Exertional Breathlessness: When It Is Poor Fitness and When It Is Disease

During exercise the muscles need many times more oxygen than at rest. To deliver it, four links must work together: the lungs (ventilation and passage of oxygen into the blood), the heart (pumping blood), the blood (haemoglobin and iron, which carry oxygen) and the muscles themselves (their ability to take up and use oxygen). A weak link anywhere makes the body reach the limit of its capacity sooner, and the brain registers this as breathlessness. For that reason the same complaint, 'I run out of breath', can have very different causes.

This explains why the work-up rarely ends with one test. Spirometry will assess how open the airways are but says nothing about the heart. An echo of the heart will show its structure and function at rest but does not always detect a problem that appears only on exertion. A blood count will reveal anaemia but not poor fitness. That is why doctors start with the most common and simplest-to-detect causes and only then turn to exercise tests, which let them see all the links at once.

'Is it just poor fitness?' is what most people with breathlessness ask themselves. A good answer comes from a French study that looked for causes of unexplained exertional breathlessness in people whose NT-proBNP was not raised and whose breathing tests were normal, that is, exactly where simple tests usually end and the impression remains that 'nothing came up'.

Two conclusions follow from these numbers. First, deconditioning is the most common explanation of unexplained breathlessness when heart and lung tests at rest are normal (47%), which is good news because it can be improved with training. Second, roughly one in six people had early heart failure detected only in exercise tests, and a normal NT-proBNP did not exclude the diagnosis. The limitations must be remembered: this is a small, selected group (mainly women over 60), so the percentages do not transfer to young, slim men who get winded after running.

NT-proBNP is a hormone released by the heart when its walls are stretched. It is one of the basic blood tests when heart failure is suspected, and doctors use it mainly to exclude disease: a low result makes acute heart failure unlikely. We describe this test in our separate entry on troponin and NT-proBNP. It is worth knowing, however, how well the test works and where it stops being enough.

What does this mean for a person who has been short of breath for several months? First, these data concern acute breathlessness in an emergency department, not gradually worsening, chronic breathlessness on exertion, so they do not transfer directly to the family doctor's office. Second, a low NT-proBNP very reliably excludes acute heart failure, but its specificity is low: a raised result does not prove heart failure, because it also rises with age, kidney disease or atrial fibrillation. Third, as we saw above, a normal NT-proBNP does not exclude early heart failure with preserved ejection fraction, which becomes apparent only on exertion. That is not a flaw in the test, only its limitation, worth bearing in mind.

Two factors often stay in the shadows when breathlessness is investigated: excess body weight and iron deficiency. Obesity increases the work of breathing and loads the heart, but it also makes diagnosis harder: according to an expert review, obesity lowers natriuretic peptide levels and reduces the sensitivity of the resting echocardiogram, so heart failure is diagnosed with delay.

The second often overlooked factor is iron. Iron-deficiency anaemia (low haemoglobin) reduces the blood's capacity to carry oxygen, and this shows quickly on exertion. More interesting is that iron deficiency can impair performance even without anaemia, which we describe in our entries on ferritin and on iron deficiency without anaemia. An observational study from a Belgian dyspnoea clinic showed that the problem is not theoretical.

A practical conclusion: for breathlessness it is worth ordering not only a blood count (haemoglobin) but also ferritin and transferrin saturation. A low result explains part of the cases and is a cause that can usually be treated, but remember that the reason for the iron deficiency (for example blood loss, diet, absorption) also needs to be established. The data from the study above are observational and concern selected patients of a specialist clinic, so they must not be read as encouragement to supplement iron on your own.

The standard diagnostic path begins with a careful history: since when the breathlessness has lasted, on what effort, whether it comes with wheezing, cough, leg swelling, palpitations or chest pain, whether the person smokes, which medicines they take, whether they are losing weight or feeling tired. On that basis the doctor decides where to start. The table below organises the tests that usually appear, and what each can and cannot show.

When these tests give no answer and the breathlessness still limits life, the doctor may refer for an exercise test with gas exchange measurement (CPET). The test, on a treadmill or cycle ergometer, shows which link of the oxygen chain fails first: heart, lungs or muscles. For many people it helps to understand that the weakest link is simply lack of training, and that news has therapeutic value. In the study described earlier even CPET did not identify a cause in 28.8% of patients, so the absence of a diagnosis after further tests does not mean the symptoms are imagined.

That breathlessness can be the first sign of cancer is rarely mentioned, and it can be a source of anxiety. It is therefore worth looking at the data: a Danish register study covered a huge group of patients who had a hospital contact for breathlessness and compared the frequency of cancer with the general population.

How to read this? First, the relative risk rose but the absolute risk stayed small: at one year cancer was diagnosed in 3.8% of people, so the vast majority of patients with breathlessness had no cancer. Second, the study concerns a hospital contact (so more marked breathlessness), not the typical puffing of someone who has simply put on weight. Third, the authors conclude that cancer should also be considered in the work-up of breathlessness, not that every case of breathlessness requires a search for cancer. For the reader the message is: persistent, worsening breathlessness, especially in a smoker or accompanied by weight loss, coughing blood or chest pain, is not something to 'wait out'.

Equally wrong is the opposite assumption that 'a young, slim person cannot have heart or lung disease'. That is why doctors do not dismiss complaints of breathlessness even when results are normal, but widen the work-up step by step, from the simplest tests towards exercise tests.

Exertional breathlessness is best treated as a diagnostic question, not a verdict. The data show that most often deconditioning stands behind it, but it is neither the only nor always the simplest cause: in a study of people with unexplained breathlessness, even with a normal NT-proBNP, early heart failure lay behind it in about one in six, iron deficiency was common in a sizeable share, and in some people no cause was found despite wide testing. A sensible order is therefore: history and notes, basic tests, and if there is no answer, an exercise assessment, with vigilance for red flags in the background. For people without serious disease the best news is that fitness can be improved by gradual, regular training, at first in agreement with the doctor.