Pulse oximetry is one obvious monitoring tool to identify hypoxemia and hypoxia. I find that one frequent area of confusion relates to understanding the important difference between arterial partial pressure of oxygen (PaO2) and oxygen saturation (O2 sat).

I often teach classes for RNs who are orienting to our preoperative and recovery areas. Hypoxemia and hypoxia occur commonly among our perioperative patients so I spend a lot of time on recognizing early signs of respiratory distress such as tachycardia, tachypnea, cyanosis, agitation, and changes in mental status.

Many RNs do not understand the important difference between oxygen saturation and PaO2. Multiple studies have identified this as a knowledge gap. One study of pediatric nurses showed that while 84% of the clinicians felt they had received adequate training, only 40% correctly identified how a pulse oximeter worked, and only 15% had a correct understanding of the oxyhemoglobin dissociation curve. This is such a key concept that we all must take pains to ensure our staff understands how to use this valuable monitoring tool. Some of the material below is from my book Anyone Can Intubate.

What Is Oxygen Saturation?

Oxygen saturation  is the percent of Hemoglobin (Hgb) binding sites in the blood that are carrying oxygen. Hemoglobin is a chemical molecule in the red blood cell (RBC) that carries oxygen on specific binding sites. Each Hgb molecule, if fully saturated, can bind four oxygen molecules. Depending on conditions, Hgb releases some percentage of the oxygen molecules to the tissues when the RBC passes through the capillaries.  We can measure how many of these binding sites are combined, or saturated, with oxygen. This number, given as a percentage, is called the oxygen saturation or simply O2 Sat, commonly pronounced “Oh Two SAT”. it is also referred to as SPO2. When all the Hgb binding sites are filled, Hgb is 100% saturated.

What Is Arterial PaO2

Pa02, put simply, is a measurement of the actual oxygen content in arterial blood. Partial pressure refers to the pressure exerted on the container walls by a specific gas in a mixture of other gases. When dealing with gases dissolved in liquids like oxygen in blood, partial pressure is the pressure that the dissolved gas would have if the blood were allowed to equilibrate with a volume of gas in a container. In other words, if a gas like oxygen is present in an air space like the lungs and also dissolved in a liquid like blood, and the air space and liquid are in contact with each other, the two partial pressures will equalize.

The Oxygen-Hemoglobin Dissociation Curve Shows the Difference

To see why this is relevant, look at the oxygen-hemoglobin dissociation curve.

As the partial pressure of oxygen rises, there are more and more oxygen molecules available to bind with Hgb. As each of the four binding sites on an Hgb molecule binds to an oxygen molecule, its attraction to the next oxygen molecule increases and continues to increase as successive molecules of oxygen bind. The more oxygen is bound, the easier it is for the next oxygen molecule to bind, so the speed of binding increases and the oxygen saturation percentage rises rapidly on the curve. As all of the binding sites fill up, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. This tendency makes it easy for Hgb to rapidly pick up oxygen in the lungs as it passes through.

As PaO2 falls, the Hgb saturation also falls as Hgb releases oxygen to the tissues in the areas of lower oxygen supply. Notice that around a saturation of 90%, that the dissociation curve drops off quickly. This is because Hgb binding sites become less attracted to oxygen as it is bound to fewer oxygen molecules. This property allows Hgb to rapidly release oxygen to the tissues.

Deoxygenated blood returns to the heart to be pumped to the lungs and the cycle repeats.

Since a normal PaO2 is between 90-100 mmHg, some people may think that an O2 saturation of 90 is normal as well — after all 90 was a pretty good grade to get in school. However, this interpretation is very wrong.

An O2 sat of 90% corresponds to a PaO2 of 60 mmHg. This is the minimum oxygen concentration providing enough oxygen to prevent ischemia in tissues. Once the O2 sat falls below 90%, the PaO2 drops quickly into the dangerously hypoxic range as fewer and fewer oxygen molecules are bound to Hgb. We want to try to keep O2 saturation above 90%.

Is An Oxygen Saturation Of 100% Always Normal?

No, it’s not. Let’s take an example of a patient breathing 50% FiO2 who has a PaO2 of 100.  A simple formula to estimate what the arterial oxygen concentration should be is to multiply the inspired oxygen concentration by 5. Someone breathing room air at 21% oxygen should have a PaO2 of about 100. So if the patient is breathing 50%, we know that his PaO2 should be about 250. It’s not, so therefore something is very wrong. But if you look at the Oxygen-Hemoglobin Dissociation Curve, a PaO2 of 100 and 250 both have an O2 sat of 100% because both provide enough oxygen molecules to fill all of the Hgb binding sites. So in this case O2 saturation doesn’t help us very much.

Are Oxygen Saturation Readings Always Accurate?

No. As good as they are they can have problems. Movement can cause inaccurate readings. This is especially common in small children.

Another problem is that poor perfusion from extreme vasoconstriction, hypotension, hypovolemia, and septic shock can all decrease peripheral blood flow.This sometimes makes it impossible for the sensor to measure the concentration correctly, or at all. When peripheral flow is disturbed, the body tends to try to protect central blood flow to the head. You can often put the sensor on the ear lobe and get a more accurate reading. Disposable sensors can also be placed on the forehead, bridge of the nose, and can also be pinched around the corner of the mouth (making sure that the light and the detector are directly opposite each other.) Note that getting a good reading in this situation reassures you that the patient is oxygenating, but it doesn’t relieve you of trying to solve the perfusion problem.

Carbon monoxide (CO) also binds to hemoglobin but the oxygen saturation monitor can’t tell the difference between CO and O2. The presence of CO fools the monitor into reading high. The patient with CO poisoning appears flushed and pink. However, CO can’t provide oxygen to tissues and PaO2 may be very low.

Children should have sensors appropriate to their size. when wrapping a tiny finger, always make sure that the light and the sensor are directly opposite each other.

If O2 Sat Can Miss Big Problems Like This, Why Use It Instead Of PaO2?

Oxygen saturation has become a standard of care because it’s measured using a noninvasive sensor placed on the skin. The monitor is small, portable for use in the field, operating rooms, and in patient hospital rooms to provide continuous, real time monitoring of the patient. It’s a trending tool  to ensure saturation stays above 90%.

Measurement of PaO2 requires drawing and testing an arterial blood sample —something that requires a trained provider, a lab, and time. While it’s extremely useful in the hospital setting, you’re not always going to have it in an emergency. Waiting for an ABG can sometimes delay clinical decisions.

I have been practicing anesthesia for 35 years and the use of pulse oximetry revolutionized patient safety when we started using it. Oxygen saturation is one of the most valuable tools I have. It can give you early warning about many things, including:

• pulmonary function
  • hypoventilation, apnea
  • atelectasis
  • bronchospasm
• misplacement of endotracheal tube
  • mainstem intubation
  • inadvertent extubation
• changes in perfusion

It’s critically important as a trending tool. Our staff needs to know that numbers in the low 90s, while acceptable, indicate significant changes in oxygenation that need to be monitored and addressed. It’s imperative that all of our staff understands what oxygen saturation means.

May Be The Force Be With You

Christine E. Whitten MD

author of Anyone Can Intubate: a Step-by-Step Guide, 5th Edition &
Pediatric Airway Management: a Step-by-Step Guide

Reference

• Popovich DM, et al. Pediatric health care providers’ knowledge of pulse oximetry. Pediatr Nurs 2004;30(1):14-20.
• Attin M, et al. An educational project to improve knowledge related to pulse oximetry. Am J Crit Care 2002;11(6):529-34.
• Howell M. Pulse oximetry: an audit of nursing and medical staff understanding. Br J Nurs 2002;11(3):191-7.