At the end of last week, Tank started having "passing out" episodes, so I took him to the vet last night. While the vet at first thought it was heart disease (not uncommon in Chihuahuas and treatable), after taking his labs he found that Tank has kidney levels that are off the chart (and a mildly enlarged heart). The problem is that when you treat the kidneys the heart doesn't like it, and when you treat the heart the kidneys don't like it. Also, kidney disease isn't really treatable. There may not be much we can do.
The vet was too worried to do anything last night and referred me to a specialist where I have an appointment tomorrow. Please send my little guy positive thoughts!