Units, units, units

So at my last ultrasound, I was having my level of amniotic fluid checked to make sure everything is alright. I was told everything is fine .... and I am quite sure that it is. And yet ......

I was taking a sneak peak at my own chart while waiting for the doctors and I noticed that the volume was reported in cm. Yes cm. Not ccm. So I asked the ultrasound technician what the volume was measured in. She replied that it was in cm. I pointed out that cm was not a unit of volume but her response was a flippant "Yes it is". Hmmmm. I chose not to argue. But I wondered what world she was from.

A nurse told me the same thing. "cm is a unit of volume." I hope they are a little bit more careful in their units before administering my drugs.

I then asked another ultrasound technician who told me that units are important and, as long as they put the right measurements in the right fields, a formula will give the correct volume .... in cm. Double hmmmm. Really? How does one know when they have a gross error? This makes me a bit nervous. Magic formulas always make me nervous. Especially magic formulas that give results of volume in units of length.

I finally asked my doctor. She was at least chagrin, replying that she used to understand the reason, but that she couldn't remember. Fine. Hopefully she'll be able to supply an answer for me at my next appointment ... if for nothing more than to satisfy my curiosity.

As a trained engineer, this concerns me. The first question I was trained to ask myself after completing any calculation is "Do I have the correct units?" The second question is, "Does this value make sense?" The third question is, "Can I explain it?" So these four educated people, who are taking my baby's health in their hands, have violated all three of my principles for calculation.

To measure fluid levels, the technician divides your uterus into four quadrants and then measures the height of each quadrant vertically, and roughly in the center of its x-plane. I say roughly because these locations are done by eye. Now keep in mind they are trying to get an approximation of an extremely irregularly shaped 3D volume from a 2D image. I am uncertain how they determine where on the z-plane to take the cross sectional measurement. However, these lengths are (rightly) measured in cm. The measurements are then put into a formula that supposedly plunks out the estimated volume .... in cm. Excluding the incorrect units, I contemplate the uncertainty of these measurements. How accurate can the method be for such an irregular shape as the shape of my fluid inside my uterus and around my baby? Is there a standard magnification these measurements are taken at? What is expected for user error? How much does this depend on the mother's weight? What if the baby is moving during measurements? Etc., etc., etc. At best, I would think we are on the order of 50% error.

So I am merely wondering about how the measurement is made and why volume is reported in cm. I am not really concerned about my actual test. Darren is fine. And I am doing alright, too. I would hope that someone, somewhere and at sometime understood the problem and gave it the best shot an engineer can give in order for hospitals to universally use this method.

But, the whole thing reminds me of a engineering joke we used to tell in college: A king decides to improve prosperity of his country by increasing the dairy production and he forms a council of a biologist, sociologist and an engineer. The biologist says "we should undertake a program of selective breeding to enhance the milk production traits of our cows." The sociologist says "we should study the behavior of our cows, how they interact with their keepers, surroundings, and each other, to minimize their stress which reduces milk production." The engineer goes to the blackboard and says "Let's assume the cow is a sphere ...."