Presented at the Neonatal Society 2006 Spring Meeting.
Quine D, Wong CM, Boyle EM, Jones JG, Stenson BJ
Neonatal Unit, Simpson Centre for Reproductive Health, Royal Infirmary of Edinburgh, UK
Introduction: An objective definition of bronchopulmonary dysplasia (BPD) is required to interpret trial outcomes and provide a baseline for prognostic studies. Reduced ventilation/perfusion ratio (VA/Q) and increased right to left shunt can be determined non-invasively by plotting oxygen saturation (SpO2) against inspired oxygen pressure (PIO2) (1,2). Reduced VA/Q and the associated raised alveolar carbon dioxide (PACO2) cause a right shift of the SpO2 against PIO2curve (3).
Aims: To describe the reduced VA/Q and shunt in infants with BPD.
Methods: 21 preterm infants with BPD were studied at around 36 weeks CGA, after local ethics committee approval for study and written informed parental consent was obtained. PIO2 was varied in steps to cause SpO2 to vary between 86% and 94%. Pairs of PIO2 and SpO2 values were plotted and analysed to derive reduced VA/Q and shunt.
Results: Characteristics of the infants studied included in table 1. In every infant the SpO2 vs PIO2 curve was shifted to the right of normal due to reduced VA/Q and associated raised PACO2. The mean (SD) right shift was 16.5 (4.7) kPa (normal 6kPa). Plots from 4 infants are shown Graphs 1-4. Varying degrees of shunt were also present but these were less important in determining SpO2 within the studied range. The degree of right shift was strongly predictive of the PIO2 required to achieve any SpO2 within the range 86-94%, (R2>0.9), permitting shift and VA/Q to be determined from a single pair of PIO2 and SpO2 values in this range. The mean (SD) difference between the shift calculated from individual data pairs and the entire data series for each infant was 0.24 (1.25) kPa, a Bland Altman plot of the differences is included, graph 5.
Table 1: Characteristics of the infants studied
Graphs 1-4: SpO2 vs PIO2 plots from 4 infants
Graph 5: Bland Altman plot of the differences between shift calculated from individual data pairs and the entire data series for each infant.
Conclusions: The predominant gas exchange impairment in BPD is a reduced VA/Q ratio. This can be described by the right shift of the SpO2 vs PIO2 relationship. This can be derived from a single pair of PIO2 and SpO2 values and provides a simple method for defining BPD that grades disease severity.
References
1. Jones JG, Jones SE. J Clin Monit Comput 2000;16:337-350.
2. Kjaergaard S et al. Intensive Care Med 2003;29:727-734.
3. Olszowka A, Wagner P. Numerical analysis of gas exchange, in Pulmonary Gas Exchange, volume 1, pp 263-306. Academic press, inc., New York 1980.