Dear Dr. Roach: How are sleep apnea, sighing and heart failure related? Recently, I was told that I have severe sleep apnea and that I must use a CPAP machine. I have noticed that during the day I frequently take a spontaneous deep breath. Is that sleep apnea while awake? I have coronary heart disease and have three stents. Will the CPAP machine help me in any way? — A.P.

Dear A.P.: It's complicated.

By far, the most common type of sleep apnea is obstructive sleep apnea. This happens in people who are overweight, or in people whose neck anatomy predisposes them to the condition. During sleep, the upper airway collapses, preventing proper inspiration. This can be noticed by a sleep partner as choking, gasping or just not breathing for a prolonged time. The person eventually will wake up enough, due to lack of oxygen, to be able to take in a good breath, but will then fall back asleep. The process may happen hundreds of times a night, with the person often unaware. Sleepiness during the day (from never sleeping properly at night) and snoring are the most common symptoms of obstructive sleep apnea. CPAP (continuous positive airway pressure), which uses high pressure to keep the airway open, is the most common initial treatment for obstructive sleep apnea.

In severe sleep apnea, the oxygen level in the blood can be so low for so long that the heart and lungs can be damaged, eventually leading to a type of heart failure of the right side of the heart.

However, the situation is even more complex. People who have other kinds of heart failure, such as from repeated heart attacks, can develop the less common form of sleep apnea, central sleep apnea. Although this also may be treated by CPAP, it is critical to treat the underlying heart failure as well. Your doctor can tell you whether you have central or obstructive sleep apnea.

Spontaneous deep breaths, sighs, are a part of normal human physiology. Sighs have several purposes, but the most important is to help keep the lungs properly expanded. Sighs might be more common in people with heart failure, but I do not know of any correlation with sleep apnea.

Dear Dr. Roach: My question involves my dentist insisting on my taking four antibiotics before any procedure even as minor as a cleaning. I have had a very successful knee replacement. Does this overuse of antibiotics contribute to the increase of "superbugs" that now appear to be very threatening? — S.D.

Dear S.D.: Four expert groups now recommend against the use of antibiotics prior to dental cleanings in people with prosthetic joints. Your dentist should know that the use of antibiotics does not reduce the risk of developing an infection in the joint you had replaced. The downsides include not only the development of resistance you mention, but also allergic reactions and a wide variety of uncommon other reactions. I can't comment on your particular case, as you might have an unusual situation I don't know about. However, most people are more likely to have harm than benefit from antibiotic treatment prior to routine dental cleaning, even if they have a joint replacement.

In contrast to joint replacements, there are a handful of people with heart problems who need antibiotics prior to dental procedures. Most of these involve prosthetic heart material, a heart transplant, congenital heart disease or prior history of valve infection.

Readers may email questions to ToYourGoodHealth@med.cornell.edu.