PaO2 vs SaO2

pulse oximetry -- SaO2 with normal value >95%

PaO2 stands for the partial pressure of O2 in arterial blood, measured in mmHg.

SaO2 is the arterial O2 saturation.

PaO2 = measures oxygen in the blood (80-100)

SaO2 = measures tissue perfusion (pulse ox >95%)

Nagmamahal, RN said:

PaO2 = measures oxygen in the blood (80-100)

SaO2 = measures tissue perfusion (pulse ox >95%)

Both definitions provided here are simplistic.

The PaO2, like I said, is the partial pressure of O2 in arterial blood-- typically measured via ABG.

SaO2 doesn't measure tissue perfusion. It measures as a percentage the amount of hemoglobin molecules which are oxygenated (oxyhemoglobin) in arterial blood.

I'm not sure why these concepts are not stated more clearly in nursing education.

Although a late response, I'm sure others want to know a few details that make the difference between Pa02 and Sa02 more clear and relevant:

Oxygen travels in blood via to mechanisms:

1) Bound to hemoglobin

2) Dissolved in plasma

SaO2 (and its indirect measurement SpO2) describe the amount of oxygen bound to hemoglobin in arterial blood. The term "saturation" likens hemoglobin to a sponge that becomes saturated with oxygen. The measurement is given as a percentage.

PaO2 describes the amount of oxygen dissolved in arterial blood plasma. The measurement is given as a pressure value (mmHg).

There is a relationship between the two numbers, which is described in the oxyhemoglobin dissociation curve. (yes, THAT oxyhemoglobin dissociation curve.) Without getting too far into it, roughly speaking all other things being equal PaO2 and SaO2 relate as follows:

SaO2 50% = Pa02 26.6 mmHg (aka P50); 70=40; 90=60; 95=80; 99=100

These numbers change depending on which way the curve shifts.

I'm studying this thing right now cause tomorrow morning i got an oral exam on it... i'm at the first year of nurse university here in Italy.

I thought it would be helpful to post the oxyhemoglobin dissociation curve pic from my slides

Enjoy! :)

Same here, just wondering if someone was interested in seeing the values right from the diagram

And, just to split hairs even more, since we're already walking down that road... I believe that the SaO2 is an "assumed" value, meaning that it actually measures the hemes that are saturated, but only assumes they are saturated with O2. CO poisoning will artificially elevate the SaO2 level, because CO has more affinity to the hemes (or vice versa, doesn't matter here for the sake of this argument) than O2 does, and the measurement is not actually the number of O2 molecules that are bound, but how many "seats on the bus" are taken up.

Doubt that will be on NCLEX, but good to know as real live nurses!

Thank you for that last mention. Since PaO2 is assumed during routine testing, how would it specifically be tested? Does an arterial blood sample need to be taken to get the ABGs? Early in my career I had a jerk doctor ask me 10 minutes after he wrote an order to put a pt on 3L O2 because her PulseOx was 88ish. I told him the PulseOx and he repeated 3 times successively louder each time "what is the PaO2!!?" I asked "if labs had to be drawn and he yelled back, "No, You go tell me what it is...NOW!" I had to get the charge nurse to talk to him. I just never thought to ask again.