Biomedical Engineering Reference
In-Depth Information
that are common in children with sleep apnea, the end-tidal PCO
2
will be elevated
(>45 mm Hg) [22].
Respiratory effort monitoring is necessary to classify respiratory events. A simple
method of detecting respiratory effort is detecting movement of the chest and abdo-
men. This may be performed with belts attached to piezoelectric transducers, imped-
ance monitoring, respiratory-inductance plethysmography (RIP), or monitoring of
esophageal pressure (reflecting changes in pleural pressure). The surface EMG of the
intercostal muscles or diaphragm can also be monitored to detect respiratory effort.
Probably the most sensitive method for detecting effort is monitoring of changes in
esophageal pressure (reflecting changes in pleural pressure) associated with
inspiratory effort [24]. This may be performed with esophageal balloons or small
fluid-filled catheters. Piezoelectric bands detect movement of the chest and abdomen
as the bands are stretched and the pull on the sensors generates a signal. However, the
signal does not always accurately reflect the amount of chest/abdomen expansion. In
RIP, changes in the inductance of coils in bands around the rib cage (RC) and abdo-
men (AB) during respiratory movement are translated into voltage signals. The
inductance of each coil varies with changes in the area enclosed by the bands. In gen-
eral, RIP belts are more accurate in estimating the amount of chest/abdominal move-
ment than piezoelectric belts. The sum of the two signals [RIPsum
×
AB)] can be calibrated by choosing appropriate constants
a
and
b
. Changes in the
RIPsum are estimates of changes in tidal volume [29]. During upper-airway narrow-
ing or total occlusion, the chest and abdominal bands may move paradoxically. Of
note, a change in body position may alter the ability of either piezoelectric belts or
RIP bands to detect chest/abdominal movement. Changes in body position may
require adjusting band placement or amplifier sensitivity. In addition, very obese
patients may show little chest/abdominal wall movement despite considerable
inspiratory effort. Thus, one must be cautious about making the diagnosis of central
apnea solely on the basis of surface detection of inspiratory effort.
Arterial oxygen saturation (SaO
2
) is measured during sleep studies using pulse
oximetry (finger or ear probes). This is often denoted as SpO
2
to specify the method
of SaO
2
determination. A desaturation is defined as a decrease in SaO
2
of 4% or
more from baseline. Note that the nadir in SaO
2
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). Various
measures have been applied to assess the severity of desaturation, including comput-
ing the number of desaturations, the average minimum SaO
2
of desaturations, the
time below 80%, 85%, and 90%, as well as the mean SaO
2
and the minimum satu-
ration during NREM and REM sleep. Oximeters may vary considerably in the num-
ber of desaturations they detect and their ability to discard movement artifact. Using
long averaging times may dramatically impair the detection of desaturations.
=
(
a
×
RC)
+
(
b
10.10
Adult Respiratory Definitions
In adults, apnea is defined as absence of airflow at the mouth for 10 seconds or lon-
ger [21, 22]. If one measures airflow with a very sensitive device, such as a
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