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

Aims-To make non-invasive measurements of right to left (R-L) shunt and red uced ventilation/perfusion ratio (V-A/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) d ays old. Shunt and reduced V-A/Q were derived from their effects on the rel ation between inspired oxygen pressure (PIO2) and arterial oxygen saturatio n measured with a pulse oximeter (Spo(2)). Pairs of PIO2 v Spo(2) data poin ts were obtained by varying PIO2 in a stepwise fashion. A computer algorith m 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 movemen t of the curve, whereas reducing V-A/Q to < 0.8 shifted the curve to the ri ght. The right shift gives a variable that is inversely related to V-A/Q, t he PIO2 - P(c) over barO2 difference, where P(c) over barO2 is mixed capill ary oxygen pressure. Results-Ten of the 12 infants on the first study day h ad large shunts (range 5.9-31.0%, median 19.9%, normal < 8%) and large PIO2 - P(c) over barO2 differences (range 9.7-64.4 kPa, median 19.8 kPa, normal < 7 kPa) equivalent to a median V-A/Q of 0.2 (normal median V-A/Q = 0.8). Sequential improvement in shunt and V-A/Q were shown in most infants after treatment. Sudden large changes in these variables were shown in two infant s. Conclusion-This simple non-invasive method distinguishes between shunt and reduced V-A/Q in neonates with pulmonary failure.