Ventilator Wash-Out Volume: A Consideration in Endotracheal Suction Preoxygenation

Abstract

Mechanically ventilated patients requiring endotracheal suction are often preoxygenated with an inspired oxygen fraction (FI02) of 1.0 for a brief period prior to the suction procedure. Because the "wash-out volume" required to displace an existing gas mixture with pure oxygen throughout the ventilator circuit has not previously been measured, we determined this volume for each of five commonly used ventilators. Using an oxygen analyzer at the distal end of the tubing circuit and varying tidal volume (VT), we found wash-out volumes ranging from 6.3 1 for the Monaghan 225 ventilator at VT 700 ml to 18.01 for both the Bennett MA-1 and Ohio 560 ventilators at VT 1,500 ml. The Bourns BEAR-1 and Servo 900-B ventilators gave intermediate volumes. Depending upon tidal volume and cycling frequency, a new FI02 may not actually be delivered to the patient for as long as 2 minutes after the ventilator controls are changed. Hypoxemia due to endotracheal suctioning may not be prevented if the ventilator is used for preoxygenation and allowance is not made for wash-out volume.