Control of breathing in man; insights from the 'locked-in' syndrome

Control of breathing was studied in a patient with a lesion in the ventral pons; no volitional behaviour, including voluntary breathing acts, was possible (locked-in syndrome, LIS). Spontaneous breathing via a tracheostomy maintained a normal PET(CO2) of 39-40 mmHg. Variability of ventilatory parameters awake was similar to that seen in five tracheostomized control subjects during stage IV sleep but much smaller than during resting wakefulness. Emotion associated with laughter caused disturbances of breathing. The ventilatory response to CO2 was normal and was associated with 'hunger for air' when the PET(CO2) was 49-50 mmHg. Mechanical ventilation to reduce PET(CO2) by as little as 1 mmHg resulted in apnoea when the ventilator was disconnected; breathing resumed when PET(CO2) crossed the threshold of 39-40 mmHg. These results demonstrate the functional dependence of the human medullary respiratory oscillator on a threshold level of P(CO2) in the absence of cortico-bulbar input, even during wakefulness. The absence of such input may explain the regularity of breathing.