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

Abstract

Background: Right to left shunt and regional hypoventilation (reduced ventilation/perfusion ratio (V A /Q)) have different effects on the curve relating inspired oxygen (P I O 2 ) to oxygen saturation measured by pulse oximetry (SpO 2 ) and can be derived non-invasively from measurements of SpO 2 and inspired oxygen pressure (P I O 2 ) using complex models of gas exchange. We developed a simpler computerised ''slide-rule'' method of making these derivations. Aims: To describe the slide-rule method and determine agreement between measurements derived with this and a more complex algorithm. Methods: Series of P I O 2 versus SpO 2 data points obtained during 43 studies in 16 preterm infants with bronchopulmonary dysplasia were analysed. Percentage shunt and the degree of right shift (kPa) of the P I O 2 versus SpO 2 curve compared with the oxyhaemoglobin dissociation curve (a measure of V A /Q) were determined for each dataset with both methods, and the results were compared using the method of Bland and Altman. Results: The computer slide-rule method produced results for all 43 datasets. The more complex model could derive results for 40/43 datasets. The mean differences (95% limits of agreement) between the two methods for measurements of shunt were 21.7% (26.5 to +3.5%) and for measurements of right shift were 0.3 kPa (22.9 to +3.6 kPa). Conclusion: The slide-rule method was reliable for deriving shunt and right shift (reduced V A /Q) of the P I O 2 versus SpO 2 curve when compared with the more complex algorithm. The new method should enable wider clinical application of these measurements of oxygen exchange.

Influence of pulmonary factors on pulse oximeter saturation in preterm infants

“…All of the infants in this report had BPD where the dominant gas exchange abnormality is a reduced V A /Q 7 12 13 19. Earlier in the clinical course, when the alveoli are more unstable and there is more intrapulmonary shunting, the same method could be applied but the slopes would be predicted to be flatter.…”

“…We reanalysed the data gathered from preterm infants with BPD in our previous report using the new fully computerised system 12 17. The infants were studied between September 2006 and December 2007 in the neonatal unit of the Simpson Centre for Reproductive Health, Royal Infirmary of Edinburgh.…”

Toxicodynamics of rigid polystyrene microparticles on pulmonary gas exchange in mice: Implications for microemboli-based drug delivery systems

“…Sapsford and Jones’ original model was developed and used on 9 healthy, free breathing volunteers and 35 patients undergoing major surgery that were mechanically ventilated (Sapsford and Jones, 1995). Subsequently, it has been used to study pre-term infants and conscious adults with different underlying pulmonary diseases (Jones et al , 2008; Rowe et al , 2010). Although there have been improvements in the original modeling technique including incorporating P a CO 2 or using multiple compartments, the original assumptions have not changed (Jones et al , 2008; Karbing et al , 2011).…”

“…By simply administering O 2 at a single, fixed percentage (e.g., 15%) any detectable change to V A /Q could be quickly inferred. In fact, Jones has recommended using a number of pre-drawn curves for common V A /Q and shunts to quickly and easily determine V A /Q and shunt rather than the need for running the complete mathematical model (Rowe et al , 2010), thereby showing that an even further simplification of this model may have clinical importance. Regardless of future improvements, the current model is more than adequate for clearly demarcating the significant degree of pulmonary toxicity or loss of function.…”

“…Changes in the estimated ventilation-perfusion ratio (V A /Q) and shunt (i.e., pulmonary gas exchange) were investigated following intravenous administration of rigid MPs in order to assess the toxicodynamics of a microemboli-based drug delivery system. Currently there are a variety of commonly used techniques or indices used by intensive care clinicians to assess pulmonary gas exchange including: PaO 2 /F I O 2 , arterial-venous difference, nitrogen washout, multiple inert gas elimination (MIGET) and ventilation-perfusion scans (Rowe et al , 2010). Each method has its limitations since they cannot be performed in mice or they require complex techniques and/or costly equipment.…”