Despite being relatively common, many people are unaware of the symptoms of heart failure. One surprising symptom, caused by fluid getting backed up in the lungs, shows up when a person with the condition lays on their back. If you notice this, experts say it’s time to get screened for heart failure and heart disease. Read on to find out which symptom to look out for, and how to determine the severity of your condition. RELATED: If You Notice This When You Cough, It May Be a Sign of Heart Failure. Those with heart failure frequently report a symptom known as orthopnea, characterized as shortness of breath while lying down. Some people also experience shortness of breath during physical activity as a result of heart failure.ae0fcc31ae342fd3a1346ebb1f342fcb “The reason you may have orthopnea is that when you lie flat, the blood that ordinarily pools in the veins of your legs is reentering your bloodstream,” says University of Michigan Health. “If you have heart failure, your heart may not be able to keep up with the increased amount of blood returning to the heart, so fluid builds up inside the lungs and causes shortness of breath,” their site explains. This can have serious implications. A 2002 study in the European Heart Journal Supplements found that roughly 90 percent of heart failure patients eventually die from cardiovascular causes. “Fifty percent die from progressive heart failure, and the remainder die suddenly from arrhythmias and ischemic events,” the study authors wrote. If you suspect heart failure, it’s important to take proactive steps to improve your heart health immediately.  RELATED: Eating This Makes You 46 Percent More Likely to Die From Heart Disease. Michigan experts note that for most patients with orthopnea, the severity of the problem will vary depending on your sleep position. Discomfort and breathlessness typically increase as you lay flatter on your bed. This may actually help you to figure out how serious your condition is. “To gauge the severity of this symptom, doctors often ask people how many pillows they need to lie on to avoid feeling short of breath in bed. For example, “three-pillow” orthopnea is worse than “two-pillow” orthopnea, because you have less tolerance for lying flat,” they say. If you notice that you can’t seem to catch your breath without a steep incline, it’s definitely time to call the doctor for a heart screening. If your shortness of breath wakes you up in the night, you may have a related condition called paroxysmal nocturnal dyspnea (PND). “Some people with heart failure wake up in the middle of the night with severe shortness of breath,” explain the University of Michigan experts. “This may occur with waking up coughing and/or wheezing, having a rapid heart rate, and a feeling of being suffocated,” they add. They note that typically, those who experience PND don’t feel increased shortness of breath when they first lie down. However, after lying down for several hours, their symptoms may appear suddenly. “People who experience PND often have to sit up on the side of the bed, and they may feel a need to open a window to get more air. The shortness of breath generally goes away after a few minutes of sitting up,” they explain. While this condition is distinct from similarly presenting sleep apnea, experts say that both conditions are linked to heart failure and merit heart health screening. In fact, sleep apnea is known to increase the risk of heart failure by 140 percent, and the risk of coronary heart disease by 30 percent, according to the Sleep Foundation. For more health news sent directly to your inbox, sign up for our daily newsletter. There are several other symptoms of heart failure that should be on your radar, according to the Mayo Clinic. Those symptoms may include fatigue, weakness, swelling in the lower extremities, persistent cough or wheezing, rapid weight gain, nausea, difficulty concentrating, and swelling in the abdomen. Those who experience heart failure as a result of heart attack tend to also experience chest pain. Heart failure can be chronic or acute, meaning the symptoms may linger over time or begin abruptly, they explain. If you notice shortness of breath while lying down, or begin displaying other signs of heart failure, talk to your doctor immediately. RELATED: Having This Many Drinks a Week Slashes Your Heart Attack Risk, New Study Finds.