The squiggly line on many finger pulse oximeters
by John R. Goodman BS, RRT, FAARC
Sometimes technological advances seem to happen almost overnight, and some- times it seems to take forever before someone has the “I could have had a V8!” moment. For people on oxygen 24 hours per day, a digital (or finger) pulse oximeter is an essential piece of kit. For anyone new to oxygen use, the ease, accuracy and price of today’s finger pulse oximeters make it a simple process of walking into your nearest Walgreens or similar store and buying what is on the shelf. For those who like to cruise the Internet, there are literally thousands of models available at a keystroke.
As I sit here updating this article, the lowest price I could find for a basic oximeter was just $8.81 with free shipping and a one year warranty. That means for about 73¢ a month (or 2.4¢ a day), you have one of the most sophisticated monitoring devices available.
A Little History Lesson
If it weren’t for ever-higher flying air- craft, the oximeter might have taken many more years to be invented. Credit for the very first oximeter is normally given to Karl Matthes in the 1940s. Matthes was a German scientist who was studying the effects of low blood oxygen levels in pilots at very high altitudes. His oximeter was a bit crude and difficult to apply to the pilots, but it did work and, believe it or not, the same principle is used in all oximeters to- day. A few years later a U.S. scientist named Glenn Millikan invented a much easier way to measure a pilot’s oxygen saturation using an ear probe to take measurements. Millikan also coined the term “oximeter.”
It wasn’t until 1964 that Hewlett Packard (HP) introduced the first commercialized ear oximeter to be used in a hospital’s intensive care unit. At this point oximeters were massive by today’s standards, requiring a great deal of calibration and upkeep, and they were very expensive. In fact, the first HP oximeter required its own cart to move it around. It cost $10,000 (or $78,582 in 2017 dollars).
Attaining Accuracy and Ease of Use
Since the Sixties, the oximeter slowly improved in accuracy, ease of use, included the addition of other clinical components and, of course, the prices have come way down. As physicians, scientists and engineers began to refine the whole field of oximetry, most of the technical advances were in the area of making the oximeter more accurate, easier to read, and making its footprint much smaller. Finally in 1995 the Nonin company introduced the first true digital pulse oximeter. This was truly the “I could have had a V8!” moment. The Nonin digital pulse oximeter provided people on oxygen in their homes exactly what was needed for a burgeoning home oxygen market. In the following 20 years, many manufacturers developed their own competitive line of finger pulse oximeters. A huge worldwide market stimulated research and development, and along with lower prices, other useful displays began to be built into succeeding generations of oximeters. So it may seem that small, affordable oximeters were always available, but the reality is they have only been around for the past 20 years or so.
Oh my, have oximeters come down both in size and price since this original HP back in 1964!
(Above) The Nonin Digital Pulse Oximeter started the paradigm shift in the routine monitoring of oxygen saturations in the home.
The Shift in Home Oxygen Monitoring
The Nonin Digital Pulse Oximeter started the paradigm shift in the routine monitoring of oxygen saturations in the home.
One of these developments had to do with the cardiac side of the oximetric equation. We all naturally focus on the two main readouts of oxygen saturation and heart rate or pulse. However, depending on which actual model of oximeter you buy, it might also display information regarding the effect of your blood as it moves through your circulation with each beat of your heart. This has a long medical term to describe it and is called a photoplethysmogram (PPG). In everyday use we simply call it the “waveform.”
The waveform is an overall reflection of the volume of blood that is circulating with each beat of your heart. This is easy to see and it gives additional information regarding your heart. It is normally shown as a (squiggly) line along with the readings for oxygen saturation and heart rate.
Oximeters with Waveform and without Waveform
The waveform gives moment by moment information allowing detection of an irregular heart rhythm. This is not an ECG tracing. It has nothing to do with the electrical system of the heart. The wave- form also gives information regarding the strength of the signal itself. It is displayed as amplitude. In other words, the actual size of the waveform may indicate a problem that might cause inaccurate readings of saturation. This may be caused by a decrease in blood flow through the finger (called low perfusion), improper position of the oxim-eter on the finger, or it can even be due to dirty sensors or LED lights.
For most people on home oxygen, it is the shape of the waveform that is easiest to see virtually every time you are checking your oxygen saturation. The normal waveform is most commonly described as “saw tooth” shaped – the way a good rip saw looks if you just look at the teeth.
The more your own waveform looks saw tooth-like, the more likely you are to be get- ting reproducible and accurate recordings of your saturation and heart rate.
If you see a saw tooth pattern whose amplitude takes up at least half of the display area, you are probably getting the most accurate readouts of saturation and heart rate. In fact, a number of oximeters will simply not display any numbers until it “recognizes” a normal waveform configuration. Others might show dashes, lights or other indications that some condition is not being met according to that particular oximeter’s measuring algorithm.
All in all, the waveform is an important feature of slightly more advanced oximeters, and one that is commonly overlooked.
Considering that oxygen is by far the most important drug you are taking, and you can have this technology for pennies per day, it is one of the best investment you are ever likely to make.