In this article, we shall look at the physiology of ventilation – the process of inspiration and expiration, how this differs between quiet and forced breathing, and their clinical correlations. Inspiration occurs via active contraction of muscles – such as the diaphragm – whereas expiration tends to be passive, unless it is forced. The processes of inspiration (breathing in) and expiration (breathing out) are vital for providing oxygen to tissues and removing carbon dioxide from the body. In a bilateral paralysis, the patient may require ventilatory support such as non-invasive positive ventilation, or, in more severe cases, intubation and invasive ventilation. In a unilateral diaphragmatic paralysis, patients usually do not require ventilatory support, unless they already have significant lung disease or are symptomatic. Firstly, the underlying cause must be identified and treated (if possible). Management of diaphragmatic paralysis is two-fold. Lung function tests will show a restrictive deficit. If both sides are paralysed (known as bilateral diaphragmatic paralysis), the patient may experience poor exercise tolerance, orthopnoea and fatigue. The affected side of the diaphragm moves upwards during inspiration, and downwards during expiration. A unilateral diaphragmatic paralysis is usually asymptomatic and is most often an incidental finding on x-ray. Paralysis of the diaphragm produces a paradoxical movement. Neuromuscular disease – such as Multiple Sclerosis or Motor Neurone Disease.Guillian-Barre syndrome – auto-immune induced muscle weakness, often triggered by infection.Compression – due to a tumour within the chest cavity.Mechanical trauma – ligation or damage to the nerve during surgery.If the nerve becomes damaged, paralysis of the diaphragm can result. The phrenic nerve provides motor innervation to the diaphragm. Innermost intercostal – depresses the ribs.Ĭlinical Relevance: Diaphragmatic Paralysis.Internal intercostal – depresses the ribs.Anterolateral abdominal wall – increases the intra-abdominal pressure, pushing the diaphragm further upwards into the thoracic cavity.These muscles act to decrease the volume of the thoracic cavity: Latissimus dorsi – elevates the lower ribs.Īctive expiration utilises the contraction of several thoracic and abdominal muscles.

Serratus anterior – elevates the ribs (when the scapulae are fixed).Pectoralis major and minor – pulls ribs outwards.Sternocleidomastoid – elevates the sternum.All of these muscles act to increase the volume of the thoracic cavity: Active InspirationĪctive inspiration involves the contraction of the accessory muscles of breathing (in addition to those of quiet inspiration, the diaphragm and external intercostals). When the volume of the thoracic cavity decreases – the volume of the lungs decreases and the pressure within the lungs increases.įorced breathing is an active mode of breathing which utilises additional muscles to rapidly expand and contract the thoracic cavity volume.When the volume of the thoracic cavity increases – the volume of the lungs increases and the pressure within the lungs decreases.As the thoracic cavity and lungs move together, this changes the v olume of the lungs, in turn changing the pressure inside the lungs.īoyle’s law states that the volume of gas is inversely proportional to pressure (when temperature is constant). This seal ensures that when the thoracic cavity expands or reduces, the lungs undergo expansion or reduction in size accordingly.ĭuring breathing, the contraction and relaxation of muscles acts to change the volume of the thoracic cavity. This is usually filled with pleural fluid, forming a seal which holds the lungs against the thoracic wall by the force of surface tension. The space between the outer surface of the lungs and inner thoracic wall is known as the pleural space.

Muscles of inspiration