Assessment of the catheter tip pressure transducer for use in infant lung function tests

A catheter tip pressure transducer was testedin vitro to determine its stability, linearity and frequency response. It was then compared simultaneously with an oesophageal balloon in two trained adult subjects, using measurements of end-expiratory oesophageal pressure and the ratio of oesophageal pressure to mouth pressure changes during airway occlusion. This ratio should be unity, since during airway occlusion there is no flow and pressure changes should be evenly distributed throughout the closed system. The catheter tip pressure transducer was used in 13 infants, who were all studied initially with an oesophageal balloon, during the course of routine lung function tests. Measurements of pulmonary resistance, compliance, end-expiratory oesophageal pressure and the ratio of oesophageal pressure change to mouth pressure change were made with both instruments. The ratio of oesophageal to mouth pressure change and pulmonary resistance were elevated and dynamic compliance reduced when measured with the catheter tip pressure transducer, showing that the catheter tip pressure transducer was overestimating the size of pressure swings within the oesophagus. The absolute value of end-expiratory oesophageal pressure was always more positive when measured with the catheter tip pressure transducer than with the balloon, suggesting that absolute pressure was also incorrectly monitored. It is recommended that the catheter tip pressure transducer should not be used in studies of infant lung mechanics until these problems have been resolved.