Source: Annual Congress 2002 - Lung mechanics: from laboratory to bedside
Disease area: Airway diseases

Abstract

INTRODUCTION:- The gold standard measurement of resistive work of breathing (rWOB) is invasive, requiring oesophageal pressure monitoring. Work is calculated from the integral over time of transpulmonary pressure x flow. We propose that the integral of flow x flow x Rrs could be used as an alternative non-invasive estimate of rWOB, where Rrs is within-breath resistance measured by forced oscillometry (FOT). Here we compare this measurement with the gold standard in the context of a histamine challenge test (HCT).
METHOD:- Five asthmatics underwent an HCT. At each concentration, measurements were made of FEV1, transpulmonary pressure, flow and Rrs (at 5Hz). The rWOB value was obtained by averaging the results for breaths over a minute.
RESULTS:- Data from1368 breaths was analysed. All subjects reached their PC20 point, the range in rWOB being 0.1 to 1.8J/L measured by oesophageal manometry and 0.25 to 0.8J/L by FOT. The coefficient of variation of the FOT measurement was 11% (23% for oesophageal manometry). Whilst the absolute agreement of the two methods was good over the midrange (Bland and Altman analysis), it deteriorated at the extremes. FOT overestimated low and underestimated high values. However, <1.2J/L, correlation of the two methods was excellent (R=0.91).
CONCLUSION:- FOT can measure relative changes in rWOB in asthmatics during a histamine challenge but as used here cannot accurately predict the absolute value. For rWOB<1.2J/L, linear regression analysis could provide a transformation with which to correct the FOT results and improve the agreement. This remains to be tested against further data. For rWOB>1.2J/L, the poor agreement is most likely due to upper airway shunt effects degrading the FOT results.

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M. K. Johnson, R. Carter, M. J. Birch, J. Kinsella, R. D. Stevenson (Glasgow, London, United Kingdom). Use of within-breath Rrs values at 5 Hz to estimate resistive work of breathing during histamine challenge tests. Eur Respir J 2002; 20: Suppl. 38, 186

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