Chest
Volume 117, Issue 2, February 2000, Pages 447-453
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Clinical Investigations
Mechanism of CO2 Retention in Patients With Neuromuscular Disease

https://doi.org/10.1378/chest.117.2.447Get rights and content

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 facioscapulohumeral 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 [Ti]; respiratory frequency [Rf]) and volume (tidal volume [Vt]) components of the respiratory cycle.

Results

A rapid shallow breathing pattern, as indicated by a greater Rf/Vt ratio and a lower Ti, was found in study patients compared to control subjects. Eldyn was greater in study patients, while MIP, MEP, and Pplsn were lower. Paco2 inversely related to Vt, Ti, and Pplsn (p = 0.012, p = 0.019, and p = 0.002, respectively), whereas it was directly related to Rf, Rf/Vt, Eldyn, and Eldyn (%Pplsn) (p < 0.004 to p < 0.0001). Also Eldyn (%Pplsn) inversely related to Ti, and the latter positively related to Vt. In other words, increase in Eldyn (%Pplsn) was associated with decrease in Ti, and the latter was associated with lower Vt and greater Paco2. 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 Paco2.

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.

Section snippets

Subjects

Twenty consecutive patients (10 men) with a mean age of 47.6 years (range, 23 to 67 years) were studied: 11 patients with limb-girdle dystrophy (LGD), 3 with Duchenne muscular dystrophy, 2 with amyotrophic lateral sclerosis, 1 with Charcot-Marie-Tooth syndrome, 1 with Becker muscular dystrophy, 1 with MD, 1 with facioscapulohumeral dystrophy, and no respiratory complaints. Nine were ambulatory, and 11 were wheel-chair bound. The standard criteria were used to select patients.20, 21

None of the

Results

Clinical, anthropometric, and respiratory function characteristics of the patients (and control subjects) are shown in Table 1. As shown in Table 1, vital capacity (VC) was reduced in 11 patients, as was total lung capacity in 9. The means of MIP (47.8 ± 28.3 cm H2O; range, 11 to 127 cm H2O; p = 0.00001) and MEP (49.5 ± 26.2 cm H2O; range, 15 to 104 cm H2O; p = 0.00002) were significantly lower than in control subjects. In 11 patients and 10 patients, the values of MIP and MEP, respectively,

Discussion

We have found that in patients with NMD, Eldyn (%Pplsn) is the strongest predictor of the variance in Paco2. Increased Eldyn (%Pplsn) was associated with a decreased Ti, which truncates Vt, thereby leading to chronic CO2 retention (Paco2).

One can argue that this type of study should pertain to patients with more advanced disease. In fact, nine of the patients had Paco2 values < 42 mm Hg, which does not indicate a definite CO2 retention. However, a sustained V˙e at a level required to

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