EVALUATION IN ANIMALS OF A SYSTEM TO ESTIMATE TRACHEAL PRESSURE FROM THE ENDOTRACHEAL-TUBE CUFF

Objective. Flow through an endotracheal tube (ETT) causes a pressure l oss across the tube. This loss results in a difference between pressur e measured at the airway and pressure measured in the trachea. This di fference can lead to errors when calculating pulmonary mechanics and w hen setting ventilators. We have tested a method of estimating trachea l pressure from the pressure in the ETT cuff. Methods. Pressure transd ucers were placed in the proximal ETT connector, in the trachea, and i n the ETT cuff (through the inflation port). Instantaneous periods of zero flow, detected with a flow meter, were used to calculate the slop e and offset of the line relating cuff pressure to tracheal pressure. The system was tested on the bench using a ventilator and lung simulat or and in 2 dogs and 5 pigs. Tests were performed at various cuff pres sures, trachea diameters, ETT sizes, respiratory rates, tidal volumes, and airway obstructions. Results. In bench tests, our estimate of tra cheal pressure was within -4.0 +/- 2.6% of the actual tracheal pressur e (mean +/- standard deviation [SD]). In animal tests, our estimation of tracheal pressure was within -0.6 +/- 5%. In all bench test measure ments and in 40 of 42 animal measurements, the error was less than 1 c m H2O. Conclusions. The cuff estimation technique gives real-time, con tinuous, noninvasive tracheal pressure measurements in intubated anima ls with cuffed ETTs.