The diaphragm is the major inspiratory muscle of humans. In this conceptual review, we sought to (a) present an overview of diaphragmatic action in normal humans; (b) explain why chronic obstructive pulmonary disease (COPD), the most common chronic pulmonary disease, interferes with the normal action of the diaphragm; (c) present the concept of diaphragmatic fatigue; (d) discuss the relationship between severe COPD and diaphragmatic fatigue; (e) present work from our laboratory indicating that at the cellular and molecular level, severe COPD elicits adaptations that would be expected to increase the fatigue-resistance of the human diaphragm; (f) carefully define the group of patients who exhibit these adaptations; (g) comment on the nature of the stimulus that elicits the fast-to-slow fiber type transformations associated with severe COPD; and (h) relate the diaphragmatic adaptations elicited by COPD to sports medicine.
OVERVIEW OF DIAPHRAGMATIC FUNCTION IN NORMAL HUMANS
Figure 1 (top) presents a schematic of diaphragmatic function in normal humans. The figure indicates that the diaphragm can be conceptually divided into a region whose upper boundary is adjacent to the lung—we call this area the diaphragmatic dome— and a region that is directly apposed to the rib cage—we call this region the zone of apposition. The figure shows that the fibers in the zone of apposition are parallel to the craniocaudal (i.e., head-to-feet) axis of the body. Therefore, contraction of these fibers will cause downward displacement of the diaphragmatic dome and thereby increase the volume of the thoracic cavity; we refer to this inspiratory action of the diaphragm as the insertional component. This downward displacement of the diaphragm causes an increase in abdominal pressure, and this increase in pressure is transmitted to the zone of apposition, where it results in an upward and outward movement of the lower rib cage; we refer to this inspiratory action of the diaphragm as the appositional component.
EFFECTS OF COPD ON DIAPHRAGM FUNCTION.
Figure 1 (bottom) presents a schematic view of the deleterious effect of COPD on diaphragmatic function. First, the figure shows that at the end of a normal expiration, the volume of gas in the thorax is greatly increased; we use the term hyperinflation to refer to this increase of lung volume due to COPD. This hyperinflation markedly attenuates the zone of apposition and thereby interferes with both insertional and appositional components of diaphragmatic inspiratory function. In addition, in COPD, the diaphragmatic fibers have a lateral-to-medial orientation, and therefore contraction of these diaphragmatic fibers limits the expansion of the lower rib cage.
CONCEPT OF DIAPHRAGMATIC FATIGUE
Muscle fatigue (12) is a condition in which there is loss in the capacity to develop force and/or velocity of a muscle under load; the condition is reversible with rest. In seminal studies carried out more than two decades ago, Roussos and Macklem (14) demonstrated that the diaphragm, the major inspiratory muscle of humans, can undergo fatigue in humans. The site of muscular fatigue can occur anywhere in the motor pathway between the cerebral cortex and the diaphragmatic muscle fibers per se; however, previous workers have demonstrated that the clinically important type of fatigue (i.e., low-frequency diaphragmatic fatigue) occurs within the diaphragmatic myofibers (15). Hereafter, we use the term diaphragmatic fatigue to refer exclusively to low-frequency diaphragmatic fatigue.
RELATIONSHIP BETWEEN SEVERE COPD AND DIAPHRAGMATIC FATIGUE
Bellemare and Grassino (3) used their diaphragmatic time-tension index to show that during resting breathing, COPD patients are closer to the development of fatigue than are normal subjects. Therefore, with the increasing diaphragmatic energy utilization associated with exercise (i.e., any increase in physical activity), one might expect COPD patients to exhibit exercise-induced diaphragmatic fatigue. Using the technique of bilateral phrenic nerve stimulation, we and others have demonstrated that various types of exercise can elicit diaphragmatic fatigue in normal subjects (see Levine et al. (8) for a list of these references). Surprisingly, there has been little mention in the literature of exercise-induced diaphragmatic fatigue in COPD subjects. Indeed, Polkey et al. (13) reported that diaphragmatic fatigue did not develop in patients with severe COPD during treadmill exercise to “exhaustion.”
We do not interpret the data of Polkey and colleagues as indicating that patients with severe COPD never develop exercise-induced diaphragmatic fatigue. Rather, we interpreted their observations as indicating that it is unusual for patients with severe COPD to develop exercise-induced diaphragmatic fatigue. Three categories of mechanisms exist to account for this surprising finding. (a) Some sensation of increased ventilatory effort caused these patients to stop exercising before the development of diaphragmatic fatigue. (b) These severe COPD patients terminated exercise due to some sensation unrelated to shortness of breath; for example, recent work indicates that approximately one third of patients with COPD stop exercising due to leg muscle sensations (7). (c) Severe COPD elicits adaptations in the diaphragm that render it relatively resistant to fatigue. For the past several years, we have been carrying out studies to test the third of these hypotheses, and this work on a possible increase in diaphragmatic fatigue resistance constitutes the major focus of this review.
OUR APPROACH TO TESTING THE HYPOTHESIS THAT SEVERE COPD ELICITS ADAPTATIONS THAT INCREASE THE FATIGUE RESISTANCE OF THE DIAPHRAGM
In our studies, we applied the approach of Monod and Scherrer (10) to the diaphragmatic myofibers. During either rest or nonfatiguing work, ATP generation is equal to ATP consumption. In contrast, during fatiguing exercise, ATP consumption exceeds ATP generation. Therefore, an increase in fatigue resistance of a muscle can be effected by either a decrease in the rate of ATP consumption or an increase in the rate of ATP generation. In this review, we present molecular and cellular evidence that the human diaphragm adapts to severe COPD by both mechanisms (see Figure 2).
Decreases in Rate of ATP Consumption
Myosin Heavy Chains
In theory, a decrease in diaphragmatic ATP utilization represents one possible method for increasing fatigue resistance. Because the myosin heavy chain (MHC) accounts for approximately 70% of the ATP consumption by diaphragmatic myofibers, a change toward an MHC with a lower ATPase activity would decrease ATP consumption by the diaphragmatic myofibers. The human diaphragm has two major categories of MHCs: slow (i.e., type I) and fast (i.e., type II) MHCs. Much work indicates that the slow MHC has a lower ATPase rate than the fast MHCs. Accordingly, we hypothesized that severe COPD elicits adaptations in diaphragmatic MHCs characterized by a fast-to-slow transformation of MHCs (i.e., type II to type I).
We tested our hypotheses by carrying out measurements on biopsy samples of the right costal diaphragm in patients with severe COPD who were undergoing lung volume reduction surgery, a new surgical therapy for far-advanced emphysema. We compared the MHC proportions in these biopsy samples with those obtained from the diaphragms of age- and gender-matched subjects who were undergoing surgery for solitary pulmonary nodules; this latter group of patients had virtually normal pulmonary function tests.
Figure 3 shows SDS-polyacrylamide gels for three of the COPD subjects and three of the control subjects. All of the control gels show three distinct bands, whereas the upper band is faint to absent in the COPD gels. We used immunoblotting to definitively demonstrate that the lowest band represented MHC I and the middle band represented MHC IIa; these immunoblotting studies showed the upper band was an MHC that did not react with either the anti-MHC I or the anti-MHC IIa antibodies; therefore, we designated this MHC as MHC IIx. Visual inspection of the gels suggests that COPD is associated with increases in the proportion of MHCs accounted for by slow MHC (i.e., MHC I). Figure 4 provides quantitative data showing that COPD diaphragms contained a greater proportion of MHC I and lesser proportions of MHCs IIa and IIx than control diaphragms. These SDS-polyacrylamide gel results are consistent with our hypothesis that COPD elicits a fast-to-slow transformation of diaphragmatic MHCs. More recently, Mercadier et al. (9) reported similar findings.
Immunocytochemistry constitutes another method for comparing COPD and control diaphragms with respect to proportions of MHCs. The most widely used method for typing muscle fibers is that of Brooke and Kaiser; this method classifies myofibers, as well as assigns names to these fibers, based on the MHCs that they contain. Normal human diaphragms contain three predominant fiber types: type I, which contains only MHC I; type IIA, which contains only MHC IIA; and a hybrid fiber, which expresses both IIa and IIx and is termed type IIax.
At the present time, there is general agreement that antibody NOQ7.54D reacts only with type I fibers. Figure 5 (top) compares representative COPD and control diaphragms with respect to the proportion of fibers that react with the anti-slow MHC antibody; visual comparison of the representative COPD and control diaphragm indicates that the COPD diaphragm contains a greater proportion of slow (type I) fibers than the control diaphragm.
We also incubated our sections with monoclonal antibody SC-71. This antibody is specific for MHC IIa and therefore will react with both IIa and IIax MHCs (i.e., with all fast fibers in the human diaphragm). Figure 5 (bottom) compares serial sections of the same representative diaphragms with respect to the proportion of fibers reacting with the SC-71 antibody; the panel shows that the representative COPD diaphragm contained a lesser proportion of fast fibers than the representative control diaphragm.
Figure 6 shows a statistical comparison of COPD and control diaphragms with respect to fiber type proportions. The figure indicates that the above-noted immunocytochemical differences on the representative sections are highly statistically significant in the group comparisons of COPD and control diaphragms (11). In summary, we have used both SDS-polyacrylamide gels and immunocytochemistry to demonstrate that severe COPD elicits diaphragmatic adaptations characterized by a fast-to-slow transformation of MHCs and fiber types.
INCREASED CAPACITY FOR ATP GENERATION
In human muscle fibers, citric acid cycle (i.e., Krebs’ cycle) oxidations followed by phosphorylation is the major pathway for ATP generation. Therefore, previous workers have used the activity of representative Krebs’ cycle enzymes to make predictions of aerobic capacity to generate ATP. Succinic dehydrogenase (SDH) is a representative Krebs’ cycle mitochondrial membrane-bound enzyme that catalyzes the oxidation of succinate to fumarate. In preliminary studies, we carried out standard biochemical measurements of SDH activity in COPD and control diaphragms; we noted that SDH activities of the COPD diaphragms were almost twice those of controls. Therefore, using SDH activity as a marker of oxidative phosphorylation, COPD diaphragms would appear to have a greater capacity for oxidative phosphorylation than control diaphragms.
HOW DO COPD-INDUCED ADAPTATIONS RENDER THE DIAPHRAGM MORE RESISTANT TO FATIGUE?
At the outset , we acknowledge that our data in no way demonstrate that the diaphragms of patients with severe COPD exhibit increased fatigue resistance. Indeed, we believe that further studies relating clinical measurements of diaphragmatic fatigue to the measurements that we have carried out on diaphragmatic biopsies are necessary to produce definitive data on this point. However, our data indicate several possible mechanisms that would tend to increase fatigue resistance in COPD diaphragms. First, due to the increased activity of mitochondrial oxidative enzymes, the COPD myofibers will have an increased ability to aerobically supply ATP to the MHCs; therefore, during increases in diaphragmatic ATP utilization, such as during exercise, COPD myofibers will have less of a need to utilize glycolysis and the creatine phosphokinase pathways (i.e., phosphocreatine shuttle) for ATP generation. This is important because increases in glycolytic flux are associated with an increased generation of hydrogen ions, whereas increases in flux through creatine phosphokinase pathway are associated with increases in the activity (i.e., concentration) of inorganic phosphate. Increases in either hydrogen ion or inorganic phosphate activity are known to impair force generation or shortening due to a direct depressant effect on the cross-bridge cycling phase of myofiber contraction (6). In addition, some evidence exists that increases in hydrogen ion activity can interfere with myofiber contraction by impairing excitation-contraction coupling (6). Second, for any given increase in either hydrogen or phosphate ion activity, type I fibers will exhibit less impairment of contraction than will type II fibers; therefore, due to its greater proportion of type I fibers, the COPD diaphragm will be more resistant to fatigue mediated by either increases in hydrogen or inorganic phosphate activity (6).
CHARACTERISTICS OF PATIENTS SHOWING ADAPTATIONS ELICITED BY SEVERE COPD
First, previous investigators have reported that moderate COPD does not elicit either fast-to-slow fiber type adaptations or increases in activity of mitochondrial oxidative enzymes. Our unpublished data show similar results. Because COPD elicits decreases in expiratory flow rates and increases in the volume of air remaining in the lungs after a maximal forced expiration (i.e., residual volume), these measurements are used as markers of the severity of COPD. All of the COPD subjects that we have reported on had an FEV1.0 that was less than 30% of the predicted normal and a residual volume that was greater than 200% of the predicted normal. The American Thoracic Society defines severe COPD as having an FEV1.0 of less than 50% of the predicted normal (1). Therefore, all of the COPD patients that we have reported on had severe COPD; indeed, we believe that only severe COPD elicits the diaphragmatic adaptations that we have described.
NATURE OF THE STIMULUS THAT ELICITS THE DIAPHRAGMATIC ADAPTATIONS ASSOCIATED WITH SEVERE COPD
Our current knowledge of the pathogenesis of severe COPD is that this lung disease goes through the following sequential stages: normal lung, mild COPD, moderate COPD, and severe COPD. As one progresses from a normal lung to severe COPD, both airflow limitation and hyperinflation progressively increase over the course of several decades; this progression results in a 4- to 5-fold increase in resting diaphragmatic work, as assessed by the diaphragmatic time-tension index (TTdi) (4,5). We hypothesize that once a threshold value of TTdi is reached, this continuous (i.e., 24 h/d) increase in diaphragm work rate accounts for the diaphragmatic adaptations associated with severe COPD.
RELEVANCE TO SPORTS MEDICINE
Can endurance training, such as that used in sports medicine, account for the diaphragmatic adaptations associated with severe COPD? Endurance training of human limb muscles can produce increases in the activities of mitochondrial oxidative enzymes; however, at the present time, we know of no situation where any type of endurance training has elicited a fast-to-slow (i.e., type II to type I) transformation of MHCs and fiber types. However, sports medicine experts point out that endurance training may have the ability to effect this type of transformation, “but so far no lengthy human training study has unambiguously demonstrated such a shift” (2). We respectfully suggest that the MHC and fiber type adaptations noted in our COPD diaphragms represent an example of fast-to-slow fiber type transformations elicited by long-term endurance training.
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Keywords:© 2001 American College of Sports Medicine
muscle fatigue; myosin heavy chain; slow fibers; fast fibers; fiber type transformation; mitochondrial oxidative enzymes; endurance training