[Reason analysis of ineffective triggering caused by a continuous nebulization airflow in mechanical ventilation]

To analyze the ineffective triggering caused by nebulization in the way of respiratory mechanics.

A test-lung and an oxygen-driven jet nebulizer were connected to the circuit in a PB840 ventilator. The test-lung was pulled outwards in manual way till an inspiration was effectively triggered separately in different flow-trigger modes [flow-trigger sensitivity (V_Trig) 3 L/min and 5 L/min] and pressure-trigger modes [pressure-trigger sensitivity (P_Trig) 2 cmH₂O and 4 cmH₂O, 1 cmH₂O = 0.098 kPa] with the nebulizer being closed and opened in turn. The corresponding relationship and characteristics between the flow and pressure in the circuit under different triggering conditions were observed by adjusting the curve amplitude in the screen. The minimum alveolar pressure (Pa) which could cause an effective triggering and the variation span of Pa during the triggering period were analyzed in respiratory mechanics.

(1) In flow-trigger mode: Pa was pulled down from positive end-expiratory pressure (PEEP) or intrinsic positive end-expiratory pressure (PEEPi) to "PEEP-V_TrigR" (R meant airway resistance) without nebulization, and the span of Pa was "V_TrigR" or "PEEPi-PEEP+V_TrigR". Pa was pulled down from PEEP or PEEPi to "PEEP-(V_Trig+N)R" (N meant nebulization airflow) with nebulization, and the span of Pa was "(V_Trig+N)R" or "PEEPi-PEEP+(VTrig+N)R". (2) In pressure-trigger mode: Pa was pulled down from PEEP or PEEPi to "PEEP-P_Trig-1R" without nebulization, and the span of Pa was "P_Trig+1R" or "PEEPi-PEEP+P_Trig+1R". Pa was pulled down from PEEP or PEEPi to "PEEP-P_Trig-(N+1)R" with nebulization, and the span of Pa was "P_Trig+(N+1)R" or "PEEPi-PEEP+P_Trig+(N+1)R".

Nebulization airflow increases the difficulty of inspiratory triggering in mechanical ventilation. PEEPi makes it more difficult.