There are many causes of shortness of breath. Increasing age and lack of physical activity cause some degree of breathlessness. Two main medical causes are heart and lung disorders. If the heart muscle is not working properly, pressure builds up within the heart chambers and in the lungs, causing the sensation of breathlessness. If the muscle damage is mild, breathlessness is only felt with significant exercise. Increasing muscle damage causes breathlessness even with mild activity and is more noticeable when lying down in bed, such that a patient may start to sleep with more pillows than before. The most common cause of breathlessness in the Western world is ischaemic heart disease i.e. furring up of the coronary arteries (atherosclerosis).

If an artery blocks and causes a heart attack, the heart muscle that was supplied by that artery will die, healing with time to leave a scar. The larger the scar, the greater the effect on the remaining healthy heart muscle and the more likely it is that a patient may feel breathless.

Other causes of breathlessness include high blood pressure (hypertension), malfunctioning heart valves (which may narrow down and/or leak) and various abnormalities of the heart muscle itself, which may be inherited e.g. hypertrophic cardiomyopathy or acquired e.g. following a viral infection of the heart or following a long period of alcohol excess.