With the use of some simple calculations it can be demonstrated that the choice of cannula and initial glyceryl trinitrate (GTN) infusion rate for patients with acute left ventricular failure (LVF) requires some careful thought. If a GTN infusion is commenced at a rate of 1 ml/h, a critically unwell patient with a large cannula - for example, a grey cannula - will have to wait over 6 min for the drug to enter the body. This compares with 1.5 min for a pink cannula at the same infusion rate. If a large-diameter cannula is chosen for these patients, then a fast initial infusion rate should also be chosen to ensure that the GTN begins to act quickly. The rate can later be adjusted depending on clinical conditions.