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A2007 - A Non-Invasive Approach for Calculating Work of Breathing in Obstructive Sleep Apnea Patients Using Computational Fluid Dynamics and Cine CTs
Author Block: G. Mylavarapu1, R. J. Fleck2, K. McConnell3, R. S. Amin4; 1Pulmonary Medicine, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States, 2Radiology, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States, 3Pulmonary Medicine, Cincinnati Childrens Hospital Medical Center, Cincinnati, OH, United States, 4Pulmonary Medicine, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH, United States.
RATIONALE: An effective Continuous Positive Airway Pressure (CPAP) therapy reduces the Work of breathing (WOB). WOB measures energy expenditure in a respiratory cycle by airway muscles. In a healthy subject, the normal value for WOB is ~150mJ. Though CPAP increases the mean airway size over a breathing cycle, it may also augment airway dynamics (rate of opening/closing). The role of airway dynamics on the efficacy of CPAP is less well-understood. We hypothesize that increased upper airway dynamics lead to increased WOB. Conventional measurement of WOB requires insertion of a gastro-esophageal catheter in the patient, limiting its practical use and for pediatric research. Towards that end, we developed a non-invasive methodology to calculate WOB using dynamic airway scans, nasal pressure, volumetric flow rate data and Computational Fluid Dynamics (CFD) simulations.
METHODS: In this pilot study, we compared WOB in two Obstructive Sleep Apnea (OSA) patients with a very dynamic (Patient-A) and less dynamic (Patient-B) airway. CINE Computed Tomography (CT) axial scans of the upper airway with a slice increment of 0.25mm, pixel spacing of 0.34mm, 512x512 spatial resolution and a 200msec temporal resolution were obtained in both patients. Three-dimensional airway geometries at each phase of the breathing cycle were reconstructed from their cine CT scans. Fractional Collapse(FC), a measure of airway dynamics is calculated as (maximum-minimum/maximum) pharyngeal airway volume in a breathing cycle. A collapsed pharynx would have FC=1 and a static airway, FC=0. Nasal pressure and volumetric flow rates were also measured using pressure transducer and pneumotachometer respectively. Tidal volumes at each phase were obtained by integrating digitized flow signal. CFD was performed in reconstructed airway geometries at each phase with corresponding nasal pressure and flow rate as boundary conditions to predict the esophageal pressure. Esophageal pressure is often taken as a substitute for pleural pressure even in conventional WOB measurements. WOB measured as pressure X tidal volume (area under the pressure-volume curve) in a breathing cycle was used to quantify respiratory effort.
RESULTS: Patient-A with severe OSA (AHI 27 events/hour), has a very dynamic airway with FC of 0.84 with a calculated WOB of 752.4mJ. Patient-B with mild/moderate OSA (AHI 5.3 events/hour) has a lesser dynamic airway with FC of 0.41and calculated WOB of 34.5mJ.
CONCLUSIONS: A novel, non-invasive computational approach for estimating WOB is presented. Calculated WOB is an order of magnitude higher for Patient-A with a very dynamic (obstructive) airway when compared to Patient-B.