Here is a brief description of pacemakers - what they do, how they are inserted, and what it means to have one.

It has long been known that aerobic exercise can increase HDL levels. Now there's clinical evidence that shows just how much exercise is needed to produce HDL elevations. Read about exercise and HDL levels here.

In recent years, people have been encouraged to estimate their body mass indexes (BMI) to help assess cardiac risk. BMI is meant to be an estimate of whether you are of normal weight (for your height), underweight, overweight or obese. However, recent evidence suggests that, at least for people in the "overweight" category, the overall level of cardiac fitness and the amount of abdominal fat may be more important than BMI. Read more about it here.

A study published this week in the Archives of Internal Medicine suggests that women who have very mild hypothyroidism (i.e., an underperforming thyroid gland) may be at increased risk for heart disease.

The study looked at levels of Thyroid Stimulating Hormone (TSH) in over 17,000 women from Norway, and found that over an 8-year period, women whose TSH levels were in the "high normal" range had significantly higher cardiovascular risk than women whose TSH levels were lower.

TSH is a hormone that sets the throttle for the thyroid gland; that is, if the thyroid gland is not producing enough thyroid hormone, TSH levels increase to make the thyroid gland work harder. So, elevated TSH levels indicate that the thyroid gland is underperforming (so it needs to be "whipped"). In fact, elevated levels of TSH is the best way to diagnose hypothyroidism.

The so-called "normal range" for TSH levels is already controversial. Many have pointed to evidence that what most now consider to be the normal range for TSH is actually too wide, and that people with TSH levels in the "high-normal" range (which generally indicates "low-normal" thyroid gland function) should actually be considered as frankly hypothyroid. This new study certainly adds fuel to that argument.

This study does not prove that "high-normal" TSH levels actually cause an increase in cardiac risk, but it does suggest a correlation. A clinical trial will be needed to test the proposition that treating "high-normal" TSH levels in women (by giving thyroid hormone) might reduce cardiac risk.

For an in-depth discussion of this issue, read this article by Mary Shomon, the About.com Guide to Thyroid Disease.

PVCs are a common form of cardiac arrhythmia, and their significance can be confusing to both patients and their doctors. Read about PVCs, how to tell if they're medically significant, and how they are treated, here.

Several studies have now shown that cocoa-flavenols-containing foodstuffs (which, to most of you, might be more familiar as "dark chocolate"), can improve vascular health. Cocoa flavenols can make blood vessels more elastic, improve insulin sensitivity, and reduce blood pressure.

Early studies documenting the benefits of dark chocolate used "doses" of 100 grams of the stuff, which amounted to 500 calories. So, while patients had better vascular health they tended to get fat.

Later studies showed that as little as 6 grams (about 30 calories) were also effective in reducing blood pressure.

Now, the Mars candy company (launching into clinical science), in a study published in the Journal of Nutrition, has shown that their fortified brand of dark chocolate - CocoaVia - not only reduces blood pressure, but also lowers total and LDL cholesterol levels. They conducted a very well-designed randomized clinical trial in 49 patients, comparing their product (which has been fortified with cholesterol-reducing plant sterols), with standard, unfortified dark chocolate. The results of the study showed that blood pressure was significantly reduced in both groups, but the CocoaVia group also had significant reductions in cholesterol levels. Patients in this study consumed 2 bars of CocoaVia (about 200 calories) per day. They were reported to have gained no weight over the 4 weeks of the study - but notably, all patients in this study were also placed on the American Heart Association diet along with their candy-munching.

The chief significance of this study is that it has produced yet another piece of evidence to show that consuming dark chocolate can help with blood pressure reduction. We already knew that plant sterols reduce cholesterol levels. Those who want both beneficial effects (blood pressure and cholesterol) can either buy CocoaVia bars, or buy some other form of dark chocolate and plant sterols. (Hint: Check the respective prices of these two options.)

Amiodarone (Cordarone) is both the most effective and the most toxic antiarrhythmic drug ever developed. It's range of toxicities is so odd and potentially so harmful that one might wonder how a drug like this ever got approved. As it turns out, the history of amiodarone - its development, its usage, and how the drug came to be used in the United States - is as strange as the drug itself. Click here for the whole, strange history of amiodarone.

The importance of taking Plavix (clopidogrel - a drug that helps prevent blood clotting) after receiving drug-eluting stents (DES) has received a lot of attention over the past few years. Not long after DES came into common usage, reports surfaced indicating that there is a long-term risk of sudden occlusion of the coronary artery with these stents, produced by the sudden formation of blood clots at the site of the stent. Sudden blockage of a coronary artery, of course, can have catastrophic results including heart attack and sudden death. The long-term use of Plavix is thought to substantially decrease this risk.

Unfortunately, long-term Plavix therapy is not benign. Indeed, trauma or surgery while on Plavix can have devastating consequences. Some cardiologists refuse to allow their DES patients to stop Plavix even long enough to have elective surgery - placing the patient in an untenable position. So a major remaining question with the use of DES is: How long should Plavix therapy be maintained?

In an extraordinary hearing of the FDA in late 2006, the recommendation was to continue Plavix therapy for at least 12 months after DES. But recognizing that there was little data on which to base this recommendation, a strong plea was made to conduct a randomized clinical trial as rapidly as possible to answer the Plavix question once and for all.

Unfortunately, as reported by Shelley Wood for theHeart.org, nearly a year and a half after this FDA hearing that study is not even close to getting off the ground, let alone yielding useful results. (Wood's report is linked here, but registration is required.) Wood describes continued difficult "negotiations" (some might call it squabbling) between researchers at the Cardiovascular Safety and Research Consortium (located at Duke University), various government agencies, and manufacturers of DES, over the goals, design, size and duration of the proposed study. From the sounds of it, the study that everybody's waiting for won't be providing anything like definitive answers, for many years to come, on how long DES patients need to remain on Plavix.

This means that if your doctor is recommending DES, for the foreseeable future you're still going to need a long talk with your doctor about Plavix - specifically, how long will the doctor want you to take it, and what is to be done if it turns out you need surgery 8 months from now? Unfortunately, it appears the Plavix question will be looming over the use of DES for a long time to come.

Palpitations - an unusual awareness of the heart beats - are a common symptom. Here's a discussion of what causes palpitations, and how they should be evaluated.

In the wake of the continuing soap opera (featuring congressional hearings and the publication of hot corporate e-mails) over Vytorin that has resulted from publication of the ENHANCE trial, here's some positive news on the cholesterol-therapy front.

This week, Astra-Zeneca announced they were closing the JUPITER trial because the study drug, Crestor (rosuvastatin), showed unequivocal benefit. JUPITER had enrolled over 15,000 men and women with no evidence of heart disease and normal-to-low LDL cholesterol levels, but whose CRP levels were elevated. (CRP, a marker of inflammation, has been associated with increased cardiac risk. The statin drugs have been shown to reduce CRP levels.) The participants were then randomized to receive either Crestor or placebo.

The JUPITER trial had been scheduled to continue for some time, but this week the study's Independent Data Monitoring Board (a panel of experts whose job is to control the study's data without interference from the sponsor), reported that study participants randomized to Crestor were showing an unequivocal reduction in cardiovascular mortality and morbidity as compared to those on placebo, and recommended that the study be stopped. It will be some time before the actual results from the JUPITER study are compiled and presented publicly.

Based on what we now know, JUPITER is likely to become another piece of evidence that the known benefits of statin drugs may not be limited to their lipid-lowering capabilities, but may at least in part derive from some of their other effects. Read about cholesterol, statins and everything here.