Impairment of pulmonary functions

Pulmonary function is a conglomerate of ventilation, diffusion, and exchange of oxygen and carbon-dioxide. Plethysmography, spirometry, and blood gas analysis are useful tools in the evaluation of pulmonary function. Pulmonary function tests namely FEV₁, FVC, FEV₁/FVC ratio and TLC interpret impairment of lung function. In general, forced expiratory volume in one second (FEV₁) grades the impairment of pulmonary function. Clinical symptoms, post-bronchodilator FEV₁, reversible airflow obstruction, medication requirement, and peak expiratory flow rate for asthma control define impairment in asthma. The ratio of post-bronchodilator forced expiratory volume in one second to forced vital capacity less than 0.70 and/or below the 5th percentile of predicted value and decreased FEV₁ serve as criteria to define impairment in COPD. A decrease in TLC below the 5th percentile, a decrease in FEV₁ percentage predicted, normal FEV₁/VC ratio, and degree of reduction of D_Lco percentage predicted determine impairment in restrictive lung diseases. PaCO₂ more than 50 mm of Hg, pulmonary hypertension, respiratory failure, and right heart failure serve as parameters to assign impairment in hypercapnic respiratory failure. The level of spinal cord injury, vital capacity, cough reflex and the need for assisted ventilation determines impairment in respiratory muscle paralysis. Apnea, hypopnea, respiratory disturbance index (RDI), cardiac arrhythmia, right heart failure and cor pulmonale serve as factors to quantify impairment in apnea/hypopnea syndrome. Impulse oscillometry (IOS) is a noninvasive method to measure airway impedance comprising of resistance and reactance. IOS eliminates the constraints such as forceful manoeuvre employed in spirometry. Hence it is preferable to use IOS in children and persons who cannot perform forceful manoeuvre to assess airway hyperreactivity and obstruction.