Heart Slowdown Rate Predicts Post-MI Mortality Risk

"This new measure can detect high-risk patients even if the current gold-standard measure fails," said Georg Schmidt, M.D., of the Munich University of Technology.

The variability of the heart rate is affected by both vagal and sympathetic modulation of the sinus node, and a fall in vagal activity indicates an increased risk of death, Dr. Schmidt and colleagues noted in the May 19 issue of The Lancet. But standard measures of variability don't distinguish between vagal and sympathetic effects.

The researchers hypothesized that an "approximate distinction" of the different effects might be made by measuring the so-called deceleration capacity and acceleration capacity of the heart, using a Holter heart-rate monitor.

The issue is important, because it's known that prophylactic insertion of an implantable cardioverter defibrillator (ICD) can save the lives of patients at high risk for arrhythmic death, said Sami Viskin, M.D., of Tel Aviv University, in an accompanying commentary.

But relying on LVEF to define who's at risk (defined as a LVEF at 30% or less) means surgeons have to insert 17 devices to save one life, Dr. Viskin noted. Adding other measures to the equation can define a more highly selected population and reduce the number needed to treat, Dr. Viskin said, but that leaves out many people who remain at relatively high risk.

To test their idea, Dr. Schmidt and colleagues studied a cohort of 1,455 patients in Munich who had recently suffered a myocardial infarction. They conducted Holter monitoring and calculated deceleration capacity and acceleration capacity as well as standard measures, such as LVEF and standard deviation of normal-to-normal intervals.

The results were correlated with mortality during a median follow-up of 24 months, during which 70 people died of all causes.

The researchers found that people with a low deceleration capacity were more likely to die than those with a high deceleration capacity, regardless of their LVEF status. In those who survived the follow-up, the mean deceleration capacity was 5.9 milliseconds, whereas in those who died it was 2.8, a difference that was statistically significant at P<0·0001.

From the Munich data, they developed three risk categories:

• Low risk was defined as a deceleration capacity of greater than 4.5 milliseconds.
• Intermediate risk was between 2.5 milliseconds and 4.5.
• High risk was below 2.5 milliseconds.

Using those categories, the researchers prospectively (and in a blinded fashion) evaluated two other cohorts of post-MI patients—one in London including 656 people and one in Oulu, Finland, including 600 people—and found similar correlations between deceleration capacity and mortality.

The precision of the test was evaluated using the area under the receiver-operator characteristics curve (AUC), which is a measure of the specificity and sensitivity of a test. The researchers found:

• In the London cohort, the mean AUC of deceleration capacity was 0.80, compared with 0.67 for LVEF and 0.69 for standard deviation of normal-to-normal intervals.
• In the Oulu cohort, the mean AUC was 0.74 for deceleration capacity, 0.60 for LVEF, and 0.64 for standard deviation of normal-to-normal intervals.
• All the comparisons were statistically significant at P<0·0001.

It's still not clear whether the measurement of deceleration capacity actually reflects what's going on with vagal modulation, the researchers said, but "our findings show that pathologies that slow the heart down are more clinically important than those that speed it up."

Because the test is consistently better at predicting mortality—and because it's relatively cheap and easy to use—the researchers argued that it will be useful to identify patients at low risk "in whom further diagnostic workout is not warranted" and to improve the selection of those who could benefit from an ICD.

This study however provided no data to show that specific treatments provided based on the use of this predictor will improve patients' outcome.

As noted by Dr. Viskin, the ultimate goal of risk stratification is not merely to identify individuals at high risk of dying but to detect those who are particularly likely to benefit from a specific intervention.

Because ICD insertion is effective for stopping arrhythmic death, a crucial point in the management of patients after MI is deciding when to implant. This decision should be based on findings of randomized trials, because interventional studies sometimes in the past have disproved arguments derived from observational studies.

"Prediction and prevention of sudden death are two different matters, Dr. Viskin said.