Effects of expiratory trigger setting on respiratory parameters of nonchronic obstructive pulmonary disease patients

Objective

To observe the effects of expiratory trigger (ET) setting on the following respiratory parameters in nonchronic obstructive pulmonary disease (COPD) patients: respiratory rate (RR), frequency to tidal volume ratio (f/Vt ratio), tidal volume (Vt), minute volume (Ve), oxygen saturation (SpO₂), duty cycle (Ti/Ttot).

Materials and methods

Twenty-four stable patients were evaluated. All patients were ventilated in pressure support ventilation, with pressure support between 8 and 12 cmH₂O (to obtain Vt between 7 and 8 ml/kg), positive end-expiratory pressure between 5 and 7 cmH₂O, fraction of inspired oxygen (FiO₂) ≤ 40% (to obtain SpO₂ ≥ 96%). The expiratory trigger was set at 1%, 25%, 50% and 70%, for a 5-minute period each. The RR, f/Vt ratio, Vt, Ve, SpO₂, and Ti/Ttot were measured at each percentage of ET. Analysis of variance for repeated measures was used to analyze variations during the four ET values and to verify variations out of the comfort zone, defined as: RR >30 bpm, f/Vt >100 breath/min/l, Vt <300 ml, Ve >10 l, SpO₂ <90%, Ti/Ttot >0.45. The Bonferroni test was used to identify which values were significantly different among the multiple comparisons. A probability of less than 0.05 was considered significant. The ventilator used was the Bennet 840.

Results

All respiratory parameters presented significant variations when the comparisons were made from 1% to 70% of ET (P = 0.0001), and 0.0003 for Ti/Ttot. The Vt, RR and f/Vt ratio presented significant increase in the percentage of patients that showed these parameters out of the comfort zone (P = 0.0025, P = 0.0002 and P = 0.007, respectively). No respiratory parameter presented significant variations when the comparisons were made from 1% to 25% of ET.

Conclusion

In non-COPD patients, the use of ET at 1% or 25% has no effect on the respiratory parameters. The increase of ET to 50% or more can worsen the respiratory parameters and lead to rapid shallow breathing, suggesting that these values should be avoided in non-COPD patients.