Note how quickly Hgb loses oxygen below 90% saturation.Īs the partial pressure of oxygen rises, there are more and more oxygen molecules available to bind with Hgb. An O2 sat of 90% corresponds to a PaO2 of 60 mmHg. The Oxygen-Hemoglobin Dissociation Curve shows the percent of oxygen binding to Hgb per mmHg. To see why this is relevant, look at the oxygen-hemoglobin dissociation curve. The Oxygen-Hemoglobin Dissociation Curve Shows the Difference 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. 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. 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 all the Hgb binding sites are filled, Hgb is 100% saturated. This number, given as a percentage, is called the oxygen saturation or simply O2 Sat, commonly pronounced “Oh Two SAT”. We can measure how many of these binding sites are combined, or saturated, with oxygen. Depending on conditions, Hgb releases some percentage of the oxygen molecules to the tissues when the RBC passes through the capillaries. Each Hgb molecule, if fully saturated, can bind four oxygen molecules. Hemoglobin is a chemical molecule in the red blood cell (RBC) that carries oxygen on specific binding sites. Oxygen saturation is the percent of Hemoglobin (Hgb) binding sites in the blood that are carrying oxygen. Some of the material below is from my book Anyone Can Intubate. This is such a key concept that we all must take pains to ensure our staff understands how to use this valuable monitoring tool. 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. Many RNs do not understand the important difference between oxygen saturation and PaO2. Multiple studies have identified this as a knowledge gap. 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. I often teach classes for RNs who are orienting to our preoperative and recovery areas. 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). Pulse oximetry is one obvious monitoring tool to identify hypoxemia and hypoxia.
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