Strychnine poisoning could not be misdiagnosed as a cardiac event. Strychnine attacks the central nervous system causing seizures so intense they often break limbs.
When Hetty recommends Sam try laudanum, Sam retorts that she'll pick some up from the alchemist's. Apothecaries were the older equivalent of modern chemists/ pharmacies, while practitioners of alchemy were the precursors of modern experimental scientists, not dispensers of medicines.