Pressure transducers

Numerous devices have been described which can measure pressure changes in different segments of the upper airway during an obstructive episode. Different numbers of transducers can be used to measure pressures at different levels of upper aerodigestive tract ranging from the nasopharynx to the oesophagus. The transducers are attached to a catheter which is introduced though the nose in a similar fashion to a nasogastric tube. This device can be left in-situ during sleep thus allowing an overnight recording.

One of the more recent devices illustrated in figures 5 and 6 and known as Apnea-Graph AG200 (MRA, Medical UK) seems quite promising in that it is capable of combining polysomnography data with pressure recording thus providing the clinician with information regarding the severity of OSA as well as giving some idea regarding the anatomical obstructive segment in the individual patient. Essentially, it relies on measuring pressure and airflow simultaneously at different levels in the pharynx. It stores and analyses the cardio-respiratory data of a patient with simultaneous recording of two different sites in the upper airway using a micro-pressure and temperature transducer catheter. Tvinnereim11 et al published an encouraging study illustrating the importance of using this pressure catheter evaluation before embarking on surgical treatment. Singh12 et al also demonstrated some usefulness of this technique, though they had some reservations about the ability of this device to accurately detect hypopharyngeal obstruction. They compared the Apnea-Graph to polysomnography. In addition they assessed correlation in some of these patients pharyngeal obstruction data to that seen whilst performing drug induced sleep endoscopy (DISE) and concluded the latter to be superior as it allowed visualisation of the upper airway and was also more useful in indentifying lateral wall collapse. They also commented that in their group of patients, some found it difficult to tolerate the catheter for the whole night and stressed that as the catheter moves during respiration the transducers would also move thus the accuracy of the levels identified could be questioned. Another point to note is that this device has fixed transducers on a catheter and has a fixed reference transducer and does not take in to account that all patients are morphologically different and therefore the positioning will not be identical in all patients.

Fig. 5. The Apnea-Graph device with its components: a pulsoximeter and the fine bore nasal catheter with four transducers
Fig. 6. Silver 'reference' marker indicating the correct position of the Apnea-Graph catheter

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