Pulse oximetry is one obvious monitoring tool to identify hypoxemia and hypoxia. 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. Therefore, I spend a lot of time on recognizing early signs of respiratory distress such as:

• tachycardia
• tachypnea
• cyanosis
• agitation
• 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,

• only 15% had a correct understanding of the oxyhemoglobin dissociation curve.

We all must take pains to ensure our staff understands how to use these valuable monitoring tools. 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

PaO2, put simply, is a measurement of the actual oxygen content in arterial blood. Partial pressure is the pressure a specific gas exerts in a mixture. When gases like oxygen are dissolved in liquids, partial pressure refers to the pressure that gas would have if the liquid equilibrated with the gas in a container. If a gas exists in both an air space and a liquid in contact, their partial pressures will eventually equalize.

The Oxygen-Hemoglobin Dissociation Curve Shows the Difference

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

As oxygen partial pressure increases, more oxygen molecules bind to hemoglobin (Hgb). Each bound oxygen increases Hgb’s affinity for the next, causing a rapid rise in oxygen saturation until most sites are filled. Once saturated, additional binding slows and the curve plateaus. This property enables Hgb to quickly load oxygen in the lungs.

As PaO2 decreases, Hgb saturation drops as oxygen is released to tissues with lower oxygen supply. Around 90% saturation, the dissociation curve steepens because Hgb becomes less attracted to oxygen with fewer molecules bound, enabling rapid oxygen release to 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 oxygen saturation (O2 sat) of 90% equals a PaO2 of 60 mmHg, which is the lowest concentration that still supplies tissues with enough oxygen to avoid ischemia. If O2 saturation drops below 90%, PaO2 falls rapidly into a dangerously low range because hemoglobin binds much less oxygen. For this reason, maintaining O2 saturation above 90% is essential.

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 can all decrease peripheral blood flow. This can occur from extreme vasoconstriction, hypotension, hypovolemia, or septic shock. Poor perfusion sometimes makes it impossible for the sensor to measure the concentration correctly, or at all. With poor perfusion, the body tends to try to protect blood flow to the head. You can often put the sensor on the ear lobe and get a more accurate reading. You can place disposable sensors on the forehead, bridge of the nose. One can also pinch one 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 is 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’ve been practicing anesthesia for 35 years.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

Oxygen saturation is a vital indicator. Staff should know that readings in the low 90s, though acceptable, signal important changes that require monitoring. Everyone must understand its significance.

May Be The Force Be With You

Christine E. Whitten MD, author:

Anyone Can Intubate: A Step-by-Step Guide, 5th Edition
Pediatric Airway Management: A Step-by-Step Guide
Basic 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.