Computational fluid dynamics endpoints for assessment of adenotonsillectomy outcome in obese children with obstructive sleep apnea syndrome

Abstract

Background: Improvements in obstructive sleep apnea syndrome (OSAS) severity may be associated with improved pharyngeal fluid mechanics following adenotonsillectomy (AT). The study objective is to use image-based computational fluid dynamics (CFD) to model changes in pharyngeal pressures after AT, in obese children with OSAS and adenotonsillar hypertrophy.

Methods: Three-dimensional models of the upper airway from nares to trachea, before and after AT, were derived from magnetic resonance images obtained during wakefulness, in a cohort of 10 obese children with OSAS. Velocity, pressure, and turbulence fields during peak tidal inspiratory flow were computed using commercial software. CFD endpoints were correlated with polysomnography endpoints before and after AT using Spearman׳s rank correlation (rs).

Results: Apnea hypopnea index (AHI) decreases after AT was strongly correlated with reduction in maximum pressure drop (dPTAmax) in the region where tonsils and adenoid constrict the pharynx (rs=0.78, P=0.011), and with decrease of the ratio of dPTAmax to flow rate (rs=0.82, P=0.006). Correlations of AHI decrease to anatomy, negative pressure in the overlap region (including nasal flow resistance), or pressure drop through the entire pharynx, were not significant. In a subgroup of subjects with more than 10% improvement in AHI, correlations between flow variables and AHI decrease were stronger than in all subjects.

Conclusions: The correlation between change in dPTAmax and improved AHI suggests that dPTAmax may be a useful index for internal airway loading due to anatomical narrowing, and may be better correlated with AHI than direct airway anatomic measurements.

Keywords: Airway resistance; Computer simulation; Humans; Magnetic resonance imaging; Pediatrics.

Figure 1

3D upper airway model of subject 4 pre-surgery based on reconstructed segmented MR axial images. Anatomical locations along the airway model are shown for reference. A_min is the location of minimum cross-section where tonsils and adenoids constrict the pharynx; in this subject A_min is located between the tonsils in the retrolingual pharynx. Inset: midline sagittal MR image of subject 4.

Figure 2

Pressure contours on the UA surface of subject 1 before and after AT surgery (lateral view; note larger pressure scale in pre-surgery image). Anterior-posterior restriction in the nasopharynx and oropharynx is reduced after AT. Pressure drop was low (about 50 Pa) in the nasal passages of this subject. Before surgery there was a rapid drop in pressure (averaged over the airway cross-section) to P_min = −620 Pa in the retrolingual pharynx where tonsils restrict the airway, then mild pressure recovery to −400 Pa. After surgery the minimum cross-section area in the overlap region increased, and as a result the minimal pressure increased to P_min = −60 Pa. The pressure in the retrolingual pharynx is well represented by P_min in each model.

Figure 3

Pressure contour on the UA surface of subjects 4 (top) and 7 (bottom), before and after AT surgery. Surgery relieved both lateral and anterior-posterior (not visible in this view) restriction of the airway. These two subjects actually had decreased UA volume after surgery, due to decreased retrolingual airway volume. But increased minimum cross-section area and more gradual narrowing caused increased P_min and increased retrolingual airway pressure in both subjects.

Figure 4

Correlation between the percentage increase in minimum cross section area and the percentage decrease in AHI.

Figure 5

Correlation between the percentage decrease in airway pressure from choanae to area minimum, dP_TAmax, and the percentage decrease in AHI. A. All subjects. B. Subjects with >10% improvement in AHI after surgery (moderately to completely successful surgery). Correlation is approximately linear in this subgroup of patients.