Results
Of 6461 VACHS patient records reviewed, 415 met the inclusion criteria. Patients were divided according to their mMRC score: 65 had mMRC score of 0, 87 had an mMRC score of 1, 2 had an mMRC score of 2, 146 had an mMRC of 3, and 115 had an mMRC score of 4. The population was primarily male (98.6%) and of Hispanic ethnicity (96.4%), with a mean age of 72 years (Table 1). Most patients (n = 269, 64.0%) were prior smokers, while 135 patients (32.5%) had never smoked, and 11 (2.7%) were current smokers. At baseline, 169 patients (41.4%) had interstitial lung disease, 39 (9.6%) had chest wall disorders, 29 (7.1%) had occupational exposure, 25 (6.1%) had pneumonitis, and 14 (3.4%) had neuromuscular disorders.
There was a statistically significant relationship between mMRC score and hospitalization and FEV1 but not TLC (Table 2). As mMRC increased, so did hospitalizations: a total of 168 patients (40.5%) were hospitalized; 24 patients (36.9%) had an mMRC score of 0, 30 patients (34.0%) had an mMRC score of 1, 2 patients (100%) had an mMRC score of 2, 54 patients (37.0%) had an mMRC score of 3, and 58 patients (50.0%) had an mMRC score of 4 (P = .04). Mean (SD) TLC values increased as mMRC scores increased. Mean (SD) TLC was 70.5% (33.0) for the entire population; 68.8% (7.2) for patients with an mMRC score of 0, 70.8% (5.8) for patients with an mMRC score of 1, 75.0% (1.4) for patients with an mMRC score of 2, 70.1% (7.2) for patients with an mMRC score of 3, and 71.5% (62.1) for patients with an mMRC score of 4 (P = .10) (Figure 1). There was an associated decrease in mean (SD) FEV1 with mMRC. Mean (SD) FEV1 was 76.2% (18.9) for the entire population; 81.7% (19.3) for patients with an mMRC score of 0, 80.9% (18) for patients with an mMRC score of 1, 93.5% (34.6) for patients with an mMRC score of 2, 76.2% (17.1) for patients with an mMRC score of 3, and 69.2% (19.4) for patients with an mMRC score of 4; (P < .001) (Figure 2).
The correlation between mMRC and FEV1 (r = 0.25, P < .001) was stronger than the correlation between mMRC and TLC (r = 0.15, P < .001). The correlations for DLCO (P < .001), DLCO/VA (P < .001), hemoglobin (P < .02), and PaO2 (P < .001) were all statistically significant (P < .005), but with no strong identifiable trend.
Discussion
The patient population of this study was primarily older males of Hispanic ethnicity with a history of smoking. There was no association between body mass index or smoking status with worsening dyspnea as measured with mMRC scores. We observed no significant correlation between mMRC scores and various factors such as comorbidities including heart conditions, and epidemiological factors like the etiology of lung disease, including both intrinsic and extrinsic causes. This lack of association was anticipated, as restrictive lung diseases in our study predominantly arose from intrinsic pulmonary etiologies, such as interstitial lung disease. A difference between more hospitalizations and worsening dyspnea was identified. There was a slightly higher correlation between FEV1 and mMRC scores when compared with TLC and mMRC scores concerning worsening dyspnea, which could indicate that the use of FEV1 should be preferred over previous recommendations to use TLC.10 Other guidelines have utilized exercise capacity via the 6-minute walk test as a marker of severity with spirometry values and found that DLCO was correlated with severity.11
The latest ERS/ATS guidelines recommend z scores for grading the severity of obstructive lung diseases but do not recommend them for the diagnosis of restrictive lung diseases.12 A z score encompasses diverse variables (eg, age, sex, and ethnicity) to provide more uniform and consistent results. Other studies have been done to relate z scores to other spirometry variables with restrictive lung disease. One such study indicates the potential benefit of using FVC alone to grade restrictive lung diseases.13 There continues to be great diversity in the interpretation of pulmonary function tests, and we believe the information gathered can provide valuable insight for managing patients with restrictive lung diseases.
Limitations
Only 2 patients reported an mMRC score of 2 in our study. This may have affected statistical outcomes. It also may reveal possible deficits in the efficacy of patient education on the mMRC scale. This study was also limited by its small sample size, single center location, and the distribution of patients that reported an mMRC favored either low or high values. The patients in this study, who were all veterans, may not be representative of other patient populations.
Conclusions
There continue to be few factors associated with the physiological severity of the defective oxygen delivery and reported dyspnea of a patient with restrictive lung disease that allows for an accurate, repeatable grading of severity. Using FEV1 instead of TLC to determine the severity of a restrictive lung disease should be reconsidered. We could not find any other strong correlation among other factors studied. Further research should be conducted to continue looking for variables that more accurately depict patient dyspnea in restrictive lung disease.
Acknowledgments
This study is based upon work supported by the Veterans Affairs Caribbean Healthcare System in San Juan, Puerto Rico, and is the result of work supported by Pulmonary & Critical Care Medicine service, with resources and the use of its facilities.