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Published ahead of print on February 20, 2004, doi:10.1164/rccm.200309-1287OC
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American Journal of Respiratory and Critical Care Medicine Vol 169. pp. 1034-1040, (2004)
© 2004 American Thoracic Society

Nitric Oxide Diffusing Capacity and Alveolar Microvascular Recruitment in Sarcoidosis

Anagha R. Phansalkar, Chad M. Hanson, Ahmed R. Shakir, Robert L. Johnson, Jr. and Connie C. W. Hsia

Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas

Correspondence and requests for reprints should be addressed to Connie C.W. Hsia, M.D., Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390–9034. E-mail: connie.hsia{at}utsouthwestern.edu


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
We measured diffusing capacities for carbon monoxide (DLCO) and nitric oxide, lung volume, and cardiac output by a rebreathing technique at two alveolar O2 tensions (PAO2) at rest and exercise. Membrane diffusing capacity for CO (DMCO) and VC were estimated from DLCO by the Roughton-Forster (RF) method and also from simultaneous lung diffusing capacity for NO and DLCO measured at one O2 tension (modified RF method). Estimates by these methods agreed closely in normal subjects (Tamhane et al., Chest 2001;120:1850–1856). Using these methods, we studied patients with stages II–III pulmonary sarcoidosis to determine (1) whether the modified RF method accurately estimates DMCO and VC in parenchymal disease and (2) whether sarcoidosis alters recruitment of diffusing capacity with respect to cardiac output. In patients, DMCO and VC estimated by the two methods agreed closely. DMCO was disproportionately reduced relative to VC at any given cardiac output, and the slope of the relationship between DLCO and cardiac output was moderately, though significantly, below normal. We conclude that in sarcoidosis (1) the modified RF method provides comparable estimates of DMCO and VC as the standard RF method and (2) the limitation to diffusive gas transport resides primarily in the membrane barrier, although recruitment of microvascular reserves is also modestly impaired.

Key Words: membrane diffusing capacity • nitric oxide • capillary blood volume • exercise • Roughton-Forster relationship

Lung diffusing capacity for carbon monoxide (DLCO) measures uptake of CO across alveolar–capillary membrane and its subsequent binding to hemoglobin. Contribution of the membrane and erythrocyte to overall CO uptake can be partitioned into membrane diffusing capacity for CO (DMCO) and VC, respectively, from DLCO measured at two alveolar O2 tensions (PAO2) by the Roughton-Forster (RF) method (1). Adding a small amount of nitric oxide (NO) to the test gas allows simultaneous measurement of DL for NO (DLNO). Because NO binds hemoglobin 280 times faster than CO, DLNO primarily reflects DM (DLNO {cong} DMNO) (2, 3). In normal subjects, a strong correlation exists between DLNO and DMCO estimated by the RF method (3); however, it is not known whether the relationship holds in the presence of parenchymal lung disease where gas diffusion coefficients in lung tissue may be altered and significant diffusion–perfusion heterogeneity might exist. Our first hypothesis is that a strong correlation between DLNO and DMCO also exists in patients with pulmonary sarcoidosis. If so, it would allow a more accurate and direct assessment of DLCO, DLNO, DMCO, and VC in patients using a modified RF method in which all measurements are obtained in a single maneuver at the same PAO2 and cardiac output (3).

Whether measured for O2, CO, or NO, diffusing capacity normally increases from rest to exercise in a linear relationship with respect to cardiac output (4); linear relationships are also observed for DMCO and VC. The increase reflects recruitment of alveolar–capillary reserves for gas exchange that are not fully used at rest, that is, unfolding of membrane surfaces with increased tidal volume, opening and distension of capillaries with increased pulmonary perfusion, and a more homogeneous erythrocyte flow pattern within and among capillaries. In lung disease, the pattern of DL recruitment determines the propensity for developing arterial hypoxemia (5). For example, in interstitial pulmonary fibrosis (6), alveolar–capillary reserves are diminished, but in addition, the remaining reserves cannot be recruited normally due to a thickened blood–gas barrier as well as a reduced surface area and distensibility of the capillary bed. Consequently, DLCO fails to increase as cardiac output increases, and arterial oxygen saturation declines on exercise (6). In contrast, after pneumonectomy when the remaining lung is normal, that is, a simple loss of alveolar–capillary units without membrane thickening or reduced microvascular compliance, existing reserves can be recruited normally to mitigate the development of arterial hypoxemia during exercise (5, 7). In chronic congestive heart failure, DLCO at a given cardiac output is reduced, but DLCO recruitment is normal within the restricted range of cardiac output; hence, arterial hypoxemia is generally not a major feature-limiting exercise (5, 8).

In pulmonary sarcoidosis, patchy alveolar deposition of noncaseating granulomas often selectively obliterates capillaries, accompanied by variable inflammation that may also cause membrane thickening. The extent to which these pathologic changes alter microvascular recruitment is unknown; no previous study had taken into account the effect of cardiac output on DL and its components. Our second hypothesis is that despite a loss of alveolar–capillary surfaces, microvascular recruitment in the remaining lung is well preserved in patients with stages II–III pulmonary sarcoidosis but without overt fibrosis. We assessed the relationships of DLCO, DLNO, and DMCO with respect to cardiac output, as well as the partition of DMCO and VC at rest and during exercise in patients for comparison with normal subjects. Some of the results of these studies have been previously reported in the form of abstracts (9, 10).


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
Twenty-five nonsmoking patients with stages II–III pulmonary sarcoidosis (39.6 ± 9.2 [SD] years) referred from the Sarcoidosis Clinic (Parkland Health Systems, Dallas, TX) were studied at rest; 16 patients consented to and underwent exercise studies. None had ischemic heart disease, neurologic disease, or hospitalization within 6 months. Eighteen healthy nonsmokers (33.9 ± 16.5 years) were studied, including 12 subjects previously reported (3). All subjects signed an informed consent. The institutional review board at the University of Texas Southwestern Medical Center approved the protocols.

Apparatus
Subjects exercised on a bicycle ergometer (Ergometrics-800; Sensormedics, Yorba Linda, CA) breathing through three-way pneumatic valves (#8500; Hans Rudolph, Kansas City, MO) (4, 11). Airflow (VMM2; Interface Associates, Aliso Viejo, CA), ventilatory parameters (Sensormedics Vmax229), and electrocardiogram were recorded continuously. Transcutaneous O2 saturation was measured in patients (N-180; Nelcor, Carlsbad, CA). At a selected end-expiration, subjects inspired to total lung capacity one breath of a test mixture (0.3% methane, 0.4% acetylene, and 0.3% CO in 30% or 99% O2) and rebreathed the mixture via an anesthetic bag for 16 seconds; NO (40 parts per million) was added just before rebreathing and thoroughly mixed. Gas concentrations at the mouth were monitored by infrared gas analyzers (Sensors, Saline, MI) and chemiluminescence NO analyzer (Model 280; Sievers, Boulder, CO).

Lung volume was estimated from methane dilution and DLCO, DLNO, and cardiac output from the linear portion of exponential CO, NO, and acetylene disappearance, respectively (12, 13). Septal tissue volume was estimated from the extrapolated acetylene intercept. CO uptake by erythrocytes ({Theta}CO, ml · [min · mm Hg · ml blood]–1) was calculated from the mean PAO2 during rebreathing and hemoglobin concentration ([Hb]) in g · dl–1 (1, 14):

(1)

DMCO and VC were calculated from DLCO at two levels of PAO2 (RF method) (1):

(2)

DLCO was expressed at standard conditions (DLCO-std, [Hb] = 14.6 g/dL, PAO2 = 120 mm Hg). In a modified RF method, VC was also calculated from DLNO and DLCO measured simultaneously while rebreathing the 30% O2 test mixture, assuming negligible erythrocyte resistance to NO uptake (1/{Theta}NO {cong} 0) so that Equation 2 applied to NO becomes: DLNO {cong} DMNO (15, 16) and DMCO = DLNO/2.42. The factor 2.42 was taken empirically from our own aggregate data (Figure 2B).

Protocol
After spirometry, maximal O2 uptake was determined by continuous incremental exercise. Workload increased by 10 W every minute (patients) and 30 W every 2 minutes (normal subjects) until exhaustion or voluntary termination. On another day, normal subjects exercised for approximately 4 minutes at 25, 50, and 80% of maximal workload; patients were studied at one to three of the previously mentioned workloads as tolerated. At each workload, rebreathing maneuver was performed during the fourth minute and repeated while breathing the test mixture containing either 30 or 99% O2. When using 99% O2, subjects prebreathed 100% O2 for 1 minute to equilibrate inspired gas with resident alveolar gas. Subjects rested for 10–20 minutes between exercise bouts or until heart rate and respiratory rate returned to baseline. Duplicate measurements at each workload were averaged. Rebreathing rate was 30 breaths/minute at rest and was spontaneously chosen during exercise. Hematocrit and [Hb] (OSM3, Radiometer, Copenhagen, Denmark) were determined from a venous blood sample.

Statistical Analysis
Data were normalized by body surface area and compared by factorial analysis of variance (Statview, version 5.0; SAS, Cary, NC). Diffusing capacities were analyzed with respect to cardiac index. Slopes and intercepts of individual correlation or regression relationships were compared between groups, as described by Zar (17). A p value of less than 0.05 was significant.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Subject demographics are shown in Table 1 . The average disease duration in patients is 6.82 years (range of 3 months to 20 years). All but three patients were taking prednisone; the average daily dose of prednisone at the time of study was 13.3 mg. In the patients compared with normal control subjects, hematocrit was significantly lower, and [Hb] was slightly lower. FVC, FEV1, and FEF25–75% were reduced proportionally to approximately 65% of normal, consistent with a restrictive ventilatory defect. Peak O2 uptake and peak workload were reduced to 50% of predicted. The maximal heart rate achieved was significantly lower in patients.


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  		TABLE 1. Demographics, spirometry, and peak oxygen uptake

 
Table 2 shows ventilation data at rest and at 80% of maximum workload, the highest exercise intensity that could be sustained for 3–4 minutes. Minute ventilation and tidal volume were significantly reduced in the patients compared with control subjects, whereas respiratory rate was elevated at rest and heavy exercise. At each workload, O2 uptake and CO2 output were also significantly reduced, whereas respiratory quotient was similar to that in normal control subjects. In patients, transcutaneous O2 saturation declined significantly from rest to exercise (by repeated-measures analysis of variance).


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  		TABLE 2. Ventilatory data

 
Data obtained during rebreathing are shown in Tables 3 and 4 . Presence of NO in the test gas mixture did not alter the measurement of DLCO, cardiac output, or lung volume in either group. Mixing efficiency during rebreathing was slightly lower in patients. The number of breaths required for achieving 90% equilibration between the test gas and resident alveolar air was 3.8 ± 1.1 in sarcoidosis patients and 2.9 ± 0.4 in normal subjects at rest (p < 0.005) and 3.6 ± 1.1 and 2.4 ± 0.3, respectively, during exercise (p < 0.001). At each workload, end-inspiratory and end-expiratory lung volumes, DLCO and DLNO, cardiac index, stroke index, and heart rate were all significantly reduced in patients compared with control subjects. Septal tissue volume, estimated from the time zero intercept of the acetylene disappearance curve, was also significantly lower in patients.


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  		TABLE 3. Rebreathing data

 

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  		TABLE 4. Membrane diffusing capacity and capillary blood volume

 
The relationship of cardiac index to O2 uptake was similar in patients and control subjects within the measurement range (Figure 1) . Independent relationships of DLNO to DLCO (expressed at standard conditions) were not significantly different between groups. Regression equations were: DLNO = 3.45 DLCO – 2.69 (r2 = 0.879) for patients and DLNO = 3.59 DLCO + 5.14 (r2 = 0.747) for normal subjects. Combining data from both groups considerably improved the correlation (DLNO = 4.16 DLCO – 6.82, r2 = 0.918) (Figure 2A) . Independent relationships of DLNO to DMCO were not significantly different between groups; hence, data were combined to yield the following relationship (DLNO = 2.42 DMCO – 1.87, r2 = 0.865) (Figure 2B). Similarly, the relationships between VC estimated by the standard RF method and that estimated by the modified RF method were not different between groups; combined data show a correlation with slope = 0.961 (r2 = 0.636) (Figure 3) . In patients, the range of excursion in these parameters was severely restricted, and thus, there was little overlap in data points between groups. Average DMCO and VC estimated by these two methods at different workloads are shown in Table 4.



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  		Figure 1. The relationship between cardiac index (CI) and oxygen uptake (O2) was not significantly different between patients with sarcoidosis and normal control subjects. Regression equation through pooled data: normal subjects, CI = 5.10 O2 + 2.64, r2 = 0.802; patients, CI = 7.00 O2 + 1.410, r2 = 0.733.

 


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  		Figure 2. The relationships between (A) lung diffusing capacity for NO (DLNO) and lung diffusing capacity for carbon monoxide (DLCO) and (B) between DLNO and membrane diffusing capacity for CO (DMCO) were not different between patients (closed symbols) and normal subjects (open symbols). DLCO was expressed under standardized conditions (DLCO-std) of hemoglobin concentration ([Hb]; 14.6 mg/dl) and alveolar O2 tension (PAO2; 120 mm Hg). Regression lines are shown through the pooled data. DLNO = 4.16 DLCO – 6.82, r2 = 0.918. DLNO = 2.42 DMCO – 1.87, r2 = 0.865.

 


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  		Figure 3. VC estimated from simultaneous measurements of DLNO and DLCO by the modified Roughton-Forster (RF) method was correlated to that estimated by the standard RF method in patients (closed symbols) and normal subjects (open symbols). Regression line is shown through the pooled data. VCDLCO–DLNO = 0.961, r2 = 0.636.

 
Figure 4 shows recruitment of DLCO (Figure 4A), DLNO (Figure 4B), and VC (Figure 4C) relative to a rising cardiac index from rest to exercise. VC was estimated from DLCO at two different PAO2s using Equations 1 and 2. At any given cardiac index, DLCO, DLNO, and VC were significantly lower in patients than in control subjects; the reduction in DLNO was greater than that in VC. Average slopes of DLCO, DLNO, and VC with respect to cardiac index were 46%, 37%, and 33% lower, respectively, in patients than in control subjects, reaching statistical significance for DLCO (p < 0.0001) but not for DLNO or VC.



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  		Figure 4. Recruitment of DLCO (A), DLNO (B), and VC (C) with respect to cardiac index (CI) from rest to exercise in patients (closed symbols, dashed lines) and normal subjects (open symbols, solid lines). At a given cardiac index, both DLCO and DLNO were significantly lower in patients than in normal subjects. DLCO was expressed under standardized conditions (DLCO-std) of [Hb] (14.6 mg/dl) and PAO2 (120 mm Hg); VC was estimated by the RF technique. Averages of individual slopes and intercepts are given here. Normal: DLCO = 1.49 CI + 9.49; DLNO = 2.87 CI + 56.18; VC = 6.80 CI + 31.60. Patient: DLCO = 0.80 CI + 4.61 (p < 0.0001); DLNO = 1.82 CI + 18.87 (p > 0.05); VC = 4.54 CI + 23.21 (p > 0.05); p values indicate a comparison of slopes and intercepts of individual regression lines between groups.

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Summary of Findings
In patients with stages II–III pulmonary sarcoidosis, simultaneous measurement of DLNO and DLCO at a single O2 tension (modified RF method) provides comparable estimates of DM and VC at rest and exercise as that provided by two separate measurements of DLCO at different O2 tensions (standard RF method). This agreement is consistent with our previous report comparing the two methods in normal subjects (3) and validates the assumption that DLNO provides a close approximation of true DM. Using these methods, we obtained the first data to assess directly recruitment of alveolar microvascular reserves from rest to exercise in sarcoidosis. In patients, DLCO, DLNO, and DMCO at a given cardiac index were reduced by 50–60% and VC by approximately 30% below normal, suggesting that the limitation to diffusive gas transport resides predominant within the membrane barrier, whereas capillary reserves are better preserved. Recruitment of alveolar–capillary reserves during incremental exercise, indexed from the slope of the increase in DLCO with respect to cardiac output, is only modestly impaired in patients compared with normal control subjects.

Critique of Methods
Because the ventilatory defect in sarcoidosis is predominantly restrictive and the rebreathing technique inherently minimizes effects of uneven ventilatory distribution compared with single breath techniques, observed abnormalities in ventilatory mixing were rather modest in our patients.

The RF relationship (Equation 2) should apply to NO as well as to CO. Equation 1 is less relevant to NO because NO reacts as rapidly with deoxyhemoglobin as with oxyhemoglobin and DLNO is not altered by PAO2. Because reaction velocity between NO and hemoglobin is approximately 283 times that between CO and hemoglobin, we assume that 1/{Theta}NO is negligible. If true, DLNO provides a direct estimate of DMNO. Others have made the same assumption (16, 18, 19) in spite of the fact that Carlsen and Comroe (20) estimated a finite value of {Theta}NO = 4.5 min–1 · mm Hg–1. There is ongoing controversy about the effective value of {Theta}NO. Part of the resistance to NO uptake measured by Carlsen and Comroe (20) resides in unstirred layers surrounding erythrocytes in a rapid flow apparatus. When unstirred layers were eliminated by dithionite as done by Yamaguchi and colleagues (21) when measuring O2 release from intracorpuscular hemoglobin, {Theta}O2 increased approximately 40% above that measured when unstirred layers are present by Staub and colleagues (22) with the same apparatus used by Carlsen and Comroe (20). The effect of unstirred layers should increase with the velocity of gas uptake by erythrocytes, that is, greater for {Theta}NO than for {Theta}O2. Thus, although the true value of {Theta}NO remains uncertain, it should be considerably higher than estimated by Carlsen and Comroe (20).

Given that conductance of the alveolar membrane and capillary blood for CO is approximately equal and can be partitioned by the standard RF method, whereas alveolar NO uptake is predominantly limited by diffusion across the membrane (16, 18, 23), we might expect the ratio of DMNO/ DMCO to be related to the ratio of their respective Krogh constants, based on molecular weights (MW) and Bunsen solubility ({alpha}) in water, that is

(3)

Assuming 1/{Theta}NO = 0, Guenard and colleagues reported that DLNO/DMCO was 1.97 in normal subjects (15) and 1.5–1.9 in patients with chronic obstructive lung disease (24) using a single-breath method at rest. We reported that DLNO/DMCO = 2.49 in normal subjects using a rebreathing technique at rest and exercise (3). Combining resting and exercise data from normal subjects and patients yields DMNO/DMCO = DLNO/DMCO = 2.42 (Figure 2B), approximately 25% higher than the theoretic estimate. The discrepancy is not unreasonably large considering the uncertainties inherent in the assumed constants, that is, NO solubility coefficient and {Theta}CO. The solubility coefficient for NO is measured in water; experimental values in tissue or plasma are not available. NO can interact with sulfhydryl groups in plasma lipids and proteins (25) and be transported as S-nitrosothiols (26), leading to a higher than assumed solubility in plasma as well as facilitated diffusion; facilitated diffusion could easily explain the higher experimental ratio. In addition, the range of values given for {Theta}CO in the literature (1, 27–29) can easily explain a 25% variation in DMCO estimated by the RF method. Alternatively, we assumed that DMNO/DMCO = 1.93 and solved for the 1/{Theta}NO that would satisfy our experimental data; however, this approach yielded a negative 1/{Theta}NO, which is an impossibility. In a third approach, we estimated the effects of errors in the assumed 1/{Theta}NO on DLNO/DMCO. A nonzero 1/{Theta}NO would yield a DLNO/DMCO ratio below 1.93, that is, in the wrong direction to be able to explain the experimental data. Given that our experimental DLNO/DLCO (= 4.16) is within the range reported by other groups in humans (2, 15, 24) and animals (16, 18, 23) and the relationships of DLNO to DLCO as well as DLNO to DMCO are not different between patients and normal subjects, these data support the empirical use of DLNO by the modified RF method to interpret changes in DM and VC more directly in the presence of parenchymal disease.

The ability to estimate DM and VC from one-step simultaneous measurement of DLNO and DLCO represents a technical advance with significant savings of effort as well as a conceptual advance that should yield more accurate estimates. In the standard RF method, cardiac output can vary between measurements of DLCO at different O2 tensions, which have to be interpolated to obtain DLCO at two O2 tensions but the same cardiac output. In the modified RF method, all measurements are obtained at the same cardiac output and O2 tension; no interpolation is necessary. Conceptually, the RF method is based on a lumped parameter model applied to a distributed process, which inherently assumes CO uptake across alveolar membrane to be constant regardless of PAO2 (1). We have presented theoretic evidence that DMCO is in fact sensitive to O2 tension (30). At a high O2 tension when CO uptake by capillary hemoglobin is low, CO flux into erythrocytes becomes more uniformly distributed around the cell surface, and the mean air-to-hemoglobin diffusion path length approaches a maximum. At a low O2 tension when CO uptake by hemoglobin is high, the pattern of CO flux is redistributed preferentially to the portion of erythrocyte surfaces nearest to the capillary membrane and the mean air-to-hemoglobin diffusion path length approaches a minimum (30). Hence, effective DMCO is higher when O2 tension is low and lower when O2 tension is high. The net result is systematic overestimation of DMCO and underestimation of VC by the RF method; the error is exaggerated by a high capillary hematocrit. The magnitude of this error in vivo has not been defined; however, the modified RF method avoids this error altogether and should improve the accuracy of estimated DMCO and VC.

Uneven distribution of diffusing capacity with respect to perfusion also develops in advanced parenchymal disease (31) and can further impair functional recruitment by reducing the efficiency of diffusive gas equilibration across the blood–gas barrier at a given pulmonary blood flow (32). Such diffusion–perfusion heterogeneity should have a greater impact for NO than for CO uptake, potentially leading to systematically lower DLNO/DLCO and DLNO/DMCO ratios in patients than in normal subjects. In our patients, the DLNO/DLCO and DLNO/DMCO relationships were slightly but not significantly lower than normal; therefore, the presence of diffusion–perfusion heterogeneity in these patients does not cause a significant error to DLNO and DLCO in the modified RF method.

Recruitment of DL
In pulmonary sarcoidosis, radiograph staging and lung volumes provide poor predictors of functional impairment (33). Exercise capacity if often impaired even when resting lung function is relatively intact (34), primarily related to a reduced DLCO (35). A resting DLCO below 55% of normal predicts a greater likelihood for developing arterial hypoxemia during exercise (33). Few studies have evaluated DM and VC in sarcoidosis, and none has examined recruitment with exercise. Sharma and Mohler (36) reported that patients with sarcoidosis failed to increase the ratio of DLCO to alveolar volume from sitting to supine, suggesting a diminished ability to recruit or distend alveolar capillaries. Dujic and colleagues (37) reported 11% and 27% reductions in the ratio of DLCO to alveolar volume in stages II–III sarcoidosis, respectively. Our findings are also consistent with that by Saumon and colleagues (38) (i.e., both alveolar membrane and capillaries are affected in stages II–III disease, but DM tends to be affected to a greater extent than VC).

Given a high prevalence of vascular involvement in pulmonary sarcoidosis (39), the milder impairment of VC seems surprising. In autopsy series, vascular involvement consists of granulomatous angiitis as well as microangiopathy (40) and is directly related to granulomatous involvement in the parenchyma as well as in the walls of surrounding lymphatic capillaries (39). Histologic involvement is typically patchy, with discrete sites of microvascular inflammation, whereas adjacent vessels are relatively unaffected. A small increase in harmonic mean thickness of the alveolar–capillary barrier has been observed but is insufficient to explain the functional reduction in DL; at the same time, septal interstitial tissue is quantitatively increased at the expense of the capillaries (41). It is likely that in sarcoidosis diversion of blood flow away from foci of capillary inflammation/obliteration to uninvolved areas recruits the remaining normal capillaries to compensate for microvascular function, resulting in a milder reduction of VC than DM. For a comparable level of radiologic infiltrates, exercise impairment is less severe in patients with sarcoidosis than with cryptogenic fibrosing alveolitis (42); in the latter patients, Hughes and colleagues (6) showed minimal recruitment of DLCO associated with a markedly widened alveolar–arterial O2 tension difference on exercise. Patient demographics, [Hb], and PAO2 are not given in the study by Hughes and colleagues (6); assuming that these parameters are comparable, we have compared their average DLCO data with our present data (Figure 5) . The slope of the relationship between DLCO and cardiac output is considerably lower in interstitial fibrosis than in sarcoidosis. These distinct patterns of response in two interstitial diseases support the concept of DLCO recruitment as a functional index of microvascular integrity. The characteristic diffuse alveolar inflammation and obliterative fibrosis in cryptogenic fibrosing alveolitis significantly impair microvascular recruitment (6), whereas the characteristic patchy and discrete granulomatous deposition in sarcoidosis tends to spare microvascular recruitment until the late stage when obliterative fibrosis also develops. A variable degree of coexistent interstitial fibrosis in our patients likely accounts for the modest although statistically significant reduction in the slope of the DLCO–cardiac output relationship relative to control subjects.



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  		Figure 5. Recruitment of DLCO with respect to cardiac output is compared among patients with sarcoidosis (this study: closed diamonds and dashed line, sarcoidosis; open diamonds and soild line, normal), cryptogenic interstitial pulmonary fibrosis (data from Hughes and colleagues [6]: closed squares and dashed line, interstitial fibrosis; open squares and solid line, normal) and their respective normal control subjects. Data are mean ± SEM.

 
We conclude that in stages II–III pulmonary sarcoidosis, the modified RF technique incorporating DLNO and DLCO in a single rebreathing maneuver provides comparable estimates of DM and VC, as that provided by the standard RF technique at rest and during exercise, that is, similar agreement as previously shown for normal subjects (3). In patients, DM is disproportionately reduced relative to VC, suggesting that thickening of the blood–gas diffusion barrier contributes more to gas transport limitation than obliteration of alveolar capillaries. Although statistically impaired relative to normal subjects, recruitment of DLCO with respect to cardiac output is better preserved in sarcoidosis than in patients with cryptogenic fibrosing alveolitis. This observation likely reflects differences in the characteristic patterns of alveolar histologic involvement in these two diseases but should be verified in future studies by direct comparison of the two patient groups. Thus, recruitment of DLCO can provide useful information for understanding alveolar pathophysiology and for assessing the functional integrity of alveolar capillaries.


   Acknowledgments
 
The authors thank Dr. William F. Miller, Dr. Pedro N. Paez, and the staff of the Sarcoidosis Clinic for their help with subject recruitment and other aspects of the study. This article is dedicated to the memory of our friend Dr. Pedro N. Paez.


   FOOTNOTES
 
Conflict of Interest Statement: A.R.P. has no declared conflict of interest; C.M.H. has no declared conflict of interest; A.R.S. has no declared conflict of interest; R.L.J. has no declared conflict of interest; C.C.W.H. has no declared conflict of interest.

Received in original form September 16, 2003; accepted in final form February 20, 2004


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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