Partial Pressure of Oxygen (PaO₂) and Pulse Oximetry (SaO₂) (Nursing)

by Rhonda Lawes, PhD, RN

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    00:00 Now we talked about the PaO₂ in the introduction, but it's the partial pressure of dissolved oxygen in arterial blood. Now it's the major determinant in the sat. PaO₂ involves an invasive blood sample. The sat can be done with a pulse oximetry that's not invasive. Normally, the PaO₂, remember that's a blood sample, ranges from 80-100. In a perfect textbook world, that's what we're looking for. Now, as the person ages, their normal PaO₂ might be a little lower because it tends to decrease as we age. Now remember, you can only measure this with arterial gases. A PaO₂ requires an arterial blood sample. So, ABGs are invasive but they're more accurate than what pulse oximetry reading. So if the patient's PaO₂ on room air is too low, this is a sign of impaired gas exchange. We've got a ventilation perfusion imbalance. Now we're not going to go into detail in VQ here because we've talked through that in other areas, but that just means either I don't have enough blood flowing by the lungs to carry the oxygen to the tissues and brings here to back or I'm not getting enough air in. That's all a ventilation perfusion imbalance means. So you can have a lung tissue problem, asthma, COPD, atelectasis which is a collapsed lung, or even if the patient is oversedated. Patient has received too many opioids or any medication that suppresses the respiratory system, they will breathe slower and more shallow. So that will decrease their PaO₂. Now let's talk about the pulse ox just a little bit. I want to give you a bit of an introduction to that because oxygen combines with hemoglobin and it forms oxyhemoglobin. Now what a pulse oximetry can do, you can use a specific waveforms of light and it compares the oxyhemoglobin versus the deoxyhemoglobin.

    02:00 So that's hemoglobin that has in combined with oxygen. So that's what a pulse ox does, uses a slight from the top and the bottom, generally put on a patient's finger with a little clip and it compares oxyhemoglobin, that's oxygen and hemoglobin, and deoxyhemoglobin to give us a percentage number that represents the percentage of saturation. That's why it's called a pulse ox. Now, the sat is a ratio of those oxyhemoglobin that we talked about to deoxyhemoglobin.

    02:31 Pause for a minute and think. Which one of this contains oxygen? Is it oxyhemoglobin or deoxyhemoglobin? Right, the drawing is a good clue. The oxyhemoglobin is the one where oxygen has combined with hemoglobin. So, patients like pulse oxes because it's less than basis in ABGs but remember it's an indirect measure. So if I have 2 measures to look at, if I'm looking at the pulse ox saturation or the PaO₂, the arterial sat, I'm going to trust the arterial one as being more accurate. Okay, now there's the little clip that we showed that you put on a patient's finger, see there's a light on the top, there's a sensor in the bottom, we already talked about a lot more pleasant for the patients because you don't need to have a blood sample. But here's a note I want you to remember. If the patient doesn't have good circulation here, it's not going to give us an accurate reading. So if a patient has poor perfusion in their fingers for any reason like, say, Raynaud's disease and that's one of the symptoms of Raynaud's disease is poor perfusion, you're not going to want to put a pulse ox on their finger.

    03:40 Remember, normal ranges for a pulse ox are approximately 95%-100%, but the healthcare provider might adjust that number based on any underlying disease and lung disease that the patient has or if there's COPD, etc. So you can see in the picture that different color represents the lack of circulation in the patient's fingers.

    About the Lecture

    The lecture Partial Pressure of Oxygen (PaO₂) and Pulse Oximetry (SaO₂) (Nursing) by Rhonda Lawes, PhD, RN is from the course Supplemental Oxygen (Nursing).

    Included Quiz Questions

    1. Pulse oximetry (SaO2)
    2. Capillary refill < 3 seconds
    3. Skin color
    4. Auscultation of clear lung sounds
    1. Poor perfusion in fingers and toes will not give an accurate SaO2 reading.
    2. Supplemental oxygen is needed STAT if the reading is less than 90%.
    3. The nurse should move the pulse oximeter to the client's great toe.
    4. A normal SaO2 reading is 89–100%.

    Author of lecture Partial Pressure of Oxygen (PaO₂) and Pulse Oximetry (SaO₂) (Nursing)

     Rhonda Lawes, PhD, RN

    Rhonda Lawes, PhD, RN

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