RELATIONSHIP BETWEEN RESTING HYPERCAPNIA AND PHYSIOLOGIC PARAMETERS BEFORE AND AFTER LUNG VOLUME REDUCTION SURGERY IN SEVERE CHRONIC OBSTRUCTIVE PULMONARY DISEASE

	To the Editor :

Shade and colleagues (1) reported that in 33 patients, the Pa_CO2 decreased approximately 2 mm Hg when measured 3 to 6 mo following lung volume reduction surgery. They also noted that the patients with the highest Pa_CO2 levels prior to surgery had the greatest reductions, and that the changes in Pa_CO2 were well correlated with the changes seen in the FEV₁, the maximum inspiratory pressure, the diffusing capacity, and the RV/TLC.

In an earlier study of 46 patients (2) we found that the Pa_CO2 decreased 3 mm Hg, 3 mo following lung volume reduction surgery, but we found no correlations between the change in Pa_CO2 and the preoperative Pa_CO2, or the change in the FEV₁, the diffusing capacity, the RV, or the TLC following the operation.

Shade and colleagues neither noted these disparities nor tried to explain them in the Discussion section of their article. We would be interested in their doing so.

Department of MedicineUniversity of Colorado Health Sciences CenterDenver, Colorado

Joshua O. Benditt

Respiratory Therapy and Lung Volume ReductionSurgery ProgramUniversity of Washington Medical CenterSeattle, Washington

1. Shade, D. Jr., F. Cordova, Y. Lando, J. M. Travaline, S. Furukawa, A. M. Kuzma, and G. J. Criner. 1999. Relationship between resting hypercapnia and physiologic parameters before and after lung volume reduction surgery in severe chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 159: 1405-1411 [Abstract/Free Full Text].

2. Albert, R. K., J. O. Benditt, J. Hildebrandt, D. E. Wood, and M. P. Hlastala. 1998. Lung volume reduction surgery has variable effects on blood gases in patients with emphysema. Am. J. Respir. Crit. Care Med. 158: 71-76 [Abstract/Free Full Text].

	From the Authors:

Drs. Albert and Benditt (1) previously reported in 46 patients with severe COPD that Pa_CO2 decreased 3 mm Hg three months following LVRS but failed to find correlations between the changes in Pa_CO2, Pa_O2, or the AaPO₂ and the preoperative Pa_CO2, Pa_O2, AaPO₂, FEV₁, residual volume, total lung capacity, or diffusing capacity. Furthermore, they found no correlation between the changes in Pa_CO2, Pa_O2, or AaPO₂ and the changes in FEV₁, RV, TLC, or DL_CO occurring as a result of the operation.

In contrast, we (2) studied 33 consecutive severe COPD patients and reported that resting Pa_CO2 levels pre-LVRS correlated with baseline FEV₁, RV/TLC and maximum minute ventilation, maximum tidal volume and dead space at maximum exercise. Previous authors have also shown a correlation of Pa_CO2 levels in COPD patients with degree of hyperinflation, airflow obstruction, inspiratory muscle strength, dead space, and breathing pattern (3, 4). Resting Pa_CO2 level pre-LVRS showed no significant correlation with percent of ideal body weight, or DL_CO/VA in our study. To examine factors responsible for the decrease in Pa_CO2 observed post-LVRS, we found by single and multiple linear regression analysis that decreases in Pa_CO2 significantly correlated with increases in FEV₁, RV/TLC, DL_CO, and maximum inspiratory mouth pressure (PI_max). Furthermore, decreases in Pa_CO2 post-LVRS also correlated with changes in maximum minute ventilation and maximum tidal volume during symptom-limited treadmill exercise.

Our data was obtained in 33 consecutive patients undergoing LVRS with pulmonary function testing performed in one laboratory, with the same technician in most cases, studying the same patient before and after LVRS. All patients were screened by the same group of pulmonary physicians before pulmonary function testing to ensure clinical stability. All pulmonary function tests reported are those performed after bronchodilator administration. Pulmonary function studies, exercise testing, and arterial blood gas analysis were done on the same day or over two consecutive days. All LVR S was performed by the same cardiothoracic surgeon. We believe that this carefully controlled approach is responsible for the correlations found in our study. Previous studies have shown that 20% of COPD patients have changes in gas exchange in response to optimization of therapy, necessitating the presence of a stable state before measurement of other variables (5).

In the paper by Albert and colleagues, the description of pulmonary function test performance, and the relationship of testing to arterial blood gas performance were not provided. Moreover, follow-up studies were only collected in 61% of the patients who were eligible for their study. As the authors themselves suggested, a "more complete or more prolonged follow-up might have altered the mean changes in the Pa_CO2, Pa_O2, and/or the AaPO₂ observed."

GERARD J. CRINER

Pulmonary and Critical Care Medicine Temple Lung Center Philadelphia, Pennsylvania

1. Albert, R. K., J. O. Benditt, J. Hildebrandt, D. E. Wood, and M. P. Hlastala. 1998. Lung volume reduction surgery has variable effects on blood gases in patients with emphysema. Am. J. Respir. Crit. Care Med. 158: 71-76 .

2. Shade, D. Jr., F. Cordova, Y. Lando, J. M. Travaline, S. Furukawa, A. M. Kuzma, and G. J. Criner. 1999. Relationship between resting hypercapnia and physiologic parameters before and after lung volume reduction surgery in severe chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 159: 1405-1411 .

3. Rochester, D. F., and N. M. T. Braun. 1985. Determinants of maximal inspiratory pressure in chronic obstructive pulmonary disease. Am. Rev. Respir. Dis. 132: 42-47 [Medline].

4. Begin, P., and A. Grassino. 1991. Inspiratory muscle dysfunction and chronic hypercapnia in chronic obstructive pulmonary disease. Am. Rev. Respir. Dis. 143: 905-912 [Medline].

5. Nocturnal Oxygen Trial Group. 1980. Continuous or nocturnal oxygen therapy in hypoxemic chronic obstructive lung disease. Am. Intern. Med. 93: 391-398 .