At rest, breathing in (inspiration) is an active process in which the diaphragm muscle contracts, increasing the volume of the thoracic cavity and drawing air in. Breathing out (expiration) is passive in normal resting conditions; on exertion, the internal intercostal muscles and abdominal wall muscles are used to push the diaphragm upwards, reducing the volume of the thoracic cavity and pushing air out. If the chest wall mechanics are altered due to the effects of cancer or other co‐morbidities the threshold for breathlessness will be lower. This is further explained in Chapter c07: Moving and positioning.

Difficulty breathing may cause strong emotional responses, releasing hormones that stimulate the neural respiratory drive centrally. Panic, anxiety, gloomy thoughts and worry may increase overall muscular tension, which reduces the efficiency of breathing and increases the metabolic rate and energy requirements. The worry and anxiety of carers may contribute to this and inadvertently add to the drive to breathe (Bausewein et al. [13]).

Breathlessness may also be a result of poor cardiovascular fitness that may develop as people become less active following a cancer diagnosis and treatment (Parshall et al. [218]). This deconditioning may cause a reduced ability to participate in previously undertaken activities. (Deconditioning is addressed in Section c27-sec-0267, Physical activity for people with cancer.) Poor nutritional status may contribute to reduced muscle strength and output (Section c27-sec-0131, Nutritional status). Poor sleep and fatigue may also contribute to breathlessness. Poor pacing or rushing activity may also contribute to reduced functioning (Section c27-sec-0291, Cancer‐related fatigue and sleep). The loss of functioning experienced by those with breathlessness contributes to the ‘thinking’ aspects and can compound breathlessness.