Learning Outcome

The nurse will:

• Obtain a basic understanding of the heart's conduction system.
• Understand the processes that result in sinus bradycardia.
• Recognize the clinical signs and symptoms of sinus bradycardia.
• Implement appropriate nursing interventions for the care of the patient with sinus bradycardia.

Introduction

The sinoatrial node (SA) is the default pacemaker and therefore a crucial component of the heart's conduction system. It is located subepicardially and is crescent in shape. The sinoatrial node is innervated by vagus and sympathetic nerves. The sinoatrial nodal artery supplies blood to the sinoatrial node, it branches off the right coronary artery in 60% of cases, whereas in 40% of cases it comes off the left circumflex coronary artery.[1][2] Sinus bradycardia is a cardiac rhythm with appropriate cardiac muscular depolarization initiating from the sinus node generating less than 60 beats per minute (bpm). Diagnosis of sinus bradycardia requires visualization of an electrocardiogram showing a normal sinus rhythm at a rate lower than 60 bpm. Where a normal sinus rhythm has the following criteria[3][4]:

• Regular rhythm, with a P wave before every QRS.
• P wave is upright in leads 1 and 2, P wave is biphasic in V1.
• The maximum height of a P wave is less than or equal to 2.5 mm in leads 2 and 3.
• The rate of the rhythm is between 60 bpm and 100 bpm.

Causes

Sinus bradycardia has many intrinsic and extrinsic etiologies[5]][6][7][8][9][10].

Inherent Etiologies 

• Chest trauma                                                            
• Ischemic heart disease
• Acute myocardial infarction
• Acute and chronic coronary artery disease
• Repair of congenital heart disease
• Sick sinus syndrome
• Radiation therapy
• Amyloidosis
• Pericarditis
• Lyme disease
• Rheumatic fever
• Collagen vascular disease
• Myocarditis
• Neuromuscular disorder
• X-linked muscular dystrophy
• Familial disorder
• Inherited channelopathy

Extrinsic Etiologies

• Vasovagal simulation (endotracheal suctioning)
• Carotid sinus hypersensitivity
• Beta-blockers
• Calcium channel blockers
• Digoxin
• Ivabradine
• Clonidine
• Reserpine
• Adenosine
• Cimetidine
• Antiarrhythmic Class I to IV
• Lithium
• Amitriptyline
• Narcotics
• Cannabinoids
• Hypothyroidism
• Sleep apnea
• Hypoxia
• Intracranial hypertension
• Hyperkalemia
• Anorexia nervosa

Risk Factors

In clinical practice, adults over the age of 65 and young athletes of both sexes are commonly known to present with sinus bradycardia. One in 600 adults over the age of 65 has sinus node dysfunction, but more research needs to be done to collect epidemiologic data within the United States and globally for patients with sinus bradycardia.[11]

Assessment

Majority of patients with sinus bradycardia do not have symptoms. Healthy young adults and athletes tend to have an increased vagal tone which keeps them in sinus bradycardia at rest. Also, patients above the age of 65 tend to have sinus bradycardia during sleep secondary to the aging of the sino-atrial node. Using history to relate to the symptoms of a patient with sinus bradycardia on an electrocardiogram is essential to come to the correct diagnosis. Those who present with symptoms may present with fatigue, exercise intolerance, lightheadedness, dizziness, syncope or presyncope, worsening of anginal symptoms, worsening of heart failure or cognitive slowing. When taking a history a health care provider must include relevant questions. Such as any recent medication changes, medication overdoses, chest pain, shortness of breath, history of prior myocardial infarction, symptoms of intermittent palpitations, history of chest trauma, rash or recent tick bite, current or past diagnosis of streptococcus pharyngitis, family history of sinus bradycardia, family history of muscular dystrophy. Moreover, physical exam findings should be correlated with the history.[12][13]

Evaluation

The most significant component of evaluation for a patient who presents with signs and symptoms of sinus bradycardia is history and physical assessment. These should include vital signs (respiratory rate, blood pressure, temperature, and heart rate) and an electrocardiogram. During assessment it should be established whether the patient is hemodynamically unstable, evaluation for this includes high blood pressure, altered mental status or difficulty breathing. If the patient is healthy, athletic and has no symptoms then sinus bradycardia may be a normal finding for the patient. On the contrary, in older individuals, sinus bradycardia may point towards an unhealthy sinus node. Patients with congestive heart failure often have sinus bradycardia. These patients may have compromised blood supply from the right coronary artery or left circumflex artery to the sinus node secondary to some underlying ischemic heart disease.[14]

Medical Management

A patient in sinus bradycardia should be evaluated for hemodynamic instability. If found to be hemodynamically unstable the patient can be treated with intravenous (IV) atropine. If the patient's symptoms and heart rate do not improve, the patient is a candidate for a temporary pacemaker. If the patient on arrival is hemodynamically stable but has signs and symptoms of acute myocardial infarction, they should be treated for an acute myocardial infarction appropriately. If there are no signs or symptoms of acute myocardial infarction in a hemodynamically stable patient, then workup should be initiated for an infectious cause (including chest x-ray, blood cultures, urinary analysis, viral panel) together with thyroid function tests. If a patient is found to have an infectious cause or a thyroid abnormality, the patient should be appropriately treated for these underlying causes and re-evaluated.  The patient's medication list should also be reviewed for possible causes of bradycardia. If a patient has comorbid conditions that require him to be on certain medications which may be causing his sinus bradycardia then in the patient may be a candidate for a permanent pacemaker.[15]

Nursing Management

Nursing management includes:

• Careful physical assessment of the patient.
• Assessment of vital signs, with special attention to heart rate.  Especially, prior to administering a medication which may have an effect on heart rate or blood pressure.
• If the heart rate is less than 60 beats per minute notify the physician, or advanced practice provider, prior to administering medications which may slow the heart rate.
• Immediately notify the physician, or advanced practice provider, if the patient experiences shortness of breath, hypotension, or chest pain. 
• Immediately notify the physician, or advanced practice provider, of other changes in patient condition. [15]

Coordination of Care

Educating patients at risk for this rhythm and making a closed loop communication between them and their providers can help further improve the management of these rhythms.

Health Teaching and Health Promotion

Multiple resources are available for nurses to help educate patients about this rhythm and its potential symptomatic complications. A patient who comes to the hospital or a clinic and has this rhythm identified should be provided with educational pamphlets if they are available at the facility.

Pearls and Other issues

There is a growing clinical consensus to lower the diagnosis threshold of sinus bradycardia to less than 50 bpm as there is a significant population size with a resting heart rate between 50 to 60 bpm. At present, the diagnostic consensus remains at a rate lower than 60 bpm with only the American College of Cardiology/American Heart Association/American College of Physicians–American Society of Internal Medicine (ACC/AHA/ACP–ASIM) Task Force recommending that it be diagnosed at 50 bpm.[4]