Monitoring steam sterilisation processes in hospitals

Steam sterilisation is the method of choice for the sanitising of medical devices within a hospital setting. The process is currently controlled and monitored by three parameters — pressure, temperature and time — and every steam steriliser is able to show these exact values.

Biological and chemical indicators and process challenge devices located inside the chamber and the individual barrier systems can be used to give additional information about the conditions during the process and prove to the user that the medical devices have gone through a sterilisation process before they open the sterile barrier system.

Pockets of air act as a barrier that prevents steam from penetrating the load, so we also rely on checks like the daily Bowie-Dick. This test demonstrates proper air removal from the chamber of a pre-vacuum steriliser and indicates penetration of steam into the standardised test pack.

Additionally, a leak test is performed to ensure no air from outside the chamber is leaking into the chamber. Optionally, an air detector can be fitted to the steriliser to determine whether non-condensable gases are present in the chamber. All these tests, measuring devices and parameter values are essential to ensuring patient safety during surgical procedures.

So far, so good.

But is this enough? Is what we have been doing for all these years still in line with the ongoing evolution of surgical instruments?

We are controlling and measuring some of the vital parameters that determine the effectiveness of steam sterilisation, but we need to consider the need for controlling the presence and density of saturated steam.

As a sterilising agent, saturated steam is a balance of water vapour in a state of equilibrium between its liquid phase and its gas phase. A continuous supply of saturated steam is required for effective steam sterilisation. Excess moisture carried in suspension can cause damp loads, while too little cannot prevent the steam from becoming superheated during expansion into the steriliser chamber. On top of this there is a risk of non-condensable gasses. These non-condensable gases will not liquefy under the conditions of saturated steam sterilsation. So without the presence of saturated steam at an instrument level, it is uncertain that sterilising conditions are met.

On a regular basis, at least once a year, we validate our loads and processes. Steam quality tests are often performed once a year. During validation, thermometric tests at instrument level are used ensure that the required sterilisation conditions are achieved. The measured temperatures are checked and compared with the steam pressure to ensure there is no superheated, supersaturated steam or non-condensable gases present which affects the required temperature/time exposure. But how do we ensure these conditions in every cycle in between validations?

To date, all the test and checks described above are based upon the assumption that saturated steam is present in every process. And with that assumption we use indicators and test devices. However, those measurements are all showing the absence or presence of factors that influence the steam quality but are not telling us the one thing we really want to know: do we actually have saturated steam, and what is the density of that steam in all our cycles during the day?

We run the risk of relying upon monitoring technologies developed in the 1930s, biological indicators and the Bowie-Dick towel pack test from the 1960s. In the last 50 years, surgical techniques and instruments have evolved and become more complex. This especially applies to the mix of instruments with complex design, narrow channels and compositions of stainless steel, aluminium and plastics placed in preformed fixation within instrument trays. The physics and basics of steam sterilisation has not changed since the towel packs, but the complexity of medical devices that need to be sterilised has changed significantly.

In international standards, like EN 285, steam sterilisation conditions are two simple parameters: a predefined temperature for a predefined holding time. The pressure measured as the third value is there to verify the theoretical steam temperature (ISO 17665-1:2006, table C 1 annex C, temperature and pressure of saturated steam for use in moist heat sterilisation) and not a critical parameter. When the steam is not saturated, temperature and pressure are not related at all.

We put our trust in the three values we all know: temperature, time and pressure. Are we forgetting something?

The saturation of the steam in every processed load! This is the missing fourth dimension in steam sterilisation.

Why don’t we start measuring this?

*René Vis has been working in the field of decontamination for over 30 years. He is now working for Miele Professional, where he is the Sterilisation Product Manager and the Manager of CSSD Projects International.

René was manager of CSSD from 1986–2001 at the University Medical Centre in Amsterdam and spent six years on the board of the Dutch National sterilisation association, Sterilisatie Vereniging Nederland (SVN). During this time he was an active member of the Dutch National Standardization Institute (NEN), where he led various working groups and committees. In 2006 he joined Synergy Health as operational manager CSSD and he outsourced the first hospital CSSD in the Netherlands.

Image credit: ©stock.adobe.com/au/Walenga Stanislav