Reliable Time to Estimate Subglottal Pressure

Summary 

Measuring subglottal pressure (P_s) with complete interruption can be problematic due to unsteady plateaus in supraglottal pressure data traces during balloon valve interruption. Subjectively determining when the graph plateaus neglect the effects of laryngeal, auditory, and other physical reflexes may alter patient effort and glottal configuration. If the P_s estimation was made at a consistent time before the onset of reflexes, the recorded pressure would not be dependent on subjective analysis by a clinician, and intrasubject data would be more precise. Previously collected data using the airflow interruption system have shown consistency at approximately 150 milliseconds after balloon valve inflation. To evaluate the validity of estimating P_s at this point, a theoretical and a physical model were applied. A theoretical ideal gas model of capacitance calculated the time necessary for supraglottal pressure to equilibrate with P_s. Using a mechanical pseudolung which served as a constant pressure source, known subresistor pressures were compared to the pressure measured by the interruption device. Both models confirmed the validity of measuring P_s consistently at 150 milliseconds into the 500-millisecond interruption. In human trials testing 25 subjects, mean intrasubject standard deviation using this optimal time constant was 0.66±0.37cmH₂O, and 1.11±0.48cmH₂O when performing plateau analysis (P<0.0005). This novel modification to the clinically feasible interruption model for P_s estimation demonstrates a marked improvement in the reliability of balloon valve interruption while maintaining the validity demonstrated in previous studies.

Key Words: Subglottal pressure, Airflow interruption, Reliable time