SCD is an abbreviation with several common meanings in medicine and health. The three you’re most likely looking for are sudden cardiac death, sickle cell disease, and sequential compression device. Each refers to something completely different, so here’s a clear breakdown of all three.
Sudden Cardiac Death
Sudden cardiac death is defined as death from a cardiac cause that occurs within one hour of symptoms starting, or within 24 hours of the person last being seen alive and healthy. It is not the same as a heart attack. The American Heart Association describes the distinction this way: a heart attack is a “circulation problem” caused by a blocked artery cutting off blood flow to part of the heart muscle. Sudden cardiac death is an “electrical problem” where the heart’s rhythm malfunctions so severely that the heart stops pumping blood entirely. The person loses consciousness and their pulse within seconds.
The electrical malfunction typically takes the form of a dangerous heart rhythm called ventricular fibrillation, where the heart’s lower chambers quiver chaotically instead of contracting. In about half of out-of-hospital cases, the heart’s electrical activity has already flatlined (asystole) by the time first responders arrive. Roughly 2 million sudden cardiac deaths occur worldwide each year, an incidence of about 100 cases per 100,000 people annually. The risk is low in childhood but climbs steeply with age, reaching 200 per 100,000 in people in their 70s.
For people identified as high risk, an implantable cardioverter-defibrillator (ICD) can continuously monitor heart rhythm and deliver a corrective shock if a lethal arrhythmia starts. Current guidelines consider an ICD appropriate for people whose heart pumps 35% or less of its blood volume per beat, whether the cause is a previous heart attack or another form of heart muscle disease. ICDs are also recommended for people with certain inherited heart conditions. For those who have already survived a cardiac arrest, an ICD serves as secondary prevention to stop it from happening again.
Sickle Cell Disease
Sickle cell disease is a genetic blood disorder present from birth. A child develops SCD when they inherit two copies of an abnormal hemoglobin gene, one from each parent. The defective hemoglobin (called hemoglobin S) causes red blood cells to become rigid and curve into a crescent or “sickle” shape instead of remaining round and flexible.
These misshapen cells create two core problems. First, they die much earlier than normal red blood cells, leading to a chronic shortage of red blood cells (anemia). Second, their stiff, sticky shape causes them to clump together and clog small blood vessels. When blood flow gets blocked this way, it triggers intense pain episodes called vaso-occlusive crises, which are the hallmark symptom of the disease. Beyond pain, the blocked blood flow can lead to serious complications including stroke, acute chest syndrome (a life-threatening lung condition), and recurring infections.
The global burden of SCD is substantial and growing. The number of people living with the disease increased by about 41% between 2000 and 2021, from roughly 5.5 million to 7.7 million worldwide, according to the Global Burden of Disease Study. Most cases are concentrated in sub-Saharan Africa, though significant populations exist in India, the Middle East, and among people of African descent in the Americas.
Gene Therapy for Sickle Cell Disease
In December 2023, the FDA approved two gene therapies for SCD patients aged 12 and older who experience recurrent pain crises. Casgevy uses a gene-editing tool to modify a patient’s own stem cells so they produce a different form of hemoglobin that prevents sickling. In clinical trials, 29 out of 31 evaluable patients (93.5%) went at least 12 consecutive months without a severe pain crisis after treatment. Lyfgenia works by a different method, inserting a functional gene into stem cells so they produce a hemoglobin that behaves normally. In its trial, 88% of patients achieved complete resolution of pain crises in the months following infusion. Both therapies require a patient’s stem cells to be collected, modified in a lab, and infused back after chemotherapy to clear out the old marrow.
Sequential Compression Device
A sequential compression device is a medical sleeve, usually wrapped around the lower legs, that rhythmically inflates and deflates to prevent blood clots. If you’ve had surgery or been hospitalized, you may have worn these as the puffy, calf-hugging wraps that periodically squeeze your legs. They work by mimicking the natural pumping action of your calf muscles, which normally push blood back toward the heart when you walk. When you’re stuck in bed, that pump stops working, and blood pools in the deep veins of your legs, raising the risk of a deep vein thrombosis (DVT).
SCDs are standard equipment for surgical patients, trauma patients, and anyone spending extended time in an ICU. Research shows that using these devices cuts the risk of blood clots by more than half compared to no mechanical prevention at all. They’re especially valuable for patients who can’t safely take blood-thinning medications due to bleeding risk, though they’re also used alongside blood thinners for added protection.
There are a few situations where compression devices should not be used. Patients with severe peripheral artery disease (where blood flow to the legs is already critically low) should avoid them, as the squeezing can worsen an already compromised blood supply. They’re also not appropriate for people with severe heart failure or significant nerve damage from diabetes that has eliminated sensation in the legs, since the patient wouldn’t feel if the device were causing harm. Notably, current evidence suggests that compression is not necessarily off-limits in patients with an existing blood clot, contrary to older thinking, though careful medical judgment is needed.