Lung function in newborn infants with tachypnea of unknown cause.

Twenty-one infants with tachypnea (f greater than 60/min) lasting more than 2 h and diagnosed as mild respiratory disease or pulmonary maladaptation according to previously presented criteria were studied during the course of the disease and after clinical recovery. Lung physiology (total and alveolar ventilation, efficiency and distribution of ventilation, functional residual capacity, and lung mechanics) was studied in combination with clinical data. The pathophysiological findings were characterized by increased total ventilation but normal alveolar ventilation, reduced efficiency of ventilation but more even distribution of ventilation (nitrogen elimination pattern) during disease than after clinical recovery, hyperinflation, reduced dynamic lung compliance but unaffected specific lung conductance. Infants with low gestational ages were most severely affected and had longer duration of disease than full-term infants. Our findings suggest that this condition is caused by small airway disease. Disturbances in normal pulmonary adaptation with abnormal retention of lung fluid is the most probable cause.