An MD&DI February 1997 Feature
STERILIZATION
Speeding EtO-Sterilized Products to Market with Parametric Release (continued)
Headspace Gases
Water Vapor. When process parameter integration is confirmed by the use of BIs, regulatory bodies allow water vapor to be calculated in terms of relative humidity (%Rh) rather than directly measured. Knowing the pressure at the beginning of humidification (Pi), the pressure at the end of humidification (Pt), and the chamber temperature at the end of steam injection (°Tdc), one can calculate the %Rh by referring to the pressure of saturated steam (Pss) at °Tdc in standard steam tables.5
But %Rh is not a completely reliable measure of water vapor. Pressure rise during the steam injection phase does not necessarily mean that pure steam has been introduced. In fact, a ruptured seal on the inlet side of the steam control valve can cause ambient air to be sucked into the vessel in place of steam. This would provide a normal pressure rise profile, but no rise in humidity. If BIs are being used afterward, they will reveal that such an error has occurred. But to ensure that this parameter is correct so that parametric release can be possible, the mass of water vapor per unit volume must be measured directly.
Sterilant Gas. EtO concentration must also be measured directly rather than calculated from other values in parametric release.
Using BIs allows the sterilization control system to depend on indirect measurement methods. The most common of these methods are pressure rise inside the vessel from sterilant addition and the change in weight of the gas supply cylinder. In these methods, minimum pressure rise and cylinder weight change are established as parameters during validation, and then verified after each production run.
However, equipment malfunctions may allow "false" pressure rises and cylinder weight changes to occur. Leaks along the sterilant gas feed lines, leaks in the gas vaporizers, and malfunctioning valves can all lead to a normal pressure rise inside the vessel and a normal decrease in cylinder weight without the introduction of the required amount of EtO to the vessel.
