Impact of adding a filter for protection from toxic inhalational compounds to the ventilation circuit of mechanically ventilated patients.

Background: Standard-issue Chemical-Biological-Radio-Nuclear (CBRN) gasmasks, as used for protection from non-conventional warfare agents or toxic industrial compounds, cannot be used by ventilated patients, leaving them exposed to toxic agents inhaled via their ventilators. This study was conducted to determine the safety of a CBRN filter added to the patient circuit of a ventilator, as a method for affording inhalational protection to ventilated patients.

Methods: A Landrace pig was ventilated sequentially with 3 types of ventilators according to 17 different ventilation protocols, with and without a CBRN filters added in-line to the ventilation tubing for each protocol. For each protocol, physiological parameters, including oxygen saturation, inspired CO₂, end tidal CO₂, inspired oxygen, respiratory rate, and pulse rate, as well as airflow parameters including peak inspiratory pressure, positive end expiratory pressure and tidal volume were measured. The impact on the ventilator's trigger/sensitivity function was evaluated in vitro using a Michigan test lung.

Results: On average, the addition of the CBRN filter resulted in a 16 ml (5 %) decrease (range 0-50 ml) in the tidal volume, a 1.7 cm H₂O (10 %) decrease (range 1-3 cm H₂O) in the peak inspiratory pressure, and a 0.1 cm H₂O (3 %) decrease (range 0-1 cm H₂O) in the positive end expiratory pressure delivered to the animal. Some ventilators compensated for these airflow changes while others did not, depending on the design of the ventilator's pressure/flow sensing mechanism. Significant rebreathing occurred when the filter was positioned directly on the animal's endotracheal tube, but not when positioned on the air outflow port of the ventilator. In vitro measurements showed that the addition of the CBRN filter added a mean pressure gradient of 0.45 cm H₂O to the trigger/sensitivity function of the system.

Conclusions: In-line addition of a CBRN filter to ventilation tubing is a feasible strategy for affording inhalational protection to ventilated patients.