PARTITIONING OF TOTAL PULMONARY RESISTANCE IN HORSES

The partitioning of total pulmonary resistance (RL) into upper airway resistance and lower airway resistance (R1) was studied in 8 Thoroughb red geldings. In addition, the phase shift and amplitude distortion of 3 catheters used for pressure measurements in this study were evaluat ed under static and dynamic conditions. Flow rate was obtained from a heated pneumotachograph attached to a tight-fitting mask placed over t he nose. Electronic integration of the flow signal gave tidal volume. Transpulmonary pressure (PL) was obtained from calculation of the diff erence between the esophageal balloon catheter pressure and mask press ure. Lateral tracheal pressure was measured from a polyethylene cathet er placed percutaneously in the middle portion of the trachea. Lower a irway pressure (PL) was calculated as the difference between esophagea l pressure and lateral tracheal pressure. Similarly, upper airway pres sure was defined as the difference between lateral tracheal pressure a nd mask pressure. Pressures are reported as the difference between the maximal and the minimal pressures recorded during a respiratory cycle . Airway resistance was calculated, using the isovolume method, at 50% of tidal volume. There were individual and group variations in P1 and P1/PL, although P1 accounted for more than 60% of PL in all horses. I n 6 horses, RI was more than 50% of RL whereas in 2 horses, RI was onl y 30 and 34% of RL. Amplitude distortion was minimal for the 3 cathete rs under static conditions in the in vitro study. Under dynamic condit ions, amplitude distortion varied according to the catheter studied, t he frequency, and the resistance of the system. There were no phase di fferences under static conditions at low frequency. However, phase dis crepancy, which was variable through the cycle, was observed for some catheters at high frequency under static and dynamic conditions. It wa s concluded that, until measuring techniques are standardized in horse s, variations in the partitioning of RL are likely to be obtained betw een studies and between animals within studies.