The regular vet and I should have thought to do a CBC/chem panel in April when she was in for the urine issue. If a cat isn't sick, one does not need a CBC/chem panel. But with proven constipation (the xray showed it at the time) it may have been a good idea to check her values for dehydration. And because we know Myrna's condition, she should have them regularly. Now, regularly for her means once a year unless otherwise called for. It's been a year since she had a full panel (it was a year ago in April that she had her last congestive heart failure incident) and her renal values were last checked last August. I had the cardiologist do a CBC/chem panel only because it had been a year. I'm glad I did. Dehydration can be a serious issues. It can lead to many other issues than just with the colon (constipation). It can lead to trouble and failure of renal, liver, heart and other body functions as the body loses electrolytes and other minerals it needs to function, and can lead to loss of blood pressure (less water, less blood flow, less pressure) and to lethargy-in much the same way as dehydration affects humans.
Here's a link from "Pet Well Being" website that explains it:
So, give your cat water on a regular basis in their wet food. Give cats with urinary or bladder issues extra water in their food and by mouth especially in hot weather. Give extra water on a regular basis if your cat gets gassy/constipated from time to time. Your vet can give you plastic syringes (sans needles) which you can use to put water into the cat's mouth. Those are very helpful and make the job easier.