Pulmonary Function Tests (PFT) Adult (70 kg male): 20-25% lower in women Tidal volume (Vt): 500 ml Inspiratory reserve volume (IRV): 3100 ml Expiratory reserve volume (ERV): 1200 ml Residual volume (RV): 1200 ml Vital capacity (VC): 4800 ml Inspiratory capacity (IC): 3600 ml Functional residual capacity (FRC): 2400 ml Total lung vapacity (TLC): 6000 ml FEV1: 84% FEV2: 94% FEV3: 97% To evaluate the patient's respiratory status, especially in patients experiencing shortness of breath or other breathing difficulty To evaluate the therapy for or progression of obstructive and restrictive lung disease As parameters for weaning patients from mechanical ventilation and as part of preoperative evaluations To assess airway function over time and to evaluate various therapeutic intervention Functional residual capacity: Chronic Obstructive Pulmonary Disease Forced expiratory volume: Chronic Obstructive Pulmonary Disease Tidal volume: Atelectasis, fatigue, Pneumothorax, pulmonary congestion, restrictive lung disease, tumors Inspiratory reserve volume: Asthma, exercise, obstructive pulmonary disease Expiratory reserve volume: Ascites, kyphosis, obesity, pleural effusion, Pneumothorax, pregnancy, Scoliosis Residual volume: Elderly individuals, obstructive pulmonary disease, Functional residual capacity: Adult respiratory distress syndrome Forced expiratory volume: Restrictive pulmonary disease Inspiratory capacity: Restrictive pulmonary disease Vital capacity: Diaphragm restriction, drug overdose with hypoventilation, neuromuscular diseases, restrictive thoracic movement. Fatigue Lack of patient cooperation Smoking Abdominal distension or pregnancy Poor seal around mouthpiece Medications