Quantitative Sleep Monitoring - Quantitative EEG Analysis Methods and Clinical Applications - page 267

10.9 Respiratory Monitoring

The three major components of respiratory monitoring during sleep are airflow,
respiratory effort, and arterial oxygen saturation [21, 22]. Many sleep centers also
find a snore sensor to be useful. For selected cases, exhaled or transcutaneous PCO 2
may also be monitored.
Traditionally, airflow at the nose and mouth was monitored by thermistors or
thermocouples. These devices actually detect airflow by the change in the device
temperature induced by a flow of air over the sensor. It is common to use a sensor in
or near the nasal inlet and over the mouth (nasal-oral sensor) to detect both nasal
and mouth breathing. Although temperature-sensing devices may accurately detect
an absence of airflow (apnea), their signal is not proportional to flow, and they have
a slow response time [23]. Therefore, they do not accurately detect decreases in air-
flow (hypopnea) or flattening of the airflow profile (airflow limitation).
Exact measurement of airflow can be performed by use of a
pneumotachograph. This device can be placed in a mask over the nose and mouth.
Airflow is determined by measuring the pressure drop across a linear resistance
(usually a wire screen). However, pneumotachographs are rarely used in clinical
diagnostic studies. Instead, monitoring of nasal pressure via a small cannula in the
nose connected to a pressure transducer has gained in popularity for monitoring air-
flow [23, 24]. The nasal pressure signal is actually proportional to the square of
flow across the nasal inlet [25]. Thus, nasal pressure underestimates airflow at low
flow rates and overestimates airflow at high flow. In the midrange of typical flow
rates during sleep, the nasal pressure signal varies fairly linearly with flow. The
nasal pressure versus flow relationship can be completely linearized by taking the
square root of the nasal pressure signal [26]. However, in clinical practice, this is
rarely performed.
In addition to changes in magnitude, changes in the shape of the nasal pressure
signal can provide useful information. A flattened profile usually means that airflow
limitation is present (constant or decreasing flow with an increasing driving pres-
sure) [23, 24]. The unfiltered nasal pressure signal also can detect snoring if the fre-
quency range of the amplifier is adequate. The only significant disadvantage of
nasal pressure monitoring is that mouth breathing often may not be adequately
detected (10%-15% of patients). This can be easily handled by monitoring with
both nasal pressure and a nasal-oral thermistor.
An alternative approach to measuring flow is to use respiratory inductance
plethysmography. The changes in the sum of the ribcage and abdomen band signals
(RIPsum) can be used to estimate changes in tidal volume [27, 28]. During positive-
pressure titration, an airflow signal from the flow-generating device is often
recorded instead of using thermistors or nasal pressure. This flow signal originates
from

a pneumotachograph or other flow-measuring device inside the flow

generator.
In pediatric polysomnography, exhaled CO 2 is often monitored. Apnea usually
causes an absence of fluctuations in this signal, although small expiratory puffs rich
in CO 2 can sometimes be misleading [7, 22]. The end-tidal PCO 2 (value at the end of
exhalation) is an estimate of arterial PCO 2 . During long periods of hypoventilation