It is essential to measure movement of the chest wall and abdomen during polysomnography to determine whether respiratory efforts are present, and whether these efforts are synchronous. Esophageal manometry is the gold standard and only recommended method for directly measuring respiratory effort but it is seldom used clinically (particularly in children) because of patient discomfort.
There are several different devices available for monitoring respiratory effort indirectly by recording thoracoabdominal excursion, including strain gauges, respiratory inductance plethysmography (RIP), piezoelectric sensors, impedance plethysmography, and polyvinylidene fluoride (PVDF) sensors.
PVDF is a piezoelectric plastic polymer that creates an electric charge when stretched. When placed into a portion of a belt, PVDF will generate varying signals depending on the tension on it. The magnitude of the signal does not necessarily reflect the shape or length of the belt.1
RIP belts consist of a sinusoidal wire coil insulated in elastic. Dynamic stretching of the belts creates waveforms due to change in self-inductance and oscillatory frequency of the electronic signal. Changes in thoracic/abdominal radius, cross-sectional area, and circumference all are directly related to changes in lung volume. Thus, with RIP the entire belt is the sensor, and so with normal tidal breathing amplitude of the waveform correlates with tidal volume.2 Strain gauges, impedance, PVDF, and piezoelectric sensors are typically located in a single portion of the belt and depend on the remainder of the belt to transfer force to them. RIP measures the inductance of the belt as a whole, and so a change in length of any section of the belt will affect the measurement in proportion to the magnitude of change and length of belt. An additional advantage of RIP is the option to calibrate it with actual changes in volume.
In 2012 and again in 2016, the American Academy of Sleep Medicine's sleep apnea definitions task force recommended the use of RIP based on a greater amount of evidence and longer history of use. Subsequent studies led the task force to conclude that “PVDF sensor effort belts appear to adequately detect respiratory effort in adults,” and the task force judged the use of PVDF in adults as acceptable for monitoring respiratory effort and for the identification of apneas and hypopneas when the thermistor and nasal pressure transducer signals are not reliable, based on “limited published evidence.”3–5
Studies in adults cannot be directly extrapolated to children, however. The pediatric montage and scoring rules are different, and children have fewer respiratory events and a higher prevalence of hypopneas. The article by Griffiths et al. in this issue of Journal of Clinical Sleep Medicine compares the use of PVDF belts with RIP for respiratory motion monitoring in children during polysomnography.6 It was hypothesized that the two sensors would be equivalent in detecting respiratory events, and that there would be less artifact from belt motion with PVDF sensors. One of the authors has served as a consultant to the company that produces the PVDF belts.
The subjects included many children typically referred for polysomnography, ages 2 to 17 y, with a slight preponderance of boys, with common conditions such as obesity and asthma. Children younger than 2 y, who are more likely to have more compliant chest walls that promote thoracoabdominal asymmetry (i.e., “paradoxing”), where the accuracy of the sensors would be most severely tested, were not included in the study.7
Griffiths et al. compared scoring using a full respiratory montage including RIP to one with PVDF substituted for RIP. The scorers were blinded to belt type and there was excellent interscorer reliability. Griffiths et al. considered a difference in total count of obstructive or central apneas or hypopneas of at least five events to be clinically significant.
The authors found no increase in artifact using RIP over PVDF.
Total AHI and scoring of obstructive apneas were comparable between PVDF and RIP, based on Bland-Altman analysis. Information regarding central apnea scoring was not reported for the main analysis. It is surprising that not a single central hypopnea was identified in any of the 50 patients. However, the number of obstructive hypopneas was statistically and clinically significantly different between the two sensors. Obstructive hypopneas are the most common respiratory event in children and are also the most subject to scoring variation, because of their reliance on the scorer visually recognizing a 30% decrease in pressure tracing amplitude and determining whether the chest wall and abdomen are in phase (suggesting a central hypopnea) or out of phase (confirming an obstructive hypopnea). This variation may be the reason the 95% confidence interval for the lower limit of agreement in the obese children crossed −5.
This study has several strengths. There was a variety of patients and those with common medical problems were not excluded. Interscorer reliability was assessed and was excellent. The scorers were blinded as to belt type. The statistical methods were solid. However, the primary determinant of apnea is the presence or absence of airflow. Thus, the type of belt used should have little effect on scoring the total number of apneas. The belts characterize whether an apnea is central or obstructive, but that is a rather low standard to meet. The challenge is identifying and characterizing hypopneas. It is surprising that not a single central hypopnea was identified in 66 reviews of 50 studies. That likely represents institutional training bias. However, the increased scatter beyond the predetermined limit of agreement suggests that the sensors are not equivalent in detecting obstructive hypopneas. This is even after excluding children younger than 2 y who have the most compliant chest walls and may be most susceptible to subtle events (or non-events). Future studies should include younger children, and more direct measures of the presence and quantity of airflow such as pneumotachometry. The ability of the RIP sum channel to identify events if other airflow channels are not available, and the ability to characterize central from obstructive events (particularly hypopneas) should be considered.
The authors have indicated no financial conflicts of interest.
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