INTRODUCTION

Top Abstract Introduction References

INHALATION OF SMALL AEROSOL boluses delivered to shallow volumetric depths in the lung has been used to study particle clearance from the bronchial airways (16, 19). This technique may also be useful for delivering aerosolized drugs to the bronchial epithelium when alveolar deposition is undesirable (2). An aerosol bolus is a discrete volume of air containing particles sandwiched within an inhaled volume of particle-free air (17). The depth to which a bolus penetrates into the lung is determined by the volume of the bolus and the volume of air inhaled after its insertion into the airstream. An injection at the end of a breath tends to deliver the aerosol to the extrathoracic and conducting airways. The inhalation is generally followed by a period of breath holding to maximize particle deposition on airway surfaces. The volumetric front depth (VFD) is an estimate of the penetration of particles into the respiratory tract and represents the volume inspired from the point at which the first particles enter the mouth to the end of inhalation (16, 17, 19).

We recently analyzed the intrathoracic deposition and subsequent clearance of particles from small boluses (40 ml) delivered to a shallow VFD in the lung (70 ml; Ref. 3). These boluses were delivered near the end of a tidal volume breath beginning at functional residual capacity (FRC) and ending at 70% total lung capacity (TLC). One striking and unexpected finding in this study was a left (L)-right (R) lung asymmetry in particle deposition, with ~50% more particles depositing in the L vs. R lung. In theory, to minimize alveolar penetration of the aerosol bolus, it is desirable to obtain a uniform distribution of the bolus transport into the lung. Asynchronous filling of different lung regions may effectively reduce the anatomic dead space (ADS), allowing penetration of the bolus to more peripheral regions, including the alveoli.

The mechanism responsible for the observed left-right asymmetry in bolus deposition was unclear in our previous study (3). Left-right asymmetries have not generally been noted when ¹³³Xe boluses have been used in regional ventilation studies (1, 7). However, our experiments were different from these regional ventilation studies in that we delivered much smaller boluses (40 ml) at the very end of inhalation. A pilot study by Ilowite et al. (11) also found left-right asymmetries in deposition when the aerosol boluses were delivered at 90% TLC. They also found that small ¹³³Xe gas boluses, delivered in the same manner as the aerosol boluses, were unevenly distributed between the L and R lungs (2). It may be that, as the lung approaches end inhalation at high lung volumes, the L and R lungs are expanding nonuniformly. We hypothesized that delivering the boluses at lower end-inspiratory lung volumes might alleviate this effect and result in a more even distribution between the two lungs.

Our purpose in this study was to determine the effect of lung volume on uniformity and efficiency of bolus deposition in the lung and subsequent retention of the deposited particles. To accomplish this purpose, we had healthy subjects inhale radiolabeled, monodisperse 40-ml aerosol boluses to a VFD of 70 ml into the lung at lung volumes of 85, 70, and 50% TLC end inhalation. Particles were 3.5-µm mass median aerodynamic diameter (MMAD), which is similar to therapeutic particles delivered by many nebulizers (3-5 µm; Refs. 2, 20).

	METHODS

Experiments were performed in a group of 11 healthy subjects, age 20-43 yr, with normal pulmonary function. The methods described below are the same as those used in our previous study (3). In each subject, we measured forced expiratory volumes and flows and lung volumes, inspiratory capacity, and expiratory reserve volume by spirometry. Airway resistance and FRC were measured by body plethysmography. The subjects had no smoking history, no history of lung disease, and no recent (within the previous 4 wk) history of acute respiratory infection or viral illness. Informed consent was obtained from each volunteer; the study had the approval of the University of North Carolina Committee on the Protection of the Rights of Human Subjects.

ADS

¹³³Xe Equilibrium Scan

Radiolabeled Aerosol Bolus Inhalations

All subjects were studied at least twice, once at 70% TLC end inhalation and then at either 50 or 85% TLC. Three subjects were studied at all three lung volumes. As a result, n = 7 subjects for each of two cohorts: 85% vs. 70% TLC and 70% vs. 50% TLC. The inhalations were performed at least 1 wk apart. For each study day, the subjects began by sitting with their backs against the gamma camera while a 15-min background image was recorded for later correction of lung images. On each study visit, the subjects then inhaled the radiolabeled, monodisperse aerosol (3.5-µm MMAD) boluses (40 ml) delivered by a Pari bolus-delivery system (respiratory aerosol probe; Ref. 22) via the mouthpiece described in ADS. Each bolus was inhaled to a 70-ml VFD during inhalation to either 85, 70, or 50% TLC by using a constant inspiratory flow rate (250 ml/s) followed by an 8-10 s breath hold to maximize particle deposition in the airways. A total of 10-20 boluses was inhaled by each subject at a given lung volume. After steady-state tidal breathing, inhalations to 70 and 85% TLC began at FRC. Inhalations to 50% TLC began at slightly lower lung volumes by having subjects exhale below their FRC (<500 ml) on the breath before the bolus inhalation. During the bolus inhalations, the relative aerosol concentration and respired volumes were measured by photometry and a pneumotachograph at the mouth to determine the VFD of each inhaled bolus (16). When ~10-15 µCi of activity were deposited in the subject's lungs, as determined by a single NaI detector placed against the subject's back, inhalations ceased. In most cases, the inhalation took <15 min; however, in some subjects with lesser fractions of the bolus depositing, inhalation took as long as 20 min.

The subjects were then seated with their back to the gamma camera (in the same position as for the ¹³³Xe scan described in ¹³³Xe Equilibrium Scan), and an initial posterior deposition scan (the sum of two 2-min scans) was recorded. Continuous 2-min images were recorded for a period of 2 h and 30 min. After the initial deposition scan, i.e., the third image, the subjects immediately ate and drank to wash deposited activity from their oropharynx and esophagus into the stomach. Before the fifth 2-min image, the subjects, still seated, turned with their chests to the camera to obtain an anterior image of their deposition pattern, i.e., the fifth image. At 24 h postdeposition, the subjects returned for a 30-min posterior camera scan.

Effective Air Space Dimension (EAD) Measurements

Data Analysis

Regional lung deposition of the inhaled bolus. Using region-of-interest (ROI) gamma camera analysis, we determined the uniformity of deposition throughout the lungs (3, 11) by using the first 2-min deposition scan. Outline ROIs of the L and R lungs created from the ¹³³Xe equilibrium scan were equally divided into an upper (U) and lower (L) region (i.e., UL, LL, UR, and LR). The ^99mTc activity in each of the four regions was divided by total ^99mTc activity for all regions combined. A similar ratio for ¹³³Xe equilibrium activity was also calculated for each region. Each ^99mTc ratio was then divided by the corresponding ¹³³Xe ratio for each region to account for differences in lung volume in the four regions, e.g., UL/total = (A_{Tc UL}/A_{Tc total})/(A_{Xe UL}/A_{Xe total}), where A is activity, Tc is ^99mTc, and Xe is ¹³³Xe. If the deposited particles from the bolus were evenly distributed throughout the lungs, this ratio for each region should be close to 1.0. An increase or decrease in deposition out of proportion to the volume in that region will increase or decrease the ratio to >1.0 or <1.0, respectively. We then defined the SD (as a measure of variance) for the average ratio for all four regions as the evenness index (EI) for bolus deposition (3, 11). If all regions have ratios of 1.0, i.e., a uniform distribution, then EI = 0. EI should increase with increasing unevenness as the four ratios diverge from 1.0. Because we had previously observed L vs. R lung asymmetry in deposition (3), we also calculated the L-to-R lung ratio (L/R) for each subject's deposition scan, normalized to a similar ratio for the ¹³³Xe equilibrium scan to account for lung volume differences between the two lungs.

Fractional deposition of the bolus. By filter techniques, aerosol photometry, and gamma camera analysis, we estimated the fraction of the inhaled boluses that actually deposited in intrathoracic airways [intrathoracic deposition fraction (IDF)]. During the inhalation of the boluses, a filter (Pall BB50T) was placed on the expiratory port of the Pari bolus delivery system to collect total activity exhaled by the subject (A_ex). After the subject completed inhalation of the aerosol boluses, a test (inspiratory) filter was attached to the mouthpiece of the respiratory aerosol probe system, and a single 40-ml bolus was drawn onto the filter with a 1-liter syringe (A_filter) to measure the aerosol delivered (inhaled) by a single bolus. In addition, the peak aerosol number concentration for each bolus was measured by laser photometry at the mouth during both the subject's bolus inhalations (Conc_in) and this postinhalation filter sample (Conc_filter). The total inhaled activity (A_in) could then be determined as

(1)

where n is the total number of boluses inhaled by the subject. The total deposition fraction (DF) for each subject could then be determined as

(2)

The aerosol number concentration measured by photometry on expiration (Conc_ex) either was generally too noisy or was affected by exhaled water condensate (9) to be useful in determining DF.

Pulmonary retention of deposited particles. A rectangular region bordering the R and L lungs (defined by the ¹³³Xe equilibrium scan) was used to determine, by computer analysis, the lung retention as a fraction of the initial counts (background and decay corrected) in each lung over the gamma camera scanning period of 2 h (R₂). Retention for the whole lung (left and right regions combined) was also determined at 24 h postdeposition (R₂₄). Because the boluses were very small and DFs might be low in some individuals, we established a criteria for net counts above background at 24 h to accept the retention value at this time. As the radioactivity in the lung approaches background activity (by either clearance or decay), its measure becomes highly variable (i.e., a large %SD of measured counts) over a given measurement period (6). For R₂₄ data to be acceptable (for both lungs combined) in a given subject, the %SD (6) of the net counts could not exceed 20%. This error, %SD, associated with the net counts (i.e., background corrected) at 24 h was calculated as the square root of the sum of total and background counts divided by the difference in these measured counts (expressed as a percentage).

Statistical analysis. Comparison of parameters between the two lung volumes within a cohort was made by paired t-test (Systat for Macintosh). Differences in parameters between the two different cohorts were determined by ANOVA.

	RESULTS

Table 1 summarizes the regional deposition data (EI, L/R, and C/P) of the inhaled boluses for the two cohorts studied. There was no difference in the EI, although at higher lung volumes there was a tendency for EI to be greater (i.e., more uneven deposition). At 85% TLC, EI was much more variable among the subjects studied (coefficient of variation = 0.72) than when the bolus was delivered at 70% TLC (coefficient of variation = 0.16 in that cohort). As we had reported earlier (3), there was clearly a difference in L vs. R lung deposition, especially at the higher lung volumes (85% TLC).

Figure 1 illustrates the deposition scans of one subject who participated in both the 85 vs. 70% TLC and the 70 vs. 50% TLC cohort. On the left, her posterior deposition scan at 85% TLC shows a predominance of the deposition in the L compared with the R lung (L/R = 2.44). When she inhaled the bolus to the same volumetric depth at 50% TLC, the deposition was more evenly distributed between the L and R lungs (L/R = 1.28).

View larger version (47K): [in this window] [in a new window]   Fig. 1.   Initial posterior deposition scans for 1 subject who inhaled boluses to 70-ml volumetric front depth at 85% (left) and 50% total lung capacity (TLC; right) end inhalation. Left-to-right lung ratio activity equals 2.44 at 85% TLC (left) and 1.28 at 50% TLC (right).

Mean L/R values for the posterior (first image) and the anterior (fifth image) are given in Table 1. Distribution between the L and R lungs became more uniform at the lower lung volume (i.e., 50% TLC). Except for the L/R at 50% TLC, all posterior L/R values were significantly >1.0, indicating significantly greater deposition in the L compared with the R lung. The anterior image L/R values were significantly less than the posterior L/R values for the two cohorts combined (P < 0.01 by paired t-test analysis). Nevertheless, the anterior L/R values at 85 and 70% TLC in the first two cohorts were still significantly >1.0, indicating greater L lung deposition at these higher lung volumes, regardless of which image was considered. There were no differences in C/P as a function of lung volume in the two cohorts. However, in the 85-70% TLC cohort, there was a significantly greater C/P in the L compared with the R lung (Table 1). When all measurements were combined for both cohorts, there also was a significantly greater C/P for the L vs. R lung (P < 0.02 by paired t-test analysis).

Table 2 summarizes the mean fractional deposition data (DF and IDF) of the inhaled boluses for the two study cohorts. The lung volume at end inhalation tended to affect IDF only when going from 85 to 70% TLC (P = 0.055), i.e., IDF was increased at the lower lung volume. There was no difference in IDF for the 70 vs. 50% TLC cohort. As expected, ADS (phase 2 of nitrogen washout) was significantly decreased as end-inhalation lung volume decreased. As suggested by values at 70% TLC in each group, the subjects in the 85 vs. 70% TLC cohort tended to have smaller ADS and EADs than those in the 70 vs. 50% TLC cohort. Although there was a tendency for EADs to decrease with lung volume within each cohort, the differences were not statistically significant.

The retention data for the deposited boluses in each study cohort are shown in Table 3. Surprisingly, despite the differences in L vs. R lung distribution (L/R in Table 2), there was no difference in R₂ for the L vs. R lung or as a function of lung volume in either of the groups studied. On the other hand, the L-R similarity in retentions is consistent with the C/P values being fairly similar between L and R lungs (Table 2; Ref. 3). There was also no difference in whole lung R₂₄ as a function of lung volume, again despite the asymmetry in deposition at the higher lung volume. Similarly, there was no correlation between L/R and R₂₄ (r = 0.14, n = 20) when data from the two groups were combined. Two subjects were excluded from R₂₄ analysis because of insufficient activity in their whole lung (see METHODS). These two individuals had the lowest IDF among the subjects studied.

	DISCUSSION

As in our previous study (3), we found a striking difference in deposition between the L and R lungs (L/R) for small boluses delivered to very shallow depths into the lung. In the present study, we found that this asymmetry in deposition was a function of end-inspiratory lung volume. For boluses delivered at 70 and 85% TLC end inhalation, there was significantly greater deposition in the L vs. R lung (Table 2). This was true for either posterior or anterior camera images. However, the asymmetry was greater in the posterior scans, suggesting that deposition in the L lung tended to be more posterior, whereas the R lung deposition tended toward the anterior portion of the lung. The anterior image may have been more affected by increased ^99mTc gamma attenuation by the heart on the left side. Because we did not have an anterior image for the ¹³³Xe equilibrium scan (for normalization purposes) and had to use the posterior L/R for ¹³³Xe, the anterior L/R may be underestimated by this effect.

At 50% TLC end inhalation, the deposition was more evenly distributed between the two lungs, i.e., L/R not different from 1.0. In addition to our previous study (3), an earlier pilot study by Ilowite et al. (11) showed left-right asymmetries in deposition when the boluses were delivered at 90% TLC. In their study, they found that ¹³³Xe gas boluses also distributed asymmetrically and similar to the aerosol boluses (2). It is not clear why such asymmetry in bolus ventilation occurs at the higher lung volumes. Similar findings have not been found with larger radio-gas boluses used in regional ventilation studies (1, 7). However, Dollfuss et al. (7) used a column of scintillation detectors that did not distinguish between the L and R lungs. Bake et al. (1) did employ scintillation counters over each lung, but they, like others who have utilized gas boluses to determine regional lung ventilation, introduced the boluses at the beginning of inhalation from FRC as opposed to the end of inhalation as we did. Our results seem to imply that, as the lung approaches end inhalation at and above 70% TLC, the L and R lungs are expanding nonuniformly. One explanation may be that the base of the R lung is opposed by the relatively rigid liver, whereas the L lung base is opposed by the more distensible stomach, allowing the L lung to expand more easily at higher lung volumes. L vs. R lung differences in anatomy and geometry of the bronchial airways may also contribute to the increased efficiency of deposition in the L lung. The mean linear intercept at these shallow bolus depths, i.e., the distance through which particles settle to deposit, may be less in the L than in the R lung. In the asymmetric Horsfield model of the lung (10), the main stem and lobar bronchi tend to be oriented more horizontal to the direction of gravity in the L vs. R lung, i.e., shorter settling distances for particles to deposit. The greater C/P in the L vs. R lung (when all data were combined) suggests that there was a greater deposition efficiency in the large, central airways of the L lung. However, the fact that left-right asymmetry was diminished as lung volume decreased suggests that geometry differences were probably not the primary explanation for this finding. Our data suggest that, to achieve an even distribution in the lungs, the aerosol boluses should be delivered at lung volumes near FRC.

Under our experimental conditions, ~25% of the inhaled boluses actually deposited in the IDFs (Table 2). The IDF decreased when the bolus was delivered at the highest lung volume (85% TLC) compared with IDF for 70% TLC in that cohort. This was likely due to the significant increase in ADS at 85% vs. 70% TLC (Table 2). We showed previously (3) that IDF at a fixed volumetric lung depth was dependent on the size of ADS, i.e., IDF decreases with increasing ADS. As with regional deposition, the IDF of shallow boluses is maximized when the boluses are delivered at end-inspiratory lung volumes near FRC.

R₂ or R₂₄ was also unaffected by the uneven deposition (Table 3), nor was there a significant correlation between L/R and whole lung R₂₄ for all measurements combined. This suggests that asymmetric delivery of these very shallow boluses did not lead to significantly greater portions of the bolus penetrating into alveolar regions of the lungs. In fact, as discussed above, the central vs. peripheral (C/P) distribution of deposited boluses within the IDF was slightly greater in the L vs. R lung. If the bolus had penetrated deeper in the L lung, then we would have expected the opposite, a lesser C/P in the L vs. R lung. At all lung volumes, the size of the mean EAD (>5 mm; Table 2) at 70-ml VFD suggests that the center of the bolus resided in intermediate-sized bronchial airways (segmental bronchi).

In conclusion, for delivery of shallow, inhaled aerosol boluses to the bronchial airways, for either experimental or therapeutic purposes, subjects should inhale to lung volumes close to FRC. Inhalations to higher lung volumes result in heterogeneous deposition, especially in L vs. R lung distribution. This asymmetry in bolus distribution at high lung volumes suggests that the L and R lungs are expanding nonuniformly as the lung approaches TLC. It appears that, for these very shallow boluses, the asymmetry in bolus deposition has little effect on subsequent particle clearance measurements. However, for purposes of drug delivery (2, 15) to the bronchial airways, maximizing the fractional deposition and distribution homogeneity of the bolus may be important for optimizing the drug's therapeutic effect.