It's long been suspected that sleep
apnea goes hand in hand with more serious conditions.
That suspicion has a much more solid basis this month,
after a slew of research findings implicated sleep apnea
in diabetes, heart disease, early cardiovascular death,
pre-eclampsia and even car crashes.
The profile of the typical sleep
apnea sufferer is already a familiar one in most cardiologists'
waiting rooms: male, middle-aged and overweight. It's
common for these patients to have several cardiovascular
risk factors. But attendees at this year's conference
of the American Thoracic Society (ATS) in San Francisco
at the end of May heard that the risk of heart disease
in sleep apnea patients remains elevated even when standard
risk factors are taken into account.
RESTLESS
HEARTS
Sleep apnea — defined as an apnea-hypopnea index
score over 15 on polysomnography — can increase
the risk of heart attack or death by 30%, according
to a study led by Dr Neomi Shah of Yale University Medical
School. The study followed 1,123 patients over five
years, and found that risk increased throughout that
period.
Patients with the most severe sleep
apnea (apnea-hypopnea index score over 30), ran a risk
of heart attack or death nearly 30% higher than those
with milder apnea symptoms.
Patients in the apnea group had
higher blood pressure and body mass index than controls,
although lipid profiles, arrythmias and smoking status
were similar in the two groups. Before these risk factors
were controlled for, sleep apnea showed a hazard ratio
of 1.56 for heart attack or death, with a significance
of P=0.007. After controlling for cardiovascular risk
factors, the hazard ratio fell to 1.4, and the finding
remained statistically significant at P=0.048.
Dr Shah says in a telephone interview
with NRM that the magnitude of the added risk
is "clinically important," similar to that conferred
by a previous history of heart attack.
In obstructive sleep apnea, the
airway temporarily collapses, leading to partial arousal
of the sleeper. This can happen dozens or even hundreds
of times in a night's sleep. The sudden, repetitive
decreases of oxygen trigger the sympathetic nervous
cascade commonly known as the fight-or-flight response,
says Dr Shah. This can raise blood pressure and increase
heart rate.
CPAP
FOR CVD?
Sleep apnea is hardly a rare condition, but most people
who have it don't even know it. "Estimates for the prevalence
of apnea in the US range around 24% in men and 9% in
women. The vast majority are undiagnosed and untreated,"
says Dr Shah. "There's some evidence to make us believe
that when sleep apnea is appropriately treated, the
risk of heart disease can be lowered."
The gold standard treatment for
sleep apnea, continuous positive airway pressure (CPAP),
has been shown to reduce blood pressure somewhat in
a meta-analysis of small trials published last month
in the Archives of Internal Medicine.
Among heart failure patients, more
than half of whom typically suffer from sleep apnea,
the survival benefit conferred by CPAP is no longer
really disputed. A series of research articles in the
Journal of the American College of Cardiology this year
has documented the negative effects of sleep apnea on
survival in heart failure, and the benefits of treating
the condition with CPAP.
But in the broader population,
the evidence is still very much at the stage of highlighting
the risks, rather than suggesting solutions. Dr Shah
agrees that this is the next step: "There were several
proposals at the conference to take this further by
seeing whether apnea treatment could achieve reductions
in cardiovascular risk," she says. "Obviously that's
going to require randomized controlled trials."
DIABETES
TOO
Dr Shah's Yale colleague Dr Nader Botros led a study
which followed 593 patients at the VA Connecticut Health
Care System referred for evaluation of sleep-disordered
breathing, and looked at their risk of developing diabetes.
Over six years' follow-up, those
who'd been diagnosed with sleep apnea were more than
two-and-a-half times as likely to develop diabetes.
The more severe the apnea, the greater the risk.
Cortisol, a product of the sympathetic
nervous response, is known to stimulate insulin resistance
and glucose intolerance, says Dr Botros, but he also
doesn't rule out a direct effect of oxygen shortage.
"Our next step will be to determine whether the treatment
of sleep apnea can improve an individual's diabetic
parameters and consequently the negative health effects
of diabetes," he says.
GESTATIONAL
APNEA
The same basic story was repeated in a study of sleep
apnea in pregnancy, also presented at the ATS conference,
which found — you guessed it — a link to both
hypertension and diabetes in expectant mothers.
"The repetitive decrease in oxygen
that occurs during the night in someone with sleep apnea
heightens the body's 'fight or flight' state, which
can raise blood pressure," explains lead researcher
Dr Hatim Youssef, of the University of Medicine and
Dentistry of New Jersey. "The body also secretes more
hormones such as cortisol and epinephrine, and the body
responds by producing more glucose coupled with a decreased
sensitivity to insulin, which can lead to diabetes."
DRIVING
WHILE DOZY
As if all that weren't bad enough, Dr Alan Mulgrew of
UBC's Sleep Disorders Program presented research to
the ATS conference suggesting that sleep apnea sufferers
run greatly elevated risks of car accidents, presumably
because of daytime tiredness.
Comparing 800 apnea sufferers to
800 healthy controls, Dr Mulgrew found that the former
group had twice as many accidents — 250 over a
three year period compared to 123 in the control group.
Worse yet, each accident in the sleep apnea group was
about twice as likely to involve serious personal injury.
"We were surprised not only by
how many of the sleep apnea patients' crashes involved
personal injury, but that some patients had fairly mild
sleep apnea and were still having serious crashes,"
says Dr Mulgrew. "Based on these findings, I now consider
driving risk when deciding on treatment for patients
with mild sleep apnea."
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