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Diaphragm Length and Neural Drive after Lung Volume Reduction Surgery

Prince of Wales Medical Research Institute and University of New South Wales, Sydney, Australia

Correspondence and requests for reprints should be addressed to Simon Gandevia, M.D., Prince of Wales Medical Research Institute, Barker Street, Randwick NSW 2031, Australia. E-mail: s.gandevia{at}unsw.edu.au

Rationale: Patients with chronic obstructive pulmonary disease have shorter inspiratory muscles and higher motor unit firing rates during quiet breathing than do age-matched healthy subjects. Lung volume reduction surgery (LVRS) in patients with chronic obstructive pulmonary disease improves lung function, exercise capacity, and quality of life.

Objectives: We studied the effect of LVRS on length and motor unit firing rates of diaphragm and scalene muscles.

Methods: Diaphragm length was estimated by ultrasound and magnetometers, and firing rates were recorded with needle electrodes in patients (five females and seven males) with severe chronic obstructive pulmonary disease, before and after surgery.

Measurements and Main Results: Pre-LVRS total lung capacity was 135 ± 10% predicted (mean ± SD), and FEV₁ was 30 ± 12% predicted. After surgery, median firing frequency of diaphragmatic motor units fell from 17.3 ± 4.2 to 14.5 ± 3.4 Hz (p < 0.001), and scalene motor unit firing rates were reduced from 15.3 ± 6.9 to 13.4 ± 3.8 Hz (p < 0.001). Tidal volume and diaphragm length change during quiet breathing did not change, but at end expiration, the zone of apposition length of diaphragm against the rib cage (L_Zapp) increased (30 ± 28%, p = 0.004). Improvements in quality-of-life measures and exercise performance after surgery were related to increased forced vital capacity and L_Zapp.

Conclusions: Increased diaphragm length resulted in lower motor unit firing rates and reduced breathing effort, and this is likely to contribute to improved quality of life and exercise performance after LVRS.

Key Words: chronic obstructive pulmonary disease • electromyography • emphysema • pneumonectomy • ultrasound

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