|
|
FOR IMMEDIATE RELEASE
Contact: Donna Krupa
Phone: 703.527.7357
Cell: 703.967.2751
djkrupa1@aol.com
Influence of Gender on Upper Airway Structure and
Function During Obstructive Sleep Apnea
Study Appears in November Edition of the Journal of
Applied Physiology
November 25, 2001 -- Bethesda, Md.— The November edition of
Journal of Applied Physiology, one of the 14 peer-reviewed journals
published by the American Physiology Society (APS), spotlights recent
research findings designed to improve and understand human well-being and
health. The summary below, from the current edition, focuses on Obstructive
Sleep Apnea (OSA), a medical condition that is life threatening and affects
millions of Americans.
Background
Obstructive Sleep Apnea Syndrome (OSAS) is characterized by recurrent
upper airway collapse and obstruction. Upper airway structure and function
are believed to be primary determinants of the degree of upper airway
collapsibility. Gender differences in upper airway structure and function
are believed to explain the clear male predominance of OSAS in the general
population.
In an investigation of the influence of gender on upper airway structure
and function, James A. Rowley, Xusong Zhou, Isabelle Vergine, Mahdi A.
Shkoukani, and M. Safwan Badr, all of the Sleep Research Laboratory, John
D. Dingell Veterans Affairs Medical Center, and Division of Pulmonary,
Critical Care and Sleep Medicine, Department of Medicine, Wayne State
University School of Medicine, Detroit, Michigan, conducted an investigation
using two protocols involving men and women.
Upper airway collapsibility can be measured in several ways. Most
researchers have focused on pharyngeal resistance, an indirect measure of
upper airway caliber. Another measure is critical closing pressure (Pcrit),
a concept based on modeling the upper airway as a simple collapsible tube.
Although Pcrit has been compared between subjects with and without sleep
apnea, there have been no reports on differences in Pcrit with gender.
Methodologies
In the study, these researchers examined two measures of upper airway
mechanics that have been previously shown to characterize upper airway
mechanics and function (pharyngeal resistance and Pcrit). In particular,
they tested the hypothesis that (1) pharyngeal resistance during sleep would
be larger in men and (2) Pcrit would be higher (i.e., the airway would be
more susceptible to collapse) in men compared with women.
Protocol 1: Measurement of Upper Airway Resistance
-
Data analysis: Measurements of upper airway
resistance (Rua) were collected from studies performed on 33 men and
27 women. These subjects had either no sleep complaints or mild snoring on
history. Studies in which apneas and hypopneas were demonstrated were
excluded from analysis. In the majority of subjects (85 percent), breaths
for analysis were chosen from a five-minute segment of stable stage
2 sleep. For the remaining subjects, a total of five minute stable stage
2 sleep was chosen from multiple segments. Airflow and supraglottic
pressure were recorded, and for each breath, a pressure-flow loop was
generated, from which resistance was measured at two points. Resistance
levels were computed and analyzed. For each subject, the percentage of
inspiratory flow limitation (IFL) breaths was calculated as the number of
IFL breaths divided by total breaths. Approximately 50-100 breaths were
analyzed per subject.
-
Statistical analysis: A simple t-test was used to
compare men and women for age, body mass index (BMI), neck circumference
(NC), resistance at a fixed flow of 0.2 l/s (RL), and
resistance at peak airflow (Rpk). Multiple linear regression was utilized
to determine whether gender, age, BMI, or NC predicted the resistance at
0.2 l/s and resistance at peak airflow during non-rapid eye
movement (NREM) sleep. Because neither resistance at 0.2 l/s nor
resistance at peak airflow was normally distributed, the research team
first transformed the values to the natural logarithm. Multiple logistic
regression analysis was used to determine whether gender, age, BMI, NC, or
RL predicted whether a subject had >10 percent of the analyzed breaths
showing flow limitation.
Protocol 2: Measurement of Pcrit
-
Measurement: Measurement of Pcrit was performed in
eight men and eight women recruited from the general population. The
subjects were free of sleep complaints, including snoring. If, during the
course of the study, the subject demonstrated sleep-disordered breathing,
the study was terminated because only normal subjects were needed. In
addition to the above measurements, pressure was also monitored at the
nasal mask (Pn) in each subject. Pcrit measurements were performed in a
manner similar to a previously published report on this measurement.
-
Protocol: Patients were allowed to fall asleep
breathing at atmospheric pressure. During periods of stable stage 2 sleep,
Pn was abruptly reduced, maintained for two breaths, and then raised back
to atmospheric. Complete airway collapse was achieved in all subjects.
Results Of Influence of Gender on Upper Airway Structure and
Function
The study found no gender differences in upper
airway resistance during stable stage 2 NREM sleep and corroborates recent
work demonstrating no gender difference in the changes in upper airway
resistance (Rua) between the awake state and NREM sleep. These findings
agree with previous work by these researchers which found no gender
difference in Rua during stage 2 NREM sleep.
The study found no gender difference in Pcrit,
which suggests that the upper airway of men is not more prone to collapse
than that of women.
Conclusions
The gender difference in OSA prevalence could be secondary to differences
in upper airway structure, function, or the neurochemical control of
breathing. The evidence suggests there is no gender difference in upper
airway size, whether measured directly by imaging methodologies or
indirectly by Rua. Evidence also suggests that the degree of airway
narrowing secondary to interventions such as negative pressure or resistance
loading is not different between the genders. However, there remains a
fundamental difference between the upper airway of men and women as
evidenced by the increased susceptibility to the development of flow
limitation and increased upper airway compliance in men compared with women.
The authors believe that different methodological approaches to upper
airway function are measuring different aspects of upper airway function.
Further research to explore the fundamental differences between various
methodologies used to measure collapsibility and the implications of the
differences for sleep-disordered breathing are therefore indicated.
Finally, because it has also been shown that men are more susceptible to
hypocapnic hypopneas than women, further investigations into how the
neurochemical control of ventilation interplays with mechanical and
structural factors could be helpful in determining potential mechanisms for
the gender prevalence difference in OSA.
A Unifying Hypothesis
The authors propose a hypothesis to explain the divergent findings
regarding upper airway patency in men and women. First, nonneuromuscular
factors -- primarily pharyngeal wall structures and properties -- are the
likely dominant factor to explain within-breath changes in patency during
breathing. They postulate that the anatomy of the male upper airway renders
it more susceptible to within-breath narrowing under eupneic conditions.
This postulate is supported by their unpublished findings that men have
higher nasopharyngeal compliance compared with women during eupneic
breathing and are more likely to develop inspiratory flow limitation in
response to inspiratory resistive loading. It is also supported by a recent
finding of increased soft tissue volume in the necks of men compared with
women.
Second, neuromuscular factors are the dominant factors when the upper
airway is challenged and these factors are not different between men and
women. The similarity in Pcrit and the similarity in genioglossus response
to inspiratory loading support this postulate. The authors emphasize that
this interpretation is a speculation awaiting further experimental proof.
The authors also note that they found that an increased resistance
predicted a lower likelihood of the presence of flow-limited breaths. This
finding would indicate that a more narrow airway would be less prone to flow
limitation, which is in contrast to the long-held assumption that a more
narrow airway is a more easily collapsed airway.
Source: Journal of Applied Physiology (November 2001)
-end-
The American Physiological Society (APS) was founded in 1887 to foster
basic and applied science, much of it relating to human health. The
Bethesda, MD-based Society has more than 10,000 members and publishes 3,800
articles in its 14 peer-reviewed journals every year.
***
Editor’s Note: For the full
text of the research cited above, or to set up an interview with a member of
the research team, please contact Donna Krupa at 703.527.7357 (direct dial),
703.967.2751 (cell) or djkrupa1@aol.com.
|
|
