Another way to look at the oxygen saturation issue -- pulse ox measures the amount of oxygen that is being carried on an individual "chunk of hemoglobin". You could have a patient with a 100% sat level who is severely hypoxic and quite cyanotic, if they have a low hemoglobin count to begin with, because even if each chunk is carrying a full load (100%) of oxygen, there still won't be enough oxygen to go around to all the tissues that need it. You can't relate O2 sat levels to the amount of oxygen that is circulating in the body unless you know that the hemoglobin levels are satisfactory.
Taking the truck analogy farther, let's say that a hypothetical body needs 90 cases of oxygen to maintain normal function, and a normal body has 100 pickup trucks driving around delivering oxygen and then refilling their load at the oxygen manufacturing plant, each truck carrying their full capacity of one full case of oxygen. Capacity carried by each truck = 100% load capacity = 100% O2 sat.
Then five pickup trucks get flat tires and are out of commission, so you now only have 95 pickups each carrying one full case. You now have 95 cases available in the body, but each truck is still carrying 100% of its load capacity, so O2 sat is still 100%, even though the actual number of cases has decreased by 5% (started with 100 cases, now only have 95 in circulation). But that's okay, because we had a bit of a cushion built in -- we had enough trucks to carry 100 cases, but we really only need 90 cases to get by.
Then there is a defective part on that model of truck, and twenty trucks all blow a head gasket at the same time. You're now down to only 75 trucks going around the body, but each truck still has a full case of oxygen, so O2 sat is still at 100%, even though there are only 75 cases available now instead of 100 cases like we started out. But with only 75 cases available, and with our previous statement that the body needs 90 cases in order to maintain normal function, we're going to start seeing some problems... we're running on an overall deficit because twenty-five percent of our trucks are out of commission. So even though all the remaining trucks are running at 100% capacity and we have a 100% O2 saturation level, we're turning blue because we are 15 cases short of the 90 cases we need for normal functioning.
So now imagine that there's a problem at the oxygen manufacturing plant as well, and production is slowed considerably -- they can't even keep up with the demands of the 75 trucks that are still on the road. When a truck comes in for a fresh load, they can only give them 85% of a case. Now each truck is only carrying 85% of a load, so the O2 saturation level drops to 85%. But since we had fewer trucks (75) to begin with, that means that there are only 64 cases of oxygen available in the body. So even with an 85% sat level reading, we're only running at 64% of actual capacity.
Hopefully this little story helps with mentally separating the O2 sat number from the actual volume of oxygen that is available to the body to use. Pulse ox readings are great, but always look at your patient first, the monitor second... remember, someone can be completely blue with a 100% O2 sat reading!