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Respiratory monitoring using an air-mattress system

Authors:
  • Yarl Technologies Pty Ltd, Australia

Abstract

This paper describes a non-invasive respiratory monitoring system using an air mattress. The air-mattress system features multiple air compartments to monitor movements of the thorax and the abdomen separately. To evaluate the performance of the air-mattress system, four subjects were selected for the study and their separate ribcage and abdominal movements were monitored simultaneously by respiratory inductive phlethysmography belts and the air-mattress system. The sensitivity and accuracy indices of the air-mattress system for the detection of hypopnoeas scored remarkably well (above 90%). In addition, it was noted that the mean error in the measurement of the respiratory rate between the two systems was very small. This paper shows that the air-mattress system can be a reliable non-invasive respiratory monitoring system to detect simple abnormalities in breathing, such as respiration rate and hypopnoeas.
A NOVEL APPROACH TO RESPIRATORY MONITORING USING AN AIR-MATTRESS SYSTEM
Patrick Chow, Gangadharan Nagendra, John Abisheganaden, Y T Wang
School of Electrical and Electronic Engineering, Singapore Polytechnic
Department of Respiratory Medicine, Tan Tock Seng Hospital
Introduction
Monitoring of human respiration usually involves attaching some sensors or transducers to the human body. However, in
situations where long term monitoring of respiration of sleeping patients are required, patients are more reluctant to be
strapped down onto the bed. Many non-invasive and non-constrained methods have been proposed to monitor respiration
[1]-[4] with varying degrees of success reported. In the air mattress method proposed by [3] and [4], a single pressure
transducer is used to measure changes in air pressure inside a single air compartment of an inflatable mattress. This method
provides just a single channel output. Information on separate motions of the ribcage and abdomen, which indicates
paradoxical breathing [5] are not available. This is also true for other methods mentioned above. This paper describes a
novel approach to monitor respiration using an air mattress that can measure both the ribcage and abdomen movements
separately. Its design and principle of operation are discussed and its performance is measured against a conventional
respiratory belt system.
Methods
During normal quiet breathing, both the thorax and abdomen expand during inspiration and return to their resting
dimensions during expiration. When a person is lying on an air mattress, these movements of the thorax and abdomen are
translated as a varying pressure on the mattress. To measure these pressures independently, a multiple compartment air
mattress is designed and fabricated. The pressure inside each of these individual compartments will indicate movements
of that part of the body, with which it is in contact with.
Figure 1. Construction of the air-mattress
Figure 1 shows the construction of the air mattress. It is made of 1-mm rubber laminated with cotton fabric and each
compartment is inflated to about 75mBar of air pressure. A, B, C, and D are four separate air compartments connected to
two pairs of differential pressure transducers. The other compartments are not connected to any transducers but are inflated
for the comfort of the subject lying on the mattress.
As the sleeping subject breathes, the thoracic and abdominal movements are translated as a differential pressure change in
compartments AB and CD respectively. A very sensitive pressure transducer is used to measure the very minute thoraco-
abdominal movements. Figure 2 shows the pneumatic circuit of the set-up. The solenoid valves shown are used to equalise
the large pressure changes that might occur when the subject moves. This will prevent any damage to the differential
pressure transducers.
Figure 2. Pneumatic circuit diagram of the air-mattress system.
Four male subjects (age: mean 44.25, SD 7.01 and BMI: mean 30.2, SD 4.56) who were scheduled to undergo a standard
overnight polysomnography at the Tan Tock Seng Hospital, Singapore, were selected for the trial. The air mattress was
placed on the foam mattress of the hospital bed on which the subjects slept in the supine position, from 11.00pm to 6.00am.
The thoraco-abdominal movements of the subjects were monitored by respiratory inductive phlethysmography using belts
(Respitrace), and were used as a reference to evaluate the air mattress-based system. Recording of the respiratory data,
from both the respiratory belts and air mattress, commenced only when they had fallen asleep, as documented by the
electroencephalogram (EEG). Four channels of real-time data: - two from the standard respiratory belts and two from the
air mattress system were recorded for a one-hour period and compared. Data thus collected were divided into epochs of
ten seconds, yielding 360 epochs per channel per subject. Each epoch was manually analysed to identify periods of normal
breathing, hypopnoeas and movement artifacts.
Results
A typical 30-second segment of normal breathing is shown in Figure 3. Data from the two systems exhibit high correlation
as evident from the figure. During inspiration the chest expands and this exerts an increasing pressure on the mattress. Thus
a positive going slope of the signal denotes inspiration. Conversely, a negative going slope of the signal denotes expiration
of the subject.
30
0
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28
Thorax
Respiratory belt system
Air mattress system
30
0
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8
10
12
14
16
18
20
22
24
26
28
Thorax
Abdom
Figure 3. Top plot shows the thorax and abdomen movements recorded by the respiratory
belt system. Bottom plot shows the recording by the air mattress system.
120
0
10
20
30
40
50
60
70
80
90
100
110
120
0
10
20
30
40
50
60
70
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90
100
110
Thorax
Abdom
Respiratory belt system
Air mattress system
Thorax
Abdom
Figure 4. Breathing pattern with several epochs of apnoea.
A typical 2-minute segment of abnormal breathing with several episodes of apnoea/hypopnoea is shown in Figure 4. This
is evident in the reduction in the thoraco-abdominal movement exhibited by both systems. Intuitively, it can be seen that
the breathing patterns recorded by both systems are very similar. Abnormalities in the patterns and movement artifacts are
reported distinctly by both systems.
Thorax
Abdomen
Abnormality
H
M
H
M
No. of epoch detected by both systems
93
9
164
14
No. of epoch detected by belt system
103
9
166
15
No. of epoch detected by air mattress
100
16
176
24
Sensitivity (%)
90.29
100.00
98.80
93.33
Accuracy (%)
93.00
56.25
93.18
58.33
Table 1. Comparison of scores between the belt system and air mattress.
H: Hypopnoeas; M: Motion artifacts.
From the consolidated results in Table 1, it is obvious that the sensitivity and accuracy indices for the detection of
hypopnoeas have scored remarkably well (above 90%). This suggests that the air mattress system is as good as the standard
respiratory belt system in detecting abnormal breathing movements. However, the accuracy index for the detection of
motion artifacts performed rather badly (56% for thorax and 58% for abdomen). This is because the air mattress system
uses minute movement of the thorax and abdomen to detect breathing and hence movements of other parts of the body,
such as the legs, would also affect the signals. These movements do not affect the respiratory belt system. The mean and
standard deviation of the difference in the breathing frequencies measured by both systems were also compared. The
calculated mean error between the two systems was very small (less than 0.2).
Discussion
With the multi-compartment air mattress, we have shown that not only it can monitor the respiratory patterns but also it
can provide separate information on the ribcage and abdominal movements. Paradoxical breathing patterns may be
monitored with this novel two-channel air mattress system. We have compared the performance of the air mattress system
with that of the widely accepted respiratory belt system on a slate of patients with sleeping disorders. It has been shown
that our air-mattress system is as good as the respiratory belt system in detecting simple breathing abnormalities, such as
reduction and cessation of breathing and respiratory rate. The present air-mattress system is also more sensitive to
movement artifacts than the respiratory belt system. The detection of movements can be used to determine the wakefulness
of the patient [6]. Future research efforts would be needed to study the use of this air-mattress system in monitoring
sleep/wake activities of patients.
The phase relationship between the thoracic and abdominal signals is important for determining paradoxical breathing. On
the air mattress system, this phase relationship is highly dependent on the sleeping position of the patient. Longitudinal
shift in patient's position would cause phase shifts in the signals. Future development of the system would include some
means of identifying the exact position of the patient on the mattress.
With increasing demand for home monitoring systems, this air mattress based respiratory monitoring system will prove to
be useful in monitoring and detecting respiratory conditions in the home in the near future.
References
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We have developed a minimally intrusive system to monitor respiratory movements of sleeping subjects. This system is based on a pressure transducer which measures the changes in air pressure inside an inflatable mattress on which the subject sleeps. Using a mechanical filter to protect the transducer against the large pressure changes due to sudden movements and subject weight, we can detect the more subtle movements of the subject's chest. This paper discusses the design of the monitoring system, including the design and modelling of the mechanical filter.
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Daytime sleepiness and impaired cognitive function can be a consequence of recurrent transient arousal from sleep. Arousal is often associated with abrupt changes in the electroencephalogram (EEG), and such changes can be used as an index of sleep disturbance, but EEG analysis is laborious and requires trained observers. Possible alternative indices of arousal not reliant on EEG analysis were investigated. Recordings were made from 36 sleeping subjects who were being investigated for sleep-related breathing disorders. In each study awakenings and transient arousals according to EEG criteria were compared with activity in five potential indirect indicators of arousal: wrist movement, ankle movement, left and right tibial electromyogram, and phase change in ribcage-abdominal movement. The mean values of sensitivity to arousal ranged from only 25 to 45%. However, their high positive predictive accuracies (PPAs, 68 to 92%) indicated that activity, when present, was usually associated with arousal. Sensitivity to awakenings was higher (71-87%), though PPAs were lower (42 to 63%). For the indicator based on ribcage-abdominal phase, the number of periods of activity showed a significant relation to the number of arousals (r = 0.70, P < 0.001). It can be concluded that phase changes in chest/abdomen movement are a useful indicator of arousal associated with obstructive apnoea and related conditions. Limb activity has much lower sensitivity for transient arousal, but may be of value in indicating periods of wakefulness.