Why do patients with AFib feel breathless?

In atrial fibrillation, the atrial tissue quivers at approximately 600 cycles per minute, which means that it has no useful pumping function. The left atrium in patients with atrial fibrillation, merely acts as a passive conduit between the lungs and the left ventricle. It no longer serves to prime the left ventricle, and as a result, there may be less blood entering the heart during diastole and thus less 'priming' occurs, so cardiac output may fall. If ever you have experienced a car's turbo charger going 'on the blink' you will know that the car still GOES, but not very well, since it too suffers loss of output.  The left atrial pressure in many such cases will increase, causing oedema or fluid in the alveolar space, reducing gas exchange and contributing to the symptom of breathlessness. Any stiffening of the left ventricle that for example accompanies old age, poor glycaemic control or indeed any condition that causes thickening of the left ventricle, such as hypertension or cardiomyopathy, therefore has a big impact on the ability of the left ventricle to relax and suck blood through from the left atrium and joining pulmonary veins, creating  backup of blood into the lung tissue. For more on this topic and other related blog articles and podcasts see https://www.scvc.co.uk/naked-heart/why-do-patients-in-atrial-fibrillation-get-breathless/