Morbid obesity: PFTs

The overall respiratory problem is one of restrictive lung disease. Chest wall and lung compliance are decreased from the heavy layer of fat. Subsequent decreased pulmonary compliance leads to decreased FRC (primarily a result of lowered ERV), decreased VC, and decreased TLC. The reduced FRC can result in lung volumes below the closing capacity, which is the volume at which small airways begin to close. This can cause ventilation-perfusion mismatch, right-to-left shunting and arterial hypoxia. Dead space is unchanged. The RV and closing capacity are essentially unchanged. The FEV1 and FVC are usually decreased, but FEV1/FVC ratio is usually normal.

Obesity is a condition of excessive body fat that can lead to adverse health effects. The respiratory system is largely affected by obesity. Theoverall respiratory problem is one of restrictive lung disease. Chest wall and lung compliance is decreased from the accumulation of fat on the thorax and abdomen. Decreased pulmonary compliance leads to decreased FRC (primarily a result of lowered ERV), decreased VC, and decreased TLC. Thereduced FRCcan result in lung volumes below the closing capacity, which is the volume at which small airways begin to close. This can cause ventilation-perfusion mismatch, right-to-left shunting and arterial hypoxia. Dead space is unchanged. The RV and closing capacity are essentially unchanged. The FEV1 and FVC are usually decreased.

Under anesthesia, the FRC of the obese patient decreases about 50%as compared to 20% reduction for the nonobese patient. While supine, increased elastic resistance and decreased compliance of the chest wall further reduces total respiratory compliance. This is demonstrated by shallow rapid breathing, increased work of breathing and limited maximum ventilatory capacity.

Because obesity increases metabolic needs, oxygen consumption and carbon dioxide production increases. This leads to increased alveolar ventilation and cardiac output. Normocapnia is maintained by an increase in minute ventilation. Arterial oxygen tension is lower and chronic hypoxemia may lead to pulmonary hypertension and cor pulmonale.