Lyme disease, caused by the bacterium Borrelia burgdorferi, is known for symptoms that can range from a distinct skin rash to neurological issues. A less common but serious complication of this infection is Lyme carditis (LC), which involves the heart’s electrical system. This condition can interfere with the coordinated timing of heartbeats, potentially leading to noticeable symptoms like lightheadedness or fainting. The electrocardiogram (ECG), a non-invasive test that records the heart’s electrical activity, serves as the primary tool for rapidly diagnosing this cardiac involvement.
Understanding Lyme Carditis
Lyme carditis is an inflammatory process of the heart tissue that occurs when the Borrelia burgdorferi spirochete invades the heart muscle. This cardiac manifestation typically arises in the early disseminated stage of Lyme disease, appearing weeks to a few months after the initial infection. While relatively rare, occurring in an estimated 1.5% to 10% of untreated Lyme cases, its effects on the heart’s rhythm can be profound. The infection triggers an inflammatory response that targets the heart’s electrical pathways, particularly the atrioventricular (AV) node.
The AV node functions as the main electrical bridge, ensuring that the upper chambers (atria) and lower chambers (ventricles) contract in a precise, timed sequence. Inflammation and swelling in this region cause a bottleneck, slowing or completely blocking the electrical signal as it attempts to pass through. This creates a problem of electrical timing, and any interruption can immediately lead to a slow or erratic heartbeat.
How the Electrocardiogram Works
An electrocardiogram is a non-invasive test that translates the heart’s electrical impulses into a visible tracing of waves and spikes. Electrodes are placed on the skin to detect the voltage changes generated by the heart muscle with each beat. This tracing allows clinicians to assess the heart’s rate, rhythm, and timing, which is crucial in the context of Lyme carditis.
The tracing consists of three main components: the P wave, the QRS complex, and the T wave. The P wave represents the electrical signal spreading through the atria. The QRS complex follows, showing the electrical signal spreading quickly through the ventricles, causing them to contract and pump blood. The T wave signifies the ventricles resetting electrically. By measuring the time intervals between these components, doctors determine if the electrical conduction system is working correctly.
Specific ECG Patterns Indicating Lyme Carditis
Lyme carditis appears on an ECG as an atrioventricular (AV) block, a specific electrical conduction disorder. This block reflects the interruption of the signal traveling from the atria to the ventricles via the AV node. The classic electrocardiographic sign of this process is the prolongation of the PR interval, which is the time measured from the start of the P wave to the start of the QRS complex. A prolonged PR interval, known as a first-degree AV block, is the mildest finding and indicates a simple delay in conduction.
The AV block caused by Lyme carditis can rapidly progress through various degrees. Second-degree AV block occurs when some electrical signals from the atria fail to reach the ventricles, resulting in dropped beats and an irregular rhythm. The most severe manifestation is third-degree, or complete, AV block, where the electrical connection between the atria and ventricles is completely severed. In this scenario, the ventricles must rely on a slower, less reliable “escape” rhythm, leading to a dangerously slow heart rate that can cause fainting or sudden cardiac collapse. Patients with a PR interval greater than 300 milliseconds are considered at high risk for progressing to complete heart block.
Post-Diagnosis Confirmation and Treatment
An abnormal ECG showing AV block, particularly in a patient with recent exposure or other Lyme symptoms, strongly suggests Lyme carditis. Definitive diagnosis requires confirmation through serological testing, which detects the presence of antibodies against Borrelia burgdorferi in the blood. This blood test confirms that the conduction abnormality is a result of the bacterial infection. An echocardiogram (ultrasound of the heart) may also be performed to assess the heart’s structure and pumping function, ensuring the infection has not caused severe inflammation of the heart muscle (myocarditis).
Treatment focuses on eliminating the bacterial infection and reducing the inflammation in the AV node. Patients with high-degree AV block or a prolonged PR interval often require immediate hospitalization for cardiac monitoring and the administration of intravenous (IV) antibiotics, such as ceftriaxone. These antibiotics are typically given for 14 to 21 days, sometimes switching to oral medication once the heart block improves. For patients whose heart rate drops dangerously low due to high-degree block, a temporary pacemaker may be required to maintain a safe rhythm until the antibiotics take effect. The heart block usually resolves completely within days to weeks with appropriate antibiotic therapy, often without the need for a permanent pacemaker.