Oral Presentation Australian and New Zealand Obesity Society Annual Scientific Conference 2026

Discordance between forced oscillometry (FOT) and spirometry in adults with obesity-related asthma (144837)

Tamara Blickisdorf 1 , Lisa G Wood 1 , Sarah R Valkenborghs 1 , Anne E Dixon 2 , John D Brannan 1 , Jay C Horvat 1 , Natasha A Weaver 1 , Serene Yoong 3 , Bronwyn S Berthon 1 , Evan J Williams 1 , Alexandra C Brown 1 , Christine R Jenkins 4 , Meagan L Morrissey 1 , Peter A Wark 5 , Katie Wynne 1 , Christopher L Grainge 1 , Emad M El-Omar 6 , Lily M Williams 1 , Hayley A Scott 1
  1. The University of Newcastle, Callaghan, NSW, Australia
  2. University of Vermont, Burlington, United States of America
  3. Deakin University, Geelong, Victoria, Australia
  4. George Institute for Health, Sydney, NSW, Australia
  5. Monash University, Melbourne, Victoria, Australia
  6. University of New South Wales, Sydney, NSW, Australia

Introduction

Asthma is primarily defined by variable airway obstruction. Obesity can impose further mechanical constraints on the respiratory system, contributing to altered lung volumes, airway dysfunction, or both. Consequently, people living with obesity-related asthma may exhibit complex respiratory impairment. The forced oscillation technique (FOT) is a measure of respiratory mechanics that may detect airway dysfunction not captured by standard lung function testing (spirometry).

Aim

To classify adults with asthma and obesity according to FOT-derived mechanical patterns and explore their associations with spirometry.

Method

In a cross-sectional analysis, 61 adults (57±12 years old, 58% female) with asthma and obesity were classified into four groups according to FOT z-scores: (1) normal mechanics; (2) resistance-dominant abnormality, defined by elevated frequency dependence of resistance (R5-R20 >+1.64) without abnormal reactance; (3) reactance-dominant abnormality, defined by reduced reactance at 5 Hz(X5<-1.64) and/or elevated area under the reactance curve (AX >+1.64) with normal R5-R20; and (4) mixed resistance and reactance abnormality (R5-R20 >+1.64 plus abnormal X5 and/or AX). Lung function was assessed using Kruskal-Wallis and Fisher’s exact test.

Results

FOT classified 11 participants (18%) as normal, 2 (3%) with increased resistance, 12 (20%) as reactance-dominant, and 36 (59%) as having mixed resistance-reactance abnormalities. Spirometric measures of lung function (FEV1/FVC, FEV1 and FVC) differed across the four classifications (all p<0.001), with median FEV1/FVC and FVC z-scores below the normal range (-2.42, -2.02 respectively) in the mixed classification only. The distribution of conventional spirometric abnormalities differed by FOT classification (Fisher’s exact p=0.028). Among participants with abnormal FOT, 25% had normal spirometry.

Discussion

FOT identified heterogeneous respiratory mechanics in adults with obesity-related asthma, with combined resistance-reactance abnormalities predominating. Abnormal FOT despite normal spirometry suggests that respiratory effects of obesity may not be fully captured by conventional lung function testing. Identifying these mechanical patterns may support more targeted assessment of obesity-related asthma.