Hello & welcome to Just Answer! If you have any questions after reading my answer, be sure to let me know.
First of all, let me convey my deepest sympathies for your loss,
While hindsight can be 20/20, I believe that it can be tough to put blame on anyone in this situation, since as you have your self stated, he had a normal EKG and ECHO just 4 days before his death.
It is not possible to say that doing a pulmonary function test would have changed the management in any meaningful way for certain.
I do agree that persistent breathing problems after a valve replacement do demand a detailed follow up, but it does appear that the medical care taken was not negligent,
An autopsy might provide further answers to the exact cause of death, and let us know if the doctors missed something that they could have prevented,
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