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Pulse Oximetry: User Beware!

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Use of pulse oximetry by healthcare professionals may seem as simple as attaching the probe to a patient's finger. But there are a few caveats for nurses to be aware of when reading the results.

As a long-time respiratory therapist, I have worked in multiple settings such as nursing homes, prison and psychiatric settings as well as hospitals. I have watched as healthcare professionals have used - and misused - pulse oximetry. Consequently, it is important to know the potential pitfalls and concerns that can be associated with pulse oximetry and how to correctly interpret the results.

Pulse oximetry is very easy to use. Generally, a caregiver simply attaches the probe to the finger of the patient and pushes the start button. Unfortunately, there are many potential pitfalls in the interpretation of the readings, and some of them can have serious consequences.

Case Study

I was working as a respiratory therapist in the ED of my local hospital when a patient was brought in with smoke inhalation secondary to a house fire. The ED doctor's first order was to "get an oximetry reading on him."

In this case the oximeter probably would have read 95 percent or higher and so could have misled the physician and staff into not giving needed oxygen. I informed the doctor that the oximetry reading was going to be of little value because the pulse oximeter reads carboxyhemoglobin the same as oxyhemoglobin. In other words, the oximeter might have read 95 percent but perhaps 10 percent of that (or more) could be due to carbon monoxide in the blood; therefore, it would tell us very little regarding the oxygenation of the patient.

In this example, we needed (and obtained) a blood gas, which told us that the patient, indeed, had a very high carboxyhemoglobin level. His oxygen level was very low, and he was placed on 100 percent oxygen.

Smoking Also Can Skew Results

This problem is also true of patients who smoke and get an oximeter reading, as they can have elevated levels of carbon monoxide and thus have a spuriously high oximeter reading. I have come across this in certifying patients for home oxygen delivery where the saturation level must be below 85 percent for them to qualify. After smoking, the patient's oximeter reading was over 90 percent, but a subsequent blood gas showed that the actual oxygenation was much lower.

Many healthcare staff who routinely use the oximeter do not know about the pulse oximeter reading the carboxyhemoglobin as oxygen. I have seen this in numerous healthcare settings. Pulse oximetry technology is tremendously helpful, noninvasive and easy to use. Unfortunately, this can be deceiving, as there are certain potential pitfalls, of which healthcare providers must be cognizant. Without training, it's all too easy to miss things, often to the detriment of the patient.

Precautions to Observe

The pulse oximeter picks up the motion of the pulse and determines by color (wavelengths of light ? blue gives off a different wavelength than red) what percent of hemoglobin in the artery is saturated with oxygen (red). However, the following precautions should be observed:

1. Pulse oximetry gives a reading of the percentage of hemoglobin saturated with oxygen. This reading will be inaccurate if the patient has just smoked (within last 4 hours), because the machine cannot discriminate between oxygen and carbon monoxide in the blood. Carbon monoxide in the blood also looks red, as does the skin of a victim of CO poisoning.

2. Results may not accurately reflect tissue oxygenation if the patient is severely anemic. For example, a patient with a hemoglobin of 6 may have a low tissue oxygenation even if the oximeter reads 99 percent. The oximeter reading is a percentage of hemoglobin saturated. It does not tell you how much hemoglobin one has or the state of tissue oxygenation.

3. Results that are not correlated with the patient's actual pulse may not be accurate. For example, the pulse oximeter reads 84 percent with a pulse of 120. However, if the patient's actual pulse when checked is only 70, the reading of 84 percent is probably not accurate.

4. Oximeter results lower than 75-83 percent may not be accurate. The pulse ox accuracy diminishes the further it is from the norm, so readings of 52 percent should be viewed as low but may not be as bad as they appear.

5. Poor peripheral perfusion also can affect the accuracy of the readings. The oximeter works by detecting the pulse and measuring the wavelength of light from the blood. If the pulse is virtually undetectable, the oximeter will not read or will read inaccurate results. This is another instance of correlating pulse oximeter values with the patient's actual palpated pulse.

6. Skin pigmentation or presence of nail polish can affect the accuracy of the readings. Polish should always be removed prior to using the pulse oximeter. Patients who have onychomycosis may not get accurate readings due to the thickness of the nail.

7. Motion artifact can also affect the accuracy of the readings. This occurs when the patient will not or cannot hold still (such as patients with Parkinson's disease) and the motion can affect the oximeter, since it picks up the pulsing (motion) of the artery to determine saturation levels. Motion artifact may be one reason for a marked difference in pulse rate on the machine versus palpated. The oximeter companies are diligently working on reducing motion artifact and having the machines compensate for it, but we are not yet at a place where it plays no role.

Look at Whole Clinical Picture

There are other pitfalls of oximetry as well. Caregivers must always verify results by looking at the whole clinical picture. A patient's skin pallor, presence of cyanosis and other factors can belie or substantiate a pulse oximeter reading. Also, since cyanosis of nail beds can be due to poor circulation, the oral mucosa should be checked for color and/or presence of cyanosis. In certain instances, such as the smoke inhalation study above, an arterial blood gas may be needed. This will give actual PaO2 and carboxyhemoglobin levels.

Even accurate oximeter readings must be used as part of the total assessment. A good example of this is the patient with acute changes in his level of consciousness (LOC). His pulse ox reading may be over 90 percent, and yet, to dismiss a possible respiratory reason for the change in LOC may be in error, since an increase in carbon dioxide in the blood can cause this change and will not necessarily be picked up by oximetry. Patients with chronic obstructive pulmonary disease can have a reasonable saturation and yet be retaining carbon dioxide, resulting in an acute change in LOC.

Pulse oximetry is an excellent tool as long as care is taken to not depend entirely on this technology in the absence of correlating clinical information.

Further information regarding the pitfalls, contraindications and device limitations can be found in the American Association of Respiratory Care (AARC) Clinical Practice Guidelines. Access these on the AARC Web site: www.aarc.org.

Mike Coen is program director, senior intensive services, Diamond Health Care, South Bend, IN. <% footer %>


 

I need to know what oximeter to buy for my grandpa his dr.(Dr.Adler) wanted him to buy one. My grandpa is on full oxygen 15L and 5 more L then most people. Is the new spo checkmate pulse oximeter oxygen heart rate better then the old pc66 handheld pulse oximeter? How are they both on breaking and wear and tear?

Rochelle Robertson,  CNA,  First CallOctober 29, 2008
oklahoma, OK




     

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