## Thursday, March 25, 2010

### How do you rate?

Rate is a math term that implies one measured quantity is being placed in relationship to or judged by a second measured quantity.

For example, speed means distance traveled over a period of time. A common unit for rate of speed is mph or kph, which means miles or kilometers (measured by odometer) per hour (measured with a watch).

Since speed is a very common term, we usually don't bother to say rate of when we speak.

A "pulse rate" is the number of beats (counted) per minute (measured with a watch). Since measuring our heartbeat is very common, we usually don't bother to say "rate" when we speak, we just say pulse.

Doctors, nurses and emergency personnel need to measure heartbeat all the time. And they may not have a computer or calculator handy. In the field, they don't even usually wait one entire minute. They count for 15 seconds and multiply their result by 4 to get beats per minute, or count for 10 seconds and multiply by 6.

Another approach is to count a few beats, then look at a scale to tell what the rate would be per minute.

Here's a watch that makes it easy for emergency medical personnel. Notice that this watch has a 4-ended second hand. That way you never have to wait more than 15 seconds for a hand to come along! I've never seen another watch like this.

Instructions are indicated on the dial. When a second hand (in this case, the orange hand) comes by the pointer at the bottom (below 6 o'clock), you start counting respirations (breaths). When you have counted  5 breaths, the orange hand will be pointing at a number on the outer scale. That indicates the respirations per minute. This scale shows a respirations range of 10-60 a minute. Normal for an adult is about 15-20.  In this picture, my respiration rate was about 14.

You do the same thing to get a pulse, but using the top and right side of the watch. Put your finger on a spot where you can feel a pulse. When a hand passes the top pointer (above 12 o'clock) you start counting. In this case, you count 15 pulsations, then look at the moving hand. If it's where this hand is pointing, it means 70 beats per minute. This scale shows a pulsations range of 50-180 per minute.  Normal for an adult is about 60-100. My pulse rate was about 70.

As I said earlier, this watch is designed to save the wearer from having to do any math. It's easy enough to count 15-20 respirations in a minute, but much harder to count to 70 or 100 in a minute. So doing a 15 second pulse count and multiplying by 4 is the normal process.

Remember that we started with the term rate. Other common uses of the word are with birth rate, unemployment rate, interest rate, exchange rate, inflation rate, and so on. And of course at this time of the year many of us may be thinking about the tax rate!