Many drugs (diphenhydramine included) are known to redistribute after death, specifically in central compartments, such as the thoracic cavity, when compared with more accurate femoral blood.
While PMR seems a simple concept, the underlying science is multifactorial and complex. On a cellular level, it begins with post-death hypoxia that halts synthesis of ATP, anaerobic glycolysis, an imbalance in the intracellular versus plasma pH, an inverse ionic gradient attracting certain molecules, the failure of the sodium pump, the release and diffusion of potassium, the breakdown of cell organelles, and intracellular acidification. Added to this is the physical breakdown of organs and structures (like blood vessels) leading to the “leakage” of molecules no longer physically and chemically maintained where they belonged in vivo. In short, despite it being very well-documented, it is a very complicated (and fascinating) process that is still being studied.
Postmortem redistribution medical expert witness specialties include forensic pathology and toxicology.