Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 17

Inspiration requires muscular work. The diaphragm is the principal muscle of inspiration. At the end of an expiration, the diaphragm sits in a high, domed position in the thorax (Fig. 1.4). To inspire, the strong muscular sheet contracts, stiffens and tends to push the abdominal contents down. There is variable resistance to this downward pressure by the abdomen, which means that in order to accommodate the new shape of the diaphragm, the lower ribs (to which it is attached) also move upwards and outwards. (When airway resistance is present, as in asthma or chronic obstructive pulmonary disease [COPD], the situation is very different; see Chapters 2 & 11.) The degree of resistance the abdomen presents can be voluntarily increased by contracting the abdominal muscles; inspiration then leads to a visible expansion of the thorax, rather than a distension of the abdomen (try it). The resistance may also be increased by abdominal obesity. In such circumstances, there is an involuntary limitation to the downward excursion of the diaphragm and, as the potential for upward movement of the ribs is limited, the capacity for full inspiration is diminished. This inability to fully inflate the lungs is an example of a restrictive ventilatory defect (see Chapter 3).

Figure 1.4 Effect of diaphragmatic contraction. Diagram of the ribcage, abdominal cavity and diaphragm showing the position at the end of resting expiration (a). As the diaphragm contracts, it pushes the abdominal contents down (the abdominal wall moves outwards) and reduces pressure within the thorax, which ‘sucks’ air in through the mouth (inspiration). (b) As the diaphragm shortens and descends, it also stiffens. The diaphragm meets a variable degree of resistance to downward discursion, which forces the lower ribs to move up and outward to accommodate its new position.

Other muscles are also involved in inspiration. The scalene muscles elevate the upper ribs and sternum. These were once considered, along with the sternocleidomastoids, to be ‘accessory muscles of respiration’, only brought into play during the exaggerated ventilatory effort of acute respiratory distress. Electromyographic studies, however, have demonstrated that these muscles are active even in quiet breathing, although less obviously so.

The intercostal muscles bind the ribs to ensure the integrity of the chest wall. They therefore transfer the effects of actions on the upper or lower ribs to the whole ribcage. They also brace the chest wall, resisting the bulging or in‐drawing effect of changes in pleural pressure during breathing. This bracing effect can be overcome to some extent by the exaggerated pressure changes seen during periods of more extreme respiratory effort, and in slim individuals intercostal recession may be observed as a sign of respiratory distress.

Whilst inspiration is the result of active muscular effort, quiet expiration is a more passive process. The inspiratory muscles steadily release their contraction and the elastic recoil of the lungs brings the tidal breathing cycle back to its start point. Forced expiration, however – either volitional or as in coughing – requires muscular effort. The abdominal musculature is the principal agent in this.