Respiratory modulation of the pharyngeal airway in lean and obese mice

Abstract

Obesity is an important risk factor for pharyngeal airway collapse in obstructive sleep apnea (OSA). To examine the effect of obesity on pharyngeal airway size on inspiration and expiration, respiratory-gated MRI of the pharynx was compared in New Zealand obese (NZO) and New Zealand white (NZW) mice (weights: 50.4g vs. 34.7g, p<0.0001).

Results: (1) pharyngeal airway cross-sectional area was greater during inspiration than expiration in NZO mice, but in NZW mice airway area was greater in expiration than inspiration; (2) inspiratory-to-expiratory changes in both mouse strains were largest in the caudal pharynx; and (3) during expiration, airway size tended to be larger, though non-significantly, in NZW than NZO mice. The respiratory pattern differences are likely attributable to obesity that is the main difference between NZO and NZW mice. The data support an hypothesis that pharyngeal airway patency in obesity is dependent on inspiratory dilation and may be vulnerable to loss of neuromuscular pharyngeal activation.

Figure 1

Example of the chest wall movement signal used for respiratory-gated imaging. On the respiratory trace, there is one complete breath in the continuous readout where the respiratory rate averaged over several breaths was 89 breaths/min (676 ms per breath; monitor had 4 s sweep rate refresh). The respiratory signal moved upwards during inspiration and electronically triggered the start of the MRI acquisitions. Each dot, extended with the vertical lines, represents one the 6 phasic acquisitions during a respiratory cycle. The separation between image acquisition lines was 75 ms. Lines 1 and 5 (with double arrows) indicate the timing of images acquired at approximately mid-inspiration and mid-expiration.

Figure 2

Mid-sagittal MRI of representative NZW mouse (in supine position) highlighting the locations (vertical lines) where 7 transverse axial MR images of the upper airway were obtained. A series of 6 respiratory-gated axial images were obtained at each location (Figure 4).

Figure 3

Seven axial slices obtained during expiration in the pharyngeal region from the anatomical locations noted by the 7 vertical lines in Figure 2 are shown from a representative NZW mouse. The axial slices (numbered 1-7) represent 1 mm thick MRI gradient recalled acquisitions as described in the METHODS with slice 1 from the most rostral location and slice 7 from the most caudal location. Annotations include: a 1 cm scale bar, the brain (BR), tongue (Ton) and the two, tympanic bulla (TB) on slice 6. NP and OP airways are evident in slices 3, 4 and 5 and noted with arrows in slices 4 and 5.

Figure 4

Six axial slice images from same location in the pharynx in a representative NZW (top panel) and NZO mouse (bottom panel). Slice location #6 (see Figure 2) is displayed since that is a location where differences between NZO and NZW airways were most apparent. Images in each panel are numbered (1-6) corresponding to the numbered lines (time sequence of acquisition) in Figure 1. The axial images are displayed with the animal orientation (supine) so that the tongue (T) is superior and the brain (B) is inferior in the image. The two large dark circular structures lateral to the airway (A) are the tympanic bulla (Tb) that were used as a registration marker to orient axial images from rostral to caudal loci. Note that in the NZW mice (top panel), the inspiratory-timed images (1 and 2) show reduced cross-sectional airway area compared to the expiratory images (3 to 6). In contrast, in the NZO series (bottom panel) the inspiratory-timed images (1 and 2) showed an increase in cross-sectional area, while the expiratory images (3 to 6) were smaller. Quicktime movies: Cinematic images of the dynamic behavior of the mouse pharynx at matched loci (in the hypopharyngeal region) are shown in two QuickTime movies (NZWAxial.mov and NZOAxial.mov). In the NZWAxial.mov the airway is reduced in size during inspiration (first and second images), and then enlarges in expiration (following 4 images). In the NZOAxial.mov, the airway is dilated in inspiration (first two images), and then becomes smaller during expiration (following 4 images). Representative individual images from each movie are displayed in Figure 3 (top panel = NZW, bottom panel = NZO).

Figure 5

(A-D): Mean cross-sectional area (CSA) of the pharyngeal airway in (N = 13) NZW and (N = 11) NZO mice during mid-inspiratory and mid-expiratory phases of respiration. Panel A shows NZW results measured in the nasopharynx (NP) across 7 slice locations, where filled triangles denote inspiration, and open triangles denote expiration. Panel B shows the NP airway CSA for NZO mice where filled circles denote inspiration and open circles denote expiration. In each plot data errors bars show SEM and (*), indicates a significant difference between inspiration and expiration from post hoc comparisons (step-down Bonferroni, p < 0.05). Panel C (NZW mice) and Panel D (NZO mice) compare inspiratory and expiratory CSA in the NP + OP airway. Symbols and error annotations are the same in C and D as in A and B. In some cases the slice locations were intentionally offset for clarity. In each plot data errors bars show SEM and (*), indicates a significant difference between inspiration and expiration (step-down Bonferroni, post hoc comparisons, p < 0.05).

Figure 6

(A-D): Mean cross-sectional area from Figure 4 is plotted to show comparisons between NZW and NZO mice. Panel A compares cross-sectional area (CSA) measured during inspiration in the nasopharynx (NP) across 7 slice locations between NZW (filled triangles) and NZO mice (filled circles). Panel B compared NZW (open triangles) to NZO (open circles) measured during expiration in the nasopharynx. Panel C compares NZW and NZO measured in nasopharynx plus oropharynx (NP + OP) during inspiration and Panel D shows NZW and NZO results in the NP + OP during expiration.

Figure 7

(A, B): A combined variable ‘Delta’ (expiratory cross-sectional area - inspiratory cross-sectional area) was plotted to show how the respiratory phasic pattern was different between NZW and NZO mice. Panel A shows Delta CSA for NZW (closed triangles) and NZO (closed circles) from the nasopharynx (NP) across 7 slice locations. Panel B shows Delta CSA measured from the nasopharynx plus oropharynx (NP + OP) where open triangle represent NZW points and open circles represent NZO points. Data are means with SEM and (*), for this figure indicates a significant difference between inspiration and expiration (step-down Bonferroni, post hoc comparisons, all p < 0.05)