The EEG frequencies of interest range from less than 1 cycle per second (i.e., <1 Hz) to 14 Hz. Therefore, a low-frequency filter setting of 0.3 Hz (which allows the detection of slow waves) and a high-frequency filter setting of 35 Hz (which allows the detection of alpha waves) are typically used for the EEG channels.

A low-frequency filter setting of 0.3 Hz allows the detection of the slow-rolling eye movements that occur with sleep onset, and a high-frequency filter setting of 35 Hz allows the detection of rapid eye movements (REMs) and the fast, low-amplitude EOG activity occurring throughout sleep.

A low-frequency filter setting of 10 Hz and a high-frequency filter setting of 100 Hz are used to detect EMG frequencies of the skeletal muscles (e.g., legs and chin). This range is sufficient to record the faster firing rate of the muscles (i.e., increased muscle tone) during arousals, wake, or nonrapid eye movement (NREM) sleep and the decreased firing rate of the muscles (i.e., atonia [lack of muscle tone]) during REM sleep.

A low-frequency filter setting of 0.3 Hz and a high-frequency filter setting of 70 Hz are used to detect ECG frequencies. The low-frequency filter setting of 0.3 Hz (i.e., 0.3 cycles per second = 1 cycle per 3.33 seconds) allows the recording of each heartbeat (which normally takes about 0.8 seconds) and a high-frequency filter setting of 70 Hz (i.e., 70 cycles per second = 1 cycle per 0.014 second) allows the recording of a heartbeat’s individual waves (i.e., P, QRS complex, and T wave, the duration of these waves ranges from <0.10 to 0.25 seconds).

The low-frequency filter setting is 0.1 Hz and the high-frequency filter setting is 15 Hz for the respiratory effort sensors, usually, respiratory inductance plethysmography belts. (Some centers may use piezoelectric belts for recording respiratory effort; however, the AASM no longer considers these belts acceptable for recording respiratory effort.) The low setting allows the slower sinusoidal waves of inhalation and exhalation to be recorded, whereas the high setting excludes frequencies that would obscure these waves.

For the airflow sensors (thermistor or thermocouple, pressure transducer), the low-frequency filter setting is 0.1 Hz and the high-frequency filter setting is 15 Hz. As with the respiratory effort channel, these settings allow the sinusoidal waves of inhalation and exhalation to be recorded while excluding frequencies that would obscure these waves.

The retina has a negative charge (reflected by an upward deflection on the EOG channel) and the cornea has a positive charge (reflected by a downward deflection on an EOG channel). The difference between the two, called the “corneoretinal potential,” is approximately -60 mV. The corneoretinal potential makes it possible to record eye movements from sensors placed on the outer canthus of each eye. When the eyes move in the same direction, one electrode will be close to the retina and record a negative charge, whereas the other electrode will be close to the cornea and record a positive charge.

To ensure equal amplitude of conjugate eye movements, the AASM recommends placing one EOG electrode 1 cm above and 1 cm lateral to the midline of the outer canthus of one eye (typically the right eye) and one EOG electrode below and 1 cm lateral to the midline of the outer canthus of the other eye (typically the left eye). In general, both EOG electrodes are referenced
to the same mastoid site (M2); however, it is acceptable for each EOG electrode to be referenced to the contralateral ear mastoid reference (i.e., E1-M2 and E2-M1). If both electrodes are placed above or below the midline of the outer canthus, the EOG signals will deflect in phase (i.e., in the same direction) rather than out of phase (i.e., in opposite directions). When using the recommended placement, the out-of-phase deflections of the EOG signals are greater for horizontal eye movements than for vertical eye movements.

An alternate placement allowed by the AASM for the EOG electrodes is to place each electrode 1 cm below and lateral to the midline of the outer canthus of its respective eye. Both electrodes are referenced to Fpz. In this configuration, vertical eye movements are in phase and horizontal eye movements are out of phase (Figs. 35-1 and 35-2).

An advantage of using the AASM’s recommended eye placement is that all conjugate eye movements produce out-of-phase deflections. However, the deflections of vertical eye movements are greater than those of horizontal eye movements and eye blinks. As a result, it can be difficult to determine the direction of all eye movements, and eye movements with low amplitude may be missed. Using the alternate AASM eye placement, the vertical and horizontal eye movement deflections are
more prominent, compared with those of the recommended placement, and the signals for eye blinking are more readily detected (as in-phase deflections). However, with the alternate placement, the Fpz electrode may allow some EEG activity in the EOG recording, which can make it difficult to distinguish EEG activity from EOG activity, particularly during vertical eye movements. Even if electrodes are correctly placed using the recommended or alternate EOG electrode placement, several physiologic conditions can affect the recording of biopotential changes in an EOG channel.

Problems recording biopotential changes on the chin or leg EMG channels usually result from improper electrode placement, from high impedances (e.g., the skin is insufficiently cleaned), or from electrodes becoming dislodged during sleep. To reduce the likelihood of poor signal quality in the EMG channels, the technologist must show the same care in preparing the electrode sites as when applying EEG and EOG electrodes, and he or she should obtain a low-impedance reading (<10 Ohms [Ω]). The electrodes must also be securely attached to the chin or legs to prevent their being easily dislodged during the night. For example, an extra piece of tape may be placed on the leg sensors to ensure that they remain on the patient during the night. However, vigorous movement by a patient may dislodge an electrode, no matter how well secured. If the PSG recording system provides re-referencing capabilities, the EMG channel can be re-referenced to a backup electrode. If necessary, electrodes may need to be reapplied.

Problems with the ECG recording are generally caused by poorly attached or dislodged ECG electrodes. If an ECG electrode becomes dislodged during the study (Fig. 35-4), an ECG tracing may be temporarily obtained from alternate electrode derivations, provided that the PSG recording system offers re-referencing capabilities. If re-referencing is not possible, the electrode will need to be reapplied.

People with certain neurologic problems that affect the abdominal and thoracic muscles or who have chest deformities such as kyphosis (commonly called “hunchback”) may be unable to fully expand and contract the chest wall. This insufficient movement may result in reduced respiratory signals, even if the abdominal and thoracic sensors are properly placed on the patient. The technologist may, therefore, need to increase the sensitivity of a respiratory channel to record respiratory signals.

A common problem with recording airflow pressure changes is that the pressure transducer sensor may become plugged by excessive mucus, especially in a person with an upper respiratory infection or sinus infection. This obstruction will interfere with the proper recording of the signal. The technologist may need to clean or replace the sensor during the study to obtain a clear airflow signal. Another common problem with the airflow transducer sensor is that the air tube becomes kinked or the patient lies on the air tube. The technologist may need to enter the patient’s room to unkink the air tube or dislodge it from under the patient.

For these channels, it is important that the eye signals of each channel deflect in opposite directions when the person’s eyes move conjunctively (i.e., in the same direction). To mimic the conjunctive eye movements of REM sleep, ask the patient to look left and right five times, then up and down five times (Fig. 35-5). As a result of the corneoretinal potential, the signal on one eye channel should deflect upward and the signal on the other eye channel should deflect downward with each movement. (However, if one uses the alternate EOG electrode placement, horizontal eye movements produce deflections in the opposite direction and vertical eye movements produce deflections in the same direction.)

The next physiologic calibration that involves the eyes elicits alpha waves. To do this, ask the patient to close the eyes for 30 seconds and then open the eyes for 30 seconds. In most people, when the eyes are closed, alpha waves become prominent in the EEG, especially in the occipital channels. When the eyes are opened, the EEG resumes its original waking activity, which consists of a mixture of fast, low-frequency waves, primarily alpha and beta waves (the latter of which have a frequency of 18 to 30 Hz) (Fig. 35-6).

The third physiologic calibration involving the eyes involves having the patient blink the eyes five times rapidly (Fig. 35-7). During wake, a patient may blink at a rate of 0.5 to 2.0 Hz. As drowsiness develops, the blinking rate begins to slow and may be replaced by slow oscillating eye movements.

For this channel, it is important to sufficiently detect muscle tone when the person is in NREM sleep. To ensure muscle tone is being recorded appropriately, ask the patient to grit the teeth together or chew (for 5 seconds), and then relax (Fig. 35-8). There should be greater than or equal to 0.5 cm increase in the amplitude of the signal, which reflects the increase in muscle tone. For patients who have no teeth, ask them to open the mouth or yawn instead.

For this channel, it is important that the sensor detects the increased vibration of the upper airway muscles that occurs with snoring. Ask the patient to make a snoring noise or hum for 5 seconds (Fig. 35-9). If the patient is unable to make a sufficient snoring noise, an alternative is to ask the patient to make a vocalization such as counting “1, 2, 3,” or saying “A, B, C.” The signal should increase briefly with each vocalization.

For these channels, it is important to detect the leg movements that occur with periodic leg movement (PLM) disorder or restless legs syndrome. Some patients with PLM disorder may bend the legs at the knee or hip level; however, more often, a person with PLM disorder may flex the ankle or toes. Therefore, it is important that the leg sensors are able to detect the smallest leg movements (e.g., flexing of the toes) occurring during a sleep study. To mimic PLMs, ask the patient to flex the foot/raise the toes of the right (or left) foot five times and then to flex the foot/raise the toes of the opposite foot
five times (Fig. 35-10). There should be a burst of activity with each flex.

For these channels, it is important that the airflow sensor (i.e., thermistor or thermocouple) and pressure transducer sensor are detecting airflow and air pressure changes, respectively, occurring with inhalation and exhalation. It is also important that these sensors detect the lack of airflow during an apnea episode. To ensure that the airflow sensors are detecting nasal breaths and oral breaths, ask the patient to breathe through the nose only for 10 seconds and then to breathe for 10 seconds through the mouth only (Fig. 35-11). As the person inhales and exhales, the thermistor or thermocouple should generate sinusoidal signals on the airflow channel. Nasal pressure transducers typically only measure nasal airflow; therefore, the signal in this channel should be flat during oral breathing maneuvers.

To mimic apnea, ask the patient to take a deep breath in and hold it for 10 seconds. The airflow and air pressure signals should be flat because no air is passing through the nose or mouth (Fig. 35-12).

For these channels, it is important that the sensors detect the movements of the abdomen and chest during normal respiration and during an apnea. During the airflow tests, the abdominal and thoracic effort channels should have a sinusoidal signal moving in correlation with the airflow signal for inhalation and exhalation. When the patient is holding the breath to mimic apnea, the abdominal and thoracic channel signals should be flat (see Fig. 35-12).

Another respiratory feature usually tested during physiologic calibrations is paradoxical breathing—the movement of the abdomen and thorax in opposite directions—when the upper airway is fully blocked but the person continues to make respiratory efforts to breathe. Paradoxical breathing can occur during an obstructive sleep apnea (OSA) episode. To mimic paradoxical breathing, ask the person to hold the breath while making breathing movements. An alternative is to ask the person to pant. Either of these actions will cause the thoracic and abdominal channel signals to move in opposite directions (Fig. 35-13).

The ECG signal is not tested as part of the physiologic calibrations procedure. However, the technologist should note that the ECG signal has the correct polarity. The P wave should deflect upward, the QRS complex should have a down-up-down deflection, and the T wave should deflect upward. If the waves are in the wrong directions, the polarity should be corrected before lights out (Figs. 35-14 and 35-15).