POLYSOMONOGRAPHY BY AHMAD YOUNES PROFESSOR OF THORACIC MEDICINE Mansoura Faculty of Medicine
The standard parameters used to record sleep and wake : 1-electroencephalography (EEG), 2- electro-oculography (EOG), 3-electromyography (EMG), 4- airflow measurement, 5-respiratory effort measurement, 6-electrocardiography (ECG), 7- oxygen saturation, 8- snoring monitor, and sleep position evaluation.
ELECTROENCEPHALOGRAM AND THE 10-20 ELECTRODE SYSTEM The EEG records local graded electric potentials generated by the cerebral cortex and other structures, including the thalamus. The term derivation refers to a set of two electrodes (and the voltage difference between the electrodes). The term montage refers to a particular set of derivations. A referential derivation includes an active signal and a passive one. A bipolar derivation includes two active signals. Most derivations used in routine polysomnography are referential in nature. International Federation 10–20 system of electrode placement Each electrode site is designated with a letter or letters and a number. The letters FP, F, C, P, and O represent frontal pole, frontal, central, parietal, and occipital, respectively. M represents the mastoid process. Odd numbers are used to denote the left-sided electrode placements; even numbers are used to denote the rightsided electrode placements. ‘‘Z’’ denotes midline electrode placement sites
The 10–20 map is derived by using four major landmarks: The nasion (the intersection of the frontal and two nasal bones, which is the depressed area between the eyes that is just superior to the bridge of the nose), the inion (the most prominent projection of the occipital bone in the lower rear part of the skull), and the left and right mastoid areas. The electrode locations are based on these landmarks.
The 10–20 map is derived by using four major landmarks: The ‘‘10’’ and ‘‘20’’ of the system’s name indicate the distance between the adjacent electrodes are either 10% or 20% of the total front-to-back and mastoid-to- mastoid distance. To determine the potential electrode sites, the head circumference is measured with a tape measure. The normal adult circumference is 50 to 65 cm. This number is divided into the 10% and 20% measurements from the nasion to the inion on each side of the head. The nasion-to-mastoid distance is a total of 50% of the distance from the nasion to the inion
The recommended location of the electrodes was 1 cm above and slightly lateral to the outer canthus of one eye, with a reference electrode on the ipsilateral earlobe or mastoid. The second eye movement channel was to be recorded from an electrode 1 cm below and slightly lateral to the outer canthus of the other eye, referred to as the contralateral ear or mastoid (both eyes referred to same reference electrode). This arrangement produced the so-called ‘‘out of phase’’ deflections with most any conjugate eye movements, whereas EOG artifact produced inphase or single-channel deflections
Eye Movement Patterns: Eye blinks: Conjugate vertical eye movements at a frequency of 0.5–2 Hz present in wakefulness. Slow eye movements: Conjugate, fairly regular, sinusoidal eye movements with an initial deflection lasting > 500 msec. are typical of eyes closed drowsy, wakefulness, and stage N1 sleep. Rapid eye movements (REMs): Conjugate, irregular, sharply peaked eye movements with an initial deflection usually lasting < 500 msec. Whereas rapid eye movements are characteristic of stage R sleep, they may also be seen in wakefulness with eyes open Reading eye movements: Trains of conjugate eye movements consisting of a slow phase followed by a rapid phase in the opposite direction as the subject reads. due to a slow scan of the written page (left to right) followed by a rapid return to the left.
ELECTROMYOGRAM The recording of muscle activity for the purposes of polysomnography is performed using surface electrodes instead of needle electrodes. The recording provides data about muscle areas or regions as opposed to specific muscles. The electrodes are usually held in place with short pieces of tape. It is recommended to use three electrodes to record the chin EMG. One electrode is placed in the midline 1 cm above the inferior edge of the mandible, another electrode is placed 2 cm below the inferior edge of the mandible and 2 cm to the right of the midline, and the last electrode is placed 2 cm below the inferior edge of the mandible and 2 cm to the left of the midline. The standard derivation is either of the electrodes below the mandible referred to the electrode above the mandible
A 30-second tracing shows a reduction in the chin EMG on transition to stage R sleep (A). Note saw-tooth waves (B) and REMs (C).
When determining the presence of periodic limb movements of sleep Surface electrodes are placed longitudinally and symmetrically around the middle of the anterior tibialis muscle such that the electrodes are 2 to 3 cm apart. It is recommended that the legs be evaluated with separated channels. Monitoring of the upper extremities may be performed if clinically indicated.
AIRFLOW MEASUREMENT 1- Thermal sensors, or thermistors, use the difference between the temperature of exhaled breath (heated by the human body) and the ambient air. These sensors are placed at the nose and mouth in the path of inspiratory and expiratory airflow. 2- Nasal pressure transducers monitor the pressure changes that occur with inspiration and expiration. Relative to the atmospheric pressure, the airway is negative with inspiration and positive with expiration. The signal obtained is used to estimate air flow and provides a breath-by-breath graphic representation of the size of each breath, i.e. flow amplitude.
RESPIRATORY EFFORT MEASUREMENT Normally the expansion of the rib cage and enlargement of the abdomen occur at the same time or are ‘‘in phase.’’ Out-of-phase or so-called ‘‘paradoxic’’ motion of the thorax and abdomen can be seen with a loss of tone in the diaphragm or accessory muscles of respiration. Of more clinical significance, paradoxic motion can be seen with partial and complete obstruction of the airway, more commonly with the latter. It is currently recommended that either esophageal manometry or calibrated or uncalibrated inductance plethysmography be used to monitor respiratory effort, although an accepted alternative is diaphragmatic/intercostal EMG
CARDIAC RHYTHM EVALUATION BY ELECTROCARDIOGRAPHY
OXYGEN SATURATION Pulse oximetry. In this method SaO2 is determined by the passage of two wavelengths of light (650 nm and 805 nm) through a pulsating vascular bed from one sensor to another. The light is partially absorbed by the oxygen-carrying molecule, hemoglobin, depending on the percent of the hemoglobin saturated with oxygen. A processor calculates absorption at the two wavelengths and computes the proportion of hemoglobin that is oxygenated, giving it a numerical value. A thin anatomic pulse site (such as the finger tip, ear lobe, nose, or toe) is required, as is proper alignment of the sensors. With movement in sleep, the device can become dislodged. The readings can also be affected by anemia, hemoglobinopathies, a high carboxyhemoglobin level, elevated methemoglobin level, anatomic abnormalities/previous injury to the site tested, sluggish arterial flow (due to hypovolemia or vasoconstriction),and the use of nail polish.
OTHER MONITORING DEVICES Microphone, which is most commonly placed on the neck or upper chest, to record snoring sounds. Body position is also an important piece of information to monitor in the sleep laboratory. Body position is determined by a sensor, which is usually placed on the chest. Monitor the subject’s body position via camera/video
SLEEP SYSTEM SPECIFICATIONS In the current sleep medicine era, fully computerized polysomnography has replaced paper-based analog recording. Although it is still the standard to display 30 seconds of digital data on a computer screen, the digital equipment also allows compression of the time scale to allow viewing of 1 to 10 minutes of data at a time. This is particularly valuable when a bird’s eye view is helpful in determining a subject’s sleep-related breathing pattern.
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