If I have a patient with no respiratory complaints and an O2 sat of 98%+ , what am I achieving? Its impossible to break 100% O2 sat, there is no going the extra mile. (granted o2 sat is not a definitive measure of oxygenation, just using it as an example)
While you cannot break 100% saturation of hemoglobin, you can continue to increase the PaO2!
Which brings us to the cornerstone of
effective use of pulse oximetry: understanding the
Oxyhemoglobin Dissociation Curve. Basically, as you increase the partial pressures of dissolved O2 in the blood, your Hgb holds onto O2 more tightly. As you decrease the partial pressures of dissolved O2 in the blood, your Hgb more readily releases O2.
All you know (assuming no shift in the Hgb-O2 curve) with 100% SpO2 is that you have a PaO2 of at least 100 mmHg...you don't know where on the curve you are.
This is what has been shown to be harmful to your cardiac and stroke patients! You could have a PaO2 of 100, 150, 200 mmHg or even higher, you just don't know. Oxidative stress is going to increase the wider the gap is between metabolic demand and available oxygen. Going beyond ~98% (outside of preoxygenation for a procedure) isn't helpful, because you no longer know where you are on the curve.
But with say an SpO2 of 90%, again assuming no shift, you have a PaO2 of around 60 mmHg and now you're in danger of "falling off the cliff" as the curve is very steep. Partial pressures of oxygen in that range will allow the oxygen to more readily dissociate from Hgb. States like acidosis or hyperthermia will also cause your patient to shed bound-O2 more readily, even at higher SpO2's! Oops, I'm rockin' a tangent here.
Coming full circle: no respiratory complaint or distress? No need for O2.