Silent heart attacks, which have no symptoms but still involve a loss of blood flow to the heart, make up 45% of all heart attacks and triple the chances of dying from heart disease, according to a study published Monday.

The study, published in the American Heart Association’s publication Circulation, found that silent heart attacks make up nearly half of all heart attacks and increase the chance of death by all causes by 34%. The study, which examined almost 1,000 patients, also found that silent heart attacks are more common in men but more deadly in women.

Silent heart attacks are often undetected by the patient, and may not be noticed until patients have an electrocardiogram to measure their heart’s electrical activity.

“The outcome of a silent heart attack is as bad as a heart attack that is recognized while it is happening,” said lead author Elsayed Z. Soliman in a statement accompanying the release of the study. “And because patients don’t know they have had a silent heart attack, they may not receive the treatment they need to prevent another one.”

This article originally appeared on Time.com.