Bacterial heart infection has challenged doctors since the end of the nineteenth century. This illness is called endocarditis. This illness had a terrible mortality rate. Because of a very subtle difference between bacterial heart infections and all other bacterial infections, the diagnosis of bacterial heart infections remains difficult.
Almost all bacterial infections announce their presence by making the individual feel ill, and causing a fever. They almost always create a symptom that identifies which part of the body is infected. For example, a strep throat creates a sore throat, pneumonia causes chest pain, coughing and sometimes breathlessness, a leg cellulitis causes the affected leg to swell and hurt and the list goes on. The heart infection causes a feeling of illness and a fever but it cannot produce heart pain so the cause of the fever remains enigmatic. This combined with the rarity of heart infections and the presence of so many viruses and other febrile conditions makes it the last illness many doctors think of. As a defense, medical schools constantly drill medical students about this illness so that it is thought of earlier when a patient presents for care.
The Science
The heart is a difficult organ to infect because it is buried deep inside the body and has no direct connection to the outside world like the lungs or the throat or parts of bowels. The only common path for bacteria to reach the heart is by hopping a ride in the blood stream from some other part of the body that harbors bacteria. Because this is the route the bacteria take to the heart, the first part of the heart that is infected is always a surface that faces the flowing blood. This leads to the most important characteristic of heart infections. Because the infected mass faces the flowing blood, it constantly sheds bacteria into the blood. Therefore, the most reliable way to prove the presence of a heart infection is to demonstrate in the laboratory that live bacteria are growing in the blood. This technique involves taking a blood culture from the patient under sterile conditions into sterile bottles and incubating these bottles in the laboratory for several days. If bacteria grow in the laboratory, they could have only come from the patient’s blood.
The nature of the bacteria infecting the heart depends on its source. If the source was bacteria from the teeth or gums it will frequently be a streptococcus because they dominate in the mouth. If the source is the bowels it will be one of the many species that inhabits the large colon. When the infection starts during surgery the bacteria is often a staphylococcus.
So, in a logic that works in two directions if the source of the infection can be known because of a previous illness the nature of the bacteria can frequently be guessed and antibiotics can be started before the laboratory completes testing the bacteria. By reversing this logic, knowing which bacteria infected the heart frequently identifies the complimentary illness that permitted the bacteria to enter the bloodstream.
The Illness
But I have skipped a step. I have talked about the use of blood cultures to make the diagnosis of endocarditis, but the critical question is why would a physician think of taking blood cultures in the first place. We have already alluded to one situation and that is the individual who keeps running a fever without a clear cause. Because the heart is a famously silent actor it would become a suspect in this situation.
Unfortunately, endocarditis is frequently diagnosed late in its course when it causes a serious complication. One complication occurs when the infected mass in the heart chews its way through a heart valve and causes the valve to leak. These patients present to the doctor with heart failure, water in the lungs and difficulty breathing. Sometimes the stress on the heart can cause irregular heart rhythms. The sudden appearance of a damaged heart valve in a febrile patient raises the suspicion of endocarditis.
The most feared consequence of endocarditis is when the infected material rips loose from the heart and travels through the arterial blood stream like a bullet. This moving lump is called an embolus. If it lands in the brain it causes a stroke. If it blocks another artery it will damage whatever organ depends on that artery’s blood flow. Common targets for these infected blood clots are the spleen, kidneys and even return to the heart where it can cause a heart attack. So, a febrile patient with an embolic event or a deteriorating heart valve frequently gets referred for blood cultures to search for endocarditis.
While the blood cultures are growing in the laboratory an echocardiogram can be used to visualize the infected masses on the heart valves. One echocardiographic technique involves inserting an ultrasound probe into the esophagus after giving sedation. This produces wonderfully clear pictures of the valves. These images are clear enough to see the infected elements growing on and through the valves.
Treatment
The treatment of endocarditis first involves the treatment of the complication such as the heart failure or the stroke because they are such an immediate threat to life. Then antibiotics are started to eradicate the infection.
Before antibiotics the mortality from endocarditis was 100%. A terrible situation in which there were no survivors. With the introduction of intense prolonged intravenous antibiotic therapy, survival rates started to approach 50%. But it soon became evident that in many individuals the infectious mass was too large for antibiotics to penetrate and kill all the bacteria. In others the infected valve was too damaged for an antibiotic cure to be sufficient to restore the patient’s health. This started the modern era in endocarditis where patients are first treated with antibiotics. If they don’t start to improve or complications such as a very leaky heart valve threaten the patient, surgery is performed to remove the infected valve achieve a cure. This is a very dramatic approach but has created a situation where for the first time most endocarditis patients can anticipate leaving hospital cured.
Prevention
An illness this severe deserves serious efforts at prevention. Bacteria are constantly entering our blood stream when we have small traumas, dental work, and colonoscopies as a few examples. For the most part endocarditis only occurs in patients whose heart valves are not completely normal and have small scarred areas on them. These scarred areas do not have adequate blood flow and this prevents the body’s white cells from killing the bacteria as soon as they land. The fact that only a small part of the population is vulnerable to endocarditis has led to a strategy where doctors listen for heart murmurs when examining even the healthiest patients. Patient with heart murmurs are then referred for an echocardiogram that can determine if a damaged heart valve is causing the murmur These individuals then get a preventive dose of antibiotics before certain surgeries, dental work or dangerous diagnostic procedures.
Endocarditis is a difficult illness that continues to bedevil patients and challenge their doctors. There are a great many topics that I have not expanded on. Infections of artificial heart valves, fungal infections, the completely different type of endocarditis that occurs in intravenous drug abusers are just a few examples of the many other topics I have omitted in an effort to stay focused on the principles of this complicated, rare but yet not uncommon illness.
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