1. To assess whether voluntary control of breathing is impaired in patients with chronic obstructive pulmonary disease, a group of such patients performed a tracking task, requiring volitional control of respiratory muscles.
2. Eight patients (mean age 60 years; mean ratio of forced expiratory volume in 1 s to forced vital capacity = 31%) took part in the study. Five of the seven patients in whom blood gas measurements were made were mildly hypoxaemic (Pao2 = 53–71 mmHg), and one of these was hypercapnic (Paco2 = 55 mmHg). Each subject performed a compensatory ventilatory tracking task using a tracking system which comprised a fixed target displayed on a monitor screen and a cursor moving in a line bisecting the target. The position of the cursor was perturbed by a forcing function and patients were required to keep the cursor on the target by breathing in and out of a spirometer.
3. To allow for any non-specific deficiency in motor control, patients performed a similar manual tracking task, using their dominant arm to move a joystick. As a control group, 11 healthy subjects (mean age 58 years; mean ratio of forced expiratory volume in 1 s to forced vital capacity = 77%) underwent an identical experimental protocol.
4. Motor control performances were measured in terms of the error between the target position and the subject's positioning of the cursor. Indices of performance were the root mean square of the error and the averages of the zero errors (i.e. end expiration/arm movement towards the trunk) and the peak errors (i.e. end inspiration/arm movement from the trunk).
5. For manual tracking, patients with chronic obstructive pulmonary disease, were not significantly different from control subjects in terms of mean root mean square (69.3 versus 60.9), mean zero error (93.8 versus 72.2) or mean peak error (99.5 versus 82.7). For ventilatory tracking, mean root mean square was significantly higher in patients (83.4 versus 65.9). This was due to a significantly higher mean zero error (101.4 versus 66.1); mean peak error was not significantly different (68.3 versus 82.1). Comparison of flows achieved during ventilatory tracking with those possible during a maximal forced expiratory manoeuvre in the patients suggested that poor expiratory control in this group could be explained by their degree of airflow limitation rather than a deficiency in motor control per se.
6. These results provide no evidence that patients with chronic obstructive pulmonary disease have any impairment in their voluntary motor control of breathing.
