"The skeletal muscle in chronic respiratory diseases", summary of the ERS research seminar in Rome, Italy, February 11-12 1999

Noninvasive Evaluation of Skeletal Muscle Oxidative Metabolism after Heart Transplant

PURPOSE

The main aim of the present study was to investigate skeletal muscle oxidative metabolism in heart transplant recipients (HTR) by noninvasive tools.

METHODS

Twenty male HTR (age 50.4 +/- 2.6 yr; mean +/- SE) and 17 healthy untrained age-matched controls (CTRL) performed an incremental exercise (IE) and a series of constant-load (CLE) moderate-intensity exercise tests on a cycloergometer. The following variables were determined: heart rate (HR); breath-by-breath pulmonary O2 uptake (VO2); and skeletal muscle (vastus lateralis) oxygenation indices by continuous-wave near-infrared spectroscopy. Changes in concentration of deoxygenated hemoglobin (Hb) and myoglobin (Mb) (Delta[deoxy(Hb + Mb)]), expressed as a fraction of values obtained during a transient limb ischemia, were taken as an index of skeletal muscle O2 extraction. "Peak" values were determined at exhaustion during IE. Kinetics of adjustment of variables were determined during CLE.

RESULTS

VO2peak, HRpeak, and Delta[deoxy(Hb + Mb)] peak were significantly lower in HTR than in CTRL (17.1 +/- 0.7 vs 34.0 +/- 1.9 mL.kg(-1).min(-1), 133.8 +/- 3.8 vs 173.0 +/- 4.8 bpm, and 0.42 +/- 0.03 vs 0.58 +/- 0.04, respectively). In HTR, Delta[deoxy(Hb + Mb)] increase at submaximal workloads was steeper than in CTRL, suggesting an impaired O2 delivery to skeletal muscles, whereas the lower Delta[deoxy(Hb + Mb)] peak values suggest an impaired capacity of O2 extraction at peak exercise. VO2 and HR kinetics during CLE were significantly slower in HTR than in CTRL, whereas, unexpectedly, no significant differences were found for Delta[deoxy(Hb+Mb)] kinetics (mean response time: 21.3 +/- 1.1 vs 20.2 +/- 1.2 s).

CONCLUSION

The findings confirm the presence of both "central" (cardiovascular) and "peripheral" (at the skeletal muscle level) impairments to oxidative metabolism in HTR. The noninvasiveness of the measurements will allow for serial evaluation of the patients, in the presence and/or absence of rehabilitation programs.

Dyspnea and leg effort during exercise

Dyspnea and leg effort are the major symptoms limiting exercise in healthy subjects and in patients with a variety of respiratory disorders. Quantitative measurement of both symptoms may be obtained by category scales such as VAS and Borg, with the latter being widely used. Furthermore, descriptor clusters of dyspnea help to assess some of the reasons for stopping exercise. The intensity of dyspnea and leg effort are similar in different disease states; this symmetry suggests that the limiting discomfort is a function of the intensity of increased motor drive to peripheral and respiratory muscles. An alternative explanation for the factors which limit exercise is that the subjects stop exercise volitionally when the discomfort associated with continuing exercise exceeds that which they are willing to tolerate. Muscle strength contributes to the intensity of dyspnea and leg effort at a given power output: the greater the muscle force, the lower the symptom. Symptoms also correlate with intensity and duration of a task by a power function in such a way that when minimizing the intensity of a given muscular task by prolonging the duration of activity, the symptom is drastically reduced. Skeletal muscle fatigue may be a factor limiting exercise tolerance both in healthy subjects and in patients with cardiorespiratory disorders. In conclusion, symptom measurement complements physiological measurements, both being essential to a comprehensive understanding of exercise tolerance.

Contractile leg fatigue after cycle exercise: a factor limiting exercise in patients with chronic obstructive pulmonary disease

We evaluated whether contractile fatigue of the quadriceps occurs after cycling exercise in patients with chronic obstructive pulmonary disease (COPD) and whether it could contribute to exercise limitation. Eighteen COPD patients performed two constant work-rate cycling exercises up to exhaustion. These tests were preceded by nebulization of placebo or 500 microg of ipratropium bromide. Muscle fatigue was defined as a postexercise reduction in quadriceps twitch force of more than 15% of the resting value. There was an increase in endurance time postipratropium compared with placebo nebulization (440 +/- 244 seconds vs. 322 +/- 188 seconds, p = 0.06). Nine patients developed contractile fatigue after placebo exercise. In these patients, ipratropium did not increase the endurance time (394 +/- 220 seconds with placebo vs. 400 +/- 119 seconds with ipratropium) despite an 11% improvement in FEV1. In the nine patients who did not fatigue after placebo exercise, endurance time increased from 249 +/- 124 seconds with placebo to 479 +/- 298 seconds with ipratropium (p < 0.05). There was a significant correlation between the improvement in endurance time with ipratropium and quadriceps twitch force at 10 minutes after placebo exercise (r = 0.59, p = 0.01). The occurrence of contractile fatigue during exercise may explain why bronchodilation fails to improve exercise tolerance in some COPD patients.

Exercise in chronic pulmonary disease: resistance exercise prescription

Resistance exercise training has received relatively little attention as a means to reduce the muscle dysfunction and ensuing exercise intolerance seen in chronic pulmonary diseases. To date, only a few studies have examined the characteristics of skeletal muscle function or its responsiveness to strength training in patients with chronic respiratory diseases. It is clear from these studies, however, that peripheral muscle, particularly muscles of ambulation, are weak in patients with lung disease, exhibiting effort-dependent strength scores that are 70--80% of these measures in age-matched healthy subjects. The degree to which this dysfunction is accounted for by deconditioning, disease-related myopathy, or other causes is unclear. It is evident, however, that patients with chronic respiratory diseases can acquire and maintain substantial improvements in skeletal muscle function, physical function, and quality of life through participation in a well-structured program of resistance exercise training. Despite the positive, albeit limited, evidence that skeletal muscle dysfunction may be improved with resistance training, no clear guidelines are available for this purpose. This review discusses the skeletal muscle dysfunction that accompanies chronic respiratory disease and presents strategies for resistance exercise training that may be considered as part of pulmonary rehabilitation. These strategies are derived from the successful outcomes noted in studies using resistance training in patients with COPD as well as on extrapolations from extant guidelines used to develop strength, power, and endurance in healthy individuals.