Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 40

1 1.1 AThe diaphragm is the main muscle of inspiration; contraction forces the abdominal contents down, creating a relative vacuum in the thorax which ‘sucks’ air into the lungs.

2 1.2 ALung compliance is the change in lung volume brought about by a unit change in transpulmonary (intrapleural) pressure. The fibrotic lung is less compliant. The emphysematous lung is more compliant. In any lung, its capacity to stretch (expand) is reduced as volume increases ie it gets less compliant.

3 1.3 BAirway resistance in health resides principally in the central (large) airways at high lung volume (‘You only have one trachea’). As lung volume decreases, the site of greatest resistance moves peripherally to the smaller airways (their calibre diminishes). It is increased in emphysema and is proportional to r4. Increasing effort WILL lead to increased expiratory flow, but only up to a certain point, beyond which ‘peak flow’ cannot be increased no matter what the effort.

4 1.4 DGas exchange is driven by diffusion and therefore dependent on bringing the air and blood together (V/Q matching). Most of the ventilation goes to the bases, but an even greater proportion of the perfusion goes to the bases. Poor V/Q leads to a fall in PO2 but does not affect PCO2. Reduced overall ventilation causes a rise in PCO2 and a fall in PO2.

5 1.5 BThis is elevated, implying a problem with VQ matching within the lung.

6 1.6 ADuring inspiration, the diaphragm contracts and stiffens, pushing the abdominal contents down and reducing pressure in the thorax, which ‘sucks’ air in. Expiration is a relatively passive reversal of the process.

7 1.7 CReducing ventilation means less CO2 is ‘blown off’ (leading to a rise in PACO2 and PaCO2). If fresh air isn’t brought into the lungs then alveolar oxygen will not be replenished, PAO2 will fall and so must PaO2.

8 1.8 ESee Figure 1.11.

9 1.9 CThe sensitivity to changes in pH and pCO2 is so exquisite that adjustments are made before any measurable change can occur. Hypoxia does matter, but only has significant impact on the drive to breathe when pO2 falls significantly (approx. 8 kPa). A low pH can be caused by either reduced ventilation or a metabolic disturbance (in which case, it would lead to a rise in ventilation). Increased ventilation will increase PAO2 and therefore PaO2 though it won’t increase the oxygen content of the blood (much) as arterial blood is ordinarily close to fully saturated.

10 1.10 BAs lung volume is reduced, the small airways narrow and the site of principal resistance moves peripherally (i.e. to the smaller airways). Resistance is lowest (and therefore max forced flow rate is achieved) when the lungs are full. FEF25–75 provides information on the calibre of the small airways, but it can be a rather noisy signal.