People recovering from heart attacks have a lot to attend to. They face the shock of a close call; their initial recovery may take longer than they expect; and, to top it off, they get a new regime of medications, diet and exercise. It can be overwhelming.

A heart attack also can serve as a wake-up call and compelling motivation to make changes in lifestyle and behavior. Even so, many people are apprehensive about exercise after experiencing a heart attack.

Understandably so. We read in the newspaper about heart attack deaths from shoveling snow and think it's best to keep exertion to a minimum. Doctors thought the same thing early in the past century, says Michael Chen, a cardiologist at the University of Washington's Harborview Medical Center in Seattle. "Patients were put on bed rest for days to weeks, as doctors feared activity would cause more damage to the heart." Now it's well accepted that physical activity is beneficial to heart attack survivors and is key to preventing a second attack. And it begins right away.

"Phase 1 of cardiac rehabilitation starts in the hospital," Chen says, where patients are encouraged to get up and walk as soon as 12 hours after treatment for heart attack.

Still, there are barriers to starting and sticking to an exercise program. The usual culprits include time (in too-busy lives) and opportunity (a gym membership or some convenient place to bike, walk or run). Particular obstacles that can crop up after a heart attack include feelings of vulnerability and helplessness.

There also is fear.

"It's exceedingly common for someone to feel anxious after a heart attack," says Una McCann, a Johns Hopkins psychiatrist who studies anxiety and depression in the context of heart disease, "or to want to avoid situations that remind them of a recent traumatic health event such as a heart attack." For some people, fear can become pathological. Anxiety disorders, whether pre-existing or triggered by the heart attack, can interfere with patients' following their treatment plans and can impede recovery. In such cases, a mental health professional can help. Cognitive behavioral therapy, which might include relaxation techniques and reframing of automatic negative thoughts, is the first-line treatment, McCann says. In some cases, psychiatric drugs may also be prescribed.

The best way to start an exercise program is through a formal cardiac rehabilitation program. "It's the standard of care," Chen says, "and a central component of cardiac rehab is supervised exercise." In a cardiac rehab session, the patient is monitored by a nurse or an exercise physiologist while walking on a treadmill or pedaling an exercise bike. After a warm-up period, the patient is asked to increase the intensity. "Usually it's a conservative goal, such as increasing the patient's heart rate by 20 beats per minute," Chen says.

As a general rule, cardiac rehab does not involve high-intensity exercise, Chen says. Patients should be able to carry on a conversation while working out. The exercise portion of rehab usually includes light weights and stretching.

A formal cardiac rehab program has an important advantage, providing an expert to monitor your activity and offer advice. "It's really important when you're at high risk to get help," says Caroline Richardson, a family medicine doctor at the University of Michigan in Ann Arbor who researches how best to incorporate physical activity in patients with chronic diseases.

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Specific warning signs to a heart patient include chest pain or pressure, heart palpitations, dizziness and shortness of breath out of proportion to the exercise.

Cardiac rehab programs are generally covered by private insurance or Medicare. Such programs are advised for people who have had heart attack, bypass surgery, stable angina, heart valve surgery, stent insertion, heart transplant or congestive heart failure.

And yet, cardiac rehab is underused. Only one in five patients who are eligible are actually referred to such programs, according to a 2014 review of studies. In addition, many referred patients don't begin rehab, and 40 to 60 percent of those who do don't finish the prescribed course.

"The long-term compliance is not great, either," Chen says. "Patients will go to all the sessions, but then fall off of their own exercise program." But it needn't take a gym membership or personal trainer to undertake a heart-healthy exercise program. "You know, walking is probably the world's best exercise," Chen says.