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Maximum Breathing Capacity

Dynamic lung function is routinely evaluated in many pulmonary function laboratories by measuring the maximum breathing capacity or, more specifically, the maximum voluntary ventilation (MVV). This is the largest volume that can be breathed per minute by voluntary effort, and it is an estimate of the peak ventilation available to meet physiologic demands. The patient is instructed to breathe as hard and fast as possible for 12 seconds. The measured volume is extrapolated to 1 minute and is expressed in units of liters per minute. Because high rates of airflow are required for the MVV, the measurement is significantly affected by changes in airway resistance. MVV is usually reduced in patients with obstructive airway disease and correlates reasonably well with FEV₁ measured in liters (MVV FEV₁ × 35). Discrepancies between the measured MVV and that predicted by FEV₁ often indicate inconsistent or submaximal inspiratory effort.^[7] The MVV as a comprehensive test of ventilatory function is also altered by factors other than airway obstruction, including the elastic properties of the lung and chest wall, respiratory muscle strength, learning, coordination, and motivation. In healthy male adults, MVV averages 150 to 175 L/min. This extremely high level of ventilatory effort cannot be maintained for much longer than 1 minute. However, approximately 80% of the MVV can be sustained by healthy subjects for as long as 15 minutes and up to 60% for even longer periods. Abnormally low values (<80% of predicted) do not identify specific defects but do indicate gross impairment in respiratory function. The unique value of the test for the surgical candidate may lie in its dependence on intangible variables, such as cooperation, motivation, and stamina.