Lecture 39

Listen to part of a lecture from a life sciences class.

Prof: In modern hospitals and medical facilities, it is imperative that used equipment be thoroughly cleansed, or sterilized. This includes instruments such as scalpels, hypodermic needles, and artificial pacemakers. If you’ve ever had a shot, or given blood, you know just how important this is. Right? It’s crucial that the bacteria, viruses and other, uh, harmful substances on these instruments are eradicated before they are used again. All medical instruments that contact a sterile part of the body, such as blood, must be either sterilized or thrown away. What are some potential consequences of using non-sterile instruments?

S: It could cause diseases, like, uh, infections and stuff. Like hepatitis, and AIDS.

P: Correct. It can result in very serious diseases, and quite possibly [dramatic pause] death! So then, so, to ensure against that, today many medical facilities employ single-use items. After a certain instrument, such as a nee [false start] a hypodermic needle or pair of forceps, has been used, it is simply discarded. For instruments that are not discarded, hospitals use a machine called an autoclave to ensure sterility. An autoclave is a sealed device, usually made of steel. It uses pressurized steam to heat water above its boiling point, which inactivates harmful substances. A stovetop steam cooker is a type of autoclave. Hospitals use larger autoclaves that resemble washing machines or dishwashers.

For an autoclave to work, it must remain sealed. Do you know why?

S: Um, is it because if it it’s not sealed, it won’t get hot enough? Like, water can’t be heated above its boiling point in an open pot. Once it starts to boil, it doesn’t get any hotter.

P: Yes – give that man a gold star for reading his textbook! Boiling water begins to evaporate extensively, and change to water vapor, when the temperature reaches approximately 195 degrees. Evaporation cools the water, so that when it reaches 220 degrees it will still boil, but cease warming. Does that make sense? All right. Now, water heated with a seal, in a sealed vessel, can be pushed past its boiling point. Inside a vessel, evaporating gaseous vapor creates additional pressure, and the pressure, in turn, creates latent heat. When the pressure reaches a certain point, evaporation ceases. Thus, the water – some of the water – in the vessel does not evaporate. This remaining water can be heated higher than 220 degrees. Typically, it is heated to 250, which increases the pressure of the water vapor.

Latent heat, also known as the heat of transformation, absorbs this extra warmth from the water, and uses it as energy in the sterilization process. Are you with me? OK. The latent heat from this vapor is powerful enough to penetrate deep into bacteria on the instruments it is cleaning. It can penetrate through to their most heat-resistant parts, which are called endospores. This steam is very effective for sterilizing solid objects. But, uh, hollow things, hollow objects, like hypodermic needles and surgical tools, have trapped air inside of them where endospores could hide. Modern autoclaves have a powerful vacuum that sucks out this trapped air, thus ensuring that the steam thoroughly penetrates all objects in the autoclave.

When the autoclave is used properly, it will inactivate all fungi, bacteria, viruses, and bacterial spores. Autoclaves operate by maintaining the heat, er, temperature and pressure of the steam for a certain length of time. Hospitals use a variety of indicators to make sure these conditions are met. Chemical indicators are usually pieces of tape on the exterior of autoclaved packages. These will change color to show that the contents of the package have been sufficiently sterilized. Biological indicators are vials, or attests, that contain spores of a heat-resistant bacterium called Bacillus stearothermophillus. Anybody want to take a crack at spelling that? [Chuckles]. Don’t worry. It’s not important. But these vials of Bacillus stearothermophillus are placed inside the autoclave, and the spores will germinate, indicated by a change in color, if the autoclave fails to become hot enough. So chemical and physical indicators are exactly the opposite. If a chemical indicator changes color, that’s good. With biological indicators, bad. Make sense?

Physical indicators are a third type of measurement. These are, uh, typically a mixture of metals, called alloys, that um, will melt only if the temperature is hot enough for the correct length of time – in other words, if the contents of the autoclave have been sufficiently sterilized. In addition to these indicators, most autoclaves have attached gauges to measure the interior pressure and temperature. And In the most sophisticated autoclaves, these variables are controlled by a computer.

To ensure effective autoclaving, the steam must penetrate everywhere inside the machine. So it’s critical not to overload the autoclave with two many objects, and also to keep the lids of all the containers within the autoclave slightly ajar. The best way to assure complete sterilization is to place indicators within the autoclave in the hardest, um, the most difficult places to reach. Technicians place attest devices, for example, inside various containers within the autoclave. If the spores in the attests are eradicated, there is assurance that the container in which it was placed has been sterilized. Because sterilization is so vital, medical professionals don’t rely solely on autoclaving. They take care to clean instruments with special soap and hot water as soon as they are done using them. Without this step, harmful matter on the instruments could actually inhibit their successful sterilization. If this happens the matter could unintentionally protect the bacteria during the autoclaving process.