Shock is not low blood pressure. Shock is inadequate tissue perfusion — oxygen delivery failing to meet cellular demand — and it can be well established in a patient whose blood pressure still reads normal. Defining it by the blood pressure is the single most common and most dangerous error in this topic, because compensation defends blood pressure until it can no longer do so, and then it falls quickly. There are four types, and they matter because the treatment differs and some treatments are actively harmful in the wrong type. Hypovolemic shock is a volume problem. Distributive shock is a vessel-tone problem. Cardiogenic shock is a pump problem. Obstructive shock is a physical-blockage problem. Giving repeated fluid boluses helps the first, helps the second up to a point, and can drown a patient in the third. That is why this lesson leads with differentiation. Give three litres of crystalloid to a patient in cardiogenic shock and you will worsen their pulmonary oedema; give none to a haemorrhaging trauma patient and they will die of it. The assessment that tells...