What happens at polysomnography?

What happens at polysomnography?

Polysomnography (PSG) is a sleep registration of clinical neurophysiology, which makes it possible to study the quantity, structure and quality of sleep. The study lasts all night and is usually performed in specialized sleep center or a major hospital. Polysomnography (PSG)  is used to study sleep disorders such as breathing breaks during sleep, hypersomnia, insomnia, nocturnal movement disorders and parasomnias.

How the information is collected in a polysomnography?

Sleep study polysomnography measures several physiological functions of a patient. By measuring electroencephalography (EEG), eye movements / electrooculography (EOG), and the patient’s jaw muscle tension / electromyography (EMG), the patient’s sleep stages can be determined. Polysomnography also measures respiratory movements with strain gauges on the thorax and diaphragm, respiratory airflow from the nose and mouth and blood oxygen saturation. The study measures the function of the tibial muscles, the electrical activity of the heart by electrocardiography (ECG) and the position of the patient. A sleep mattress can also be used for the measurement, which can be used to measure breathing, heart function and motor skills. The examination is usually recorded, showing the patient’s movements and other activities.

How is the polysomnography sleep study performed?

The polysomnography examination is performed in the videotelemetry unit of clinical neurophysiology or, if necessary, in the ward. An adult patient arrives for the study in the evening after 6 pm with a sleep diary for two weeks and a sleep questionnaire. The patient’s identity is verified and he is introduced to the facilities and the course of the examination. Pediatric patients arrive at the videotelemetry unit at a separately agreed time in the evening, according to the child’s sleep rhythm. In infants, the study can also be performed as a daytime study. In pediatric patients, the guardian or guardians usually come into the study. Their presence is usually necessary and the parent is arranged to spend the night in the examination room.

How are measuring devices placed in the polysomnography?

The evening attendant attaches the electrodes to the patient’s head and face according to the wiring diagram. The wiring diagram has been selected by the clinical neurophysiology physician uttering the study when reviewing the patient referral. The skin of the patient’s head is cleaned with a cotton swab moistened with alcohol and rubbed with salt paste. An electrode paste is applied to the electrodes to improve conductivity and is attached to the scalp with an electrode paste and gauze pads. Finally, a mesh cap is placed on the head to hold the electrodes in place. The patient is then placed with eye movement, ECG, and jaw EMG electrodes in the correct locations on the face and body. The nurse measures the patient’s diaphragm and chest circumference with a tape measure for the breathing belts that the night nurse attaches before the patient goes to bed. In pediatric patients, the breathing belts are cut to the appropriate size from the breathing belt roll.

What happens at polysomnography?

Before going to bed, the night nurse attaches the remaining electrodes to the patient. In the center of the patient’s chest, the nurse attaches the EMBLA patient unit. The device is fastened tightly enough so that it does not move when the patient changes position during the night. The correct size breathing belts are selected according to the dimensions and are fastened around the patient around the chest and diaphragm with a snap. The breathing belt leads are attached to the patient unit according to color coding. A pressure sensor and thermistor are placed in front of the patient’s nose and mouth. The pressure sensor is placed in both nostrils so that it does not press on the walls of the nostrils. The thermistor attached to the pressure sensor is bent in front of the mouth for a distance of about 10-15 mm so that it does not touch the skin. The cord and hoses are twisted behind the ears and tightened in place under the chin. A pulse oximeter is attached to the patient’s finger and a snoring sensor is placed on the neck where the vibration caused by the snoring is best felt. EMG electrodes are attached to the patient’s feet and the sleeping mattress is placed on the bed at the chest level. In children, diaphragm EMG electrodes are also placed on the diaphragm. Finally, the electrode wires are bundled together and tied with a gauze roll into one bundle. The electrodes are connected to the connection panel.

The equipment is tested before the actual polysomnography

After attaching the electrodes and tying the wires together, the subject is asked to go to lying on bed. The hardware and software are started, followed by electrode testing. Children are first tested for the function of the diaphragm EMG channels by the child blowing a whirl or breathing deeply so that the book placed on top of the diaphragm rises up and down. The time of testing is accurately marked on the curve. In addition, movement disorders, eye movements, bite-induced muscle disturbance, function of respiratory sensors, position sensor, and foot EMG electrodes are tested. Once the tests are done, the room is darkened and the night camera is turned on. The start time of sleep waiting is marked on the curve. The nurse monitors the patient during the night and marks sleep events on to the curve. The nurse is responsible for the technical quality of the study.

How EEG electroencephalography functions in polysomnography?

Polysomnography measures electroencephalography (EEG), which records the electrical activity of the brain using electrodes placed on the surface of the head. The electrical activity to be measured arises from the simultaneous activation of thousands of parallel cerebral cortex neurons, which changes the membrane voltage of the cells. The change in the membrane voltage of nerve cell masses can be measured as voltage differences between the electrodes. Brain electrical activity consists of voltage fluctuations of different frequencies, usually between 0.16 and 70 hertz (Hz). The activity is divided into different frequency functions: alpha, beta, theta and delta. EEG is commonly used in the diagnosis and monitoring of epilepsy. EEG is also an important study in the diagnosis of encephalitis, ie inflammation of the brain, Creutzfeldt-Jakob disease and seizure events, and brain death. The EEG is also part of sleep studies, such as sleep polysomnography.

The alpha activity is at a frequency of 8–13 Hz, which occurs in the back of the brain in the area of ​​the cerebral cortex when a person is in a relaxed state of alertness with their eyes closed. Approximately 10 Hz activity also occurs in sensory cortical areas at rest, but the activity is referred to as alpha-period activity. The alpha rhythm is attenuated when the eyes are opened and sensory information enters the cerebral cortex. Operation above 13 Hz is called beta operation. It occurs when the motor cortex is at rest and also in the primary sensory cortex. Beta activity is attenuated, for example, as a result of hand movements or sensory stimuli. Faster than beta action is gamma action (approximately 40 Hz), which occurs in situations requiring special attention, but this is not usually seen in a standard EEG. The theta activity has a frequency of 4–8 Hz and is widespread in the temporal region. Especially when children fall asleep, theta activity is common. The frequency of delta activity is less than four hertz and it occurs in a healthy adult only during sleep.

What happens at polysomnography?
FIGURE 1 / 10–20 system (EEG)

EEG registration generally uses surface electrodes that are placed in place with a rubber band cap or using a cap with the electrodes attached. Electrodes are electrically conductive plates that convert the potential transmitted to the skin from the brain into an electron current flowing through the conductors to the measuring circuit. Normally, the electrodes are placed according to a 10-20 system (Figure 1), but a 10-10 system is also used. The electrodes are named with letters derived from the name of the brain blocks, frontal, temporal, parietal, and ocular lobe blocks, and the names of other locations of the electrodes. In addition, numbers are used to distinguish the right and left hemispheres of the brain. To the left of the electrodes marked with odd-numbered and right sides of the even-numbered electrodes.

The same EEG connections are used for polysomnography registration in both children and adults. In the standard study, the EEG circuits are F4, C4, O2, F3, C3, O1, A2, A1, and CZ, which is the reference electrode. In an extensive polysomnography study, the connections are F4, C4, O2, F3, C3, O1, A2, A1, P4, P3, Fp1, Fp2, F7, F8, Fz, Pz, T3, T4, T5, T6 and Cz as the reference electrode.

How sleep phases are registered?

Sleep can be divided into different sleep stages. These sleep stages include non-REM sleep classes S1, S2, S3, and S4, as well as REM sleep. The classification of sleep phases is based on 30-second sections, i.e. epochs. Epochs are classified into some sleep stage, so each epoch describes one sleep class. Sleep stages are recorded by measuring the patient’s cerebral electrical activity, eye movement, and jaw muscle tension. According to the standard sleep phase classification, EEG is registered in either C3-A2 or C4-A1 derivatives. However, additional electrodes often have to be used. The American Academy of Sleep Medicine (AASM) has published more recent guidelines for EEG registration with F4-M1, C4-M1, and O2-M1 derivatives.

Eye movements, or EOG, are registered laterally above the left eye and below the right eye with reference connections. According to the AASM guideline, the left eye electrode is placed one centimeter below the outer corner of the eye and the right eye electrode one centimeter above the outer corner. The measurement uses bipolar coupling to measure the voltage differences between the cornea and the retina of the eye. In order to determine the sleep phases, the activity of the jaw EMG is measured. Muscle tension in the jaw decreases significantly in REM sleep. Muscle tension is measured according to the AASM guideline with three electrodes placed on both sides of the chin and in the middle of the chin.

What happens at polysomnography?
FIGURE 2 / Sleep phase classification signals

Figure 2 shows the patient’s EEG signal with six connections at the top, the EOG signal with two connections in the middle stages, and the jaw EMG signal at the bottom. The picture shows a deep sleep, when K-complexes are seen in the EEG. The K-complex consists of a sharp negative wave followed by a positive wave. In deep sleep, slow delta activity is also seen in the eye movement channels. Jaw EMG shows no activity.

How breathing during sleep is registered?

Respiration is recorded by many different sensors. The use of multiple sensors supports each other, and interpretation becomes more reliable. The number of breathing sensors varies depending on the location where the measurement is performed. The most commonly measured are nasal airflow with a pressure sensor, respiratory airflow with a thermistor, respiratory movements with strain gauges, and arterial blood carbon dioxide.

What happens at polysomnography?
FIGURE 3 / Respiratory signals: thermistor, nasal pressure, Thorax and Abdomen

The nasal airflow is measured with a pressure sensor (Figure 3). Oxygen mustaches are used in the measurement, the tips of which reach a depth of about a centimeter in the nostrils. Normally, one end of an oxygen mustache would be attached to an oxygen bottle, but when measuring nasal pressure fluctuations, it would be attached to a sensitive differential pressure sensor. Nasal airflow cannot be measured if the patient breathes through the mouth. The airflow from the mouth reduces the pressure variation measured from the nose, making the nasal pressure profile difficult to assess. For this reason, AASM recommends measuring airflow from both the mouth and nose. The airflow from the mouth is measured with a thermistor that measures the temperature changes of the airflows.

The thermistor measures the airflow of the breath by determining the temperature differences between the inhalation and exhalation. It is placed in front of the nose and mouth so that it measures the airflows in both the mouth and nose (Figure 3). Changes in temperature do not linearly correlate with changes in airflows, so the thermistor cannot determine quantitative changes in airflows. The thermistor is used in situations where the patient breathes through the mouth. In this case, breathing airflows can be measured with at least one monitor.

Respiratory movements are also recorded for respiration. Most commonly, they are recorded by two strain gauges attached to the patient. Strain gauges are usually attached around the patient to chest height (thorax) and around the abdomen to diaphragm (abdomen) height (Figure 3).

What happens at polysomnography?
FIGURE 4 / Capnography Signals

What is capnography?

A capnograph is a meter that measures changes in the level of carbon dioxide in the arterial blood. The changes can be measured by aspirating the end-exhaled air from the nose and determining its carbon dioxide concentration (Figure 4). Reliable measurement requires deep exhalation. The upper signal in Figure 4 shows the volume of carbon dioxide exhaled and inhaled. The signal has a wavy shape. The lower signal in the figure shows the volume of carbon dioxide exhaled. The measurement provides an estimate of the carbon dioxide content of the arterial blood. The exact value can only be determined with a blood sample from an artery. Taking a blood sample is very difficult to perform during a patient’s sleep. The determination of the carbon dioxide content of the end-exhaled air sucked from the nose does not work if the patient inhales orally. In this case, the carbon dioxide content of the arterial blood can be determined transcutaneously from the skin surface. The principle of transcutaneous carbon dioxide measurement is to measure the diffusion of carbon dioxide through the tissues and skin from the skin surface. Heating the sensor increases the diffusion of carbon dioxide by dilating the peripheral skin arteries. The method provides a more accurate estimate of arterial carbon dioxide levels. However, the warming of the sensor must be taken into account when using the method. In transcutaneous measurement, changes in carbon dioxide concentration are slower than in the measurement of end-expiratory air in the nose. The value passes about two minutes late, so transcutaneous measurement cannot measure changes in carbon dioxide concentration after each breath.

What happens at polysomnography?
FIGURE 5 / Right and left foot EMG signals

How are nocturnal movements registered?

The patient’s nocturnal movements are recorded from the muscles of both legs with surface EMG electrodes (Figure 5). The movements of the patient’s limbs are recorded from the anterior tibialis muscles of the legs bipolarly with two electrodes. The upper signal of Figure 5, i.e. the right leg, shows movement. The movement is reflected in signal intensification and an increase in amplitude. Limb muscle function is measured to diagnose nocturnal leg movement disorder. Foot movement disorders can also be determined using an SCSB sensor or an EMFit sensor placed under the feet.

The SCSB (Static charge sensitive bed) sensor measures the patient’s breathing, body and limb movements, and heart rate. The sensor is a bed-sized plate-like sensor that is placed in the bed under a foam mattress. The SCSB sleep mattress can be used to register many different things using different filters and gains.

What happens at polysomnography?
FIGURE 6 / Sleeping mattress signals

The EMFit sensor is a newer plate-like sensor that is used more than an SCSB sensor. The EMFit sleeping mattress is smaller in size, about 30–62 cm. It is usually placed at the chest level under the mattress. The sensor is a thin and inclined electromagnetic film that generates static electricity. The EMFit sleep mattress registers breathing and motor skills for ballistocardiography. The upper signal in Figure 6 is a ballis tocardiogram and the lower signal filtered from a sleep mattress to describe the patient’s breathing. The ballisto cardiogram (Fig. 6) shows the micro-movements of the body caused by the flow of blood in the heart. It describes the mechanical function of the heart. By filtering and amplifying the signal of the EMFIT sleeping mattress, eg breathing (Fig. 6) and body movements can be seen.

The position sensor determines the patient’s sleeping positions during the night. The sleeping position is recorded by a position-sensitive sensor placed on the patient’s chest. The position sensor can be used to reliably determine in which position the patient is sleeping, on his left or right side, on his back or on his stomach.

What happens at polysomnography?
FIGURE 7 / Snoring signal

Other registrations in polysomnography?

Other issues to be registered in polysomnography research include snoring, blood oxygen saturation, and ECG, or electrocardiography. Snoring can be measured either from the patient’s neck with a sensitive vibration sensor (Figure 7) or with a snoring microphone. Snoring occurs when, during sleep, breathing causes vibration in the upper airways. In Figure 7, snoring is shown as signal density and increase in amplitude. Snoring also appears on the nasal respiration pressure profile as an opacity at the peak of inhalation.

A pulse oximeter is routinely used to measure the oxygen saturation of capillary blood. The pulse oximeter is placed on either the earlobe or the finger. A pulse oximeter can show a decrease in blood oxygen saturation during respiratory pauses.

What happens at polysomnography?
FIGURE 8 / electrocardiogram (ECG)

An ECG (Figure 8) is recorded during sleep due to adverse heart changes associated with apnea, or respiratory arrest. In Figure 8, a P-wave, i.e. atrial contraction, a QRS complex, i.e. ventricular contraction, or a T-wave, i.e. ventricular repolarization, can be distinguished in the signal. The most common changes are arrhythmias. They occur in the late stages of apnea, when the patient wakes up and tries to breathe against the blocked airways.

Interpretation of sleep polysomnography study

The interpretation of sleep polysomnography includes sleep phase classification and related parameters, as well as the calculation of various sleep-time events. The sleep phases are represented graphically in a hypnogram with the sleep phases on the y-axis and the time on the x-axis. Sleep parameters used to describe sleep quality are calculated from the sleep phase classification. These include time spent in bed, sleep latency, or the time taken to fall asleep after turning off the lights, the total time spent in sleep, and the percentages of sleep phases in total sleep time. Sleep events include waking up, abnormal breathing events, decreases in blood oxygen saturation, and leg movements. The number of events per hour of sleep is calculated as an index, ie how many events occur on average per hour of sleep.

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