Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 85

Weakness of the respiratory muscles causes a restrictive ventilatory defect, with reduced TLC and VC. Comparison of VC in the erect and supine positions is useful, because the pressure of the abdominal contents on a weak diaphragm typically causes a fall of around 30% in supine VC. Chest X‐ray often shows small lung volumes with basal atelectasis and high hemi‐diaphragms. Ultrasound screening may show paradoxical upward movement of a paralysed diaphragm during inspiration. Global respiratory muscle function may be assessed by measuring mouth pressures. Maximum inspiratory mouth pressure, P _I max, is measured during maximum inspiratory effort from residual volume against an obstructed airway using a mouthpiece and transducer device, and maximum expiratory mouth pressure, P _E max, is measured during a maximal expiratory effort from TLC. When there is severe respiratory muscle weakness, ventilatory failure develops with hypercapnia.