"Every single person on our list carries a recognised categorization - so we know the patients with long-term conditions and the seriously-ill patients, " he said.
The College of Emergency Medicine has told BBC Wales all emergency departments across the country are experiencing sustained and severe pressure, with a steady increase in seriously-ill elderly patients.
Mr Cameron will outline plans for an "early access" scheme allowing seriously-ill patients, such as those with cancer, to get new drugs up to a year before they are fully licensed.