Non-invasive assessment of shunt and ventilation/perfusion ratio in neonates with pulmonary failure

Smith, H. L.; Jones, J. G.
September 2001
Archives of Disease in Childhood -- Fetal & Neonatal Edition;Sep2001, Vol. 85 Issue 2, pF127
Academic Journal
Aims -- To make non-invasive measurements of right to left (R-L) shunt and reduced ventilation/perfusion ratio (VA/Q) in neonates with pulmonary failure and to examine sequential changes in these variables after treatment. Methods -- Twelve neonates with pulmonary failure were studied. They ranged in gestational age from 24 to 37 (median 27) weeks and were 1-39 (median 4) days old. Shunt and reduced VA/Q were derived from their effects on the relation between inspired oxygen pressure (PIO2) and arterial oxygen saturation measured with a pulse oximeter (Spo2). Pairs of Pio2 v Spo2 data points were obtained by varying Pio2 in a stepwise fashion. A computer algorithm based on a model of pulmonary gas exchange fitted a curve to these data. With PIO2 on the abscissa, an increase in shunt produced a downward movement of the curve, whereas reducing VA/Q to < 0.8 shifted the curve to the right. The right shift gives a variable that is inversely related to VA/Q, the PIO2 - P&csline;o2 difference, where P&csline;o2 is mixed capillary oxygen pressure. Results -- Ten of the 12 infants on the first study day had large shunts (range 5.9- 31.0%, median 19.9%, normal <8%) and large PIO2 - P&csline;o2 differences (range 9.7- 64.4 kPa, median 19.8 kPa, normal <7 kPa) equivalent to a median VA/Q of 0.2 (normal median VA/Q = 0.8). Sequential improvement in shunt and VA/Q were shown in most infants after treatment. Sudden large changes in these variables were shown in two infants. Conclusion -- This simple non-invasive method distinguishes between shunt and reduced VA/Q in neonates with pulmonary failure.


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