Measuring Oxygen Saturation

There are only three tests to determine oxygen saturation-- the level of oxygen in the blood. They are the arterial blood gas (ABG) test, co-oximetry, and pulse oximetry test.

• Before the oximeter, the ABG test and co-oximetry were the only two methods of measuring saturation. A technician performs an ABG by inserting a needle into an artery and taking a blood sample. The artery chosen for this test may be the radial artery at the wrist or the brachial artery on the elbow. An examination of the blood sample provides several measures, including the amount of oxygen and carbon dioxide and its pH.         
• Since the 1980s, pulse oximetry has become available as a second measurement method. Whereas the ABG test continues to be used, it has its drawbacks when it is used strictly to determine oxygen saturation. It is invasive and is administered only in a medical facility by professional staff. It cannot be administered during sleep; the patient is wide-awake and sitting up. To assume that a single reading represents the patient’s oxygen level at other times and under other conditions is expecting more of the ABG than it can deliver.

History of Oximetry


Some of the first oximeters of the 1970s weighed 25 pounds and was transported on a cart. Its sensor on the end of a long cord weighed 3 pounds, looked like a large C clamp, and was very uncomfortable when attached to the patient's ear. With that, oximetry testing became the fifth measure of vital signs, the first four being temperature, respiration, pulse rate, and blood pressure.


My First Encounter With an Oximeter

In the spring of 1998, I was in the hospital for the second time in as many months because my body could not support both my normal breathing and the coughing which accompanied my lung infection. I remember being awakened the third day to find both my primary care doctor and pulmonologist standing at the foot of my bed. Both smiling, they asked if I was ready to give up smoking. With the two bouts of coughing clear in my mind, I knew there was only one choice. I nodded my "yes."

The next thing I remember is walking very fast down the hospital hallway. One of my hands was busy keeping the back of my hospital gown closed while on the index finger of the other was a clamp with a wire coming out from it. At the other end of the wire was a nurse who was trying to keep pace with me. She was carrying a device the size of a large dictionary that she cradled in her arms. Her objective, she kept muttering was to get this device to give her the Medicare number she wanted before she ran into a wall or was tripped up by the carpeting. As we walked, her defense was to keep her eyes moving more quickly than her feet--from the numbers displayed on that device, to the connecting wire, to the floor in front of her. I later learned the number she was seeking would qualify me for supplemental oxygen under Medicare.

Medicare's Oximeter Reading Medicare covers supplemental oxygen use for those who demonstrate saturation levels at 88% or lower as measured by an oximeter. If dependent edema, polycythemia, or cor pulmonale is also present, a patient can meet the Medicare requirement with a saturation level of 89%. Some patients may not qualify for oxygen therapy at rest but can qualify for oxygen during ambulation, sleep, or exercise.

It must have been a funny sight to those in that hallway. Perhaps you too were first introduced to a pulseox while scurrying down a hospital hallway with one hand behind you, holding your gown closed.

A year later I purchased my own pulseox. It was a relatively inexpensive and, gratefully, much lighter-weight version of the device used by that nurse. In 2002, I responded to an Internet survey of oxygen users, indicating that I owned a pulseox, as did 61 percent of the other respondents.

I took my new pulseox home and quickly "trained" it, or so I thought. When I huffed and puffed, the pulseox would dutifully display a saturation reading below 90 percent and when my breathing was normal, it would show a reading above 90.


Expert Advice for the Patient?
On Saturation
But then as I became more dependent on my pulseox I became more critical of the numbers it displayed. I found times when I was not huffing and puffing and yet my pulseox displayed a saturation of less than 90 percent. I found other times I did huff and puff that it displayed saturation above 90 percent. Was my pulseox broken? Should I trust my pulseox or the huffing and puffing? Did I not fully understand the pulseox's capabilities? What was going on?

I began looking for "expert" advice, first in the manual that accompanied the oximeter and there was nothing there. A lot of advice about how to use and maintain it but nothing about what the numbers it displayed meant to my health. I next searched the Internet for helpful advice. What I found were research studies that used oximeters or training materials for respiratory therapists.

Were there no articles in which doctors or respiratory therapists talked to patients about how to use an oximeter? I found one written by Dr. Fred Furgang, but it was directed at healthy airline pilots who use oximeters when they "reach for the stars" with oxygen but no cabin pressure. I rcently for a second source entitled Tips for Using Oximetry in Home Oxygen Therapy that was written by Brent Blue, M.D.

Gradually, incidents occurred that helped me better understand the advantages and shortcomings of my huffing and puffing as a medical alert signal, as well as the advantages and shortcomings of my pulseox.

Valid Oximeter Readings Your pulseox may display outlandish readings when it first starts up. Ignore these readings. Valid readings will begin to appear about ten (10) seconds after you first see numbers displayed. Your pulseox displays rolling averages of both pulse and saturation readings. The device needs about ten seconds to accumulate enough good readings to display a valid average.

For nearly five years my pulseox has been my constant companion, dangling from its lanyard around my neck, strategically placed so it can catch the food that doesn't reach my mouth, and always available to be popped on my finger at any time. I check my "sat" several times a day for several reasons.

• I take readings after both showering and dressing in the morning. These are events I perform in a routine manner. For this reason my sat should not vary day to day when I am using the same level of supplemental oxygen. What I don't want to see are higher pulse rates and lower saturation readings. Things I have no control over--the weather, atmospheric pressure, and pollen, can cause some variations, but so can lung infections. Abnormal pulseox readings tell me to check symptoms of respiratory exacerbations--a dry nonproductive but persistent cough, excessive and/or discolored sputum, wheezing, breathlessness or chest tightness--and take proper action.
  

• I also take readings when I feel my pulseox reading will be below 88 percent. When my saturation reading is below 88 percent, I continue to monitor my saturation until it is back in the 90s. I increase oxygen output to speed the recovery process and I make note of the time it takes to recover.

Several readers have written to express their concern about how slowly the oximeter readings sometimes move from the 80s back into the 90s. If you have observed that the readings move more quickly from the low to the middle 80s and much more slowly from the middle 80s to 90, do not despair. There is nothing wrong with your oximeter. This is the way it is supposed to work. Saturation percentages are not based on numbers but on their logarithms. If you want to know more about this, look under "Theory of Operation" in the operator's manual. No lecture from me today on logarithms.

On Pulse
Up until a couple of years ago, the only reason I used the pulse reading was to compare my actual reading with the expected reading during exercise at Pulmonary Rehab. Normal pulse rate for me ranged from the 80s to as high as 120.

More recently, I have found the pulse readings became predictors of problems I was having with my heart. Abnormal readings sent me to my physician to determine what the problem was and what to do about it, if anything.

About the time I experienced significant edema--enough to require 120 mg of a diretic--I noticed my pulse rate would drop into the 40s for a time and I would become breathless until it rose again. This would occur even though my satuation was in the 90s.They tell me that arrhythemia like this are not dangerous in a normal heart.

More recently, I experienced pulse rates of 150 -250. On three occasions, this rate remained with me for over 20 minutes, which encouraged me to get into the emergency room right away. I was having SVTs (Super Ventricular Tachycardia), which are not so benign. After wearing a heart monitor for a month, it was determined that my basic problem was atrial fibrillations, a much more serious problem.

There is medication for atrial fibrillations but it did not agree with me. I therefore got a fancy pacemaker, called an implantable cardiac device (ICD), which not only regulates my heart but also has a defibrillator in it to shock life back into it should my heart stop.

I never thought that my oximeter would provide me with advanced notice of such a dangerous condition.

I have more to say about oximeters and will do so shortly in Part 2.