In this phase 2/3 study, the investigators propose to investigate the effect of sodium nitrite injection on infarct size. Eligible patients will be males aged 18 and over and females aged 55 and over, presenting within 12 hours of the onset of chest pain, who are suitable for treatment with percutaneous coronary intervention. Those who give verbal agreement to take part will receive a 2.5-5 minute injection of sodium nitrite (or placebo) immediately prior to the blocked artery being opened with percutaneous coronary intervention.