**5. 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.

positions. In any case the fact that the muscle tone variation in sleep and wakefulness must also be borne in mind. Another commonly used technique during the flexible endoscopic assessment is the Mullers4 manoeuvre. This essentially is a reversed Valsalva procedure which some patients do find difficult to perform. Furthermore, there is subjective variation in the assessment of the degree of collapse noted in different segments of the pharynx and

Xrays of the maxilla and mandible in the form of cephalometry5 may provide useful data of various parameters and dimensions controlling the upper airway. This can be particularly useful when the patient is being considered for invasive surgery such as maxillomandibular advancement or indeed when considering patients for MAS, though for the latter it is presently used for research purposes only. The limitation of this evaluation technique is that it provides a two dimensional image and that so during wakefulness. It

In contrast, computed tomography (CT) scanning and magnetic resonance imaging (MRI) provide more sophisticated imaging and allows objective cross sectional area and volumetric analysis. 6, 7 They are both more expensive than the cephalometry and the CT scans would also involve radiation. The MRI is quite noisy but is excellent at delineating soft tissue margins as well as fat deposition in the parapharyngeal space. For research reasons cine CT and dynamic MRI studies have been conducted to evaluate the upper airway but it

This form of evaluation is safe in that there is no radiation involved and it is relatively cheap. It can be performed easily during sleep and at patient's home and simultaneously with polysomnography. Multiple night recordings can be carried out and based on sound frequency spectrum, acoustic analysis can potentially discern simple snoring from OSA.8 Attempts have been made to correlate snoring sound frequency with different levels of obstruction and comparisons of this technique have been made to others such as drug

The sensitivity and specificity of this technique has often been questioned and although it can provide useful screening process, its role in helping with selecting treatment modalities is somewhat limited.10 The other problems in studies with acoustic analysis are that of variation of software and the choice of central or fundamental frequency in determining the

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.

thus the reliability of this technique may be questioned.

also exposes the patient to considerable amount of radiation.

is not considered to be practical or cost effective for routine use.

**3. Imaging** 

**4. Acoustic analysis** 

induced sedation endoscopy.9

**5. Pressure transducers** 

site of obstruction.

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

Evaluation of the Upper Airway in Patients with Snoring and OSA 71

Similarly, questions and concerns that arose about test-retest reliability and of inter-rater reliability of SNE have been elegantly addressed by studies conducted by Rodriguez-Bruno

Bispectral index monitoring (BIS) has provided an adjunct to the assessment of sleep nasendoscopy in determining the level of sedation required for snoring assessment.24 BIS (figs. 7 & 8) monitor is a neurophysiological monitoring device which continually analyses a patient's electroencephalogram during sedation and general anaesthesia to assess the level

*et* al22 and Kezirian *et* al23 respectively.

of consciousness and depth of anaesthesia.

Fig. 7. Four sensor BIS electrode attached on patient's forehead

Fig. 8. BIS Monitor reading during Sleep Nasendoscopy

The issue of assessing the patient at the correct moment has not previously been addressed and this indeed is an important point as one has to bear in mind the pharmacology and the pharmacokinetics of the different drugs used during sedation. If the patient is assessed too

Fig. 6. Silver 'reference' marker indicating the correct position of the Apnea-Graph catheter
