Assessment and monitoring of flow limitation and other parameters from flow/volume loops

Flow/volume (F/V) spirometry is routinely used for assessing the type and s everity of lung disease. Forced vital capacity (FVC) and timed vital capaci ty (FEV1) provide the best estimates of airflow obstruction in patients wit h asthma, chronic obstructive pulmonary disease (COPD) and emphysema. Compu terized spirometers are now available for early home recognition of asthma exacerbation in high risk patients with severe persistent disease, and for recognition of either infection or rejection in lung transplant patients. Patients with severe COPD may exhibit expiratory flow limitation (EFL) on t idal volume (VT) expiratory F/V (VTF/V) curves, either with or without appl ying negative expiratory pressure (NEP). EFL results in dynamic hyperinflat ion and persistently raised alveolar pressure or intrinsic PEEP (PEEPi). Hy perinflation and raised PEEPi greatly enhance dyspnea with exertion through the added work of the threshold load needed to overcome raised pleural pre ssure. Esophageal (pleural) pressure monitoring may be added to VTF/V loops for assessing the severity of PEEPi: 1) to optimize assisted ventilation b y mask or via endotracheal tube with high inspiratory flow rates to lower I : E ratio, and 2) to assess the efficacy of either pressure support ventil ation (PSV) or low level extrinsic PEEP in reducing the threshold load of P EEPi. Intraoperative tidal volume F/V loops can also be used to document th e efficacy of emphysema lung volume reduction surgery (LVRS) via disappeara nce of EFL. Finally, the mechanism of ventilatory constraint can be identified with the use of exercise tidal volume F/V loops referenced to maximum F/V loops and static lung volumes. Patients with severe COPD show inspiratory F/V loops approaching 95% of total lung capacity, and flow limitation over the entire expiratory F/V curve during light levels of exercise. Surprisingly, patien ts with a history of congestive heart failure may lower lung volume towards residual volume during exercise, thereby reducing airway diameter and indu cing expiratory flow limitation.