The heart may not be the seat of emotion as people once believed, but it is a very sensitive organ. Acutely responsive to cues from the rest of the body, it constantly adjusts its speed, racing when we need more energy and slowing down when energy demands fall.
Phyllis K. Stein, PhD, research associate professor of medicine in the Cardiovascular Division, believes that the ever-changing heart rate holds some largely overlooked clues to cardiac health.
As director of the Heart Rate Variability Laboratory, she is dedicated to decoding the heart's rhythm. Perched in front of their computer monitors, Stein and other lab members perform sophisticated analyses of ambulatory electrocardiograms (ECGs), recordings of the heart's beat-to-beat electrical activity taken while a patient goes through ordinary daily activities.
"Typically, a cardiologist will look at an ambulatory ECG to find premature beats, pauses and other big events, but there are more subtle indicators embedded in the data. I've found that if you torture them, the ECGs will admit everything," Stein says with a smile.
To conduct an ambulatory ECG test, a medical technician attaches electrodes to a patient's torso to measure the fluctuating electrical field given off by the heart with each beat. These electrical signals are generally stored in a small, portable device called a Holter recorder and downloaded to a computer for analysis.
To the trained observer, the shapes and frequencies of ECG waves can reveal abnormalities in the heart's anatomy or function, making ECGs convenient, non-invasive tools for monitoring heart health. But Stein has learned that beyond the peaks and valleys of an ECG recording lies a wealth of information about heart rate patterns that can identify patients at high risk of cardiovascular problems.
The length of the space between ECG wave-peaks indicates heart rate — the shorter the space, the faster the rate. Stein and her colleagues in the Heart Rate Variability (HRV) Lab scrutinize how that rate changes over the course of minutes, hours or an entire day.
Variation in the heart's speed generally means the heart listens well to signals from the body about oxygen and nutritional needs. Healthy young people have a lot of heart rate variability, but with advancing age, variability declines. Findings show that low heart rate variability is associated with increased cardiovascular disease and related death.
A healthy heart varies its rate in response to breathing, speeding up a little with each inspiration and slowing down a bit with each exhalation. It also varies its rhythm according to circadian rhythm, body temperature fluctuations, blood pressure changes and sleep/wake cycles. In an unhealthy heart, these responses are diminished or absent. Finding the patterns of variation buried in ECGs is where Stein and her lab excel. A few years ago, Stein discovered a new indicator of cardiac health when analyzing heart rate variability in ECG recordings of older adults. "I began to notice a unique form of heart rate variability, a chaotic, random variability," she says. "I was the first to describe this irregular pattern, which I called erratic rhythm."
It takes a certain amount of mathematical finesse to identify erratic rhythm. But it's clearly evident when Stein and her group graph out trends in the length of beat-to-beat intervals.
Intrigued by the diagnostic value of erratic rhythm, Stein's lab analyzed data taken from a large group of heart attack patients. They found that erratic rhythm was a harbinger of death — the more hours of erratic rhythm in a patient's ECG recording, the more likely that patient was to have died during the study's follow-up period. Furthermore, if the amount of erratic rhythm increased after treatment with the study drugs, that too foretold an adverse outcome.
"When I was working up the data, I'd finish one analysis and think this patient has a lot of erratic rhythm, and I'd wonder if he had died," Stein says. "When I was right, it was awful. I started to feel like the Grim Reaper."
In another study, Stein showed that older adults with erratic rhythm were more likely to die of cardiovascular disease. She believes erratic rhythm can be an important marker for existing or future heart problems. But unless a cardiologist does the kind of analysis that the HRV Lab does, erratic rhythm might look like ordinary high heart rate variability, deceptively making those at greater risk of heart disease look like they have vigorous hearts.
Stein also found that a heart rate variability pattern that can identify people with sleep apnea, a syndrome in which a person stops breathing repeatedly during sleep. Untreated, sleep apnea can cause high blood pressure, stroke and other cardiovascular disease, as well as putting people at risk for falling asleep while driving or operating machinery. Most people with sleep apnea have not been diagnosed. "There's a connection between sleep apnea and atrial fibrillation (a disorder of the upper chambers of the heart)," Stein says. "We also know that treating sleep apnea in heart failure patients can improve their heart conditions. Studies now suggest that diabetics should be checked for sleep apnea because there seems to be a link between the two disorders."
As physicians become increasingly aware of the importance of sleep apnea and as studies continue to be published, Stein hopes that ECGs obtained from Holter monitors might become a routine way to screen for sleep apnea and that all Holter recordings obtained for any purpose would also be checked for signs of sleep apnea.
Stein also studies heart rate turbulence, a measure of how the heart responds to a premature contraction of the lower chamber, which can occur in people with and without heart disease. Normally, the heart accelerates after a premature contraction and then gradually decelerates.
"The system has to be able to compensate for the weak blood flow that comes from a premature contraction," Stein says. "If it can't, something is wrong. Normal heart rate turbulence is a strong indicator of heart health."
Stein also has shown that lower heart rate variability is associated with markers of inflammation, suggesting that inflammation plays a role in the decline of cardiovascular health. In addition, she has written about how a seemingly minor fluctuation in the height of one ECG peak, a measure called t-wave alternans, detected on an ambulatory ECG recording, can identify high risk of sudden death in patients after a heart attack and even in older adults not considered at high risk.
"There is a family of possible risk factors now known for cardiovascular disease," Stein says. "Everyone agrees that age, gender, diabetes, smoking, hypertension and cholesterol levels are important. But we now are asking how heart rate turbulence, erratic rhythm and other measures of heart rate variability should be incorporated into our assessment of heart health."