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MECHANICS Of breathing/Tests of lung function Aims: How is breathing affected by changes in pressures, resistance, compliance and surface tension? How do these factors change in respiratory disease? What tests can be used to assess the function of the lungs? How do you distinguish between an obstructive and a restrictive respiratory disease ? 2 MECHANICS PRESSURES - pleural, alveolar RESISTANCE - airways COMPLIANCE - ability to expand SURFACE TENSION - surfactant 2 PRESSURES PLEURAL- Pressure between lungs and chest wall (-ve) =- 5cmH2O lung elastic recoil pressure lungs (visceral), chest wall (parietal) lung tissuel inwards whilst chest wall outwards I= more -ve; E= less -ve ALVEOLAR- pressure within alveoli I= PA -ve E= PA +ve THORAX= -ve pressure pump ARTIFICIAL VENTILATION- +ve pressure at mouth, CPAP,NPPV PNEUMOTHORAX-air entry into pleural space due to lung disease or chest wall injury; causes lung collapse, rib cage springs outward, diaphragm depressed 3 AIRWAYS RESISTANCE Raw= (mouth pressure-PA)/ airflow 40-50% of resistance in upper airways- nose, pharynx,larynx majority of the remainder (40%) in trachea and medium size bronchi(div. 1-8, 2-4mm), rest in size 1-2mm INCREASED- smooth muscle contraction, falls in lung volumes, secretions, obstructive disease, mediators, histamine, air pollution, PNS (Ach) DECREASED- smooth muscle relaxation, bronchodilators, nitric oxide, increase in lung volumes 4 COMPLIANCE Ability to distend lungs change of lung volume/change of pressure 0.2 l/cmH2O increased- age, emphysema, lung size, low volumes and low pressures decreased- oedema, atelectasis, fibrosis, at high lung volumes and pressures, high pulmonary venous pressures, 5 supine SURFACE TENSION Force- produced at a gas/liquid interface collapse of alveoli in lungs pulls fluid from capillaries into alveoli- stiff lungs smallest alveoli - largest surface tension LAPLACE’S LAW- pressure 1/radius SURFACTANT (dipalmitoyl lecithin)- complex lipoprotein surface tension produced by type II cells prevents collapse of alveoli, keeps alveoli dry normally secreted by 22 weeks in foetus lack- infant respiratory distress syndrome (RDS) RDS- atelectasis, fluid in lungs, decreased compliance ARDS - adult RDS 6 RESPIRATORY INVESTIGATIONS SYMPTOMS-chest breathlessness pain, cough, sounds, ANATOMICAL- sputum,CXR, blood, bronchoscopy, scans (PET, CT, MRI) PHYSIOLOGICAL- skin prick tests, blood gases, exercise tests, spirometry (LUNG FUNCTION TESTS) LUNG FUNCTION TESTS TESTS OF FORCED EXPIRATION FEV1, FVC, PEF, FEV1/FVC ratio Effort dependent Obstructive & Restrictive TLC, RV, FRC, Raw GAS TRANSFER- TCO, KCO FLOW-VOLUME LOOPS Vitalographs, spirometers Body plethysmographs COTES, J.E. (1993). LUNG FUNCTION. Blackwell Scientific Publications. QUANJER, Ph. H et al (1993). LUNG VOLUMES AND FORCED VENTILATORY FLOWS. EUR RESPIR. JOURNAL, SUPPL 16, 5-40. LUNG VOLUMES SPIROMETER- SPIROGRAM Tidal volume (VT) - 0.5l Inspiratory capacity(IC)-3.6l Inspiratory reserve volume (IRV)- 3.1l Expiratory reserve volume (ERV)-1.2l Functional residual capacity (FRC)- 2.4l vital capacity (VC) - 4.8l Residual volume (RV)- 1.2l Total lung capacity (TLC) - 6l VOLUMES Body size, Age, Sex, Muscular Training Ethnicity, Diseases FRC OF TLC, 10% less in females, height IMPORTANCE: 40% Minimum amount of air for gas exchange at all times Keeps alveoli & small air tubes open after expiration Provides stability of oxygen pressure (PO2) VARIATION: Obesity, pulmonary fibrosis, kyphoscoliosis, supine standing, decreased elastic recoil CONCLUSIONS 1. The work of breathing is increased by compliance and resistance & surface tension. 2. Changes in pleural and alveolar pressures are essential in creating pressure gradients and thus airflow, inflation and deflation of lungs. 3. Obstruction of airways(e.g mucus, bronchoconstriction), oedema, atelectasis and fibrosis resistance, surface tension and compliance of lungs. 4. Lung function assessed by anatomical (eg Bronchoscopy, CXR) and physiological tests (eg blood gases, expiratory tests). 5. Obstructive diseases-airways obstruction(mucus, smooth muscle enlargement, airway hypersensitivity), thus FEV1,PEF & FEV1/FVC. 6. Restrictive diseases-impairment of lung inflation( muscular, rib cage), thus VC,TLC, same or FEV1/FVC.
