Distribution of airway narrowing responses across generations and at branching points, assessed in vitro by anatomical optical coherence tomography

Abstract

Background: Previous histological and imaging studies have shown the presence of variability in the degree of bronchoconstriction of airways sampled at different locations in the lung (i.e., heterogeneity). Heterogeneity can occur at different airway generations and at branching points in the bronchial tree. Whilst heterogeneity has been detected by previous experimental approaches, its spatial relationship either within or between airways is unknown.

Methods: In this study, distribution of airway narrowing responses across a portion of the porcine bronchial tree was determined in vitro. The portion comprised contiguous airways spanning bronchial generations (#3-11), including the associated side branches. We used a recent optical imaging technique, anatomical optical coherence tomography, to image the bronchial tree in three dimensions. Bronchoconstriction was produced by carbachol administered to either the adventitial or luminal surface of the airway. Luminal cross sectional area was measured before and at different time points after constriction to carbachol and airway narrowing calculated from the percent decrease in luminal cross sectional area.

Results: When administered to the adventitial surface, the degree of airway narrowing was progressively increased from proximal to distal generations (r = 0.80 to 0.98, P < 0.05 to 0.001). This 'serial heterogeneity' was also apparent when carbachol was administered via the lumen, though it was less pronounced. In contrast, airway narrowing was not different at side branches, and was uniform both in the parent and daughter airways.

Conclusions: Our findings demonstrate that the bronchial tree expresses intrinsic serial heterogeneity, such that narrowing increases from proximal to distal airways, a relationship that is influenced by the route of drug administration but not by structural variations accompanying branching sites.

Figure 1

A 3D profile of a porcine bronchial tree acquired by anatomical optical coherence tomography (aOCT). The major portion of the bronchial tree comprises a large parent bronchus that gives rise to smaller daughter bronchi (i.e., side branches) that branch off at an obtuse angle of departure. The figure shows the airway preparation in its relaxed state and after bronchoconstriction in response to carbachol.

Figure 2

A schematic of the airway preparation indicating the measurements performed. Indentified in the figure are the parent bronchus and a connecting daughter bronchus (i.e., side branch). The distal and proximal ends of the airway preparation are also labeled. For the study, three separate analyses were performed: (Analysis 1), narrowing in the parent bronchus was measured and compared between generations (A, black line), away from regions of branching; (Analysis 2), narrowing in the parent bronchus was measured at the midpoint of branching, where the parent bronchus was seen to open into a daughter side branch (B, dotted black line), and it was compared to narrowing measured immediately proximal and distal to the branching point; (Analysis 3), narrowing within the 'mouth' of daughter side branches was measured (C, grey line) and compared to narrowing within the adjacent parent bronchus at the same site (B).

Figure 3

Cross sectional images recorded by aOCT in a proximal and distal airway. Proximal airway is Gen #3 and distal airway Gen #9. The airway epithelium (AE), probe catheter (PR) and cartilage plates (CP) are indentified. The airway preparation was scanned initially in its relaxed state and then 5, 15 and 30 min after addition of carbachol administered to the adventitial airway surface. Airway narrowing was typically greater and more rapid in distal airways.

Figure 4

Relationship between generation and airway narrowing to carbachol administered to the adventitial airway surface. Plots are from four different airway preparations. Airway narrowing was quantified by the percentage decrease in luminal cross sectional area (% CSA). Airway narrowing was increased at more distal locations indicated by positive correlations between narrowing and airway generation in each preparation. Linear equations of best fit are indicated for each airway. (Pearson's correlation analysis)

Figure 5

Airway narrowing measured in the most distal and proximal airways within the bronchial airway preparation. Airway narrowing was induced by carbachol administered to either (A) adventitial (N = 4) or (B) luminal (N = 4) airway surface. Airway narrowing was quantified from the percentage decrease in luminal cross sectional area (% CSA). Airway narrowing recordings were somewhat later in the proximal airway since aOCT scans were initiated from the distal airway (i.e., proximal recordings occurred ~1 min and 3 min later for luminal and adventitial protocols, respectively). Airway narrowing was greater in distal airways irrespective of whether the drug was applied to the adventitial (P < 0.05) or luminal (P < 0.001) surface. (Two-way ANOVA).

Figure 6

Airway narrowing within the parent bronchus at branching sites. (A) Cross sectional images of a relaxed parent bronchus at a branching site and immediately proximal and distal to the branching site. The parent bronchus opens out to the daughter airway at the centre of the branching site. Airway narrowing to adventitial (B, N = 4) and luminal carbachol (C, N = 4) was compared at the three locations identified in (A). There was no difference in narrowing of the parent bronchus between branching or non branching sites irrespective of whether carbachol was administered to the adventitial or luminal surface. (Two-way ANOVA).

Figure 7

Comparison of airway narrowing in parent and daughter bronchi. Airway narrowing was measured in response to luminally applied carbachol in the mouth of a daughter side branch and in the parent bronchus at the branching site. A total of 21 parent-daughter branching sites were measured from four different airway preparations. There was no difference in airway narrowing between parent and daughter bronchi. (Paired t-test).

Figure 8

A histological cross section of a parent bronchus at a point of branching. The entire cross section of the bronchus is shown (Left panel, Whole Airway) indicating the point at which the parent bronchus opens out into the daughter. The parent bronchial wall (PW) and the wall at the shoulder of a branching point (BP) are indicated by arrows. Middle and right panels are magnified images of the branching point and of the parent bronchial wall respectively. Substantial thickening of the cartilage plate (CP) at the branching point is apparent, which is accompanied by an increase in smooth muscle (SM) mass and a more oblique orientation of muscle cells.