Non-Invasive Measurement of Respiratory Effort

Sleep apnea is the most common disorder observed in the practice of sleep medicine and is
responsible for more morbidity and mortality than any other sleep disorder.(1) Although
characterized over 40 years ago, sleep apnea has only recently gained recognition as one of
the world's most prevalent undiagnosed disorders. Due to associated morbidity and mortality,
sleep apnea has been identified as a major public health concern. Currently, sleep apnea is
diagnosed by performing a sleep study. This requires a patient staying overnight in a sleep
laboratory while being monitored, which is often inconvenient for patients and is relatively
expensive to perform. The development of a device that allows for simple home monitoring of
patients when assessing for sleep apnea would be a significant development in the field of
sleep medicine.

The differentiation of obstructive sleep apnea (OSA) from central sleep apnea is an
important aspect of diagnosing and treating OSA. In central apneas/hyponeas, the lack of
airflow is due to lack of respiratory effort. In OSA, effort to breathe continues, but
airflow is prevented by an occluded airway. More recently, subtle obstructive events without
a defined desaturation, characterized by increasing respiratory efforts against a partially
closed airway and terminated by an arousal, have been described as Upper Airway Resistance
Syndrome (UARS). UARS can lead to daytime somnolence due to these frequent arousals.
Although nasal pressure signal has been used as a surrogate, by definition, distinguishing
UARS from OSA requires the use of esophageal manometry. This syndrome is characterized by
increased upper airway resistance (IUAR) that was defined by increasingly negative
inspiratory esophageal pressure. (2) To better understand the clinical significance of
esophageal manometry, let us briefly review some basic physiology. Taking a breath requires
that the inspiratory muscles generate sufficient force to overcome the elastic recoil of the
chest wall and lungs, frictional lung and chest wall tissue resistance, and the frictional
resistance of airflow through the airways. At the end of inspiration, the potential energy
stored in the tissues of the lungs and chest wall is available to allow rapid passive
exhalation when the inspiratory muscles cease contraction and the distending force
disappears. All intrathoracic structures are subjected to the pressures generated by
breathing. The esophagus is a thin walled muscular tube that, from the view of pulmonary
physiologists, is ideally placed in the thoracic cavity between the lung surface and the
chest wall. Measurement of differential pressure changes in the lumen of the esophagus
accurately reflect changes in intrathoracic pressure. These changes in intrathoracic
pressure are in turn reflective of inspiratory effort and have become the gold standard for
detecting and quantifying inspiratory effort.

Unfortunately, measurement of esophageal manometry requires the placement of an esophageal
catheter via the nose or mouth. This procedure in invasive, time consuming, and requires
specialized training. For this reason, esophageal manometry is not routinely performed
during sleep studies, although it is performed in patients at the WRAMC sleep lab who are
suspected of having UARS. Instead of esophageal manometry, the most frequently used method
of monitoring respiratory effort is to record thoracic cage expansion. In this procedure,
bands are placed around the rib cage and abdomen. The change in circumference of these two
compartments with breathing is measured. This procedure yields a qualitative change in
inspiratory volume and paradoxical movement of the rib cage or abdomen can indicate effort
against a closed or partially collapsed airway. This device is impractical for
self-application by patients. Given the limitations of the latter procedure and the invasive
nature of esophageal manometry, a non-invasive quantitative method for measuring respiratory
effort would be a significant advancement in sleep medicine.

Over the past five years, investigators from Advanced Brain Monitoring, Inc. developed and
validated the Apnea Risk Evaluation System (ARES). This device was initially developed to
provide a highly accurate method of in-home diagnosis of OSA that maximized patient comfort
and ease of use. The ARES Unicorder was designed as a single site (forehead) system to
acquire oxygen saturation, pulse rate, snoring, airflow (nasal pressure), and head
position/movement. Recently, investigators determined that a number of signals that were
being acquired with the Unicorder could be used to measure respiration.(1) Pulsations
observed in the red and infrared optical signals used to compute oxygen saturation and the
forehead sensor pressure signal appear to reflect central venous pressure changes. These
data has been termed the Forehead Venous Pressure/Respiratory Movement (FVP/RM) measure. If
FVP/RM data proves to correlate with esophageal manometry data, the long sought after
non-invasive quantitative measurement of intrathoracic pressure and respiratory effort would
be available. This would represent a significant advancement in the diagnostic evaluation of
sleep disordered breathing.

To better illustrate how forehead venous pressure could reflect intrathoracic pressure, let
us again review some physiology. During inspiration, the central venous pressure (CVP)
decreases, aiding the return of blood to the heart. Changes in pleural and intrathoracic
pressure due to respiration are reflected by and can be timed to changes in central venous
pressure. When recumbent, the internal and external jugular veins are open and provide the
primary cerebral and superficial forehead venous return. The communication between the
superior vena cava and the veins of the headallows intrathoracic pressure changes to be
reflected by the superficial veins of the head. In a sense, the forehead veins offer a
direct fluid filled catheter into the thoracic cavity, and when the correct compressive
force is applied against the skull, respiratory effort linked venous pressure changes can be
accurately measured.

In summary, current evaluation of UARS requires esophageal manometry testing, an invasive
and time consuming procedure for accurate diagnosis. Currently, no non-invasive quanitative
means of measuring intrathoracic pressure changes linked to respiratory effort exists. This
study is a pilot trial of a new non-invasive method of determining intrathoracic pressures.
If intrathoracic pressure data from this device is found to correlate with esophgeal
manometry data, this could have significant implications on the way in which UARS is
diagnosed. This technology also has the potential to develop a non-invasive means of
measuring CVP.

Inclusion Criteria:

- Age 18 or older,

- Referred for sleep study at WRAMC for suspected sleep disordered breathing.

Exclusion Criteria:

- Pregnancy

- Age <18

- Nasal defects precluding insertion of esophageal manometer

- Patients currently on anticoagulation