I was trying to think of a polite way earlier to say the same thing MitchF150 said as we lost a dog to congestive heart failure. The inability to walk was one of his symptoms in his last couple of days and his legs also felt cold from lack of blood circulation - then he was blue around the mouth/tongue area from lack of oxygen. It started suddenly when the end came - he was doing okay one day and the next day symptoms started and got progressively worse over the next 24 hours. We had to let him go, for his sake. It may be time.