This case report highlights the importance of preload in cardiac tamponade, as cardiac arrest with pulseless electrical activity was unintentionally induced after sublingual administration of nitroglycerin in an 83-year-old woman with unrecognised haemopericardium. Cardiac tamponade is a life-threatening condition, as accumulation of fluids in the pericardial sac diminishes preload which can ultimately lead to cardiac arrest. Primary elements in the development of critical haemodynamic tamponade are rate of fluid-accumulation and compensatory mechanisms, mainly tachycardia and vasoconstriction.