title: Peak airway pressure increase is a late warning sign of partial endotracheal tube obstruction whereas change in expiratory flow is an early warning sign.
abstract: If peak inspiratory airway pressure (Ppeak) is used to monitor airway patency, progressive obstruction of the endotracheal tube (ETT) resulting from secretions can go undetected for a prolonged period. The reason is that any increase in Ppeak depends not only on the degree of narrowing but also on the inspiratory flow () rate. Although the impact of narrowing on low inspiratory is small, its decelerating effect on the high expiratory is pronounced and, hence, easily detectable. Dividing the volume-flow curve of a passive expiration into five consecutive segments (slices) and calculating the time constants (tau(Epsilon)) of these slices allows for analyzing whether and how expiratory is impeded by a partial obstruction. In nine piglets, during volume-controlled ventilation, three grades of ETT obstruction were created with an external clamp. In all animals the tau(E) increased with ETT obstruction (mean for the first slice: 550 ms with unobstructed ETT; grade 1: 661; grade 2: 877; and grade 3: 1563 ms, respectively) and this increase was significant with grade 2 and 3 obstruction. Ppeak, by contrast, did not increase significantly (base: 13, grade 1: 14, grade 2: 15 cm H(2)O) until the most severe (grade 3: 20 cm H(2)O) obstruction was created. We conclude that partial obstruction of the ETT can be reliably monitored with the expiratory V signal and has the potential of monitoring ETT narrowing in ventilator-dependent patients independent of the inspiratory pattern applied.
authors.affiliation: From the Departments of *Surgical, and †Medical Sciences, University Hospital, Uppsala, Sweden, ‡Department of Anesthesiology and Critical Care Medicine, University Hospital Freiburg, Germany, §Department of Anesthesiology and Critical Care Medicine, Klinikum Augsburg, Augsburg, Germany.
authors.forename: Rafael
authors.initials: R
authors.lastname: Kawati
authors.forename: Marco
authors.initials: M
authors.lastname: Lattuada
authors.forename: Ulf
authors.initials: U
authors.lastname: Sjöstrand
authors.forename: Josef
authors.initials: J
authors.lastname: Guttmann
authors.forename: Göran
authors.initials: G
authors.lastname: Hedenstierna
authors.forename: Alois
authors.initials: A
authors.lastname: Helmer
authors.forename: Michael
authors.initials: M
authors.lastname: Lichtwarck-Aschoff
mesh_terms: D000403:Airway Resistance; D000818:Animals; D005260:Female; D007442:Intubation, Intratracheal / Q000009:adverse effects*; D008297:Male; D011312:Pressure; D015656:Respiratory Mechanics; D013552:Swine
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