Among the many tools at our disposal in the SPD perhaps the most misunderstood, are also the most important. Steam autoclaves are often very large, complex, noisy and blisteringly hot conundrums, the workings of which the average SPD staff rarely questions. Stalwart sentinels, destroying pathogens with crushing pressure, scalding steam and heat, these ubiquitous devices can nonetheless be thwarted by improper loading and maintenance. Steam autoclaves walk a fine line between obliterating pathogens and destroying instruments and packaging. Too little heat, steam, or pressure, and the strongest bugs can survive. Too much, and peel pouches blister and melt, plastic parts warp and even resilient electrical parts fry. While we can’t change how infectious agents die or where sensitive instruments survive, we can aim for those two ranges to overlap, the “sweet spot.” Though often overlooked, the weight of the items we put in our autoclaves is a critical factor in our success.

Instrument Weight Isn’t Just a Number…

To understand why instrument weights matter, we need to take a step back and look at weight from a scientific point of view. What we call weight is really the result of mass in earth’s gravity. “Mass” is essentially how much “stuff” an object contains. All matter has mass, but even among the great minds of physics, it’s not easily defined. Instead, let’s consider inertia. Inertia is the resistance to change in motion. The more inertia something has, the harder it is to get it moving, and conversely, to stop it. As it turns out, mass and inertia are basically interchangeable. The more mass something has, the more inertia.

While it’s easy to understand that the mass of a car or boulder is hard to move, there is similar effect when it comes to heat. Heat is essentially just the motion of atoms and molecules. The faster they move, the hotter something is. This means that the more mass (and thus inertia) something has, the longer it takes to heat up, and conversely, to cool down. It’s the reason why a 17-pound turkey takes longer to cook than a 5-pound roast chicken. When talking about resistance to changes in heat, we refer to an item’s “thermal mass”.

The Skinny On Steam Autoclaves

Let’s look at things from a more practical point of view. Steam autoclaves are essentially thermal kill chambers. Steam is an excellent medium for carrying heat, as anyone who’s cooked spaghetti knows. As the steam meets the contents of the autoclave, it transfers heat to the items. However, that heat doesn’t come free, and the steam itself loses thermal energy. When steam cools down, it condenses and turns back into liquid water, exactly like a glass of iced tea on a hot summer day. As the items absorb more and more heat, eventually it reaches above the boiling point of water, and whatever condensation formed now evaporates. The temperature sensors in the autoclave chamber monitor the heat and continue pumping in steam until it reaches a set temperature and pressure. At that point, it holds steady and maintains this condition for however long the cycle requires.

After the correct parameters have been met, the steam is evacuated, and the items cool. Because of the thermal mass of the contents, temperatures fall slowly. One benefit of this is that the high heat serves to evaporate excess condensation from the sterilizer contents.

Problems arise when we have too much thermal mass in our autoclaves. Consider a large, very heavy tray full of stainless-steel instruments. Due to thermal inertia, it takes quite a long time for these instruments to heat up. As steam condenses on these instruments, it lingers longest on the coldest parts. The excess thermal mass creates cold spots in our containers that may not reach a high enough temperature, and thus condensation on the instruments may even be present in high enough quantity to escape being fully evaporated. The result is a wet pack.

In addition to the risk of wet instruments, there is also the risk of instruments never reaching a high enough temperature to kill all present pathogens. This can result in a positive biological test, or worse, a false negative. Buried deep in a hefty tray, there may be areas that never reach a temperature capable of destroying resilient spores or other pathogens.

Of course, there is also the effect that very heavy trays have on less scientific levels. Soft wrapped containers and instruments are particularly susceptible to punctures and tears due to excessive weight. Autoclaves themselves have weight limits to prevent damage to their racks and moving parts. We must also be aware of the physical toll excessively heavy trays have on staff. Loading and unloading massive containers can lead to a host of workplace injuries. To avoid the risk of damaging our equipment and as an important workplace safety measure, it is critical to ensure that we consider container weights as part of our general safety policies.

Stepping on the Steam Autoclave Scale

AAMI ST79: 8.2.j sets the standard maximum weight for instrument containers at 25 pounds. When properly packaged, containers up to this weight can be assured to reach temperatures capable of sterilizing instruments in a practical amount of time. To maintain compliance with this standard, best practice is to measure and record the weights of all sterile packages, including loaners and linen bundles. If items are found to be over this 25-pound limit, consider removing unnecessary items, or even dividing contents into two containers.

Check the documentation for your autoclave and determine the recommended maximum load weight. CensiTrac comes equipped with the ability to record weights of containers and instruments, as well as the Load Compliance feature which automatically tallies the weights of items added to a sterilizer load, and alerts staff when the weight exceeds the maximum allowed. This feature can ensure you are not overloading your sterilizers and are thereby allowing the sterilizers to do their job; destroying pathogens and sterilizing instruments.

As sterile processing team members, we are charged with ensuring that our patients are getting the highest quality, safest possible instruments for their care. We check for a multitude of risks such as cleanliness and function. We keep diligent records. We are the experts the surgical team relies on to do their jobs safely and effectively. To that end, we must make certain that we do not overlook any avenue of failure. By staying compliant with container weights, we are blocking one more road to surgical infections.