Background: In many studies of patients with muscle weakness, chronic hypercapnia has appeared to be out of proportion to the severity of muscle disease, indicating that factors other than muscle weakness are involved in CO2 retention. In patients with COPD, the unbalanced inspiratory muscle loading-to-strength ratio is thought to trigger the signal for the integrated response that leads to rapid and shallow breathing and eventually to chronic hypercapnia. This mechanism, although postulated, has not yet been assessed in patients with muscular dystrophy. Subjects: Twenty consecutive patients (mean age, 47.6 years; range, 23 to 67 years) were studied: 11 patients with limb-girdle dystrophy, 3 with Duchenne muscular dystrophy, 1 with Charcot- Marie-Tooth syndrome, 1 with Becker muscular dystrophy, 1 with myotonic dystrophy, 1 with facioseapulohumeral dystrophy, and 2 with amyotrophic lateral sclerosis, without any respiratory complaints. Seventeen normal subjects matched for age and sex were studied as a control group. Methods: Routine spirometry and arterial blood gases, maximal inspiratory and expiratory muscle pressures (MIP and MEP, respectively), and pleural pressure during maximal sniff test (Pplsn), were measured. Mechanical characteristics of the lung were assessed by evaluating lung resistance (RL) and dynamic elastance (Eldyn), Eldyn was assessed as absolute value and as percent of Pplsn; Eldyn (%Pplsn) indicates the elastic load per unit of inspiratory muscle force. Breathing pattern was assessed in terms of time (inspiratory time [T(I)]; respiratory frequency [Rf]) and volume (tidal volume [V(T)]) components of the respiratory cycle. Results: A rapid shallow breathing pattern, as indicated by a greater Rf/V(T) ratio and a lower T(I), was found in study patients compared to control subjects. Eldyn was greater in study patients, while MIP, MEP, and Pplsn were lower. Pa(CO2) inversely related to V(T), T(I), and Pplsn (p = 0.012, p = 0.019, and p = 0.002, respectively), whereas it was directly related to Rf, Rf/V(T), Eldyn, and Eldyn (%Pplsn) (p <0.004 to p <0.0001). Also Eldyn (%Pplsn) inversely related to T(I), and the latter positively related to V(T). In other words, increase in Eldyn (%Pplsn) was associated with decrease in T(I), and the latter was associated with lower V(T) and greater Pa(CO2). Mechanical and breathing pattern variables were introduced in a stepwise multiple regression that selected Eldyn (%Pplsn) (p <0.0001; r2 = 0.62) as a unique independent predictor of Pa(CO2). Conclusions: The present study shows that in patients with neuromuscular disease, elastic load and respiratory muscle weakness are responsible for a rapid and shallow breathing pattern leading to chronic CO2 retention.
|Number of pages||7|
|Publication status||Published - 2000|
- Breathing pattern
- Neuromuscular disorders
- Respiratory muscles
ASJC Scopus subject areas
- Pulmonary and Respiratory Medicine