Extensive literature is available comparing upper airway anatomy and dentofacial structures using x-ray cephalo-metry between OSA patients and healthy controls. In siblings a significantly longer distance from the hyoid bone to the mandibular plane has been documented in those affected by SDB.⁵ Further differences were described by different working gropus. The concrete results are often difficult to compare, as the authors not only use different landmarks and parameters but also sometimes rather complex calculated indices and ratios to describe the differences they found. Therefore, the following findings in OSA patients can only be a selection: longer soft palates, reduced minimum palatal airway widths, increased thickness of the soft palate, differences in calculated craniofacial scores, increased pharyngeal lengths, retroposition of the mandible or the maxilla, micrognathia, increased mid-facial heights, and differences in hyoid bone position. In general, the differences are more pronounced in non-obese patients, suggesting that craniofacial changes play a dominant role in this subgroup. Furthermore, substantial differences in maxillofacial appearance of different ethnic groups need to be taken into account.

Various authors could demonstrate that the aberrations in craniofacial morphology they found in OSA patients were more pronounced in patients with severe OSA. Dempsey et al. demonstrated that in non-obese patients and in patients with narrow upper airway dimensions, four cephalometric dimensions were the dominant predictors of Apnea/Hypopnea Index (AHI) level, accounting for 50% of the variance.⁶ Rose et al. questioned the diagnostic relevance of x-ray cephalometry for OSA, as they found no direct correlation between skeletal cephalometric findings and OSA severity; nevertheless, they also reported a correlation with hyoid bone position.⁷

4.2 X-RAY CEPHALOMETRY AND THERAPEUTICINTERVENTIONS

One of the dominant indications for performing x-ray cephalometry has been treatment with oral appliances. Especially with regard to the evaluation of potential predictive parameters for treatment success and dental side effects, x-ray cephalometry has been the standard diagnostic tool. As early as 1995, Mayer and Meier-Ewert, two of the fathers of treatment with oral appliances in Europe, looked for cephalometric predictors of treatment success ⁸ and reported that specific cephalometric variables were indeed predictive for the therapeutic effect. Other authors have confirmed the existence of predictive cephalometric parameters, especially in relation to hyoid bone position and oropharyngeal airway dimension. Nevertheless, the problems related to different nomenclature and selection of airway parameters described above remain.

X-ray cephalometry has also been evaluated with regard to potential predictive parameters for postoperative results of UPPP alone or in combination with other approaches. To date there is no convincing evidence that skeletal measurements obtained with x-ray cephalometry could predict the outcome of UPPP. Nevertheless, lateral x-ray cephalometry is the standard tool in the preoperative evaluation of the craniofacial skeletal anatomy before maxillomandibular advancement surgery. It can be regarded as a mandatory procedure and its value is not questioned.⁹

4.3 X-RAY CEPHALOMETRY IN PATIENTMANAGEMENT

X-ray cephalometry has provided substantial insights into the pathophysiology of OSA, demonstrating significant craniofacial characteristics associated with this disease. Although the results are not easy to compare, specific cephalometric characteristics have been repeatedly mentioned as a risk factor for OSA and correlate with the severity of the disease. Selected cephalometric parameters indicate favorable results of mandibular advancement by oral appliances. Nevertheless, no cephalometric parameter exists that would reliably rule out treatment success with an oral appliance and surgical outcome cannot be predicted. This may explain why x-ray cephalometry has not become a routine procedure in the diagnostic work-up of OSA as long as maxillomandibular surgery is not planned.

5 CT SCANNING

Compared to lateral x-ray cephalometry, CT scanning significantly improves soft tissue contrast and allows precise measurements of cross-sectional areas at different levels as well as three-dimensional reconstruction and volumetric assessment. Fast scanning times and relatively quiet scanning conditions even allow a dynamic assessment of the airway during a respiratory cycle as well as measurements during natural sleep. Nevertheless, ionizing radiation remains problematic.

5.1 TECHNIQUES AND STANDARDS

Despite its widespread use in airway assessment in patients with SDB, no standardized scanning protocol exists for this indication and the nomenclature of the soft tissue structures is not uniform. In addition to a two-dimensional assessment of the upper airway, three-dimensional techniques were used to assess volumes of soft tissue structures and airway spaces. As early as 1987 ultrafast or dynamic CT was inaugurated in this field to evaluate dynamic changes of the upper airway dimensions during respiratory cycles. While the vast majority used CT imaging during wakefulness, several authors also used scanning protocols under hypnotic relaxation, sleep and sleep during apneas, and also used direct comparisons between wakefulness and sleep.

Providing insights into thepathophysiology of SDB

The majority of published data points to potential differences in upper airway structures and dimension between OSA patients and healthy controls or snorers. In general, the upper airway is described as smaller in apneic patients compared to controls, especially with regard to the retropalatal region. Cross-sectional areas were found to be significantly narrower in affected patients. Inversely, retropalatal tissue was described as being greater in OSA patients compared to controls and larger tongue and soft palates dimensions and volumes were found. Schwab et al. have pointed out the differences in upper airway configuration with an anterior–posterior configuration – a result that is in line with data obtained from magnetic resonance imaging.¹⁰

Different authors have described anatomic conditions that reflect the severity of the disease and have correlated their measurements with polysomnographic data. A high apnea index seems to be associated with large tongue and soft palate volumes, and a significant correlation of the retropalatal space and its lateral diameter with the Respiratory Disturbance Index was documented. A combination of the smallest cross-sectional area, the upper airway resistance, and the Body Mass Index was used to predict the severity of OSA and a narrower cross-sectional area and a thicker soft palate were found in severely affected patients compared to patients with only mild to moderate OSA.

With the help of dynamic and ultrafast CT further insights into airway obstruction were gained. In addition to the fact that the naso- and oropharyngeal airways were smaller in OSA patients compared to weight-matched controls, an increased collapsibility in affected patients was found. During a respiratory cycle, substantial changes in cross-sectional areas were seen in patients with SDB, the velopharyngeal segment being the narrowest and most collapsible region.¹¹ These results were essentially confirmed later, showing that patients with severe OSA have significantly narrower cross-sectional areas at the velopharyngeal level¹² (see Figure 3.1).

Fig. 3.1 Upper airway narrowing during tidal breathing as assessed with CT scanning. A. Cross-sectional image of a patient at the level of uvula in tidal breathing. B. The significant narrowing at the same level in forced expiration is seen. The region of interest (white line) was used to assess total cross-sectional areas in each image (according to Yucel et al., 2005).¹²

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5.3 EVALUATING THE EFFECTS OF THERAPEUTIC INTERVENTION

Effects of therapeutic intervention have been assessed, mostly with regard to treatment with oral appliances and surgical intervention. While a decrease in the diameters at the retropalatal and retroglossal level was seen during apnea, these cross-sectional areas were significantly enlarged with the help of the appliance. With regard to surgical treatment effects, it has been demonstrated that the upper airway increases after mandibular distraction osteogenesis in children and after maxillomandibular advancement in adults. Even more data are available for the effects of UPPP and its modifications, demonstrating that UPPP significantly increases the upper airway cross-sectional area and that the oropharyngeal enlargement seen in pharyngeal CT measures is associated with a good outcome in UPPP.

5.4 CT SCANNING IN PATIENT MANAGEMENT

CT scanning has provided valuable insights into the pathophysiology of SDB. In addition, it has often been stated that CT scanning does play or will play a major role in the management of patients with SDB. Nevertheless, CT scanning has not become part of the routine assessment of patients with SDB, especially not with regard to surgical treatment selection. The limited availability, associated costs, practical considerations and ionizing radiation remain problematic. Beneficial effects on treatment selection and thereby treatment outcome have been postulated repeatedly but could not be demonstrated to date.

6 MR IMAGING

Compared to lateral x-ray cephalometry or CT scanning magnetic resonance imaging (MRI) offers various advantages, such as excellent soft tissue contrast, three-dimensional assessments of tissue structures, and lack of ionizing radiation. The latter has made MR-imaging the imaging technique of choice in the assessment of children with SDB.

6.1 TECHNIQUES AND STANDARDS

Concerning their scientific or clinical use in the context of SDB, routine imaging techniques were initially applied following various protocols used in everyday clinical practice. For patients suffering from SDB it was attempted to determine anatomic preconditions and peculiarities for SDB. In this research, comparisons with healthy controls have been utilized, measuring two-dimensional distances and diameters of the upper airway or its related structures. In addition, three-dimensional data were obtained. Volumes were either calculated based on cross-sectional areas and slice thickness or established by various computerized models. Ultimately, ultrafast or dynamic imaging was used to visualize dynamic motion of the upper airway to assess upper airway collapse or differences in upper airway motion between patients with SDB and healthy controls. Subjects were either measured during wakefulness or during wakefulness and sleep; children were routinely scanned under sedation. Sleep in adults was either pharmacologically induced or spontaneous.

Only a small number of authors have attempted to establish distinct protocols for MRI of the upper airway in SDB; the results of the measurements were either validated with a phantom or tested for variability in repeated measures over time and with different investigators.¹³ Validation and standardization of this imaging paradigm seem essential; nevertheless, in contrast to, for example, lateral x-ray cephalometry, hardly any consensual standards exist for this indication.

6.2 PROVIDING INSIGHTS INTO THEPATHOPHYSIOLOGY OF SDB

6.2.1 OSA IN CHILDREN

Especially in children, extensive research has been done with regard to the pathophysiology of OSA. Children with or without persistent OSA after tonsillectomy and adenoidectomy were compared and it was demonstrated that enlarged lingual tonsils were present in those children with persistent disease, especially in children with Down syndrome. Further findings were an airway restriction in the vicinity of both the adenoids and the tonsils and an enlarged soft palate in the affected group. Nevertheless, the airway restriction is not limited to these areas but seems to occur throughout the initial two thirds of the upper airway. Furthermore, a close dependency between the frequency of respiratory events and the size of the tonsils and the soft palate could be demonstrated and the upper airway narrowing was more pronounced in those children with a high number of respiratory events compared to the less affected group. With regard to dynamic airway evaluation, more pronounced fluctuations in airway area during tidal breathing ¹⁴ and significant differences in airway motion¹⁵ were demonstrated in children with OSA compared to controls.

6.2.2 OSA IN ADULTS

As early as 1989, authors pointed out the significance of pharyngeal fat deposits in adult patients with OSA.¹⁶ In this group, more fat is present in the areas surrounding the collapsible segment of the pharynx and fat deposition has been described even in non-obese patients with OSA. Some authors could demonstrate a trend for larger tongues or a significantly higher tongue volume in relation to the oral cavity volume in patients with OSA compared to controls. Other anatomic conditions associated with SDB were an elliptic horizontal cross-sectional area of the pharynx and large volumes of the lateral pharyngeal walls and total soft tissue surrounding the upper airway. Nevertheless, other authors did not find any significant differences in tongue volume, soft palates or pharyngeal walls, but pointed out specific anatomic factors of the mandible in OSA patients.

Numerous authors have demonstrated that the mechanism and level of airway obstruction can be visualized by MRI, even under natural sleep ¹⁷ (Figure 3.2). The fact that patients with OSA present multiple sites of pharyngeal abnormality was demonstrated by Suto et al. as early as 1993;¹⁸ nevertheless, the authors pointed out that the levels of airway obstruction during wakefulness did not match those levels found during sleep. Other trials have shown that no pharyngeal airway narrowing was seen in the healthy subjects but a significant narrowing was seen in the OSA patients during wakefulness, and even more so during sleep. Moreover, it was demonstrated that apneic patients have a more circular occlusion, underlining the relevance of the lateral pharyngeal walls in the pathogenesis of airway obstruction.

Fig. 3.2 Complete pharyngeal collapse as detected with dynamic MRI during natural sleep. Dynamic single slice images of a 45 year old man during an apnea period. The first images show the complete naso-, oro- and hypopharyngeal obstruction; the arrows mark collapse of the different pharyngeal regions (according to Schoenberg et al. 2000).17

6.3 EVALUATING THE EFFECTS OF THERAPEUTIC INTERVENTION

Finally, MRI has been used to assess the effects or side effects of various therapeutic interventions, including surgical and non-surgical strategies. Although potential anatomic criteria for successful treatment with mandibular advancement devices were reported, MRI has not become a routine procedure in the management of patients designated for treatment with a mandibular advancement device.

With regard to surgical treatment, MRI has been used to assess potential effects of surgery on upper airway anatomy. In radiofrequency surgery, imaging has been used to visualize immediate postoperative effects on the soft palate and the tongue base, the latter leading to concrete recommendations for the technical settings of this technique.¹⁹ A standardized protocol has furthermore been used to study anatomic changes at the upper airway after radiofrequency surgery of the tongue base²⁰ and after hyoid suspension.²¹

6.4 MRI IN PATIENT MANAGEMENT

Static and dynamic MRI has substantially improved our understanding of the pathophysiology of SDB. Significant differences in upper airway anatomy and structure have been detected between patients with SDB and healthy subjects, and relevant insights have been gained in terms of the mechanisms and levels of airway obstruction. Nevertheless, MRI has not become a standard procedure either in the diagnostic work-up for patients with SDB or in the management of the disease in terms of surgical or non-surgical treatment, as a number of issues remain unresolved. MRI during sleep (especially spontaneous sleep) is possible but not easy to perform and measurements during wakefulness or induced sleep are, to a certain extent, artificial or may simply not reflect clinical conditions. Furthermore, the results of MRI, even when performed during sleep, can only provide information concerning a short period of time and are limited to the supine position. For routine clinical application, the limited availability and the associated costs are additional limiting factors.

7 VIDEOENDOSCOPY DURING SPONTANEOUSSLEEP

As early as 1978 the first report about videoendoscopic recordings of the pharynx and larynx during sleep was published. Borowiecki and colleagues described a palatopharyngeal collapse at the end of expiration and directly before inspiration in patients with OSA.²² They described different degrees of airway obstruction, often associated with a medialization of the lateral pharyngeal walls. As there was no treatment other than tracheotomy available at this time for those patients, patient selection was not an issue.

Today videoendoscopy during spontaneous sleep is performed in order to improve patient selection for the different treatments available and may also be performed in combination with overnight sleep recordings. Because video-endoscopy during spontaneous sleep allows the assessment of the upper airway during different sleep stages and lacks the side effects of sedating drugs, this method may be considered superior to videoendoscopy under sedation. However, sleep videoendoscopy is rarely performed as it usually requires nightly measurements and puts additional strain on both patient and doctor.

8 VIDEOENDOSCOPY UNDER SEDATION

8.1 IMPACT OF VIDEOENDOSCOPY UNDERSEDATION ON SLEEP, BREATHING, ANDSNORING

Videoendoscopy under sedation allows the visualization of the site and mechanism of snoring and airway obstruction in patients with SDB. Therefore it is mandatory that snoring and airway obstruction can be provoked in affected patients and that neither the endoscope itself nor the drugs used for sedation disturb or influence breathing patterns, snoring or airway obstruction during sedation. With this regard, statistically significant differences were found for the longest apnea and the portion of REM sleep as well as for acoustically analyzed snoring sounds when sleep under sedation was compared to natural sleep. Furthermore, it has to be mentioned that videoendoscopy during sedation is usually performed for 10 to 15 minutes due to practical considerations, and may therefore not reflect the conditions during an entire night of natural sleep.

Endoscopy under sedated sleep does not always succeed in inducing existing breathing disorders, and on the other hand, snoring may be provoked even in healthy patients. In a cohort study using propofol, Marais detected snoring sounds in 45% of 126 healthy, non-snoring controls.²³ When titrating propofol with target-controlled infusion, all snorers did snore reliably whereas not a single control person did at the same plasma level, amounting to a sensitivity and specificity of 100%.²⁴ Therefore, target-controlled infusion with propofol seems superior to manual titration.

8.2 DESCRIPTION OF FINDINGS

The patterns of snoring and airway obstruction that can be observed during videoendoscopy under sedation are multiform. Pringle and Croft were the first to standardize the findings according to their data obtained in a large series of patients.²⁵ Currently, different classifications coexist and, ultimately, none of them are feasible. They distinguish either between an isolated or a multisegmental obstruction, or they are modifications of existing classifications comprising the epiglottis. The majority of authors do not classify their findings but enumerate the various mechanisms and anatomic sites of snoring and obstruction. The obstructive patterns are described as being circular, antero-posterior and latero-lateral at the level of the soft palate, the tonsils, the tongue base, and the epiglottis. A combination of as many as five different concomitant sites of obstruction in primary snorers and even six in sleep apnea patients was described and an isolated site of obstruction was found in only 15 % of OSA patients.

8.3 IMPACT ON CLINICAL DECISION MAKING

In clinical routine a large tongue – defined by a modified Mallampati score of 3 or 4 – is usually considered a negative predictive parameter for a successful UPPP. However, den Herder and co-workers could not demonstrate a correlation between videoendoscopy under sedation (retrolingual obstruction) and Mallampati index (tongue size).²⁶ Pringle and Croft compared their results of the Mueller Maneuver to those obtained by videoendoscopy under sedation in a group of 50 patients and could demonstrate that treatment recommendations were not identical based on these two investigation to a significant extent.²⁵ Other authors also reported substantial differences in treatment recommendation when adding sedated endoscopy to simply using the Mueller Maneuver. Taking the limited significance of the Mueller Maneuver into account, there seems to be a potential for an improvement in treatment selection based on sedated endoscopy.

8.4 IMPACT ON THE SUCCESS RATE

Surprisingly enough, no prospective data are available to date comparing success rates of surgical intervention with and without the use of videoendoscopy under sedation. This is particularly confusing as there have been numerous advocates of sedated endoscopy presenting data and videos on countless patients with SDB, but they have not yet been able to demonstrate its usefulness with regard to surgical outcome. For example, no superiority was seen with regard to success rates compared to historic controls, despite using sedated endoscopy.²⁷ Yet an improved success rate was only found in those patients who did not even show the slightest involvement of structures other than the palate. Hessel and de Vries retrospectively reviewed snorers and sleep apnea patients after UPPP. In those patients where the soft palate was least involved in airway collapse during preoperative sedated videoendoscopy, the outcome was superior compared to the others.²⁸ In another retrospective analysis of 55 sleep apnea patients after UPPP, the same investigators did not find significantly different success rates for different sites of obstruction as revealed by videoendoscopy under sedation.

8.5 VIDEOENDOSCOPY UNDER SEDATION AND THE ROLE OF THE EPIGLOTTIS

According to our own experience videoendoscopy under sedation or in sleep is particularly helpful in detecting or excluding a possible glottic or supraglottic obstruction, most often described as a posterior movement of the epiglottis during inspiration. In those cases where an epiglottic collapse during sleep videoendoscopy was seen, a significant reduction of respiratory events may be achieved with partial epiglottectomy. Videoendoscopy under sedation or during sleep may be particularly helpful in cases of laryngeal collapse and failures of standard therapy.

8.6 SIGNIFICANCE OF VIDEOENDOSCOPY DURING SEDATION

Videoendoscopy under sedation is able to initiate snoring and upper airway obstruction during a short period of induced sleep, mostly restricted to the supine position. The severity of the underlying disorder appears comparable to natural sleep, although snoring sounds seem different and the short examination time is a significant limitation. The classification of findings can be reduced to isolated obstruction at the palate, the tongue base or the epiglottis or to combinations of these. There are subtle hints that videoendoscopy under sedation may change the indication for a limited number of surgical intervention. Nevertheless, there is not enough evidence to date that this procedure improves the outcome of snoring and sleep apnea surgery.

9 MULTI-CHANNEL PRESSUREMEASUREMENTS

Changes in inspiratory pressure in the upper airway during obstructive events can be measured with catheters. Different measuring points meaning different pressure transducers can be used from the nasopharynx through the oro- and hypopharynx down to the esophagus. Initially, pressure transducers were used mainly to investigate the mechanisms of airway obstruction in general; nowadays research is focused on the diagnostic potentials compared to standard polysomnography and on the assessment of obstruction level and its impact on the outcome of sleep apnea surgery.

9.1 TOLERABILITY OF THE PRESSURE CATHETERS

Initially, pressure recordings in the field of SDB were performed with balloon and open catheters, but esophageal balloon catheters irritated the patients significantly. The reliability of the results of micro-tip catheters was shown during wakefulness and sleep for the esophagus and for the pharynx. Well-designed studies demonstrated that catheters with no more than 2 mm diameter did not alter the sleep structure of patients suspected of SDB and the results obtained with or without catheters in place did not differ significantly.²⁹ In a large trial with 799 patients only 3% rejected the placement of the catheter and 1% refused further measurement during the night, while 96% tolerated the procedure.³⁰

9.2 RELIABILITY OF MEASURING POINTS ANDASSESSMENT OF OBSTRUCTIVE EVENTS

Multi-channel pressure catheters require a reliable positioning of the measuring points in order to attribute the site of obstruction to the anatomically defined segment of the airway. Most investigators choose the oropharyngeal sensor as a reference to be placed under visual control at the free edge of the soft palate. The level of obstruction is usually described as an ‘upper’ or ‘lower’ obstruction, meaning an obstruction at the level of the soft palate or the level of the tongue base (an isolated collapse at the level of the epiglottis cannot be assessed with this method).

Pressure catheters also can be used to measure nasal and pharyngeal airflow. This implies that they are suitable to assess increased respiratory effort as well as the AHI. Tvinnereim et al. demonstrated that the absolute number of obstructive and mixed apneas during sleep can be assessed with these catheters with minimal deviation from the results obtained by polysomnography.³¹ The overall sensitivity, specificity and negative predictive value for the detection of the different types of apneas and hypopneas may reach up to 100%. The data available indicate that multi-channel pressure recordings are suitable to assess the severity of SDB.

9.3 ASSESSING THE SITES OF AIRWAYOBSTRUCTION

When using only one measuring point placed at different levels of the upper airway it is difficult to identify a combined collapse at different levels of hypo- and oropharynx; therefore, catheters with five to six sensors are currently used. With those sensors a collapse extending over more than one segment may routinely be found, the segments may not even be adjacent and the site of obstruction may change during the night.

Investigations concerning night-to-night variability of the distribution of the obstructive sites showed that the predominant site of obstruction can be reproduced during the second night in more than 70% of the cases. Rollheim et al. compared the patterns of obstruction as obtained in the hospital with a recording at home. Although the mean AHI was significantly higher in the hospital than at home, the occurrence of palatal obstructions did not differ between both recordings.³² In patients who had less than 40% or more than 60% palatal obstructions in the first recording, this relationship was reproduced in 90% of the cases during the second recording.

9.4 IMPACT ON THE SUCCESS RATE OF SURGERY FOR SDB

Metes et al. were the first to publish data concerning the impact of pharyngeal pressure measurements on the success rate of surgery.³³ They used a catheter with one measuring point only which was pulled through the pharynx and placed at several sites along the upper airway in order to record several obstructive events at each site. The obstruction they found in this way persisted in eight of 12 patients after UPPP. Nevertheless, the success rate of UPPP did not differ between patients with predominant palatal or tongue base obstruction. Skatvedt et al. selected 16 patients with different degrees of SDB and predominant palatal obstruction detected by multi-channel pressure transducers for laser-assisted uvulopalatoplasty.³⁴ While the rate of ‘upper’ obstructions dropped from 90% to 8.8% of all apneas, the number of upper hypopneas was reduced only minimally. Osnes et al. compared the efficacy of UPPP in patients with predominantly transpalatal and subpalatal obstructions.³⁵ After UPPP, transpalatal apneas and hypopneas were reduced by 81% whereas subpalatal events only dropped by 42%. The success rate in patients with transpalatal obstruction was significantly higher than in those with subpalatal obstruction. Multi-channel pressure recordings seem to be superior to single-channel pull-through techniques in predicting surgical success of soft palate surgery.

9.5 SIGNIFICANCE OF MULTI-CHANNEL PRESSURE RECORDINGS

Pressure catheters with a small diameter of not more than 2 mm are well tolerated and have only minimal impact on sleep quality and airway obstructions. The pressure curves allow a reliable detection of respiratory events. Positioning of the measuring points by means of pharyngeal inspection seems to be sufficiently accurate for the evaluation of the palatal airway segment. There are no data suggesting that obstructions at the hypopharynx and the epiglottis can be discriminated reliably. The data available support the idea that the success rate of soft palate surgery can be improved when using multi-channel pressure recordings for patient selection.

10 CRITICAL CLOSING PRESSURE

The severity of SDB is usually described by the AHI, representing the number of upper airway obstructions during sleep. Nevertheless, it has to be kept in mind that the AHI simply describes the frequency of upper airway obstructions, not the severity of the pharyngeal collapse itself. Furthermore, measuring the severity of upper airway collapse is believed to be important when estimating the forces needed to overcome these obstructions or to maintain upper airway stability. Schwartz et al.³⁶ and Smith et al.³⁷ first measured upper airway collapsibility. They assessed airflow and airway pressure using a special nasal continuous positive airway pressure (CPAP) device with integrated pneumotachograph and pressure sensor, being able to produce positive as well as negative pressure levels. A pressure flow diagram can be drawn at different levels of airway pressure and a regression line can be calculated. Smith defined the critical closing pressure (P_crit) as being the upper airway pressure when the regression line is crossing the zero line, indicating that airflow completely stops (see Figure 3.3). Patients with obstructive apneas have a P_crit clearly above zero, in simple snorers it drops to −3 to −12 mbar, whereas in normal controls P_crit is on average below −8 mbar.

Fig. 3.3 Representative pressure flow relationships in the lateral recumbent and supine positions for one subject in non-REM sleep. Maximal inspiratory airflow (V1 max) vs nasal mask pressure (PN) is illustrated for lateral recumbent (open circles) and supine (closed circles) positions with corresponding regression lines and confidence interval. Critical closing pressure is represented by the PN at which airflow becomes zero in each body position (according to Boudewyns et al. Chest 2000;118:1031–41).

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10.1 IMPACT OF TREATMENT ON P_CRIT

Schwartz et al.³⁸ showed that substantial weight loss resulted in an improvement of P_crit and a concomitant reduction of the AHI. They investigated the effect of UPPP on P_crit in 13 patients and found a significant decrease for the entire group.³⁹ Even though a complete normalization of P_crit could be demonstrated in the subgroup of responders in contrast to non-responders they could not find any preoperative predictor of response. An identical operation had individually varying effects on P_crit.