Blood clots are in the spotlight this week after reports that they are "clogging up" the lungs of patients in intensive care with Covid-19. Not that you have to be infected with the virus to be at risk. Despite the NHS being a world leader in clot prevention, venous thromboembolism - deep vein thrombosis (normally in the leg) and pulmonary embolus (clots in the lungs)remains the main cause of preventable deaths in our hospitals.
Three factors classically contribute to the abnormal clotting that results in a deep vein thrombosis (DVT): overly "sticky" blood, injury to the lining of blood vessels and sluggish flow. Most DVTs form in the deep veins of the leg, and the most common symptom is a painful and occasionally swollen calf that is all too easy to attribute to a pulled muscle. The main risk from delayed diagnosis is that part of the clot will break off and travel to the lungs, causing a pulmonary embolus (PE). And the risk is considerable: about a third of patients with DVTs in their thighs develop a PE and one in three of them will die from it.
Sick people in hospital - whether they have Covid or not - are likely to have all three predisposing factors, particularly if they are immobile in bed after major surgery or on an intensive treatment unit (ITU) after an accident or serious infection. And to compound matters, patients with Covid also develop massive inflammation in the lungs, which leads to further clotting in the small blood vessels in the part of their lungs where gases are exchanged. Little wonder that ventilating such people has proved a significant challenge -you can boost breathing artificially with a machine, but it's not going to make a huge difference if large sections of the lungs have no blood supply. Air and blood need to meet to keep people alive.