What is Sinus Bradycardia?
The sinoatrial node, often referred to as the SA node, is like the heart’s natural pacemaker and is a key part of the system that controls how the heart beats. You can find it just beneath the outer shell of the heart, shaped like a crescent moon. In an average grown-up, this node is roughly 13.5 millimeters long. It receives signals from the vagus and sympathetic nerves. The SA node gets its blood supply from an artery known as the sinoatrial nodal artery. This artery either branches off the right coronary artery in 60% of people, or in the left circumflex coronary artery in 40% of people.
Sinus bradycardia is a heart rhythm with a rate of fewer than 60 beats per minute (bpm). This condition starts from the SA node. To confirm sinus bradycardia, a doctor will need to see your heart graph, known as an electrocardiogram, displaying a normal heart rhythm but with a rate just under 60 bpm. A normal heart rhythm has a steady pattern with a unique wave (the P wave) seen before every large wave (the QRS) on the heart graph. These P waves are typically upright in certain sections of your graph, namely leads 1 and 2, and are of two phases in another section called V1. The highest point of these P waves should be equal to or less than 2.5 mm in leads 2 and 3. The pace of this rhythm usually falls between 60 to 100 bpm.
What Causes Sinus Bradycardia?
Sinus bradycardia is a heart condition where the heart rate is slower than normal. There are many different things that could cause this condition. They are split into two categories – intrinsic (coming from within the body) and extrinsic (coming from outside the body).
Intrinsic causes could include:
Chest injury or trauma
Different heart diseases, such as ischemic heart disease where the blood supply to the heart is reduced, and acute myocardial infarction, also known as a heart attack
Coronary artery disease, which affects the blood vessels supplying the heart, and can be either a short-term or long-term condition
Procedures done to fix birth defects of the heart
An irregular heartbeat due to sick sinus syndrome
Treatment with radiation therapy for cancer
Amyloidosis, a disorder that causes an abnormal protein to build up in the body’s organs
Inflammation of the heart’s lining (pericarditis), perhaps due to infection
Lyme disease and Rheumatic fever, which are infections that can affect your heart
Collagen vascular disease, which affects the body’s connective tissue
Myocarditis, or inflammation of the heart muscle
Disorders of the nerves and muscles, muscular dystrophy which can affect the heart
Certain genetic disorders related to heart conditions.
Extrinsic causes could include:
Reactions to certain medications, like beta-blockers, calcium channel blockers (both types of heart medication), digoxin, ivabradine, clonidine, reserpine, adenosine, cimetidine, antiarrhythmic drugs, lithium, amitriptyline, and narcotics.
Other substances like cannabinoids (found in cannabis)
Body conditions like an underactive thyroid (hypothyroidism), sleep apnea, low oxygen levels in your blood (hypoxia), a high amount of pressure within your skull (intracranial hypertension), an unusually high level of potassium in your blood (hyperkalemia), an eating disorder like anorexia nervosa.
Additionally, some people might experience this due to an extreme reaction to certain stimuli, like having your windpipe suctioned during a medical procedure. People may also have an overly sensitive carotid sinus, an area in your neck that helps control your blood pressure, causing a slower heart rate.
Risk Factors and Frequency for Sinus Bradycardia
It’s quite common for adults above 65 years old and youthful athletes, regardless of gender, to have sinus bradycardia. There’s a fairly significant number of adults over 65, as many as 1 in 600, with problems relating to the sinus node. However, we need more studies to get accurate numbers on how many people are affected by sinus bradycardia in the United States and worldwide.
Signs and Symptoms of Sinus Bradycardia
Sinus bradycardia, a slower than normal heart rate, often doesn’t cause symptoms. Many healthy young individuals, athletes, and people over 65 may have it without even realizing. This condition can be detected via an electrocardiogram. While most people don’t show symptoms, some might experience tiredness, dizziness, fainting or near fainting, chest discomfort, difficulty in physical activities, worsening of heart failure symptoms, or mental sluggishness.
It’s vital for health care providers to ask for a detailed patient history, which includes questions about recent changes in medication, overdose, chest pain, difficulty in breathing, past heart attack history, irregular heartbeats, previous chest injuries, any rashes, tick bites, strep throat infection, and family history of sinus bradycardia or muscular dystrophy. These factors can help the doctors to narrow down the possible causes. The physical examination findings should align with the patient’s history, so any heard murmurs during the physical exam or skin rashes seen should also be considered.
The physical exam might also reveal following signs:
- Bluish lips, face, or limbs (Cyanosis)
- Swelling in the hands, ankles, and feet (Peripheral edema)
- Change in mental state (An altered state of consciousness)
- Difficulty in breathing (Dyspnea)
- Abnormal crackling sounds in the lungs (Rales and crackles)
Testing for Sinus Bradycardia
If someone is showing signs of a slow heart rate, called sinus bradycardia, the doctor will first take their medical history and conduct a physical exam. This exam includes measuring vital signs, such as breathing rate, blood pressure, body temperature, and heart rate. The doctor will also use a test called an electrocardiogram to study their heartbeat.
The state of the patient’s overall health and ability to maintain stable bodily functions, known as being “hemodynamically stable,” is important to establish. High blood pressure, changes in consciousness or awareness, or trouble breathing could mean a patient is not hemodynamically stable.
For healthy, athletic individuals with no symptoms, no further treatment may be necessary. However, in older people, a slow heart rate might be a sign of a sinus node (the natural pacemaker of the heart) that isn’t working well. It’s also common for patients with heart failure, a condition where the heart struggles to pump enough blood, to have sinus bradycardia. This could be because of compromised blood supply from important heart vessels to the sinus node due to underlying heart disease.
To investigate further, doctors may order lab tests to look at things like blood sugar (glucose) levels, electrolyte levels, and the levels of minerals like calcium and magnesium in the blood. Thyroid function tests, as well as testing for a protein called troponin that can indicate heart damage, might also be ordered. A drug screen to check for certain substances or toxins in the body can also be useful.
A detailed electrocardiogram (a 12-lead ECG) is necessary for diagnosing the condition.
If a patient is stable and there are no signs of a heart attack, the doctor may conduct further tests to check for infections. This can include a chest x-ray, blood culture tests, urine tests, and a viral panel. Tests to check thyroid function might also be carried out. If an infection or thyroid abnormality is found, the patient would be treated for these underlying conditions and then re-checked. If the patient no longer has symptoms and their heart rate is normal after re-evaluation, they could be evaluated for a condition called sick sinus syndrome or have a long-term heart monitoring device (called an implantable loop recorder) placed.
Treatment Options for Sinus Bradycardia
Patients with a slow heart rate or ‘sinus bradycardia’ need to be checked for signs of instability. If they are found to be unstable, they may be given a medication called atropine through an IV to help increase their heart rate. If there is no improvement, the doctor might consider using a temporary pacemaker to manage the patient’s heart rhythm.
In other situations, the patient might seem stable when they arrive but show signs of a heart attack (‘acute myocardial infarction’). In this case, they would require immediate treatment for a heart attack.
Patient’s body temperature can also affect treatment decisions. For instance, a patient who is excessively cold (‘hypothermic’) needs to be warmed up to a normal body temperature before any lasting treatment decisions can be made.
Besides these, reviewing the list of medications the patient is taking is another crucial part of managing slow heart rate. Some drugs can lead to sinus bradycardia, and if possible, these drugs should be stopped. However, in some instances, other health conditions may necessitate taking such medications. In these cases, doctors could consider a permanent pacemaker to handle the slow heart rate. If stopping the offending medication doesn’t improve the patient’s heart rate and symptoms, the doctor might still consider having them evaluated for a permanent pacemaker.
What else can Sinus Bradycardia be?
To tell the difference between a slow heart rate known as sinus bradycardia and other similar conditions, doctors look for a specific pattern on a heart monitor. This pattern is formed by different waves called P waves and QRS complexes.
If P waves cannot be seen, it could be due to heart rhythms that originate from a part of the heart known as the junction or the ventricle. In comparison, if there are more P waves than QRS complexes, it might be a sign of a delay or block in the transfer of electrical signals in the heart, which is also known as second or third-degree AV blocks.
Doctors usually consider the following heart conditions when diagnosing sinus bradycardia:
- Sick sinus rhythm
- Wandering atrial pacemaker
- Junctional escape rhythm
- Ventricular escape rhythm
What to expect with Sinus Bradycardia
The outlook is generally favorable when your healthcare provider quickly identifies the rhythm. However, for those with a condition known as sick sinus syndrome who also have a slow heart rate, known medically as bradycardia, the outlook tends to be less good – with survival rates ranging between 45 to 70% over five years.
As the number of weight loss surgeries are increasing each year, it’s been noticed that many of these patients also develop a slow heart rate. The connection between this and the rapid weight loss is not yet fully understood.
Possible Complications When Diagnosed with Sinus Bradycardia
If not diagnosed and treated quickly, symptoms like fainting, tiredness, or feeling dizzy can happen. These symptoms could indicate complications.
Common Symptoms to Look Out For:
- Fainting
- Tiredness
- Feeling dizzy
Preventing Sinus Bradycardia
Plenty of resources exist to help doctors explain to patients about this heart rhythm and the potential issues it could cause. If a patient visits a hospital or clinic and this heart rhythm is discovered, they should be given easy-to-understand leaflets, if they are available, to help them understand more about their condition.
