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Identifying and Treating Third-Degree AV Block (Complete Heart Block)

Identifying and Treating Third-Degree AV Block (Complete Heart Block)

What Is a Third-Degree AV Block?

A third-degree atrioventricular (AV) block, also known as a complete heart block, occurs when there is a communication failure in the heart between the atria and ventricles. Often, a third-degree AV block develops due to a gradual deterioration of a second-degree AV block (type I or type II). Impulses that would typically be conducted from the sinoatrial (SA) node or atrium to the ventricles through the AV node are entirely blocked. This block results in atrioventricular dissociation, where the atria and ventricles beat independently without communication or coordination with each other. 

A ventricular or junctional escape rhythm may serve as a source of electrical activity for the ventricles. However, an escape rhythm may occur intermittently, may occur but not generate adequate cardiac output, or may not happen at all (leading to cardiac arrest). Because cardiac conduction is blocked from progressing through the AV node, the SA node cannot regulate the heart rate, resulting in diminished cardiac output due to the lack of coordination between the atria and ventricles. Patients experiencing this arrhythmia may require a permanent pacemaker. This cardiac rhythm may be fatal if not treated immediately.

Third-degree AV block can occur in both adult and pediatric patients. However, this article will focus on this arrhythmia in adult patients utilizing Advanced Cardiovascular Life Support (ACLS) guidelines to navigate care of the patient. Pediatric Advanced Life Support (PALS) algorithms can guide the care of the pediatric patient experiencing a third-degree AV block.

What Are Electrocardiogram (ECG) Characteristics of Third-Degree AV Block?

What Signs or Symptoms May Be Present?

Most patients with a third-degree AV block will be hemodynamically unstable. The patient’s presentation may vary, depending on their respective comorbidities and ventricular rate during this arrhythmia.

Signs or symptoms the patient may experience with a third-degree AV block include:

What Causes Third-Degree AV Block?

Risk factors for the development of third-degree AV block include:

Treatment of Third-Degree AV Block

Patients with a third-degree AV block are often symptomatic and may be hemodynamically unstable and in distress. Unstable bradycardia occurs when signs or symptoms are due to a decreased (bradycardic) heart rate and result in poor perfusion. There are multiple signs and symptoms of unstable bradycardia, which may include altered mental status, chest pain, hypotension, acute heart failure, or shock. If the adult patient with bradycardia has a pulse, is symptomatic, and is hemodynamically unstable, the ACLS Adult Bradycardia Algorithm should be employed by the presiding medical professional to guide the evaluation and treatment of the patient.

An initial step of the ACLS Adult Bradycardia Algorithm recommends that the medical professional identifies and treats potential underlying causes of the patient’s condition. Components of this step include determining the patient’s cardiac rhythm, monitoring their vital signs, managing their airway, and providing supplemental oxygen if needed. Also, intravenous access should be established, and a 12-lead ECG should be obtained (if available). In addition, the medical professional should account for potential hypoxic and toxicological causes for the patient’s clinical condition. Depending on the patient's clinical condition and their reaction to treatment, the medical professional may need to utilize multiple interventions simultaneously, while monitoring for changes to prevent the patient’s condition from deteriorating.


For the patient without any immediately reversible causes, who demonstrates signs or symptoms of poor perfusion due to a bradyarrhythmia such as a third-degree AV block, the ACLS Adult Bradycardia Algorithm recommends administering atropine. Atropine is considered a first-line treatment. An initial dose of 1 mg is administered intravenously, and is eligible to be repeated every 3-5 minutes until a total of 3mg has been administered. However, it is important to note that atropine is often ineffective at increasing heart rate in patients experiencing a third-degree AV block because it acts on the AV node.

Dopamine or epinephrine infusions are considered second-line treatments for managing symptomatic bradycardia. Both dopamine and epinephrine may be considered temporarily supportive with this arrhythmia. However, these medications may be less effective for increasing a patient’s heart rate if they are experiencing a third-degree AV block.


Should the patient with a bradyarrhythmia (such as a third-degree AV block) continue to demonstrate persistent signs and symptoms of unstable bradycardia following the use of medications, transcutaneous pacing (TCP) may be initiated. During transcutaneous pacing, an electrical stimulus is transmitted from a power source (such as a defibrillator with a pacing function) through electrodes applied to the patient’s skin, resulting in an electrical depolarization and cardiac contraction. TCP can be performed by ACLS providers, and is considered noninvasive. Keep in mind that TCP can be painful; if the patient is conscious, they should be sedated before beginning this intervention when possible. Until expert consultation is obtained, TCP is considered an emergent intervention to bridge treatment.

Next Steps and Considerations

Most often, patients experiencing a third-degree AV block need temporary pacing during initial care and may need a permanent pacemaker placed for long-term management of this rhythm. However, pacing may not be effective if comorbid conditions contributing to the development of the third-degree AV block are not treated. For example, a patient experiencing medication toxicity should have this underlying cause addressed in order to effectively manage the cardiac arrhythmia. Another example would be a patient experiencing a third-degree AV block and acute myocardial infarction, which would require emergent cardiac catheterization to restore perfusion within the heart and increase the chances of a return to an intrinsic cardiac rhythm.

Patients who experience a third-degree AV block are at an increased risk for reduced overall perfusion as a result of decreased cardiac output associated with symptomatic bradycardia. Reduced perfusion places the patient at risk for falls and other injuries associated with syncope. Some patients with a third-degree AV block who are in critical condition and are experiencing decreased perfusion may not be able to protect their airway, and may also be at risk for aspiration and delirium.

How To Prepare for Your ACLS Exam

Test Your Skills

American Medical Resource Institute (AMRI) has multiple resources to prepare you for your certification course.

A library of case studies and practice questions are available to evaluate and reinforce your understanding of ACLS, PALS, and Basic Life Support (BLS) principles and guidelines. The medical professional utilizing ACLS or PALS to care for patients must be skilled in identifying cardiac rhythms. AMRI provides an index of cardiac rhythms that the medical professional must know as a reference. Detailed information is presented to aid in rhythm identification, recognition of signs, symptoms, and potential causes of the rhythm, as well as recommended treatment guidelines for the respective cardiac rhythm.

Initial or Renewal Certification Opportunities

As a medical professional in an urgent or emergent clinical situation, you want to feel knowledgeable and ready to lead or assist in the care of the patient. Your colleagues depend on you. Your patient depends on you. To be prepared, you want a comprehensive certification course so that you can feel confident when entering a potentially dynamic and stressful patient encounter. However, with all the professional and personal demands on your time, you also want a certification course that is both efficient in its delivery and accommodates your schedule.

AMRI provides certification options that thoroughly prepare you while accommodating your unyielding schedule so that you are ready to respond. AMRI is accredited by the National Board of Emergency Care Certifications and has been providing innovative, high-quality continuing education programs for medical professionals since 1983. Those who have completed their certification(s) with AMRI have called their courses “convenient,” “comprehensive,” and “efficient.”. Review the testimonials to learn how your colleagues have rated AMRI’s courses.

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