Peak Airway Pressure Increase Is a Late Warning Sign of Partial Endotracheal Tube Obstruction Whereas Change in Expiratory Flow Is an Early Warning Sign

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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 (V̇) rate. Although the impact of narrowing on low inspiratory V̇ is small, its decelerating effect on the high expiratory V̇ 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 (τΕ) of these slices allows for analyzing whether and how expiratory V̇ 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 τE 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 H2O) until the most severe (grade 3: 20 cm H2O) obstruction was created. We conclude that partial obstruction of the ETT can be reliably monitored with the expiratory V̇ signal and has the potential of monitoring ETT narrowing in ventilator-dependent patients independent of the inspiratory V̇ pattern applied.