Pulmonary function: the basics

Abstract 

The basics of pulmonary function testing in hospital laboratories are spirometry (FEV₁, FVC, FEV₁/FVC) and the carbon monoxide transfer factor (TLCO) and its components (KCO [∼TLCO/VA] and VA). A low FEV₁/FVC ratio indicates intrapulmonary airflow obstruction. Distinction between intrapulmonary and extrapulmonary (upper airway) obstruction is given by maximal inspiratory and expiratory flow–volume curves. Other tests of lung mechanics are measurements of absolute lung volumes (TLC, FRC, RV) and maximal inspiratory and expiratory pressures against an obstruction at the mouth. Spirometry, lung volumes and mouth pressures can differentiate obstructive (low FEV₁/FVC) and restrictive (normal FEV₁/FVC, low TLC) patterns, detect hyperinflation (high TLC and FRC) and respiratory muscle weakness (reduced mouth pressures). The TLCO and KCO measure the integrity of the blood–gas barrier, being reduced in emphysema, interstitial lung diseases and pulmonary vascular pathology. The KCO (as % predicted) is high in extrapulmonary restriction (pleural, chest wall and respiratory neuromuscular disease), and in ‘loss of lung units’ provided the structure of the lung remaining is normal. Thus, the KCO distinguishes extrapulmonary (high KCO) causes of ‘restriction’ from intrapulmonary causes (low KCO). Significant hypoxaemia (low PaO₂) normally provokes compensatory hyperventilation and lowering of the PaCO₂. Surrogates for arterial blood gases, avoiding arterial puncture, include pulse oximetry, ‘arterialized’ capillary samples and transcutaneous PO₂ and PCO₂ probes.

Keywords: arterial blood gases, flow–volume curves, KCO, lung function algorithm, lung volumes, mouth pressures: pulse oximetry, spirometry