Bio-signals

A PSG will record eight channels in our system. Two channels are for the Electroencephalogram (EEG), one measure airflow, one is for chin movements (EMG), two for eye movements (EOG), one for heart rate (EKG), one for oxygen saturation.

The channel of oxygen saturation is recorded by Nonin Onyx^® 9560 (Figure 2.2) that is a Bluetooth wireless fingertip pulse oximeter. The other channels are recorded by self-developed portable PSG (Figure 2.3).

Figure 2.2: Nonin Onyx^® 9560, Nonin Medical Inc.

A. Electroencephalogram

Electroencephalography (EEG) is the measurement of electrical activity produced by the brain as recorded from electrodes placed on the scalp. When measuring from the scalps, recorded the EEG signal is about 20-200uV for a typical adult human. And a common system reference electrode is connected to the other input of each different amplifier. These amplifiers amplify the voltage between the active electrode and the reference (typically 1,000–100,000 times, or 60–100 dB of voltage gain). The EEG is typically described in terms of rhythmic activity and transients. The rhythmic activity is divided into bands by frequency. The common band of EEG is shown as Table 2.1.[16]

Table 2.1: Common band of EEG [16]

Type Frequency (Hz)

Delta Up to 3Hz

Theta 4 – 7Hz

Alpha 8 – 13Hz

Beta 13 - 30Hz

In PSG system, the electroencephalogram (EEG) will generally use three

"exploring" electrodes and two "reference" electrodes, unless a seizure disorder is suspected, in which case more electrodes will be applied to document the appearance of seizure activity. The exploring electrodes are usually attached to the scalp near the frontal, central (top) and occipital (back) portions of the brain via a paste that will conduct electrical signals originating from the neurons of the cortex. These electrodes will provide readout of the brain activity that can be "scored" into different stages of sleep (N1, N2, N3 which combined are referred to as NREM sleep and Stage R which is rapid eye movement sleep or REM, and Wakefulness). EEG electrode position is

determined by international 10-20 system[17]. The recommended derivations are shown as Figure 2.4[18].

Figure 2.4: The recommended derivations of EEG (a) C4-M1; (b) O2-M1[18].

B. Electrooculogram

The electrooculogram (EOG) uses two electrodes; one that is placed 1 cm above the outer canthus of the right eye and one that is placed 1 cm below the outer canthus of the left eye. These electrodes pick up the activity of the eyes in virtue of the electropotential difference between the cornea and the retina (the cornea is positively charged relative to the retina). This determines when REM sleep occurs, of which rapid eye movements are characteristic, and also essentially aids in determining when sleep occurs. The recommended derivations are shown as Figure 2.5[18].

Figure 2.5: The recommended derivations of EOG[18].

C. Electromyogram

The Electromyogram (EMG) typically uses four electrodes to measure muscle tension in the body as well as to monitor for an excessive amount of leg movements during sleep (which may be indicative of Periodic Limb Movement Disorder, PLMD).

Two leads are placed on the chin with one above the jaw line and one below. This, like the EOG, helps determine when sleep occurs as well as REM sleep. Sleep generally includes relaxation and so a marked decrease in muscle tension occurs. A further decrease in skeletal muscle tension occurs in REM sleep. A person becomes partially paralyzed to make acting out of dreams impossible, although people that do not have this paralysis can suffer from REM Behavior Disorder. Finally, two more leads are placed on the anterior tibialis of each leg to measure leg movements. The EMG electrodes position is shown as Figure 2.6[18].

Figure 2.6: The EMG electrodes position[18].

D. Electrocardiogram

Though a typical electrokardiogram (ECG or EKG) would use ten electrodes, only two or three are used for a polysomnogram. They can either be placed under the collar bone on each side of the chest, or one under the collar bone and the other six inches above the waist on either side of the body. These electrodes measure the electrical activity of the heart as it contracts and expands, recording such features as the "P" wave,

"QRS" complex, and "T" wave. These can be analyzed for any abnormalities that might be indicative of underlying heart pathology. The EKG electrodes position is shown as Figure 2.7[18].

Figure 2.7: A single modified ECG Lead II use torso electrode placement[18].

E. Nasal Airflow

Nasal and oral airflow can be measured using pressure transducers, and/or a thermocouple, fitted in or near the nostrils; the pressure transducer is considered the more sensitive. This allows the clinician/researcher to measure rate of respiration and identify interruptions in breathing. Respiratory effort is also measured in concert with nasal/oral airflow by the use of belts. These belts expand and contract upon breathing effort. The current guidelines recommend the use of a thermal sensor, which is placed in ht e patient’s nostril to detect the apnea; the nasal pressure transducer is used for identifying hyzone. Ideally, both the sensor and transducer should be used. The nasal airflow sensor position is shown as Figure 2.8[19].

Figure 2.8: The nasal airflow sensor is placed to record breathing during sleep[19].

F. Pulse oximetry

Pulse oximetry helps determine changes in blood oxygen levels that often occur with sleep apnea and other respiratory problems. The pulse oximetry fits over a finger tip or an ear lobe. The pulse oximetry sensor position is shown as Figure 2.9.

Figure 2.9: The pulse oximetry sensor position.