Non-invasive assessment of shunt and ventilation/perfusion ratio in neonates with pulmonary failure
- Professor Jones, Department of Anaesthesia, Cambridge University, Level 4, Addenbrooke's Hospital, Cambridge CB2 2QQ, UKgareth{at}garjons.demon.co.uk
- Accepted 14 May 2001
Abstract
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 (Spo 2). Pairs of Pio2 vSpo 2 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 − Pc̄ o 2difference, where Pc̄ o 2 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 − Pc̄ o 2differences (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.
