Ejection fraction (EF) is the percentage of blood ejected from the left ventricle with each heartbeat. A normal ejection fraction is 55% or higher. An EF of 40 to 54% indicates mildly reduced function. An EF below 40% meets the definition of heart failure with reduced ejection fraction (HFrEF) and signals significantly impaired pumping.
Ejection fraction is most commonly measured by echocardiogram, though cardiac MRI provides the most precise assessment. After cardiac arrest, EF is a critical measurement: an EF of 35% or below is a primary criterion for ICD implantation as primary prevention in ischaemic heart disease; myocardial stunning (temporary post-arrest dysfunction) can cause a transiently low EF that recovers over weeks, so measurement timing matters; EF guides medication decisions including beta-blockers, ACE inhibitors, and aldosterone antagonists; and serial measurements track whether the heart is recovering or deteriorating over time.
Ejection fraction is typically reassessed 6 to 12 weeks after cardiac arrest or myocardial infarction, once the acute effects of stunning have resolved. An EF measured in the first few days may significantly underestimate the true baseline function. This is why ICD implantation decisions based on EF are usually deferred until after a recovery period, with a repeat echocardiogram at that point.
It is important to understand that EF is one measure of cardiac function, not the whole picture. Some people with a normal EF still have heart failure (HFpEF) due to impaired filling rather than impaired contraction. Symptoms, exercise capacity, and quality of life are equally important, and a cardiologist will explain what a specific EF measurement means for an individual’s management and risk profile.
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