[Vinnie]

The cause of death on the certificate has been given as cardiac arrest, organ failure, a septic knee and arthritis

Infection. Could have been the cause. Another possibility is a blood clot in his knee travelling to his brain. Not that uncommon.

The thought of a CIA hit causes me to smile however.

[facedown]

Another possibility is a blood clot in his knee travelling to his brain.

Generally, the effect of such an event is very rapid, like a stroke. However if it was a very small clot that only occluded a minor vessel, that could explain the drowsiness.

[The KG9 Kid]

"... Another possibility is a blood clot in his knee travelling to his brain. Not that uncommon."

Most likely. Deep Vein Thrombosis resulting in a blood clot to the brain.

Quite correct; Not that uncommon at all.