I thought this might come in handy if anyone is ever searching on this topic in the future.  First, mets can cause respiratory difficulties in at least three ways, according to the emergency vet on Saturday night.  The first is obviously the size, number or location of the mets.  But mets can also cause the lungs to fill with either fluid or air, both of which can compromise a dog's ability to breathe.  Monitoring the level of oxygen in the blood alone will not necessarily identify a problem, as a dog can maintain proper oxygenation levels with only 30% lung capacity.  An X-Ray is needed to see what is going on inside.

From Otis' discharge notes - A normal dog at rest will take 36 - 40 breathes in a minute.  The complete up and down movement of the chest counts as one breathe.  This is a dog at rest, so if it is hot or the dog has been hopping around, this test will not work. 

I was also told to look for breathe movement not just in the chest, which is normal, but all down his sides, indicating increased exertion.  Coughing is another sign of potential breathing difficulty.

I am not a vet, so am just passing along what I was told over the past two nights, but I hope it is helpful to others.