At each interval, EIT measurements were made using the Göttingen Goe-MF II EIT system (Cardinal Health, Yorba Linda, CA, USA). Sixteen electrodes (Blue Sensor BR-50-K, Ambu, Denmark) were applied circumferentially around the infant's chest at the mammary line. A 30 second reference measurement at 13 Hz scan rate was recorded. All further measurements were referenced to this measurement. All other measurements were made at a scan rate of 44 Hz for 180 seconds. A 5 mA peak-to-peak, 50 kHz electrical current was injected at each adjacent electrode pair, and the resultant potential differences were measured at the remaining adjacent electrode pairs. Subsequently, all adjacent electrode pairs were used for current injection, thus completing one data cycle. The impedance map was built using the back-projection image reconstruction algorithm [13] . It calculates the relative impedance ΔZ, defined by (Z inst -Z ref )/Z ref (where Z inst is the instantaneous local impedance and Z ref the reference impedance, determined from each cycle of current injections and voltage measurements in each pixel).