11.2 Basic Concepts of Oxygenation
Open Resources for Nursing (Open RN)
When assessing a patient’s oxygenation status, it is important for the nurse to have an understanding of the underlying structures of the respiratory system to best understand their assessment findings. Visit the “Respiratory Assessment” chapter for more information about the structures of the respiratory system.
Video Review for Oxygenation Basics
View the TED-Ed Oxygen’s Journey video on YouTube[1]
Breathing Mechanics[2]
Gas Exchange[3]
Carbon Dioxide Transport[4]
Assessing Oxygenation Status
A patient’s oxygenation status is routinely assessed using pulse oximetry, referred to as SpO2. SpO2 is an estimated oxygenation level based on the saturation of hemoglobin measured by a pulse oximeter. Because the majority of oxygen carried in the blood is attached to hemoglobin within the red blood cell, SpO2 estimates how much hemoglobin is “saturated” with oxygen. The target range of SpO2 for an adult is 94-98%.[5] For patients with chronic respiratory conditions, such as COPD, the target range for SpO2 is often lower at 88% to 92%. Although SpO2 is an efficient, noninvasive method to assess a patient’s oxygenation status, it is an estimate and not always accurate. For example, if a patient is severely anemic and has a decreased level of hemoglobin in the blood, the SpO2 reading is affected. Decreased peripheral circulation can also cause a misleading low SpO2 level.
ABG results are often obtained for patients who have deteriorating or unstable respiratory status requiring urgent and emergency treatment. An ABG is a blood sample that is typically drawn from the radial artery by a respiratory therapist, emergency or critical care nurse, or health care provider. ABG results evaluate oxygen, carbon dioxide, pH, and bicarbonate levels. The partial pressure of oxygen in the blood is referred to as PaO2. The normal PaO2 level of a healthy adult is 80 to 100 mmHg. The PaO2 reading is more accurate than a SpO2 reading because it is not affected by hemoglobin levels. The PaCO2 level is the partial pressure of carbon dioxide in the blood. The normal PaCO2 level of a healthy adult is 35-45 mmHg. The normal range of pH level for arterial blood is 7.35-7.45, and the normal range for the bicarbonate (HCO3) level is 22-26. The SaO2 level is also obtained, which is the calculated arterial oxygen saturation level. See Table 11.2a for a summary of normal ranges of ABG values.[6]
Table 11.2a Normal Ranges of ABG Values
| Value | Description | Normal Range |
|---|---|---|
| pH | Acid-base balance of blood | 7.35-7.45 |
| PaO2 | Partial pressure of oxygen | 80-100 mmHg |
| PaCO2 | Partial pressure of carbon dioxide | 35-45 mmHg |
| HCO3 | Bicarbonate level | 22-26 mEq/L |
| SaO2 | Calculated oxygen saturation | 95-100% |
Hypoxia and Hypercapnia
Hypoxia is defined as a reduced level of tissue oxygenation. Hypoxia has many causes, ranging from respiratory and cardiac conditions to anemia. Hypoxemia is a