A simplified method for deriving shunt and reduced VA/Q in infants

Lawrence Rowe ¹, John Gareth Jones ^2*, David Quine ³, Shetty S Bushan ³ and Benjamin J Stenson ³

¹ Norfolk and Norwich, University Hospital, United Kingdom
² University of Cambridge, United Kingdom
³ Royal Infirmary of Edinburgh, United Kingdom

^* To whom correspondence should be addressed. E-mail: johngareth423{at}btinternet.com.

Accepted 7 August 2009

Abstract

Background: Right to left shunt and reduced ventilation to well perfused alveoli (VA/Q) have quite different effects on the curve relating inspired oxygen (PIO2) to oxygen saturation (SpO2) and can be quantified non-invasively from measurements of SpO2 and inspired oxygen pressure (PIO2) using complex models of gas exchange. We developed a simpler and more readily applicable computerised "slide-rule" method of making these measurements.

Aims: To describe the "slide-rule" method and determine the agreement between measurements derived with this and with the more complex model.

Methods: Series of PIO2 vs SpO2 data points obtained during 43 studies in 16 preterm infants with bronchopulmonary dysplasia were analysed. % shunt and the degree of right shift (kPa) of the haemoglobin-oxygen dissociation curve (a measure of VA/Q) were determined for each dataset with both analytical methods and the results were compared using the method of Bland and Altman.

Results: The computer slide rule method produced results for all 43 data sets. The more complex model could calculate results for 40/43 datasets. The mean difference (95% limits of agreement) between the two methods for measurements of shunt were minus 1.7% (minus 6.5 to + 3.5%) and for measurements of right shift were 0.3kPa (minus 2.9 to + 3.6kPa).

Conclusion: The "slide-rule" method was a reliable technique for deriving shunt and right shift (reduced VA/Q) of the PIO2 vs SpO2 curve when compared with the more complex method. The new method should be widely applicable and should facilitate wider application of these measurements.