurance training alone, but improvement in muscle strength is less than that achieved by a strength-only training program.
Improvement in maximal strength of the respiratory muscle is important because the RPE during resistive breathing is a function of the ratio of the Pm developed to the MIP. Previous studies in COPD have shown varying results with regard to MIP. Patients with COPD have generally not shown increases in this index, although increases were found in young children with cystic fibrosis and in the normal young women studied by Clanton and associates. In previous studies and our study, the breathing strategy may have been inadequate to elicit a true training response. This may be the case de novo, or changes may develop when patients are presented with smaller orifices. By means of appropriate alterations in breathing strategy when confronted with a smaller orifice, the increase in Pm can be minimized. This change may be self-defeating, as it may negate the training stimulus.
Several of the studies which have used resistive breathing have also tested the response to overall exercise as the measure of the efficacy of inspiratory muscular training; however, only a minority of patients in these studies have shown improved exercise capacity, although they improved their inspiratory muscle endurance for resistive breathing. Because the hy-perpnea of exercise is associated with volume overload, increased breathing frequency, and a decreased Ti or increased velocity of contraction, it may be that hyper-pneic training is the more suitable approach to improve exercise performance in patients with COPD. In this study, we found that the maximal sustained ventilatory capacity, a measure of endurance for hyperpnea, did not increase after the resistive training; however, because the patients failed to improve the resistive breathing, we cannot rule out the possibility that the lack of change in hyperpneic endurance was due to an inadequate training stimulus, rather than an inappropriate form of training. This question would need to be examined in patients who showed a definite improvement in resistive breathing performance.
Inadequate Rest Time
Resistive breathing produces “low-frequency fatigue” of the diaphragm and sternomastoid muscles, and recently it was shown that there is a reduction in muscle endurance in the presence of low-frequency fatigue. As recovery from low-frequency fatigue may take several hours, it is possible that the repetition of successive testing runs within a short time prevents optimal performance because of the cumulative effect of fatigue. In our study, there was at least a 24-hour rest between tests, a sufficient time to allow complete recovery of low-frequency fatigue. In several of the previous studies, the tests of resistive breathing performance were all done on the same day, with only short rests (20 to 30 minutes) between runs. As sufficient recovery time was not available, the baseline critical orifice may have been underestimated, and this would give an erroneous impression of improvement. In a study in which the pulmonary function of the patient was comparable to ours, the critical orifice before training was larger (0.48 cm vs 0.40 cm).
In our study, determination of the critical orifice was done based on the patients subjective response to the resistive breathing. The majority of other studies in COPD also used subjective end points for determination of the critical orifice. While hypercapnea is a potential problem during resistive breathing, it only occurred in a minority of cases at the time of failure at the critical orifice in the study by Asher et al, and it is probably not a major cause of endurance failure. Simple objective measures, such as abdominal paradox or respiratory alternans, have been used, but these also are not invariably present. Power spectral analysis of the electromyogram recorded from the diaphragm and accessory muscle has been used. In most studies, spectral changes have been examined by means of the H/L ratio. Whether or not this will prove to be an effective means of detecting fatigue remains controversial. Not all patients show evidence of electromyographic fatigue despite failure at a critical orifice, and in some patients, fatigue was only detectable from scalene or intercostal muscles and not from the diaphragm. Furthermore, at the end of a fatiguing run, the H/L ratio rapidly returns to baseline values, even though low-frequency fatigue persists for several hours. The initial studies of high/low ratio were done with strict control of breathing strategy. Whether or not results from these studies are applicable to spontaneously breathing patients with COPD is as yet unclear, especially as the H/L ratio was used in the resistive training studies without control of the breathing pattern. Furthermore, there is also some doubt that the high/low ratio is the best index to follow power spectral changes, and it has been suggested that measurement of the centroid frequency is more specific. In this study the investigators showed decreases in the centroid frequency without changes in the high/ low ratio, even though patients were performing sustainable eucapnic hyperpnea. For future studies, therefore, it would seem appropriate to adopt the approach used by some investigators’ who have controlled breathing strategy during the testing and have used objective measures, such as an inability to achieve target pressures or inspiratory flow rates, as a sign of failure.
Our patients with COPD performed resistive training as suggested by the manufacturers of the inspiratory resistive device and in a manner similar to that described in several of the previous studies. Despite this, our patients failed to improve ventilatory muscle endurance or strength. We believe that in order for resistive breathing to be successful, a feedback signal of the resistive load during training is essential. By this means, both the physician and the patient would be able to regulate the training intensity and ensure a satisfactory training stimulus. Furthermore, it would prevent the patient from adopting a breathing strategy which could improve resistive breathing performance without providing direct benefits to the respiratory muscles.
Figure 7. Rating of perceived effort (RPE) calculated from equation (Pm x Ti x fb) for five patients who increased with endurance time to 15 minutes; RPE before and after training was 152 ±36 and 123 ±13 (p = 0.054), respectively. Two patients who failed to increase their endurance time had the same or higher values for RPE (solid triangles).