Patient Care

The How, What and Why of EMS Pulse Oximetry

Issue 5 and Volume 42.

I think it was in 1986 that I first encountered a new technology called pulse oximetry. I was called in for a two-hour transfer of a patient with chronic obstruction pulmonary disease (COPD) from a small rural hospital to a large tertiary center in a larger city. I was an experienced paramedic and fairly confident in my skills and knowledge.

The patient was pretty sick as best I can remember. I took a report from the nurse and the doctor came up to me and said he was sending this patient with a machine called a “pulse ox.”

“What does it do?” I asked.

“It measures their oxygen levels,” he responded. “If the oxygen saturation drops below 90%, then you should intubate
them immediately.”

I thought it was a pretty cool tool if it could tell you when to intubate! Wow!


Pulse oximeter readings can help determine if a patient is hypoxic and
help regulate the administration of oxygen. Photo Matthew Strauss

Luckily, my patient’s O2 saturation didn’t drop below 90%, but I was ready if he did!

It didn’t dawn on me at the time that I knew nothing about this new tool, but the impromptu two-minute training I received gave me a certain amount of confidence based on the pulse ox reading and what to do.

How many new tools do we purchase and put on patients without a full understanding of how the thing works? How many people use a cardiac monitor but aren’t competent in rhythm interpretation? How many people run 12-lead ECGs but don’t know how to interpret one?

And don’t even get me started about capnography! We have providers today who think the only reason to use capnography is for tube confirmation, and many are reprimanded for using special nasal cannulas to read end-tidal carbon dioxide (EtCO2) because of their cost.

But I digress. In my 40 years in EMS, I have seen this cycle play out many times: New device with limited training leads to poor understanding resulting in misuse that turns into a distraction from patient care.

Pulse oximetry has been used routinely in the medical setting longer than capnography. However, many providers may not fully understand how a pulse oximeter works. This article will give you a better understanding about how a pulse oximeter works, what the readings mean, and what role a pulse oximeter plays in emergency medicine.

Respiratory System Review

Before we dive into pulse oximetry, first we must review basic relevant anatomy and physiology of the respiratory system.

The body’s primary stimulus to breathe is increased CO2 levels. The medulla controls the ventilatory effort. Through muscle contractions, air (typically made up of 79% nitrogen and 21% oxygen) is inhaled into the lungs and fills the alveoli where gas exchange takes place. Gas exchange occurs by a process called “diffusion”-the movement of molecules from an area of high concentration to low concentration. This diffusion occurs across the alveolar capillary membrane where CO2 in the blood is exchanged for O2 from air.

As O2 travels across the respiratory membranes, it seeks out and binds to hemoglobin molecules on red blood cells. The oxygenated blood is then carried out from the lungs and into the heart where it’s pumped out as arterial blood to oxygenate cells throughout the body.

The measurement of the percentage of oxygen-
saturated hemoglobin in arterial blood is known as SaO2-a value that’s measured with an invasive procedure of an arterial blood gas. SaO2 values > 94% are considered normal.

How Pulse Oximetry Works

A pulse oximeter is a noninvasive means of measuring both pulse rate and the arterial oxygen saturation of hemoglobin at the peripheral capillary level. It consists of a portable monitor and a photoelectric sensing probe that clips onto the patient’s finger, toe or earlobe.

The photoelectrical sensing probe measures the amount of red and infrared light being absorbed as arterial oxygen reaches the capillary beds during systole, when more light is absorbed, and diastole, when less light is absorbed.

The monitor calculates the time between the peaks of light absorption and displays a pulse rate in beats per minute. It also calculates a value based on the ratio of light absorbed at systole and diastole to display a peripheral oxygen saturation percentage (SpO2). (See Figure 1.)


Figure 1: Basic pulse oximeter display

The better the sampling, the greater the difference between systolic and diastolic blood pressure in the capillary beds. A great difference makes for a more accurate reading. It’s for this reason that low perfusion states to the capillary bed being sampled will dramatically affect the accuracy SpO2 reading. In normal perfusion states, a pulse ox (SpO2) and SaO2 from blood gas readings should be very close.

Catch & Release of Oxygen

Oxygen has to be carried from the lungs and be released to the cells. Diffusion makes the oxygen move across the respiratory membranes but doesn’t make it bind or release.

Though diffusion is the force that drives the movement of molecules, it’s directly affected by several factors, including fluid in or around the alveoli, inflammation of the respiratory membrane, and many others.

Recall that the respiratory system supplies oxygen to the tissues for cellular metabolism (i.e., oxygenation) and rids the waste product CO2 from the body (i.e., ventilation). (See Figure 2, p. 52.) Oxygenation and ventilation are two separate physiological processes; however, ventilation can effect oxygenation.

Oxygenation (i.e., the delivery of O2 to the body’s cells) requires that oxygen chemically binds to hemoglobin and is released to be diffused into the tissues. When the body’s pH has a normal range of 7.35-7.45, oxygen can be bound (associated) and released (dissociated) normally from hemoglobin.

The oxyhemoglobin dissociation curve defines the point that oxygen can release (dissociate) from hemoglobin to be used by the cells and is based on normal pH and normal body temperature. (See Figure 3, p. 53.)

A high pH (i.e., alkalosis) or low body temperature (hypothermia) will cause this curve to shift to the left, and makes it harder for oxygen to dissociate from the hemoglobin molecule. In this state, the cells are deprived of oxygen and can become hypoxic.

The paradox is the pulse oximeter reading will still show an SpO2 of 100%-because the blood is still saturated with oxygen, it just isn’t being released!

Conversely, a low pH (acidosis) or high body temperature (hyperthermia) will cause a right shift of the curve, which in turn makes it more difficult for oxygen to bind very tightly to hemoglobin making oxygen more readily available to the cells.

Ventilation helps to control pH by keeping CO2 levels at a normal range. Normal CO2 usually means normal pH.

An arterial blood gas can directly measure the body’s pH, SaO2 and PaCO2, which is the pressure of carbon dioxide dissolved in the blood and how well carbon dioxide is able to move out of the body. It’s one way of determining acid-base derangement (i.e., acidosis and alkalosis).

EtCO2 is a noninvasive way to give that approximation of blood gas pH. So, as long as the CO2 is within normal limits (35-45 mmHg), it’s safe to assume the curve is working correctly and the pulse ox is accurate. The bottom line is that although a pulse ox reading is good; pulse oximetry with capnography is better!

Confused? Here’s an analogy: You place an order for an item (e.g., O2) online. It’s going to be delivered by United Perfusion Service (UPS). Under normal circumstances, the driver gets your package of O2 at the depot, loads it on the truck (i.e., associates the oxygen to hemoglobin). The driver (i.e., blood flow) then drives it to your house, checks the address and then offloads (i.e., dissociates) it from the truck and then carries it to your semipermeable front door where you receive the package of O2.

That’s how it normally works, but today UPS is running a little “alkalotic.” Perhaps because of hyperventilation (i.e., low EtCO2). The driver loads (i.e., associates) your packages on the truck, carries them to your house (i.e., cell), but when he tries to remove them from the truck, not all your packages will come off the shelf (i.e., dissociate). You miss some of your delivery this time and you aren’t happy. Or how about this? UPS is running a little “acidotic,” perhaps because of hypoventilation (i.e., high EtCO2). The driver is very busy and, in the rush, only three of your four packages are loaded onto his truck at the distribution center. When the driver gets to your house, he opens the truck to discover not all your packages are there. Again you don’t get your full delivery and aren’t happy.

Pulse Oximeter Readings

As a general rule, any pulse oximeter reading below 92% is cause for concern. A pulse oximeter reading below 90% is suggestive of hypoxemia. This means there’s lower concentration of oxygen in the blood stream than in the cells. This causes diffusion of the oxygen out of the cells and back into the blood stream, leading to tissue hypoxia and eventually death.

The ideal range for a pulse oximeter reading is 94-99%, but keep in mind that there are factors that can affect pulse oximeter readings. Conditions that can make pulse oximeter readings unreliable include:

Poor peripheral perfusion (i.e., shock, vasoconstriction, hypotension): Don’t attach the sensing probe onto an injured extremity. Try not to use the sensing probe on the same arm that you’re using to monitor the blood pressure. Be aware that the pulse oximeter reading will go down while the blood pressure cuff is inflated. Remember the blood pressure cuff will occlude the arterial blood flow affecting the reading while the blood pressure is being taken. After the cuff is deflated, the pulse ox reading should return to normal.

Hyperventilation: As you recall, an EtCO2 < 25mmHg can lead to alkalosis, causing oxygen to bind tightly to hemoglobin and not releasing it for use. This leads to tissue hypoxia with a falsely high-sometimes even 100%-pulse oximeter reading.

Hypoventilation: Remember that an EtCO2 > 50 mmHg can lead to acidosis. Acidosis causes oxygen to bind loosely and reduces the amount carried to the cells. This gives a low pulse ox reading that does not respond to O2 therapy.

Severe anemia or bleeding: This could lead to falsely high readings because of the lack of red blood cells to carry oxygen. The red blood cells that are present would all be carrying oxygen, leading to high readings unless shock sets in early. In other words, the reading is correct for the little amount of red blood cells that are available.

COPD: COPD patients often have excess red blood cells, a condition known as polycythemia. They have so many red blood cells that there isn’t enough oxygen to bind to all of them, often leading to a chronic ducky or blue “cyanotic” color of their skin. This leads to a low pulse oximeter reading that appears out of sorts with the physical exam findings.

Hypothermia: Peripheral vasoconstriction causes decreased blood flow to the probe site on the extremities.

Excessive patient movement: This can make it difficult for some pulse oximeter probes to pick up a signal.

High ambient light (i.e., bright sunlight, high-intensity light on area of the sensing probe): Some later generation devices can overcome this problem.

Nail polish or a dirty fingernail when using a fingertip pulse ox: Use acetone to clean the nail before attaching the probe. This is generally accepted practice.

Carbon monoxide (CO) poisoning: This will give falsely high readings because conventional sensing probes and the oximeters they’re attached to can’t distinguish between oxyhemoglobin and carboxyhemoglobin. If CO poisoning is suspected, you must use a specific monitor and sensor to measure levels. CO poisoning can also cause hypoxia because CO binds so tightly with hemoglobin that it takes up the space normally available for O2.

Cyanide poisoning: Cyanide poisons at the cellular level by preventing cells from using oxygen to make energy. Because the body isn’t using any oxygen, the circulating blood will usually be 95-100% saturated, but the patient will still be dying from lack of oxygen at the cellular level.

Sepsis: Infectious organisms interfere with the ability of oxygen to dissociate from the hemoglobin. While the patient may have a normal oxygen saturation, little oxygen is actually being delivered to the cells.

Using Pulse Oximetry

To use the pulse oximeter, turn on the device and clean the area where you’re going to apply the sensor (e.g., earlobe, fingertip or toe), and then attach the sensor.

Most units will display both a heart rate and SpO2 reading. Most units warm up quickly and usually give an accurate reading. Remember, however, that poor perfusion at the probe site may make the reading unreliable.

Some devices will give you a visual indicator of perfusion at the probe site-green means good. This can also be in the form of an LED or LCD bar that goes up and down with the pulse; many will display a pleth waveform.

The pleth waveform corresponds to blood flow. A well-defined pleth suggests a strong pulse and good perfusion at the probe site. With every cardiac contraction, during systole, the pulse ox pleth goes nearly straight up then starts to drop off. This is called the anacrotic limb. After the peak level, there’s a notch, known as the dicrotic notch, indicating aortic valve closure corresponding to the onset of diastole. The pleth tracing then drops to baseline, which is known as the diastolic trough.

Clearly defined waveforms make for more accurate and reliable readings. In low perfusion states, the pleth waveform will be small and ill defined. (See Figure 4.)

Since a pulse oximeter can measure perfusion at the probe site, it can be used on extremities to monitor blood flow in an injured extremity. When applying a traction splint to an extremity with a loss of circulation, for example, you can use a pulse oximeter as you pull traction to alert you when circulation (and thus, perfusion) has returned to the probe site.

In addition to pulse oximetry, capnography may provide clues as to the reasons O2 saturation is low. Hypoventilation (i.e., high EtCO2) leads to acidosis. Low perfusion means there’s poor perfusion at the pulse ox probe site.

Remember, however, that the pulse oximeter is an assessment tool; treat the patient, not the pulse ox reading.

Conclusion

Understanding our assessment tools, how they work and when to use them, gives us a better clinical picture of our patients. No one tool is definitive.

In this article we’ve broken down the very core of oxygenation, we’ve reviewed how O2 moves and is captured and released. You know how a pulse oximeter works, as well as its limitations and benefits. You’ve also learned how other technology, like capnography, can work alongside pulse oximetry for a better assessment of your patients.