The conventional hypoxic challenge test might not predict 'fitness-to-fly' equally well in all patients. Patients at risk of hypercapnia with supplemental oxygen are not well studied. Th investigators will study children who have either neuromuscular weakness or central hypoventilation and who require nocturnal or intermittent daytime ventilator support.

The study also aims to assess feasibility of a randomised controlled trial (RCT) comparing the standard hypoxic standard test to a modified test designed to better suit children with type two respiratory failure.

This study will assess feasibility of pulse oximetry (SpO2), end tidal carbon dioxide (ETCO2) and transcutaneous carbon dioxide partial pressure (TcCO2) measurement during a modified Hypoxic challenge test (HCT) in 12 children with neuromuscular weakness and 12 with central hypoventilation.

Inclusion Criteria:

• ≤18 years
• Neuromuscular weakness or central hypoventilation
• Established nocturnal or intermittent daytime ventilatory support

Exclusion Criteria:

• Unable to sit in the plethysmograph
• Cardiac anomaly with right-to-left shunt