Automatic Apnea Time Adjustments During Ventilation With Automode

Automode is a feature on Servo ventilators that automatically switches between mandatory and spontaneous breaths. Spontaneous breaths suppress mandatory breaths until apnea. The period from the last spontaneous breath to the first mandatory breath is automatically adjusted by a calculated apnea time limit based on a maximum apnea time setting, the mandatory breathing frequency setting, and the spontaneous breath count. The purpose of this study was to validate the apnea time algorithm by using simulated mechanical ventilation.

A Servo-u ventilator was connected to an ASL 5000 breathing simulator. Ventilator settings were the following: Automode (pressure control to pressure support), pressure control = 10 cm H₂O; pressure support = 5 cm H₂O; PEEP = 10 cm H₂O; breathing frequency = 10, 12, 15, 20 breaths/min; maximum apnea time = 7 and 12 s. Simulator settings were the following: resistance = 10 cm H₂O/L/s; compliance = 35 mL/cm H₂O; flow trigger model: frequency = 20 breaths/min, trigger flow = 10 L/min, trigger duration = 800 ms. Flow waveforms were recorded, and the observed apnea time limit was compared with the calculated value. The outcome variable was error, defined as the difference between observed and calculated apnea times expressed as a percentage.

The observed apnea time limit ranged from 3 to 12 s, depending on the mandatory frequency and the spontaneous breath count. The average error ranged from -2 to 0%.

The measured apnea time for simulated ventilation settings was within 2% of calculated times. Automode allowed a spontaneous frequency lower than expected based on the mandatory frequency.