1 INTRODUCTION

The field of sleep medicine has experienced an explosion of interest in recent years. This heightened focus on achieving an advanced understanding of sleep and its inherent pathologies is attributable, in large part, to significant improvement in the development of more suitable instrumentation. This, in turn, has led to more accurate measurement and comprehension of sleep processes and related parameters, although many questions regarding sleep mechanisms remain. The relative simplicity of instrumentation used by Hans Berger in the late 1920s to first describe the human electroencephalogram (EEG)¹ has been eclipsed by technology implemented today at sleep centers around the world where not only the EEG is recorded during polysomno-graphy (PSG), but usually the electro-oculogram (EOG), electromyogram (EMG), electrocardiogram (ECG), respiratory effort, nasal/oral airflow, SaO₂ levels, snoring, and body position with video monitoring are as well. In a much less standardized manner, additional parameters may be measured depending upon the clinical presentation of the patient and the technical capabilities of the individual sleep center. Thus, end tidal CO₂ (ETCO₂) may be measured in a patient with chronic obstructive pulmonary disease (COPD) or the placement of an esophageal balloon may be considered in appropriate patients to clarify the diagnosis of upper airway resistance syndrome (UARS) by measuring esophageal pressures during sleep. We have seen a gradual transformation from the traditional, pen-and-paper recording of sleep to the current, state-of-the-art, digital acquisition and reviewing of sleep (see Figs 4.1–4.4). As our knowledge of sleep and its pathologies is refined, the inexorable demand for further advancements concerning instrumentation becomes more acute, as is true in many other areas of health science in general. This chapter will serve to clarify the types of sleep studies used in the evaluation process of sleep-related breathing disorders (SRBD), provide guidelines for their acquisition pre- and postoperatively, and present typical montages used during PSG with special attention paid to the measurement of nasal/oral airflow.

Fig. 4.1 The Grass Model 1. A two-channel, analog recorder, based on the d’Arsnval galvanometer. Courtesy of Grass Technologies, An Astro-Med, Inc. Product Group.

Fig. 4.2 The Grass Model 78C Polygraph. An analog, multichannel, recorder. The first 12 rows (thin bands) are AC amplifiers and the following four rows (wider bands) are DC amplifiers. This polygraph stood around 6 feet tall and the number of channels was limited by the number of amplifiers/oscillographs that could be placed within the chassis. Courtesy of Grass Technologies, An Astro-Med, Inc. Product Group.

Fig. 4.3 The Grass Comet® PSG system. The entire AC/DC amplifier apparatus is contained within the small black box to the upper-left of the monitor. This system is capable of recording up to 50 channels. Courtesy of Grass Technologies, An Astro-Med, Inc. Product Group.

Fig. 4.4 The Grass Aura® PSG system. This portable system contains all AC/DC amplifiers and is small enough to fit in a person’s hand. This system is capable of recording up to 25, wireless, channels. Courtesy of Grass Technologies, An Astro-Med, Inc. Product Group.

2 SLEEP-RELATED BREATHING DISORDERS (SRBDs) AND RECOMMENDED TESTING

According to the American Academy of Sleep Medicine (AASM), when factors inclusive of sensitivity, specificity, likelihood ratios, and strength of evidence are analyzed, a categorization of four subtypes of sleep-monitoring procedures is the outcome.² Type 1 is the gold standard attended in-laboratory PSG, while the other three include portable monitoring methods inclusive of Type 2 (comprehensive), Type 3 (modified portable sleep apnea testing or cardiorespiratory sleep study), and Type 4 (continuous single recording, such as ambulatory overnight pulse oximetry, or dual bioparameter recording). Type 4 monitoring devices are considered unacceptable by the AASM for making the diagnosis of obstructive sleep apnea. Type 2 and 3 devices may be helpful in an attended setting used for patients without significant comorbid conditions, and if manually scored by trained personnel. It has been noted that the Type 3 devices have a tendency to underestimate the severity of OSA secondary to the absence of EEG monitoring in that arousals are not scored.³ Furthermore, it is recommended by the AASM that symptomatic patients with negative portable studies undergo attended PSG for further clarification.² According to AASM standards, a full night PSG is routinely indicated for the diagnosis of SRBDs and for continuous positive airway pressure (CPAP) titration in patients with a documented diagnosis of a SRBD for whom PAP is warranted. Such patients include those with a Respiratory Disturbance Index (RDI) of at least 15 per hour of sleep regardless of their presenting symptomatology or those with a RDI of at least five per hour of sleep with excessive daytime sleepiness. Split-night PSGs (diagnostic segment followed by CPAP titration on the same night) are considered acceptable if the Apnea/Hypopnea Index (AHI) is at least 40 events per hour of sleep and is documented by means of at least 2 hours of diagnostic recording, or if the AHI is between 20 and 40 in the presence of repetitive lengthy obstructions and major desaturations. In addition, the CPAP titration must be carried out for more than 3 hours, and the PSG must document that CPAP eliminates the respiratory events during sleep, including REM sleep with the patient in the supine position.²

Excessive daytime somnolence is often part of the clinical presentation of patients with a SRBD. There are several means by which the degree of somnolence may be measured, both subjectively (e.g. Epworth Sleepiness Scale) and objectively. The Multiple Sleep Latency Test (MSLT) is a series of nap opportunities (usually four or five) scheduled 2 hours apart beginning 1.5–3 hours after the morning awakening following PSG, which are conducted under specified conditions using accepted protocols. Patients are asked to try to fall asleep during these nap opportunities. The procedures used to perform the Maintenance of Wakefulness Test (MWT) are similar to those utilized for the MSLT with the exception that the patient is asked to remain awake under the same soporific conditions utilized for the MSLT. The MSLT is a validated objective measure of the patient’s ability or tendency to fall asleep while the MWT is a validated objective measure of the patient’s ability to remain awake under soporific conditions. Although closely related, these two procedures serve to define a patient’s sleepiness from different perspectives. The MSLT is not routinely indicated in the initial evaluation for a SRBD or in the assessment of change following treatment with nasal CPAP.⁴ However, the MWT may be used to assess an individual’s ability to remain awake if an inability to do so potentially constitutes a public or personal safety issue. Patients with a SRBD who are employed in occupations involving public transportation or safety may require assessment of their ability to remain awake. Although data regarding usefulness of the MSLT or the MWT are limited, using the MWT to assess ability to remain awake has been reported to have greater face validity than using the MSLT.⁴ However, the predictive value of either test for assessing accident risk and safety within the context of real- life circumstances is not well established. The assessment of the patient’s ability to remain awake and consequent safety risks should involve the integration of findings from the history, compliance with therapy, subjective rating scales, and in some cases, objective testing using the MWT.

3 PREOPERATIVE AND POSTOPERATIVE ASSESSMENTS

For patients undergoing upper airway surgery either for snoring or for a SRBD, the preoperative routine should be inclusive of PSG or an attended cardiorespiratory (Type 3) sleep study. In certain instances after the surgery has been completed, a follow-up PSG or attended cardiorespiratory (Type 3) sleep study is recommended. This should be considered following the surgical treatment of patients with moderate to severe OSA, to ensure satisfactory response, and in those whose clinical response is insufficient or when symptoms return despite a good initial response to CPAP treatment. In addition, follow-up PSG is indicated in other circumstances: (1) after substantial weight loss (e.g. 10% of body weight) in patients on CPAP for treatment of SRBDs to ascertain whether the pressure requirement has changed; (2) after substantial weight gain (e.g. 10% of body weight) in patients previously treated with CPAP successfully, who are again symptomatic despite the continued use of CPAP. In the latter case, the possibility of a pressure requirement change must also be entertained.

4 SPECIAL POPULATIONS

Patients with some specific medical conditions deserve mention. In general, those with a history of coronary artery disease, congestive heart failure, a history of stroke or transient ischemic attacks, significant tachyarrhythmias or bradyarrhythmias should be screened for signs and symptoms of a SRBD.² If there is a reasonable suspicion of these conditions, then a PSG is warranted. Furthermore, the application of CPAP at least during the perioperative period in many cases is prudent.

The pediatric population also merits attention. Although most children with obstructive sleep apnea present with a history of snoring and evidence of some difficulty breathing during sleep, not all of them snore. Paradoxical breathing is often prominent secondary to their very compliant rib cages. The scoring of respiratory events in adults requires a duration of at least 10 seconds, while the standard in children has been defined by a period of at least two respiratory cycles.⁵ In children, these respiratory events may or may not be followed by EEG arousals. This often leads to fairly normal sleep architecture. The respiratory events occur predominantly in REM sleep. The ordering of PSG is indicated in neonates and infants when apnea of prematurity or of infancy is suspected, or following apparent life-threatening events (ALTEs). When severe gastroesophageal reflux is suspected, PSG is also indicated. The presence of a craniofacial congenital malformation involving the face, mouth, tongue, neck or chest, such as Treacher Collins syndrome or Pierre Robin syndrome, or following surgery on a child performed to correct these anatomical abnormalities, should also prompt a consideration for PSG. Other disorders which should urge the clinician to obtain PSG include suspected seizure disorders or any disorder which can cause hypotonia.⁶