A number of devices can be added to the polysomnograph to expand its usefulness as a diagnostic tool.
Laboratory testing for sleep disorders involves the monitoring of routine parameters. Recognition of sleep and its various stages requires the use of electroencephalography (EEG) to measure the activity of the central and occipital regions of the brain, electro-oculography (EOG) to measure the movement of the eyes, and electromyography (EMG) to measure the activity of the mentalis or submentalis muscles of the chin. Placement over the masseter muscle may be used in patients with bruxism. The respiratory parameters that are monitored include airflow, respiratory effort, and oxygen saturation. EMG monitoring of the anterior tibialis muscle of the leg is also recommended to rule out periodic limb movement disorders. During polysomnography for any purpose, electrocardiography (ECG) must also be used.
These typical monitoring parameters provide rich information to the investigator of sleep disorders. The polygraph, however, is a powerful instrument, and it is capable of monitoring much more. Essentially, anything that can be amplified or that has an electrical output can be added to a polysomnogram.
It is helpful to note the sleeper’s body position, as changes in breathing may have a positional component. Some patients experience sleep-disordered breathing (SDB) only in the supine position. If a significant positional component is present in the patient’s SDB, then positional retraining to keep the patient off his/her back (such as sewing a tennis ball onto the back of the shirt worn during sleep) may be all that is required. Body position can be observed by a technologist and noted on the polysomnogram. Frequently, however, the technologist has more than one patient to monitor and may miss a body-position change, noting it some time after the actual change. Because of this potential delay in making observational notes, many technologists monitor body position using a sensor.
This sensor is a device with a mercury switch triggered by movement. The mercury moves within the switch when there is a change in position. Each side of the switch produces a different voltage, which is translated into a signal that corresponds to a particular position. The resulting DC output is calibrated so that one tracing on the polysomnogram can represent body position.
Snoring, of course, is a classic sign of SDB. To track this symptom, one can use a microphone that converts tracheal noise into an electrical signal of relatively high frequency. The microphone’s output can go directly through the polygraph’s jack box and can be amplified using AC amplifiers.
Muscle activity can be monitored through the simple addition of an electrode over the muscle for which a measurement is desired. The information from any muscle is amplified using an AC amplifier. Additional EMG leads are needed if a patient being tested is suspected of having rapid-eye-movement (REM) sleep behavior disorder. This is a condition in which the normal suppression of muscle activity seen during REM sleep is inadequate; patients may, in essence, act out their dreams. A patient who has this disorder might move any muscle independently of other muscles. Therefore, it is useful to monitor as many muscles as possible, but muscles in the forearm, such as the extensor digitorum, are typically chosen. By monitoring both legs and both arms, the clinician can see the majority of movements. Respiratory- muscle monitoring is used for patients suspected of accessory muscle use that is a factor in SDB. Sternocleidomastoid and/or scalene muscles may be monitored while bilevel positive airway pressure is being evaluated for the treatment of patients with neuromuscular or lung diseases. This monitoring helps determine the efficacy of the treatment by recording the amount of reduction in accessory muscle use achieved through the application of bilevel positive airway pressure.
Heart rate (as opposed to ECG) monitoring may improve the identification of the bradycardia/tachycardia often associated with SDB. It is sometimes difficult to appreciate subtle changes in heart rate using the ECG alone, particularly at the display speed commonly used. A tachograph is a device used to convert the R-R interval signal from an ECG to a DC signal, which can be calibrated. The electrical output of the device is determined by the rate calculated from the ECG signal. Most pulse oximeters have a rate calculation function that is determined by the pulse detector already on the device. The pulsating interruption of the light shining through the finger corresponds to the heart rate obtained from the ECG, as long as there is sufficient perfusion of the finger. Pulse oximeters may be somewhat more prone than tachographs to movement artifacts. Many pulse oximeters have DC outputs that provide the heart-rate information obtained from the device, however, and their convenience of use generally outweighs any disadvantages.
Measurement of carbon dioxide levels is useful for many patients. Whenever it is suspected that a patient’s problem is partly due to hypoventilation, carbon dioxide monitoring may be helpful. It is also very important in pediatric patients as changes in airflow may not be very pronounced during respiratory events. The primary problem associated with this type of monitoring is determining the best method of measuring carbon dioxide. A capnograph collects exhaled gas from the patient and analyzes the gas to determine the amount of carbon dioxide present in the sample. In a spontaneously breathing patient, this can be accomplished using a nasal cannula. The pressure generated during exhalation is so low that a vacuum pump is needed to extract the sample into the capnograph.
Capnography is sometimes used in place of other measures of airflow. The disadvantage of using capnography in this manner is that obstruction of the nose, whether due to significant anatomical obstruction (such as septal deviation) or redundant tissue from the palate, may create difficulties. It may also be hard to obtain stable readings when capnography is used in conjunction with positive airway pressure because high flows through the circuit may dilute the sample.
Transcutaneous monitors determine carbon dioxide levels by heating an area on the surface of the skin and measuring the amount of carbon dioxide beneath the probe. Heating the area with good capillary blood flow will allow carbon dioxide to be released to the surface of the skin. Although this eliminates the problem of high flows from positive airway pressure devices, there are other problems associated with transcutaneous monitors. The probe may burn a patient, particularly if it is not moved periodically. In obese patients, transcutaneous monitoring may be less accurate because of the diminished perfusion of areas with large amounts of adipose tissue. DC outputs from either capnographs or transcutaneous monitors may be added directly to a polygraph.
EOG is routinely employed in polysomnography, using two electrodes (one for each eye). There is a small electrical polarity difference inside the eye. The cornea is positive with reference to the retina. Movement of the eye is measured by comparing changes in electrical potential as the cornea moves closer to or away from the monitoring electrode. When two electrodes are used, both are placed on the outer canthi, with one electrode placed higher than the center of one eye and the other electrode placed lower than the center of the other eye. This method of placement allows vertical and horizontal movements of the eye to be captured, but may miss oblique eye movements. When a more complete identification of eye movement is critical to analysis, two additional electrodes, placed directly above and below the center of the eye will aid in identifying oblique eye movements. The use of these additional EOG electrodes is recommended to aid in identifying otherwise equivocal REM sleep during a multiple sleep latency test.
Esophageal pH monitoring is used for patients suspected of having gastro-esophageal reflux. Contents from the stomach flow into the esophagus for a variety of reasons, and this is often a cause of otherwise unexplained awakenings from sleep. It is also a factor in neonatal sleep disorders. A pH probe is inserted into the esophagus to measure pH levels. A pH of less than 4 indicates the presence of gastric contents in the esophagus. The amount of time that acid contents remain in the esophagus is referred to as the acid contact time, and this should be less than 4%to 55 The pH monitor is connected to the polygraph via DC amplifier.
Some patients experience seizure activity primarily during and around the time of sleep. Polysomnography performed to rule out seizures requires the use of additional EEG sites (besides those used for routine polysomnography) to aid in isolating the focus of the seizure activity. In addition, video monitoring that is synchronized with the polysomnogram is useful in correlating a questionable EEG finding with the associated behavior. Synchronized devices generally take a time cue from the polygraph and use it to synchronize sections of videotape with the polysomnogram. With the advent of digital photography, it is now possible to synchronize digital images with the polysomnogram so the entire recording is stored in a single digital file (or multiple linked files).
Blood pressure is sometimes monitored during polysomnography. This monitoring, however, is generally done in research settings because of the distracting or intrusive nature of this procedure. Automatic blood pressure monitoring devices that inflate and deflate a blood pressure cuff may be applied. The resulting data are then sent to the polygraph via DC amplifier. The inflation and deflation of the cuff may be disruptive to subjects and impair their ability to sleep. An arterial catheter may be placed in line and attached to pressurized and fluid-filled tubing. The tubing is then connected to a pressure transducer, with the output going to the polygraph via DC amplifier. Although this may be less disruptive to the patient’s sleep, it is invasive, and it may be painful, particularly during catheter insertion. Neither method of continuous monitoring of blood pressure is commonly used for clinical studies.
POSITIVE AIRWAY PRESSURE
Positive airway pressure therapy, which may be either continuous or bilevel, is frequently used to treat SDB. Information relating to the positive airway pressure level can be recorded on the polysomnogram. A pressure transducer, either attached (via tubing) to the positive airway pressure mask or placed within the circuit, can be used to monitor positive airway pressure. The transducer is connected to a DC amplifier that can be calibrated. The signal is then sent directly to the polygraph, which records pressures. Because most positive airway pressure devices now have built-in DC outputs that provide the same information, however, the sensor arrangement is generally no longer necessary. Many positive airway pressure devices also incorporate outputs for estimated flow to the patient and the total flow being delivered. The estimated patient flow rate is frequently used in lieu of other airflow monitoring devices. If the total flow is added, the difference between the two flow values provides an indication of the leakage present in the system. Many devices will also produce a calculated tidal volume output that may be used to replace other measurements of airflow.
Polysomnography is a widely used tool for testing patients with sleep disorders. Often, the standard set of measurement parameters is all that technologists encounter. It is clear, however, that the polygraph can be used to obtain a wider variety of data. In fact, a technologist may choose to add other devices to the polysomnogram for the monitoring of body position, heart rate, and blood pressure and for the detection of snoring, unusual muscle activity, oblique eye movement, gastroesophageal reflux, or seizures. This
flexibility in data gathering makes polysomnography an interesting and challenging-as well as powerful-diagnostic and research tool.
Robert Turner, RPSGT, RRT, is Clinical Supervisor of the Rose Sleep Disorders Center, Denver, and president-elect of the Association of Polysomnographic Technologists.