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Monitoring of Respiration BY AHMAD YOUNES PROFESSOR OF THORACIC MEDICINE Mansoura Faculty Of Medicine. The three major components of respiratory monitoring during sleep include. 1-Measurement of airflow, 2-Measurement of respiratory effort,
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Monitoring of Respiration BYAHMAD YOUNESPROFESSOR OF THORACIC MEDICINE Mansoura Faculty Of Medicine
The three major components of respiratory monitoring during sleep include 1-Measurement of airflow, 2-Measurement of respiratory effort, 3-Measurement of arterial oxygen saturation (SaO2). • Ancillary monitoring may include detection of snoring and recording surrogates of the arterial partial pressure of carbon dioxide (PaCO2) including end-tidal partial pressure of carbon dioxide (PETCO2) and transcutaneous partial pressure of carbon dioxide (TcPCO2). • The AASM scoring manual recommends specific sensor types and techniques to be used for recording respiration during sleep,
TECHNIQUES TO MEASURE AIRFLOW OR TIDAL VOLUME • The pneumotachograph (PNT) is the most accurate method to measure airflow during sleep studies , • This device quantifies airflow by measurement of the pressure drop across a linear (constant) resistance (usually a wire screen). The relationship between the pressure change, flow rate, and resistance is given by the following equation: Pressure change = Flow X Resistance • The PNT is worn in a mask covering the nose and mouth. • Although the PNT is commonly used to measure airflow during sleep research, this device is rarely used during clinical sleep studies.
The setup will consist of a flow head (pneumotachometer) and a transducer which will integrate volume from flow.
Thermal devices were the first to be used to monitor airflow during clinical sleep studies. • These devices actually detect changes in temperature induced by airflow (cooler inspired air, warmer exhaled air). • The changes in device temperature result in changes in voltage output (thermocouples) or resistance (thermistors). • Thermal sensors are generally adequate to detect an absence of airflow (apnea), but their signal does not vary in proportion to airflow. Therefore, thermal sensors are not an ideal means of detecting a reduction in airflow (hypopnea).
Thermal signals and PNT flow are equal at 1 L/sec. However, as airflow decreases, the thermal signals overestimate flow. • This illustrates that thermal sensors are not ideal sensors to detect hypopneas (reductions in flow). • The thermal sensor signal decreases when the nostrils are large or the thermal sensor is further from the nares. • Thermal devices composed of polyvinylidine fluoride (PVDF) film may offer a better estimate of flow. • Nasal-oral thermal sensors usually have a portion of the device placed within or just outside the nostrils with another portion over the mouth (detection of oral flow). • A major advantage of thermal sensors is that they can detect both nasal and oral airflow without the need for a cumbersome mask covering the face.
Measurement of nasal pressure (NP) provides an estimate of nasal airflow that is more accurate than one obtained with most thermal sensors. • NP is measured using a nasal cannula connected to an accurate pressure transducer . • Because the cannula tips are inside the nares and the other side of the pressure transducer is open to the atmosphere, the pressure being measured is actually the pressure drop across the resistance of the nasal inlet associated with nasal airflow. • The relationship of NP and flow is given by Equations NP = K1×(Flow)2 K1 = constant Flow = K2 × NP. K2 = constant • Changes in cannula position, periods of partial oral flow, and obstruction of the cannula by nasal secretions make the linearized NP signal a less accurate measure of flow over the entire night.
The nasal prongs signal decreases more than that of the pneumotachograph during a reduction in airflow. The linearized nasal prongs signal is very similar to that of the pneumotachograph
The AHI values from both the NP and the linearized NP signals showed excellent agreement with the AHI values determined from the PNT • The AHI values detected by the NP signal tended to be slightly higher than the linearized NP signal, but the differences were usually small. • The inter-measurement agreements (kappa) between NP and PNT and linearized NP and PNT signals were both excellent and essentially identical. • During normal unobstructed flow, the inspiratory shape (contour) of the NP signal is round , whereas during airflow limitation , the shape of the PNT and NP signals is flattened . • Airflow limitation is characteristically present during obstructive reductions in airflow (hypopnea) or snoring. • In contrast, when reductions in airflow are simply due to a fall in inspiratory effort, the NP signal amplitude is reduced but the shape is round. • The most important limitation of the NP technique is that approximately 10% of patients are “mouth breathers” and the NP signal may be misleading.
At A, the flow is rounded, whereas at C, the flattened airflow contour is associated with an increase in pressure drop across the upper airway and airflow limitation. A–C,
The AASM scoring manual recommends nasal-oral thermal sensors for detection of apnea and NP sensors (with or without square root transformation of the signal) for detection of hypopnea • Simultaneous use of both NP and nasal-oral thermal sensors is recommended and has the additional advantage of having a backup sensor if the other airflow detection device fails . • The AASM scoring manual notes that if the recommended sensor signal is not reliable, the alternative sensor can be used. • In adults, the alternative airflow sensor for apnea detection is the NP signal. The alternative sensors for hypopnea detections are oronasal thermal flow and respiratory inductance plethysmography (RIP).
The nasal pressure signal shows an absence of airflow, whereas the nasal-oral thermal sensor shows continued airflow. This pattern of airflow is due to oral breathing.
RIP is another method that can be used to detect apnea and hypopnea • The signals from rib cage (RC) and abdominal bands (AB) sensors can be summed in an uncalibrated manner (RIPsum = RC + AB) or as a calibrated signal (RIPsum = a × RC + b × AB) as an estimate of tidal volume (not airflow). • Here, RC and AB are signals from bands around the rib cage and abdomen and “a” and “b” are calibration factors determined during a calibration procedure. • If one takes the time derivative of the RIPsum signal, the result is an estimate of airflow (RIPflow). • The RIPsum during apnea has minimal deflections (approximately zero tidal volume) and during hypopnea reduced deflections (reduced tidal volume).
In the case of an obstructive apnea , the RC and AB deflections must nearly exactly cancel each other (paradox).In the case of hypopnea, there is a reduction in the RIPsum signal (low tidal volume) as well as both the RC and the AB signals. • In the case of obstructive hypopnea, there may also be paradox with chest and abdomen moving in opposite directions . • If a RIPsum signal is not available, one can detect hypopnea by a reduction in the RC and AB RIP signals. • The AHI values obtained from the RIPsum and time derivative of the RIPsum signals showed good agreement with AHI values from the PNT signal.
Obstructive apnea: Respiratory inductance plethysmography signals from the rib cage (RC) and abdominal bands (AB) are summed (RIPsum). The RIPsum is an estimate of tidal volume.
Obstructive hypopnea: Respiratory inductance plethysmography signals from the rib cage (RC) and abdominal bands (AB) are summed (RIPsum). The RIPsum is an estimate of tidal volume.
MEASURING RESPIRATORY EFFORT • Determination of respiratory effort is essential to classify apneas as obstructive (continued respiratory effort), central (absent effort), or mixed (central followed by obstructive portions). • The most sensitive and accurate method of detecting respiratory effort is by measurement of esophageal pressure. • Changesin esophageal pressure are estimates of changes in pleural pressure that occur during respiration (negative intrathoracic pressure during inspiration). • Esophageal pressure monitoring can detect rather feeble respiratory efforts even when RC and AB movements are minimal. In addition, the size of the pressure deflections provides an estimate of the magnitude of respiratory effort. • Detection of respiratory effort–related arousals (RERAs) is most accurately performed with esophageal pressure manometry.
MEASURING RESPIRATORY EFFORT Measurement of esophageal pressure can be performed using air-filled balloons, fluid-filled catheters, or catheters with pressure transducers on their tips. The technique does require special equipment and expertise and is routinely performed in only a few sleep centers. Some research sleep studies measure supraglottic pressure instead of esophageal pressure using a transducer tip placed just below the tongue base. This allows measurement of the pressure drop across the upper airway . Because this site is below the area of upper airway closure or narrowing in obstructive respiratory events, supraglottic pressure can also be used to detect respiratory effort.
Esophageal pressure deflections increase during an obstructive apnea that might at first glance appear to be central apnea (absent inspiratory effort).The chest and abdomen effort belt signals showed minimal deflections during obstructive apnea.
The most common method for detecting respiratory effort in clinical sleep studies utilized piezoelectric (PE) sensors connected to bands around the RC and AB. • Changes in the tension on the PE transducer as the RC and AB expand and contract produce a voltage that can be measured .The signal from these devices depends on the degree of tension on the transducer. • The PE belts are adequate for detection of respiratory effort in most patients but do not really quantify the changes in RC or AB volume. • Although relatively inexpensive compared with RIP effort belts, the PE effort belts may provide misleading information (false absence of respiratory effort), especially if not properly positioned and tensioned.
RIP belts provide a more accurate method of detecting changes in RC and AB motion during respiration than PE belts. • The inductance of coils in bands around the RC and AB changes during respiration as the RC and AB expand and contract. • The band inductance varies proportionately to the cross-sectional area the band encircles. • An oscillator is applied to each circuit and changes in inductance are converted into a voltage output. • The RIP bands consist of wires attached to a cloth band in a zig-zag pattern. This produces a larger change in inductance for a given change in band circumference. • Recall that if the RIP signals are calibrated, the RIPsum signal = aX RC + bX AB is an estimate of tidal volume. Here ,the constants a and b are determined during a calibration procedure.
The accuracy of the RIPsum signal can deteriorate if body position changes or the positions of the bands change during sleep. • Studied patientswith sleep apnea using both calibrated RIP belts and esophageal pressure monitoring showed that only 9% of patients were obstructive apneas sometimes misclassified as central apneas by the RIP belts. In these instances, esophageal pressure deflections were present when there was no detectable change in the RIP belt signals. • Thus, RIP effort belts are not 100% sensitive for detecting respiratory effort. However, if the bands are properly positioned and tensioned (sized) , they will detect respiratory effort (if present) in most patients. • The vast majority of sleep centers do not perform RIP belt calibration. It should be noted that the deflections of uncalibrated RIP bands do not always accurately reflect the magnitude of inspiratory effort or always show paradox during obstructive apnea and hypopnea.
Surface diaphragm EMG recording utilizes two electrodes about 2 cm apart horizontally in the seventh and eighth intercostal spaces in the right anterior axillary line. • The right side of the body is used to reduce ECG artifact. • Intercostal EMG recording often uses the right parasternal area (second and third intercostal spaces in the midaxillary line). • Inspiratory EMG activity is noted in the intercostal muscles and the diaphragm during non–rapid eye movement (NREM) sleep. • During rapid eye movement (REM) sleep, the intercostal activity is inhibited but diaphragmatic activity persists, although often diminished in amplitude during bursts of eye movements. • The AASM scoring manual recommends use of esophageal manometry or calibrated or uncalibrated RIP belts for detection of respiratory effort during sleep studies in adults and children . The measurement of respiratory muscle EMG is listed as an alternative method of detecting respiratory effort in adults.
An obstructive apnea with respiratory effort monitored by both chest and abdominal respiratory inductance plethysmography (RIP) bands and right intercostal electromyogram (EMG). The right intercostal EMG signal shows bursts coincident with inspiratory effort (and movement of chest and abdomen).A blow up of one EMG burst is shown at the bottom of the figure in a raw form and with the electrocardiogram (ECG) artifact minimized.
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.
Apneas are followed by arterial oxygen desaturations. Longer apneas are associated with more severe desaturation. SpO2 = pulse oximetry.
OXYGEN SATURATION • In sleep monitoring, an arterial oxygen desaturation is usually defined as a decrease in the SpO2 of 3% or 4% or more from baseline. • The nadir in SaO2 commonly follows apnea (hypopnea) termination by approximately 6 to 8 seconds (longer in severe desaturations) . This delay is secondary to circulation time and instrumental delay (the oximeter averages over several cycles before producing a reading).
OXYGEN SATURATION • The assessment of the severity of desaturation include the number of desaturations, the average minimum SpO2 during desaturations, the time below 80%, 85%, 90%,as well as the mean SaO2 and the minimum saturation during NREM and REM sleep. • The time with an SpO2 ≤ 88% is also commonly determined. • Oximeters may vary considerably in the number of desaturations they detect and their ability to discard movement artifact. • Using long averaging times may dramatically decrease the detection of desaturations. • The ability of oximeters to detect desaturationsis especially important in light of the definitions of hypopnea that depend on an associated desaturation. • The AASM scoring manual recommends a maximum averaging time of 3 seconds at a heart rate of 80 bpm. In patients with a slow heart rate, a slightly longer averaging time (at least a 3-beat average) may be needed.
MEASUREMENT OF PaCO2 DURING SLEEP • Documentation of hypoventilation during sleep requires measurement (or estimate) of the PaCO2. • The AASM scoring manual defines sleep-related hypoventilation in adults as an increase in the PaCO2 during sleep ≥ 10 mm Hg compared with an awake supine value. • Continuous ABGmonitoring during polysomnography to determine the PaCO2 is not practical. An ABG sample sometimes is performed at the start or the end of the study. The sample can be used to validate a surrogate measure of PaCO2 such as PETCO2 or TcPCO2. • If an ABG sample is taken just at awakening, it may be used to infer hypoventilation.
PETCO2 • Capnography consists of the continuous measurement of the fraction of CO2 in exhaled gas. • This is usually performed using an infraredsensor and, less commonly, a mass spectrophotometer. • The PCO2 is determined by multiplication of the fractionofCO2 by (Patm—47mmHg). Here, the Patm is the atmospheric pressure (760 mm Hg at sea level )and 47 mm Hg is the partial pressure of H2O in exhaled gas at body temperature. • During initial exhalation, the dead space (PCO2 = 0) reaches the sensor (phase1), then a mixture of dead space and alveolar gas (phase2),and finally, alveolar gas (phase3). The alveolarplateau occurs because the PCO2 in the air from the different alveoli differs slightly. • The differences are larger (slope of alveolar plateau steeper) in patients with lung disease. • The PETCO2 is an estimate of the mean alveolar PCO2 (and, therefore, an estimate of the PaCO2). • Of note, there is a gradient between the PaCO2 and the PETCO2 (PaCO2—PETCO2) with the PaCO2 being typically 2 to 5 mm Hg higher than the PETCO2 in normal individuals. • In lung disease, the gradient can be much larger. • In general, the PETCO2 is a valid estimate of PaCO2 only if an alveolar plateau is present.
Nonstructural risk factors • Some nonstructural risk factors include obesity, age, male sex, postmenopausal state, and habitual snoring with daytime somnolence. • Familial factors Relatives of patients with SDB have a 2- to 4-fold increased risk of OSA compared with control subjects. • Environmental exposures include smoke, environmental irritants or allergens, and alcohol and hypnotic-sedative medications. • Both hypothyroidism and acromegaly are associated with macroglossia and increased soft tissue mass in the pharyngeal region. They are associated with an increased risk of OSA. Hypothyroidism is also associated with myopathy that may contribute to UA dysfunction
PETCO2 • In the mainstream method, the sensor is located directly in the path of exhaled gas. • In the side stream method, gas is continually suctioned through a tube to a more remote sensor (in the instrument at bedside). • In the side stream approach, nasal cannulas are used to suction exhaled gas from the nares . When no CO2 is exhaled (during inspiration or apnea), the nasal cannula suctions room air (PCO2 = 0). • In the side stream method, there is a delay in exhaled gas reaching the sensor so the CO2 tracing is delayed compared with the exhaled airflow . • The exhaled CO2 tracing is sometimes used to indicate apnea (absence of exhaled PCO2). However, this is not recommended for two reasons. First, gas sampled by the nasal cannula may not detect mouth breathing, and second, small expiratory puffs rich in CO2 may still produce deflections in the exhaled CO2 trace. • Capnography is used much more frequently during pediatric than in adult sleep studies.
Imaging Studies • Modalities available for identifying the site of obstruction include lateral cephalometry, endoscopy, fluoroscopy, CT scanning, MRI. • At present, UA imaging is used primarily as a research tool. Routine radiographic imaging of the UA in the initial evaluation of SDB patients is of uncertain benefit and should not be performed unless a specific indication is present.
The exhaled CO2 tracing shows continued deflections during “inspiratory apnea” due to small exhaled puffs of air rich in CO2
TcPCO2 MONITORING • Measurement of TcPCO2 depends on the fact that heating of capillaries in the skin causes increased capillary blood flow and makes the skin permeable to the diffusion of CO2. • The CO2 in the capillaries diffuses through the skin and is measured by an electrode at the skin surface. • The measured value is corrected for the fact that heat increases the skin CO2 production as the measured value exceeds the PaCO2 measured at 37C. • Typically, TcCO2 electrodes are calibrated with a reference gas. • A thermostat controls the heating of the membrane-skin interface. • It is usually recommended that the probe of most TcPCO2 monitoring devices be moved every 3 to 4 hours to avoid skin irritation/damage. • The response time of newer TcPCO2 units has improved, but in general, the measured PCO2 may not increase rapidly enough to correlate with short respiratory events. However,TcPCO2 can be a good instrument for documenting trends in the PCO2 during the night.
Trends in the SpO2 and TcPCO2 during the night. Note the simultaneous increase in transcutaneous PCO2 and the decrease in SpO2 during episodes of REM sleep.
ACCURACY OF PETCO2 AND TcPCO2 • The measurement of PETCO2 and TcPCO2 was not found to be accurate for determining changes in PaCO2 during sleep . • PETCO2 was especially inaccurateduring simultaneous administration of supplemental oxygen or during positive airway pressure treatment as exhaled gas was sampled from a mask rather than using a nasal cannula. • The AASM scoring manual in the respiratory scoring rules for adults states that there was insufficient evidence to recommend a specific method for detecting hypoventilationduring sleep. However, it was stated that PETCO2 or TcPCO2 may be acceptable if validated and calibrated. In the pediatric rules, it states that “acceptable methods for assessing alveolar hypoventilation are either transcutaneous or end-tidal PCO2 monitoring.”
ACCURACY OF PETCO2 AND TcPCO2 • Concerning PETCO2 monitoring, the clinician should review the exhaled CO2 tracings to determine whether an alveolar plateau is present. • If an alveolar plateau is not present, the PETCO2 value may not be an accurate estimate of PaCO2. • Other problems for exhaled PCO2 monitoring include oral breathing and occlusion of the nasal cannula with secretions. • If tidal volumes are very small, a true alveolar sample may never reach the sensor. SoPETCO2 value will likely be much lower than the PaCO2. • Concerning TcPCO2 measurement, the actual tracings should also be carefully reviewed. If an abrupt change (offset) in the TcPCO2 tracing is noted, this suggests a measurement artifact is present. • Most TcPCO2 devices require calibration at the start of monitoring. • Poor application of the sensor or dislodgment of the sensor during sleep can cause a measurement artifact. • There is some advantage to the simultaneous use of both PETCO2 and TcPCO2 if tolerated. If the values show reasonable agreement during periods of stable breathing, this increases confidence in their validity. Of course, the goal standard to validate the accuracy of their measurements is a simultaneous ABG measurement.
SNORING SENSORS • Snoring is a sound produced by vibration of upper airway structures. • When snoring is present, upper airway narrowing of some degree can be inferred. • Snore sensors are usually microphones or PE transducers that are usually applied to the neck near the trachea. • Microphones can also be attached to the upper chest area or the face. • Snoring can also be seen in the NP signal as a rapid oscillation in the pressure tracing if an appropriate high frequency filter setting (100 Hz) is used and the transducer is sufficiently sensitive. • The AASM scoring manual does not provide guidance on use of snoring sensors and the signal is not part of the scoring criteria for respiratory events in adults. • Snoring is mentioned in scoring of RERAs in children.
Snoring noted both in the snore sensor (applied to the neck near the trachea) and as a vibration (oscillation) in the nasal pressure signal. Note that the nasal pressure signal also has a flattened shape. SpO2 = pulse oximetry
