We took her for an ultrasound today. Deep down inside me I knew there would be bad news. She's such a stoic, tough dog (ok, except for her irrational fears of barbeques and hoses...), so I really did know that if she's at the point that she's actually showing distress, it's bad. Once again, that inner voice was right.
She has a pretty significant tumour on the right atrium of her heart. It's causing a backflow of blood into her pericardium and abdominal cavity. Apparently heart tumours are fairly common in medium- and large-breed older dogs, and they come on quickly. That explains why we were happily running in the woods just a few weeks ago. It's not treatable, especially for a dog her age.
We had the vet drain some of the fluid from around her heart, and immediately she was more comfortable and her heart rate dropped to 100 bpm from about 175. The vet said that by draining the fluid she would probably get another week or two before she's back in distress, but we're not going to let it get back to that point and will put her down first. We want her last days with us to be happy and enjoyable.
She hasn't eaten in days, but she's had a couple of treats since she's been home this afternoon. Funny, we didn't think it would end this way... we always pictured her growing old peacefully, and running in the woods right up until her last day. We'll make sure she gets one last walk on her favourite trails, even if we have to carry all 65 pounds of her. But all this heartache - every minute of it - is worth it for the over 11 years we've had with our best friend. Like Jason said, getting her was the best decision we've ever made.