Could Loxicom have worsened my older dog's heart symptoms?

[JohnnyMarr]

Sorry, this will be long because I'm not really sure what's relevant and what's not. Ddog is 12, he has a heart murmur, but up until recently has been happy and apparently healthy.
About a month ago he started sporadically limping on his back leg, but very momentarily and would then continue running around like a loon, this continued for a week or so, until one morning he woke up and was struggling to weight bear on said leg.
I took him to the vet, who manipulated his leg a bit and suggested it was "probably" his cruciate. He said he wouldn't recommend operating, given Ddog's age and heart murmur, and prescribed very short walks, no stairs, Loxicom and paracetamol as necessary.
After a couple days taking it easy Ddog seemed to be walking normally, and I subsequently found an entire nail from his affected leg on the floor. I called the vet to ask whether it could, in fact, have been his nail that had been causing the problem, but he said he thought it was unlikely and to continue with rest and loxicom as per the cruciate "diagnosis".
Over the last week or so Ddog seems to have aged exponentially. He has been coughing, panting, lethargic and subdued and hasn't been eating as usual and is drinking a lot.
I took him back to the vet, who listened to his heart and said it's beating irregularly, no further tests were done, but Ddog now has an appointment to see a veterinary cardiologist on Wednesday. He's thrown up what little he ate yesterday overnight.
Prior to his leg issue Ddog was absolutely fine, hyper even, and also seemed fine a couple days post his initial vet visit.
The only thing I can think of is that he has had a very adverse reaction to Loxicom, and on further investigation it appears that Loxicom is contraindicated in dogs with heart conditions. I'm not sure if a murmur is classed as a "condition", but surely an irregular heart beat is? He has all the symptoms indicated re Loxicom ecacerbating heart issues.
I've obviously stopped giving it to him and will call the vet as soon as they open, but wondered if anyone has any experience of this? I'm so worried about my poor boy.

My dog cannot tolerate loxicam/metacam. It's like he's got ants in his pants. He can't settle, he just seems completely out of sorts. I reported it to my vet the second time it happened ( the first time it happened, I wasn't sure if it was the drugs). So the vet reported it to the drug company who actually did call me for feedback for side effects.

Loxicom SPC (Summary of Product Characteristics, commonly called the datasheet) says under contraindics:

"Do not use in animals suffering from gastrointestinal disorders such as irritation and
haemorrhage, impaired hepatic, cardiac or renal function and haemorrhagic
disorders."

If I saw a dog with a history of hindlimb lameness that I thought had some stifle effusion or was sore on cranial tibial thrust or attempted cranial drawer, I'd like to get it on some NSAIDs such as Loxicom. In practice I would probably use it in a dog with a heart murmur but no other signs of impaired cardiac output.

AFAIK the mention of impaired cardiac function is there not for the sake of the heart itself, but for the sake of other organs that the heart supplies, particularly the kidneys. Kidneys that are well perfused, ie in a normally hydrated animal with the cardiac "power" to keep up the bloodflow that they need , should be OK with NSAIDs.

For your dog- if I had a patient that had a known heart murmur but no signs of heart problems that suddenly started showing signs I'd worry about decompensation and fluid overload of the lungs as one possibility for the coughing; atrial stretch that hadn't previously caused any problems getting bad enough to affect the heart's own pacemaker as a possible cause of the arrthythmia; +/- rupture of a few chordae tendinae making everything worse. Heart scan should tell you a lot and help guide next steps. I hope he does OK and is able to get something that can help matters.