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Pulse Oximetry

From health fairs to heart surgery suites, everyone is using a pulse oximeter or "pulse ox," an instrument that measures the amount of oxygen in a person's blood, these days. But are we all using them correctly? Do we all really understand what we're using?

In the hands of untrained or unresponsive staff, pulse oximetry can provide false security and even deadly consequences. The truly sad thing is we can't just sit back and blame the EMT or out-of-hospital practitioner for being unskilled.

A recent Australian study reviewed data from 14 trials that examined clinicians' knowledge of pulse oximetry and found that nurses, doctors and allied health professionals (all of whom frequently used pulse oximetery - many in the critical care setting) had significant knowledge deficits about proper and safe usage.

And it wasn't just the new grads who didn't understand how to use it or what the readings meant. Senior staff had problems, too.

As a beginning point, we should note there is a difference between a pulse oximeter reading and an arterial blood gas finding.

Some Differences
Let's look at some of the differences between pulse oximetry and arterial blood gas (ABG) testing.

Pulse oximeters measure the oxygen saturation of circulating hemoglobin. An ABG measures the dissolved oxygen tension in the arterial blood.

They are related, and it could be argued they are indirect measures of each other. But they are different. An 80 mm Hg on a blood gas is normal for an adult; an 80 percent reading on a pulse ox is something else.

Next let's review how transmission pulse oximetry works. This is the kind most of us are familiar with. The oximetry probe is placed on a finger tip or earlobe. The probe sends beams of red and infrared light from one side of the sensor/probe to the other. The light is sent at two wavelengths (650nm and 805nm).

Amounts of light absorbed by the hemoglobin differ, depending on whether it is saturated or desaturated with oxygen. The microprocessor in the pulse ox calculates the absorption of the two wavelengths of light to determine the proportion of hemoglobin which is oxygenated.

Advances In Technology
Recent advances in microprocessor technology include time division multiplexing in which the lights are sent several times per second. This helps to reduce artifact. Quadrature division multiplexing is another technique in which the red and infrared signals are separated in phases rather than time sequences.

They are recombined in a later phase, and the microprocessor can differentiate the variances between the two and provide a reading that is less susceptible to artifact.

The saturation values provided by the pulse oximeter are averaged and reported over 5-20 seconds, depending on the machine. But remember pulse oximetry cannot distinguish among normal oxygen saturated, carboxyhemoglobin or methemoglobin. Other variables that can affect the function of a pulse oximeter include bright ambient light; pulse rate and rhythm; and vasoconstriction and cardiac function.

Reflection pulse oximetry works a little bit differently. The basic technology is the same as transmission pulse oximetry. But in this format, the light is transmitted and recorded by a single-sided monitor/sensor. It can be used on the forehead, abdomen or other flat body surface. There are mixed data on the effectiveness of reflective pulse oximetry.

Some reports suggest there is faster response time and fewer effects related to vasoconstriction. Other data find unstable readings and falsely low values are a concern. Perhaps the biggest concern is that while the underlying technology is the same, the systems themselves work a little differently than the ones we usually see.

So a word of caution needs to be given here. If you are running short of pulse oximeters in the ICU and you borrow one from anesthesia, make sure the staff - all the staff - knows how to use it.

Operator Error
We generally cannot fault technology when an error occurs. The problem is typically with the user. The safe, effective use of a pulse oximeter in any setting boils down to education. In many facilities, respiratory care practitioners are responsible for providing the pulse ox; but nurses and other healthcare professionals have a responsibility to request proper training to ensure the equipment is used safely.

Margaret Clark is a Georgia practitioner.

  Last Post: August 9, 2007 | View Comments(2)

Thanks for such a great piece. Three weeks ago, during a night shift at work, I took time to read the instruction manual of our unit's pulse ox machine. It looked as if no one had ever opened it and, after sending a patient to the ED based on the meter finding, I asked myself "How much do I know about this particular meter to have trusted it so much." I learned there were some things I did not know. Since then I have encouraged my teammates to read the manual, and all responses have been very positive. Your piece is a great help to me.

Philip Ballah,  LPN charge Nurse,  Ravenwood nursing and Rehab centerAugust 09, 2007
Baltimore, MD

This article would've been more useful if it would have provided how to properly and safely use the pulse oximeter.

Trena PrimaveraAugust 05, 2007


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