Use of anesthesia machines as improvised ICU ventilators may occur in locations where waste anesthesia gas suction(WAGS) is unavailable. Anecdotal reports suggest as much as 18 cmH2O positive end-expiratory pressure(PEEP) being inadvertently applied under these circumstances, accompanied by inaccurate pressure readings by the anesthesia machine. We hypothesized that resistance within closed anesthetic gas scavenging systems(AGSS) disconnected from WAGS may inadvertently increase circuit pressures.
An anesthesia machine was connected to an anesthesia breathing circuit, a reference manometer, and a standard bag reservoir to simulate a lung. Ventilation was initiated as follows: Volume Control, TV 500 mL, respiratory rate 12, I:E 1:1.9, FiO2 1.0, fresh gas flow(FGF) rate 2.0 liters per minute(LPM), and PEEP 0 cmH2O. After engaging the ventilator, PEEP and peak inspiratory pressure(PIP) were measured by the reference manometer and the anesthesia machine display simultaneously. The process was repeated using prescribed PEEP levels of 5, 10, 15, and 20 cmH2O. Measurements were repeated with the WAGS disconnected and then were performed again at FGF of 4, 6, 8, 10, and 15 LPM. This process was completed on three anesthesia machines: Dräger Perseus A500, Dräger Apollo, and the GE Avance CS2. Simple linear regression was used to assess differences.
Utilizing non-parametric Bland-Altman analysis, the reference and machine manometer measurements of PIP demonstrated median differences of -0.40 cmH2O (95%LOA: -1.00,0.55) for the Dräger Apollo, -0.40 cmH2O (95%LOA: -1.10, 0.41) for the Dräger Perseus, and 1.70 cmH2O (95%LOA: 0.80,3.00) for the GE Avance CS2. At FGF 2 LPM and PEEP 0 cmH2O with the WAGS disconnected, the Dräger Apollo had a difference in PEEP of 0.02 cmH2O (95%CI: -0.04,0.08; p=0.53); the Dräger Perseus A500, <0.0001 cmH2O (95%CI: -0.11 0.11; p=1.00); and the GE Avance CS2, 8.62 cmH2O (95%CI: 8.55,8.69; p<0.0001). After removing the hose connected to the AGSS and the visual indicator bag on the GE Avance CS2, the PEEP difference was 0.12 cmH2O (95%CI: 0.059,0.181; p=0.0002).
Displayed airway pressure measurements are clinically accurate in the setting of disconnected WAGS. The Dräger Perseus A500 and Apollo with open scavenging systems do not deliver inadvertent CPAP with WAGS disconnected, but the GE Avance CS2 with a closed AGSS does. This increase in airway pressure can be mitigated by the manufacturer’s recommended alterations. Anesthesiologists should be aware of the potential clinically important increases in pressure that may be inadvertently delivered on some anesthesia machines, should the WAGS not be properly connected.