INFLUENCE OF SITE OF TRACHEAL PRESSURE MEASUREMENT ON IN-SITU ESTIMATION OF ENDOTRACHEAL-TUBE RESISTANCE

In situ measurement of distal tracheal pressure (Ptr) via an intralumi nal side-hole catheter (IC) has been used to determine endotracheal tu be (Rett) and intrinsic patient (Rpt) resistances in intubated subject s. Because of differences in cross-sectional area between the endotrac heal tube (ETT) and trachea, fluid dynamic principles predict that IC position should critically influence these results. Accordingly, the a im of this study was to determine the effect of IC position on Rett. P tr was recorded in vitro through an IC from 2 cm inside, at the tip of , or 2 cm outside an ETT (7, 8, and 9 mm ID) situated within an artifi cial trachea (13, 18, and 22 mm ID). A reference value of Rett was als o obtained. Results were unaffected by IC position during inspiration, overestimating Rett by 7.9 +/- 0.7% (SE). In contrast, during expirat ion, Rett fell as IC position changed from outside to inside the ETT a nd was underestimated by 41.3 +/- 3.6% with Ptr recorded inside the ET T. Varying ETT or tracheal size had little effect on the relative erro r in Rett. The IC itself did increase Rett due to a reduction in effec tive cross-sectional area, the change varying directly with IC size an d inversely with ETT caliber. In vivo values in 11 intubated patients were comparable to in vitro results. In summary, IC position and size can have important consequences on in situ measurements of Ptr and sho uld be considered when clinically monitoring Rett or Rpt.