Respiratory Muscles

Richardson and coworkers assessed skeletal muscle function in patients with COPD (n = 6) during both cycle and single-leg knee-extensor exercise in comparison with that of age- and activity-matched healthy control subjects. Arterial and femoral venous blood sampling, thermodilution blood flow measurements, and needle biopsies allowed the assessment of muscle oxygen supply, utilization, and structure. Maximal work rates and single-leg

O₂max during bicycle exercise were significantly reduced in patients with COPD as compared with control subjects (0.37 ± 0.1 vs. 0.63 ± 0.1 L/minute). This difference in

O₂max disappeared during knee-extensor exercise, whereas maximal work capacity was reduced (flywheel resistance: 612 ± 81 vs. 923 ± 198 g in subjects with COPD and control subjects, respectively). Patients with COPD had a greater proportion of less efficient type II muscle fibers as compared with control subjects, whereas muscle fiber cross-sectional areas, capillarity, and mitochondrial volume density were not different between the groups. This study reveals significant changes in skeletal muscle structure and function in patients with COPD and suggests that those patients have a tendency for inefficient work economy in skeletal muscle, becoming more evident during knee-extensor exercise, when muscle function is not overshadowed by decreased lung function.

The adaptive response of inspiratory muscles to increased workloads includes increases in oxidative enzymes and changes in expression of contractile proteins, but the effect on mitochondrial function is not known. To study the in situ properties of mitochondria of respiratory muscles, Ribera and coworkers obtained biopsies of the costal diaphragm and external intercostals in 9 patients with emphysema and in 11 control subjects. Maximal oxidative capacity was 135% higher in the diaphragm and 37% higher in the external intercostals in the patients than in the control subjects. Maximal oxidative capacity was correlated with percent predicted FEV₁ (diaphragm: r = -0.64; intercostals: r = -0.67) and with the ratio of residual volume to total lung capacity (diaphragm: r = 0.64; intercostals: r = 0.63). The slow myosin heavy chain isoform was increased in the diaphragm of the patients. The authors conclude that maximal respiration of mitochondria is increased in both the diaphragm and external intercostal muscles of patients with emphysema, and that the associated improvement in the coupling of oxidation to phosphorylation suggests increased efficiency of ATP production by mitochondria.

To determine the relationship between diaphragmatic fiber type and severity of airway obstruction, Levine and coworkers did biopsies of the costal diaphragm in 40 patients with COPD (FEV₁ ranged from 16 to 118% of predicted). The relationship between the proportion of pure Type I fibers and FEV₁ was exponential (r = 0.844): the proportion showed little increase as FEV₁ decreased from 118 to 60% of predicted, but thereafter increased appreciably. Patients with severe COPD exhibited lower levels of maximal specific force in single fibers than did patients with normal lung function. Type I fibers generated lower force than did Type II fibers. The authors conclude that patients with COPD develop an increase of Type I fibers in their costal diaphragm when FEV₁ falls below 60% of predicted and that force generation is also decreased secondary to adaptations within each fiber type.

In a state of the art review article on disorders of the respiratory muscles, Laghi and Tobin discuss the effect of COPD on the respiratory muscles.

Citations 1-4 of 4 total displayed.

Reduced Mechanical Efficiency in Chronic Obstructive Pulmonary Disease but Normal Peak $V$ O₂ with Small Muscle Mass Exercise

Russell S. Richardson, Bryan T. Leek, Timothy P. Gavin, Luke J. Haseler, Sundar R. D. Mudaliar, Robert Henry, Odile Mathieu-Costello, and Peter D. Wagner
Am. J. Respir. Crit. Care Med. 169: 89 -96. First published online as doi:10.1164/rccm.200305-627OC [Abstract] [Full text]

Human Diaphragm Remodeling Associated with Chronic Obstructive Pulmonary Disease: Clinical Implications

Sanford Levine, Taitan Nguyen, Larry R. Kaiser, Neal A. Rubinstein, Greg Maislin, Christopher Gregory, Lawrence C. Rome, Gary A. Dudley, Gary C. Sieck, and Joseph B. Shrager
Am. J. Respir. Crit. Care Med. 168: 706 -713. First published online as doi:10.1164/rccm.200209-1070OC [Abstract] [Full text]

Disorders of the Respiratory Muscles

Franco Laghi and Martin J. Tobin
Am. J. Respir. Crit. Care Med. 168: 10-48. [Abstract] [Full text]

Mitochondrial Electron Transport Chain Function Is Enhanced in Inspiratory Muscles of Patients with Chronic Obstructive Pulmonary Disease

Florence Ribera, Benoit N'Guessan, Joffrey Zoll, Dominique Fortin, Bernard Serrurier, Bertrand Mettauer, Xavier Bigard, Renée Ventura-Clapier, and Eliane Lampert
Am. J. Respir. Crit. Care Med. 167: 873 -879. First published online as doi:10.1164/rccm.200206-519OC [Abstract] [Full text]

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