Respiratory airflow pattern in patients with chronic airway obstruction

Previous experimental evidence has shown that in healthy humans inspiratory airflow waveform can be optimized according to minimum rate of work criteria when the respiratory energetic requirements become a substantial fraction of the general metabolism (i.e., during exercise hyperpnea and maximum voluntary ventilation). In patients with chronic airway obstruction (CAO) the relative energetic expenditure devoted to respiration is also greatly enhanced at rest. To investigate the performance of a system also controlling airflow wave pattern in this condition we evaluated by Fourier analysis the harmonic content of respiratory flow waves recorded at rest and during exercise hyperpnea (25 and 50 W on cycloergometer) in 15 patients. The results were compared with those we previously obtained in normal subjects and with some theoretical models. It was found that, while normal subjects display at rest an inspiratory flow waveform reasonably close to a sinusoidal model and adopt a more rectangular and economical flow shape during exercise hyperpnea, patients with CAO show a rather rectangular inspiratory flow shape also at rest, without any remarkable change at higher levels of ventilation. So, in general terms, the airflow pattern employed by patients at rest entails a reduction in the rate of dynamic inspiratory work of about 12% over that required by a sinusoidal waveform, and no further advantage is observed during exercise hyperpnea. Some features of the expiratory flow wave were also analysed. As no model of the respiratory system mechanics presently developed can explain the findings obtained in CAO patients purely on the basis of their altered mechanical parameters, it has been suggested that more complex control of respiratory airflow is operating in this class of patient.