Wednesday, April 7, 2010

The Sound of Surgery

I've mentioned elsewhere that the smell can be the grossest thing about surgery. Smell is not the most nerve wracking thing, though. Sound is. Not the sound of surgeons bellowing, but rather what is normally a quite normal sound, the pulse oximeter. For those not familiar with it, the pulse oximeter is a machine that measures the amount of oxygen carried by the blood. It does this by shining a specific wavelength of red light through a relatively thin body part, usually a finger. It then measures how much of that light is absorbed. From this it is able to calculate what percentage of the red blood cells are oxygenated. The best possible "score" is 100 (unless you are a member of Spinal Tap). Numbers above 96 are considered normal. Numbers below 90 are worrisome. The machine is also able to measure the pulse by measuring the time between each wave of freshly oxygenated blood. It has become one of the basic tools of anesthesia.

It has also has the best designed sound I have ever heard. It is the modern version of Monty Python's "machine that goes ping". Every time the curve peaks the machine makes an electronic "ping". Its the background noise of every OR. Most of the time it doesn't register on our consciousness. The genius in this sound is one feature: as the oxygen saturation decreases, the the tone of the "ping" lowers. A drop of 10 points will drop the tone over an octave. Since a decrease in saturation is often accompanied by a slowing of the pulse, the machine begins to sound like its battery is dying, which is the perfect metaphor, because that's what the patient is doing.

Believe me, a serious drop in the tone of a pulse ox will get everyone in the rooms attention.

This video shows the tone change in minor way. These tone changes wouldn't grab anyone's attention but they give an idea of what I am talking about. (Tone changes about 0:27 and 1:25)


Barry said...

I think the thing that is most interesting about the pulse ox is the way our brains process the sound: although we almost never recognize the sound on a conscious level, although it tends to be treated by our brain as "background noise," as soon as it begins to drop or slow, we all become intensely aware of it without even trying.

i've always been interested to know if there are studies about how the human brain handles regular noise that can be important but most of the time is not. in most ORs, there's all kinds of beeping and blorping and background conversation and yet, just as soon as the SCD adds its own seemingly un-unique sound to the cacophony, the staff begin to act as if someone is taking a very long pair finger nails to a very black board.


Dafydd said...

I agree that that our processing of the pulse ox sound is interesting. It doesn't seem to take a lot of training to recognize it. It's a really well "designed" sound effect. I wonder if the people who made it knew would have that effect.

I also wonder if the guys who designed the SCD machine knew the effect of their machine's sound. If so they get their own circle of Hell. Don't start me on the damn beeping IV pumps, which won't shut up.