Usefulness of transcutaneous PCO2 to assess nocturnal hypoventilation in restrictive lung disorders

Nocturnal hypoventilation is now an accepted indication for the initiation of non-invasive ventilation. Nocturnal hypoventilation may be an under diagnosed condition in chronic respiratory failure. The most appropriate strategy to identify sleep hypoventilation is not yet clearly defined. In clinical practice, it is indirectly assessed using nocturnal pulse oximetry (NPO) and morning arterial blood gases (mABG). Even though continuous transcutaneous carbon dioxide partial pressure (TcPCO2 ) monitoring is theoretically superior to NPO plus mABG, it is not routinely used. We aimed to prospectively compare NPO plus mABG with nocturnal TcPCO2 for the detection of alveolar hypoventilation in a cohort of patients with chronic restrictive respiratory dysfunction.

We assessed 80 recordings of mABG, nocturnal TcPCO2 and NPO in 72 consecutive patients with neuromuscular disease or thoracic cage disorders. Nocturnal hypoventilation was defined as a mean nightime TcPCO2 ≥50 mm Hg, and nocturnal hypoxaemia as ≥30% of the night with transcutaneous pulse oxygen saturation ≤90% and/or >5 consecutive minutes with transcutaneous pulse oxygen saturation ≤88%.

Amongst the 80 recordings, 25 of 76 (32.9%) without nocturnal hypoxaemia and 16 of 59 (27.1%) without hypercapnia on mABG showed nocturnal hypoventilation on TcPCO2 . Amongst recordings showing both normal NPO and mABG, 16 of 52 (30.8%) had a mean TcPCO2 ≥50 mm Hg. Nocturnal hypoxaemia was associated with nocturnal hypoventilation in all recordings. However, 5 of 21 (23.8%) recordings that showed an absence of nocturnal hypoventilation at the chosen threshold showed hypercapnia on mABG.

Morning arterial blood gases and NPO alone or in combination underestimate nocturnal hypoventilation in patients with chronic restrictive respiratory dysfunction of extrapulmonary origin.