Polysomnographic Recording Procedures

Eileen B. Leary

LEARNING OBJECTIVES

On completion of this chapter, the reader should be able to:

1. Identify the key prerequisites for obtaining quality artifact-free data.
2. Describe appropriate patient monitoring and documentation procedures.
3. Recognize clinical and technical issues that require a technologist response.
4. Recognize common causes of recording artifacts.
5. Identify and correct common artifacts in various recording parameters.
6. Utilize the principles of system referencing, rereferencing techniques, and other strategies to maintain recording quality.
7. Describe the processes related to ending the study and discharging the patient.

The basic concepts of obtaining high-quality, artifact-free recordings have not changed despite considerable technologic advancements over the years that have impacted how we collect, process, and view polysomnographic (PSG) data. The three major factors include the following:

Proper electrode and sensor application
Accurate signal processing
Conscientious maintenance of the recording

This chapter touches on the patient element, but really focuses on how to utilize the available tools offered by today’s digital sleep recording systems to obtain a high-quality, artifact-free sleep study.

ELECTRODE APPLICATION

The first step toward obtaining an artifact-free polysomnogram is the proper application and correct placement of the electrodes and sensors. Although all data collection systems have mechanisms to filter data, they are meant to fine-tune the signal being collected, not disguise or repair a nonexistent or bad signal related to poor electrode application. Poor electrode application cannot be fixed through aggressive filtration and signal manipulation because excessive signal filtering will obscure or alter the data.

Careful attention should be paid to each of the following steps during the hookup process to attain a quality recording:

Precise head measurements for the electroencephalogram (EEG) electrodes
Careful site preparation, scrubbing an area no larger than the size of the electrode cup
Utilization of high-quality electrodes (typically, gold cup electrodes)
Secure adhesion of the electrodes to the skin
Low and relatively equal electrode impedances (<5 kΩ as measured by an AC impedance meter)
Proper management of the electrode wires

For a more detailed explanation of electrode application techniques, see Chapter 36, Preparing the Patient for Polysomnography.

SIGNAL PROCESSING

Once the electrodes and sensors are in place, the resulting signal must be processed in a standardized manner to ensure consistent output across individuals as well as sleep centers. Signal processing includes amplification of the signal, filtration, analog-to-digital conversion, and signal display.

Basic considerations for proper signal processing include the following:

The use of high-quality amplifiers
Sturdy electrical connections
Adequate electrical shielding
Proper electrical grounding of equipment
Appropriate filter settings for each recorded parameter
Adequate sampling rates for each recorded parameter
Correct signal polarity
Adequate computer screen resolution (digital screen and video card must be at least 1,600 × 1,200)
Documentation of all signal manipulations

The American Academy of Sleep Medicine (AASM) updates their guidelines (1) regularly. Every technologist is responsible for reviewing the current standards, including requirements for digital PSG recording, because they represent the essential requirements for performing and scoring polysomnograms.

Today, it is recommended that a digital PSG system include the following features:

A mechanism to perform visual (on-screen) electrical calibrations for all channels by applying a negative 50 µV DC voltage to document correct signal polarity, amplitude, and time constant
A separate 50/60-Hz notch filter for every channel
The ability to program sampling rates for each channel
The capacity to measure individual impedance levels for each electrode against a common reference (may be the sum of all other applied electrodes)
The capability to retain and display all settings used by the attending technologist, including adjustments to derivations, sensitivity, filters, and temporal resolution
A filter design that mimics conventional, analog-style frequency response curves

The AASM also suggests as an optional feature that all systems have the flexibility to choose and/or change the electrode input signal derivations without relying on a common reference electrode.

SELECTING THE APPROPRIATE MONTAGE

Today, virtually all digital sleep systems allow you to select which channels to monitor during a study by allowing customizable settings for the electrode derivation, sensitivity, filter settings, label, color, and analysis properties for each channel. The arrangement of the recording channel selections and their settings is called a montage. Multiple montages can be developed and saved for use on multiple patients. An example of a sleep apnea diagnostic montage using the AASM-recommended specifications is shown in Table 37-1 (1, 2). The technologist must ensure that the correct montage is chosen during the setup process because the montage cannot be changed once the recording is initiated.

Examples of common montages include the following:

Sleep apnea diagnostic evaluation
Positive airway pressure (PAP) titration
Seizure
Multiple sleep latency test/maintenance of wakefulness test
Specialty equipment (transcutaneous CO₂, end-tidal CO₂)

Before choosing a montage, carefully review the physician’s order to be sure you select the montage that will record the appropriate physiologic data. A quality hookup is a wasted effort if the appropriate information is not recorded and the study has to be repeated.

INITIATING THE RECORDING

It is time to initiate the recording when the montage has been selected, patient demographics have been entered in the system, and the patient is properly hooked up. Once the study is recording, system calibrations are performed for at least 30 seconds to document that the equipment is functioning properly and the correct filter settings and sampling rates have been selected.

The next step is to test the impedance levels of each electrode to confirm a secure attachment. Most sleep systems have an impedance meter built into the software, but an external AC impedance meter can be used if necessary. Ideally, all electrodes should have relatively equal and low (<5 kΩ) impedance readings. If any channel reports a high level of interference, the electrode should be reapplied or replaced before starting the recording.

After verifying that each electrode is providing an optimal signal, perform biocalibrations to assess whether the sensors are in the correct location and recording appropriate physiologic information. The patient should be walked through a series of exercises designed to provide a baseline signal for each channel. It is important to document each of these instructions in the technologist notes and allow several seconds between each exercise to allow time for the signals to return to baseline.

Once all these items have been completed, the patient is ready for bed. Turn the lights off in the bedroom and document the “lights out” information in the technologist’s notes. It is common to record details such as the bedtime, body position, and oxygen saturation at the beginning of a study.

MONITORING THE PATIENT AND DOCUMENTING DURING THE RECORDING

Once the patient is in bed for the night, the pace of the night usually changes significantly, but that does not mean the work is done. Every patient must be carefully monitored over the course of the night to ensure the patient is safe, the necessary information is collected, and there are no disruptions to data collection.

Table 37-1 Example of a Sleep Apnea Diagnostic Montage Using the AASM-Recommended Technical Specifications^a

Channel	Derivation	Type	Sampling Rates		LFF (Hz)	HFF (Hz)
			Minimal (Hz)	Desirable (Hz)
1	F3-M2	EEG	200	500	0.3	35
2	F4-M1	EEG	200	500	0.3	35
3	C3-M2	EEG	200	500	0.3	35
4	C4-M1	EEG	200	500	0.3	35
5	O1-M2	EEG	200	500	0.3	35
6	O2-M	EEG	200	500	0.3	35
7	E1-M2 (LOC)	EOG	200	500	0.3	35
8	E2-M2 (ROC)	EOG	200	500	0.3	35
9	EMG-chin	EMG	200	500	10	100
10	ECG	ECG	200	500	0.3	70
11	Snore	Snore	200	500	10	100
12	Nasal	Nasal pressure	25	100	≤0.03	100
13	Oronasal	Airflow	25	100	0.1	15
14	Chest (thoracic)	Respiration	25	100	0.1	15
15	Abdominal	Respiration	25	100	0.1	15
16	Limb	EMG	500	200	10	100
17	SaO₂	Oximetry	10	25	N/A	N/A
18	Position	Body position	1	1	N/A	N/A
^aChannel selections and settings may vary based on sleep center policy and the sleep acquisition system utilized (1).
ECG, electrocardiogram; EEG, electroencephalogram; EMG, electromyogram; EOG, electrooculogram; HFF, high-frequency filter; LFF, low-frequency filter; LOC, left outer canthus; ROC, right outer canthus.

Q30 Documentation

One element of cohesive data collection is the detailed and professional documentation, throughout the night. Some sleep centers require Q30 documentation, which is a structured technologist note entered every 30 minutes. It is possible to program some of the digital systems to prompt for a technologist note at preselected intervals with a pop-up dialog box. The content of these notes varies depending on sleep center policy. Standard comments include information such as body position, PAP setting (if appropriate), heart rate, SaO₂, CO₂ levels (if available), and any other relevant patient updates like sleep stage, snoring, or unresolved artifact. Keep in mind that the technologist notes are part of the official medical record, so they should contain facts rather than opinions or speculations about a diagnosis.

Waveform Recognition

A sleep technologist must be able to read and interpret the data being collected to effectively run a sleep study. A basic knowledge of sleep stages is necessary to differentiate wake from sleep and assess the quality of sleep throughout the night. However, monitoring a patient is more than just identifying sleep spindles or rapid eye movements. A sleep technologist must also be able to understand how the various channels of data impact one another to truly understand whether the data being viewed are a true physiologic event or artifact requiring correction.

Sleep apnea in particular is a complex phenomenon that impacts multiple biologic systems. Abnormal breathing events not only impact respiration and oxygen saturation but also affect cardiac function, muscle
tone, and, of course, sleep consolidation. Therefore, the data should be interpreted as a whole in addition to assessing each channel independently.

Assessment of Sleep Disorders

The most common reason patients undergo overnight PSG is for evaluation of sleep-related breathing disorders (SRBDs), which is an umbrella term for disorders related to abnormal breathing during sleep where breathing is repeatedly interrupted. There are several different types of sleep-disordered breathing including obstructive sleep apnea (OSA), central sleep apnea (CSA), mixed sleep apnea (MSA), and upper airway resistance syndrome. During sleep, apneas can be caused by a physical obstruction, such as is seen in OSA, the most common type of SRBD. Breathing pauses can also be caused by a disruption of the central nervous system, as is seen in CSA. Each disorder has a spectrum of severity levels ranging from mild to severe.

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