"Early next week" is turning out to mean Thursday afternoon, in reality. The cardiologist is unhappy about this, as am I, but so far none of the the local MRI-capable imaging facilities seem to be able to line up the right combination of qualified personnel and machine time any sooner.
In part, this may be due to the fact that a cardiac MRI is attempting to capture an image of a dynamic, rather than of a static, structure. The heart is gyrating frantically, whereas the standard MRI concept is about getting a high-resolution image of something that is holding still long enough to gather the data required to assemble such an image; any kind of movement distorts the image. So getting a good MRI image of the heart is significantly more difficult than getting one of, say, the liver or pancreas.
In any case, I remain in a holding pattern for what is turning out to be the entire week. The only bright spot is that on Wednesday I will be set up with a portable version of the oxygen concentrator, which will allow me to leave the house without dragging along a tank.