*2.1.4 Polysomnogram and polysomnography*

"Polysomnography" "PSG" is the recording of sleep via electrophysiological signals. Sleep recordings that appear on a sheet of paper or on a computer screen are called "Polysomnogram". Throughout one night electrophysiological signals recorded during wakefulness and sleep are as follows: "Electroencephalogram (EEG), electromyogram (EMG; jaw, arm and leg), electrooculogram (EOG), electrocardiogram (ECG), snoring, oro-nasal air flow (L/s) (liter/second) chest and abdomen movements (respiratory effort recordings), O2 saturation, and body position and real time-video-image recordings". "Penile tumescence, gastroesophageal reflux and blood pressure" are other electrophysiological signals that are recorded, despite not being performed for all patients. Polysomnography is the procedure where different physiological or pathophysiological parameters are recorded during sleep for six or more hours, evaluation of these by a medical doctor and generation of a report (**Figure 1**).

Polysomnography is performed for two main purposes: (1) Understanding physiological (normal) sleep and meanwhile demonstrating the changes that take place in the organism (for example heart rate changes can be analyzed)

**9**

**Figure 2.**

*The PSG records of electrophysiological signals.*

*Sleep Physiology and Polysomnogram, Physiopathology and Symptomatology in Sleep Medicine*

(2) Identification of abnormal events that take place during sleep; diagnosis of different sleep disturbances, guide in their treatments. PSG starts by explaining the procedure to the patient in great detail. The patient should understand that there would not be any pain involved with the procedure, that no medications would be used. The patients are informed that their natural sleep will be recorded through superficial electrodes to be placed on their bodies. The patients should be reminded that they would not be spending the night by themselves, and that a technician would be present to follow the process from a monitor. After the patient puts on his sleepwear, electrodes are placed for an overnight sleep test and calibration process is initiated. First, the calibration of PSG equipment is made. This is performed before the electrodes are placed. Afterwards the electrodes are calibrated. This is done after the electrodes are placed in the electrode box. Lastly, physiological calibration is performed. This calibration is performed via the electrodes that transmit physiological changes through EEG, EOG, and EMG, leg movements, chest and abdomen movements. The PSG records of electrophysiological signals features are as follows (**Figure 2**): *EEG recordings during PSG*: Gold and silver electrodes are used for sleep EEG. It is important to clean the skin where the electrodes are to be placed. This is performed to decrease the resistance between the skin and the electrodes. Another important issue is to use substances that would increase the conductivity of the electrodes and to make sure that electrodes stay in place

*DOI: http://dx.doi.org/10.5772/intechopen.82754*

**Figure 1.** *Examination of a polysomnogram by a medical doctor.*

*Sleep Physiology and Polysomnogram, Physiopathology and Symptomatology in Sleep Medicine DOI: http://dx.doi.org/10.5772/intechopen.82754*

(2) Identification of abnormal events that take place during sleep; diagnosis of different sleep disturbances, guide in their treatments. PSG starts by explaining the procedure to the patient in great detail. The patient should understand that there would not be any pain involved with the procedure, that no medications would be used. The patients are informed that their natural sleep will be recorded through superficial electrodes to be placed on their bodies. The patients should be reminded that they would not be spending the night by themselves, and that a technician would be present to follow the process from a monitor. After the patient puts on his sleepwear, electrodes are placed for an overnight sleep test and calibration process is initiated. First, the calibration of PSG equipment is made. This is performed before the electrodes are placed. Afterwards the electrodes are calibrated. This is done after the electrodes are placed in the electrode box. Lastly, physiological calibration is performed. This calibration is performed via the electrodes that transmit physiological changes through EEG, EOG, and EMG, leg movements, chest and abdomen movements. The PSG records of electrophysiological signals features are as follows (**Figure 2**): *EEG recordings during PSG*: Gold and silver electrodes are used for sleep EEG. It is important to clean the skin where the electrodes are to be placed. This is performed to decrease the resistance between the skin and the electrodes. Another important issue is to use substances that would increase the conductivity of the electrodes and to make sure that electrodes stay in place

**Figure 2.** *The PSG records of electrophysiological signals.*

*Updates in Sleep Neurology and Obstructive Sleep Apnea*

sleep spindles that are formed [11–14].

of a report (**Figure 1**).

*2.1.4 Polysomnogram and polysomnography*

*Examination of a polysomnogram by a medical doctor.*

alpha waves. That is why it is possible that alpha waves appear from the spontaneous negative feedback impulses in the diffuse thalamocortical system that also includes brain stem activating system. Delta waves include all the waves in EEG that have a frequency of less than 3–5 per second. They appear during very deep sleep, they also appear in the experimental animal studies where cortex has been separated from the thalamus with a subcortical section. Therefore, delta waves can appear in the cortex independent of the activities in the lower parts of the brain. Sleep spindles are produced by the thalamus. They appear as 12–15 Hz oscillations in between slow waves during NREM sleep in human EEGs. The production mechanism of these oscillations is related to the degree of hyperpolarization in thalamocortical cells. While shifting from wakefulness to sleep, the membrane potentials of thalamocortical cells are exposed to a progressive hyperpolarization, thus synaptic responsiveness decreases and sensory information transfer is prevented. When a sufficient level of hyperpolarization is achieved, we start seeing rhythmic bursting in nucleus reticularis neurons belonging to thalamus at a frequency interval which is in correlation with sleep spindle. Furthermore, slow wave oscillations due to membrane hyperpolarization also take place. It is accepted that sleep homeostasis is significantly affected by the size and characteristics of the

"Polysomnography" "PSG" is the recording of sleep via electrophysiological signals. Sleep recordings that appear on a sheet of paper or on a computer screen are called "Polysomnogram". Throughout one night electrophysiological signals recorded during wakefulness and sleep are as follows: "Electroencephalogram (EEG), electromyogram (EMG; jaw, arm and leg), electrooculogram (EOG), electrocardiogram (ECG), snoring, oro-nasal air flow (L/s) (liter/second) chest and abdomen movements (respiratory effort recordings), O2 saturation, and body position and real time-video-image recordings". "Penile tumescence, gastroesophageal reflux and blood pressure" are other electrophysiological signals that are recorded, despite not being performed for all patients. Polysomnography is the procedure where different physiological or pathophysiological parameters are recorded during sleep for six or more hours, evaluation of these by a medical doctor and generation

Polysomnography is performed for two main purposes: (1) Understanding physiological (normal) sleep and meanwhile demonstrating the changes that take place in the organism (for example heart rate changes can be analyzed)

**8**

**Figure 1.**

throughout the night. Most commonly used fixer is "collodium". For PSG, it is recommended to have recordings from at least three channels. These channels are F4-M1, C4-M1 and O2-M1. M1 is placed on the left mastoid process. To these electrodes, F3, C3, O1 and M2 electrodes are attached as back up. If one of the electrodes mentioned above gets bad during the night, then F3-M2, C3-M2 and O1-M2 derivations are used. Here, M2 corresponds to the right mastoid process. Additionally, Fz-Cz, Cz-Oz and C4-M1 derivations can be used for EEG recordings. For this spare electrodes Fpz, C3, O1 and M2 need to be placed. *EOG recordings during PSG*: The objective of having electrooculography recordings during polysomnography is to identify the eye movements. This recording is in fact the recording of the voltage difference between cornea and retina. With the movement of the eye, the distance from the retina and cornea to the electrodes changes and creates a dipole. This change is recorded with EEG. Two EOG electrodes are used in PSG recordings. E1 is located 1 cm below the left lateral canthus E2 is placed 1 cm above the right lateral canthus. Both electrodes are referred to the electrode (M2) placed on the left mastoid process. That means EOG is recorded from two channels as E1-M2 and E2-M2. Alternatively, E1-Fpz and E2-Fpz can be used. *EMG recordings during PSG*: In polysomnography, electromyography (EMG) is of importance in detecting R stage (REM sleep). This recording is different than a classical EMG. It is performed to assess striated muscle tone. Using three electrodes is recommended for this recording; only two of these are used to do the recording. Superficial electrode is placed on the mandible and is referred to either of the electrodes placed below this. In polysomnography it is sufficient to have only one channel for EMG recording. *Respiratory Effort Recordings in PSG*: In polysomnography, there needs to be at least three respiratory parameters recorded: "oro-nasal air flow (L/s) chest and abdomen movements and O2 saturation". Recording of stopping air flow (apnea) is performed by oronasal thermal sensors. For the recording of respiratory effort, inductance plethysmography method is used. For the measurement of O2 saturation, pulse oximetry that identifies the O2 saturation of hemoglobin in the capillary blood would be the most appropriate method. *Recording of Periodical Leg Movements (PLM) during Sleep in PSG*: Superficial electrodes are placed on the anterior tibial muscle. Two electrodes should be placed to each leg (active, passive) and one channel should be recorded from each leg adding up to two channels. Peak to peak amplitude of the EMG activity that is generated in the absence of movement should be 4–6 microvolts, at least four movements should take place with 5–90 seconds intervals and each of these movements should take 0.5–5 seconds to fulfill the criteria for PSG diagnosis of periodic limb movement disorder. *Multiple Sleep Latency Test-MSLT*: This is the electrophysiological signal recording method used to objectively evaluate daytime sleepiness. This is a polysomnographic recording obtained during daytime and a half to 3 hours after waking up from a nocturnal polysomnogram. It should consist of five tests of 20 minutes duration, which are 2 hours apart from each other. Two basic data for evaluation: (1) Time of sleep onset (2) Appearance of REM sleep throughout the recording process. REM of short duration that appears during recording is defined as SOREM (Sleep Onset REM). MSLT; is part of the clinical evaluation for narcolepsy and idiopathic hypersomnia.

#### *2.1.5 Recording and scoring of sleep*

Scoring of sleep corresponds to staging of sleep. For the staging of sleep polysomnography recordings are separated into 30 second-long intervals (epoch); each epoch

**11**

**Figure 3.** *Stage-N1.*

*Sleep Physiology and Polysomnogram, Physiopathology and Symptomatology in Sleep Medicine*

is scored with a sleep stage. Sleep stages are as follows: "Stage N1 (or NREM1), Stage N2 (or nREM2), Stage N3 (or NREM3), Stage R (REM), Stage W (wakefulness)". Each 30 second interval needs to match with one of these stages. Three main electrophysiological signals are used when sleep stages are identified: "EEG, EMG, EOG". There three parameters are evaluated for each epoch and one sleep stage is matched with each 30 second interval. There are certain rules to be respected when staging of sleep is performed: *Stage N1*: it is generally regarded as the stage where the shift from wakefulness to sleep happens (sleep initiation stage). If more than one half of an epoch consists of low amplitude and mixed frequency EEG activity that replaces alpha rhythm, this part is staged as Stage N1. In an individual who does not have any alpha activity during wakefulness, seeing only one of the following three features in EEG is sufficient to categorize that part as Stage N1: (1) Baseline activity in EEG should be at least 1 Hz lower than that is seen during wakefulness. (2) Observing sharp vertex waves on EEG (in central regions with duration of shorter than 500 ms, waves with sharp edges). (3) Slow eye movements should appear in EOG (lasting longer than 500 ms, conjugated, regular, and sinusoidal) (**Figure 3**). *Stage N2*: it is the longest portion of sleep both in terms of duration and proportion. In EEG, the presence of either a sleep spindle or K-complex results in naming that stage as Stage N2 (**Figure 4**). Stage N2 sleep starts in this manner and despite the absence of K complex and/or sleep spindles, the stage continues being N2 until further changes take place in the epoch (Stage W, N3, R or arousals). Only when there is a progress to Stage W, Stage 3 or Stage R or when an arousal appears, Stage N2 is concluded. *Stage N3*: It is called as slow wave sleep. In EEG, it is seen as delta activity with a frequency of 0.5–2 Hz and with amplitude that is more than 75 microvolts. During this stage there can be sleep spindles and K complexes. The only criterion for Stage N3 is the presence of slow waves (delta wave oscillations) in more than 20% of this part (**Figure 5**). *Stage R*: indicates REM sleep. In terms of central nervous system functioning this is a completely different stage of sleep. Three features are required in the epoch to be called Stage R (1). In EEG the wave activity is of low amplitude

*DOI: http://dx.doi.org/10.5772/intechopen.82754*

#### *Sleep Physiology and Polysomnogram, Physiopathology and Symptomatology in Sleep Medicine DOI: http://dx.doi.org/10.5772/intechopen.82754*

is scored with a sleep stage. Sleep stages are as follows: "Stage N1 (or NREM1), Stage N2 (or nREM2), Stage N3 (or NREM3), Stage R (REM), Stage W (wakefulness)". Each 30 second interval needs to match with one of these stages. Three main electrophysiological signals are used when sleep stages are identified: "EEG, EMG, EOG". There three parameters are evaluated for each epoch and one sleep stage is matched with each 30 second interval. There are certain rules to be respected when staging of sleep is performed: *Stage N1*: it is generally regarded as the stage where the shift from wakefulness to sleep happens (sleep initiation stage). If more than one half of an epoch consists of low amplitude and mixed frequency EEG activity that replaces alpha rhythm, this part is staged as Stage N1. In an individual who does not have any alpha activity during wakefulness, seeing only one of the following three features in EEG is sufficient to categorize that part as Stage N1: (1) Baseline activity in EEG should be at least 1 Hz lower than that is seen during wakefulness. (2) Observing sharp vertex waves on EEG (in central regions with duration of shorter than 500 ms, waves with sharp edges). (3) Slow eye movements should appear in EOG (lasting longer than 500 ms, conjugated, regular, and sinusoidal) (**Figure 3**). *Stage N2*: it is the longest portion of sleep both in terms of duration and proportion. In EEG, the presence of either a sleep spindle or K-complex results in naming that stage as Stage N2 (**Figure 4**). Stage N2 sleep starts in this manner and despite the absence of K complex and/or sleep spindles, the stage continues being N2 until further changes take place in the epoch (Stage W, N3, R or arousals). Only when there is a progress to Stage W, Stage 3 or Stage R or when an arousal appears, Stage N2 is concluded. *Stage N3*: It is called as slow wave sleep. In EEG, it is seen as delta activity with a frequency of 0.5–2 Hz and with amplitude that is more than 75 microvolts. During this stage there can be sleep spindles and K complexes. The only criterion for Stage N3 is the presence of slow waves (delta wave oscillations) in more than 20% of this part (**Figure 5**). *Stage R*: indicates REM sleep. In terms of central nervous system functioning this is a completely different stage of sleep. Three features are required in the epoch to be called Stage R (1). In EEG the wave activity is of low amplitude

**Figure 3.** *Stage-N1.*

*Updates in Sleep Neurology and Obstructive Sleep Apnea*

throughout the night. Most commonly used fixer is "collodium". For PSG, it is recommended to have recordings from at least three channels. These channels are F4-M1, C4-M1 and O2-M1. M1 is placed on the left mastoid process. To these electrodes, F3, C3, O1 and M2 electrodes are attached as back up. If one of the electrodes mentioned above gets bad during the night, then F3-M2, C3-M2 and O1-M2 derivations are used. Here, M2 corresponds to the right mastoid process. Additionally, Fz-Cz, Cz-Oz and C4-M1 derivations can be used for EEG recordings. For this spare electrodes Fpz, C3, O1 and M2 need to be placed. *EOG recordings during PSG*: The objective of having electrooculography recordings during polysomnography is to identify the eye movements. This recording is in fact the recording of the voltage difference between cornea and retina. With the movement of the eye, the distance from the retina and cornea to the electrodes changes and creates a dipole. This change is recorded with EEG. Two EOG electrodes are used in PSG recordings. E1 is located 1 cm below the left lateral canthus E2 is placed 1 cm above the right lateral canthus. Both electrodes are referred to the electrode (M2) placed on the left mastoid process. That means EOG is recorded from two channels as E1-M2 and E2-M2. Alternatively, E1-Fpz and E2-Fpz can be used. *EMG recordings during PSG*: In polysomnography, electromyography (EMG) is of importance in detecting R stage (REM sleep). This recording is different than a classical EMG. It is performed to assess striated muscle tone. Using three electrodes is recommended for this recording; only two of these are used to do the recording. Superficial electrode is placed on the mandible and is referred to either of the electrodes placed below this. In polysomnography it is sufficient to have only one channel for EMG recording. *Respiratory Effort Recordings in PSG*: In polysomnography, there needs to be at least three respiratory parameters recorded: "oro-nasal air flow (L/s) chest and abdomen movements and O2 saturation". Recording of stopping air flow (apnea) is performed by oronasal thermal sensors. For the recording of respiratory effort, inductance plethysmography method is used. For the measurement of O2 saturation, pulse oximetry that identifies the O2 saturation of hemoglobin in the capillary blood would be the most appropriate method. *Recording of Periodical Leg Movements (PLM) during Sleep in PSG*: Superficial electrodes are placed on the anterior tibial muscle. Two electrodes should be placed to each leg (active, passive) and one channel should be recorded from each leg adding up to two channels. Peak to peak amplitude of the EMG activity that is generated in the absence of movement should be 4–6 microvolts, at least four movements should take place with 5–90 seconds intervals and each of these movements should take 0.5–5 seconds to fulfill the criteria for PSG diagnosis of periodic limb movement disorder. *Multiple Sleep Latency Test-MSLT*: This is the electrophysiological signal recording method used to objectively evaluate daytime sleepiness. This is a polysomnographic recording obtained during daytime and a half to 3 hours after waking up from a nocturnal polysomnogram. It should consist of five tests of 20 minutes duration, which are 2 hours apart from each other. Two basic data for evaluation: (1) Time of sleep onset (2) Appearance of REM sleep throughout the recording process. REM of short duration that appears during recording is defined as SOREM (Sleep Onset REM). MSLT; is part of the clinical evaluation

**10**

for narcolepsy and idiopathic hypersomnia.

Scoring of sleep corresponds to staging of sleep. For the staging of sleep polysomnography recordings are separated into 30 second-long intervals (epoch); each epoch

*2.1.5 Recording and scoring of sleep*

**Figure 4.** *Stage-N2.*

and mixed frequency (2). In jaw EMG, there is a low basal EMG activity which is the lowest of all stages (3) In EOG rapid eye movements are observed (rapid eye movements—REM; these are eye movement shorter than 500 ms in duration, when are conjugate, irregular, sharp spike eye movements). In epochs that follow Stage R, even in the absence of rapid eye movements, if the other two rules remains, it continues to be classified as Stage R (**Figure 6**).

**13**

is uncomfortable.

**Figure 6.** *REM sleep.*

*Sleep Physiology and Polysomnogram, Physiopathology and Symptomatology in Sleep Medicine*

American Academy of Sleep Medicine (AASM) has published the rules for scoring sleep, sleep associated events as well as respiratory events. Based on these rules, abnormal respiratory occurrences that are observed during sleep are "apnea, hypopnea, arousal associated with respiratory effort, hypoventilation and Cheyne-Stokes breathing". Electrophysiological signal recordings that are required for interpreting respiratory problems in PSG are: "O2 saturation, nasal/ oronasal air flow (nasal cannula, thermistor), thoracic, abdominal respiratory effort, EEG recordings (absolutely required to identify arousal), body position, tracheal microphone, ECG, leg EMG recordings". To detect respiratory effort the following methods are used: (1) *Measurement of thoracic and abdominal movements*, this is the most widespread method used in the sleep laboratories to detect the respiratory effort. (2) *Respiratory muscle EMG*, standard electrodes are placed into intercostal spaces. These can be mixed with ECG recordings. This is the oldest method available. (3) *Pleural pressure*, esophageal pressure is measured to measure the inspiratory effort. Patients cannot tolerate esophageal balloons. Newer, thin, piezoelectric transducers with catheter tips are better tolerated. Esophageal pressure measurements are very helpful in two instances: (a) Identification of central apnea and hypopnea with a high sensitivity (b) The diagnosis of upper respiratory tract resistance syndrome. However this method

*Apnea scoring*: Apnea starts with a net loss of breathing amplitude when signals coming from nasal cannula as an alternative to oronasal thermistor stops and it ends with the start of the first breath that comes close to the basal value. Apnea is scored based on the following criteria: (1) 90% or more of a decrease in the peak signal of the thermal sensor compared to the basal amplitude. (2) Having an incident of at least 10 seconds duration. (3) At least 90% of the incident fulfilling the amplitude decrease criteria required for apnea scoring.

*DOI: http://dx.doi.org/10.5772/intechopen.82754*

*2.1.6 Recording and scoring of breathing during sleep*

**Figure 5.** *Stage-N3.*

*Sleep Physiology and Polysomnogram, Physiopathology and Symptomatology in Sleep Medicine DOI: http://dx.doi.org/10.5772/intechopen.82754*

**Figure 6.** *REM sleep.*

*Updates in Sleep Neurology and Obstructive Sleep Apnea*

continues to be classified as Stage R (**Figure 6**).

and mixed frequency (2). In jaw EMG, there is a low basal EMG activity which is the lowest of all stages (3) In EOG rapid eye movements are observed (rapid eye movements—REM; these are eye movement shorter than 500 ms in duration, when are conjugate, irregular, sharp spike eye movements). In epochs that follow Stage R, even in the absence of rapid eye movements, if the other two rules remains, it

**12**

**Figure 5.** *Stage-N3.*

**Figure 4.** *Stage-N2.*

#### *2.1.6 Recording and scoring of breathing during sleep*

American Academy of Sleep Medicine (AASM) has published the rules for scoring sleep, sleep associated events as well as respiratory events. Based on these rules, abnormal respiratory occurrences that are observed during sleep are "apnea, hypopnea, arousal associated with respiratory effort, hypoventilation and Cheyne-Stokes breathing". Electrophysiological signal recordings that are required for interpreting respiratory problems in PSG are: "O2 saturation, nasal/ oronasal air flow (nasal cannula, thermistor), thoracic, abdominal respiratory effort, EEG recordings (absolutely required to identify arousal), body position, tracheal microphone, ECG, leg EMG recordings". To detect respiratory effort the following methods are used: (1) *Measurement of thoracic and abdominal movements*, this is the most widespread method used in the sleep laboratories to detect the respiratory effort. (2) *Respiratory muscle EMG*, standard electrodes are placed into intercostal spaces. These can be mixed with ECG recordings. This is the oldest method available. (3) *Pleural pressure*, esophageal pressure is measured to measure the inspiratory effort. Patients cannot tolerate esophageal balloons. Newer, thin, piezoelectric transducers with catheter tips are better tolerated. Esophageal pressure measurements are very helpful in two instances: (a) Identification of central apnea and hypopnea with a high sensitivity (b) The diagnosis of upper respiratory tract resistance syndrome. However this method is uncomfortable.

*Apnea scoring*: Apnea starts with a net loss of breathing amplitude when signals coming from nasal cannula as an alternative to oronasal thermistor stops and it ends with the start of the first breath that comes close to the basal value. Apnea is scored based on the following criteria: (1) 90% or more of a decrease in the peak signal of the thermal sensor compared to the basal amplitude. (2) Having an incident of at least 10 seconds duration. (3) At least 90% of the incident fulfilling the amplitude decrease criteria required for apnea scoring.

Based on respiratory effort, there are three types of apnea: (1) Obstructive apnea: apnea during a respiratory effort. (2) Central apnea: apnea criteria and the absence of respiratory effort during the period where air flow stops. (3) Mixed apnea: apnea criteria that start with an absence of respiratory effort and follows with a continuing increase in respiratory effort. *Hypopnea scoring*: It is performed based on following criteria: (1) More than 30% of a decrease in the signal amplitude of the nasal cannula compared to the baseline. (2) The episode lasting for at least 10 seconds. (3) 4% or more of a decrease in the O2 saturation compared to the baseline saturation. (4) At least 90% of the incident should satisfy the amplitude decrease parameters accepted for hypopnea. *Apnea index (AI)*: defines the number of apneas that occur within an hour while sleeping. *Apnea-hypopnea index (AHI)*: defines the total number of apneas and hypopneas combined during an hour while sleeping. *Respiratory effort related arousal (RERA) scoring*: arousal resulting from 10 seconds or longer of flattening of the inspiratory portion of nasal pressure which does not fulfill the criteria for hypopnea or apnea. *Respiratory disturbance index (RDI)*: it defines the summation of apnea, hypopnea and RERA incidents that appear in an hour during sleep. *Hypoventilation scoring*: if PaCO2 increases more than 10 mmHg during sleep compared to levels obtained in supine position during wakefulness this is scored as hypoventilation. *Cheyne-Stokes breathing is score*: when there are at least three consecutive crescendo and decrescendo breathing changes, with at least one of the following criteria: (1) five or more central apnea or hypopnea during an hour of sleep (2) breathing amplitude changes in crescendo-decrescendo style in a consecutive manner lasting at least 10 minutes (3) Cheyne-Stokes breathing cycle lasting for 60 seconds in general, but variable. *O2 saturation measurement*: measurement of O2 saturation is generally performed with pulse oximetry. It detects the O2 saturation of hemoglobin in the capillary blood through the emission and absorption of light generated from a source. *O2 desaturation index (ODI)*: it defines the number of oxyhemoglobin desaturation incidents per hour seen during sleep [15–18].
