Does dead space ventilation always alleviate hypocapnia? Long-term ventilation with plain tracheostomy tubes.

Long-term tracheostomy-ventilated patients have better speech with a cuffless tracheostomy tube and a large tidal volume. Moderate day time hyperventilation from a pressure-limited ventilator is necessary in these patients to avoid hypoxia during sleep due to the variable insufflation leak. This study sought to confirm whether a dead space of 3 ml.kg-1 could help to provide normocapnic hyperventilation during waking time without causing hypercapnia and hypoxaemia during sleep. Transcutaneous blood gas studies were performed on 11 patients with high tetraplegia undergoing pressure-limited pulmonary ventilation with room air. Recordings were made for 120 min each when awake and asleep, with and without dead space. The mean derived arterial PCO2 without the dead space was 2.95 kPa awake and 3.21 kPa asleep, whilst the corresponding tensions with dead space were 3.39 kPa and 3.79 kPa. These small increases associated with the dead space, both awake and asleep, were statistically significant. There was a statistically, though not clinically significant decrease in oxygen tension when the patients without dead space went to sleep. The fact that the carbon dioxide tension was higher during sleep when dead space was in situ indicates that, despite the insufflation leak in these patients, there is significant rebreathing back through the dead space. Amelioration of hypocapnia during waking and sleeping is achievable using a dead space extension in these patients.