Heart blocks are cardiac rhythm problems caused by delayed or blocked electrical conduction between the atria and ventricles. In a normal heartbeat, the electrical impulse starts in the SA node, moves through the atria, reaches the AV node, and then travels into the ventricles. This pathway allows the atria and ventricles to work together in a coordinated way.
When a heart block occurs, the signal slows down or fails to pass through the AV node or lower conduction system. The ECG changes depend on how much conduction is delayed or blocked. Some heart blocks are mild and may cause no symptoms. Others can reduce cardiac output, cause dizziness or syncope, and become medical emergencies.
For nursing students, ECG learners, paramedics, and healthcare professionals, heart blocks are essential rhythm topics. They are commonly tested in exams and seen in clinical practice. The most important clues are the PR interval, P wave relationship to QRS, dropped QRS complexes, and whether the atria and ventricles are communicating.
What Is a Heart Block?
A heart block is a delay or interruption in the electrical signal that travels from the atria to the ventricles.
The atria are the upper chambers of the heart. The ventricles are the lower chambers. The AV node works like a gate between them. It slows the signal briefly so the ventricles have time to fill before they contract.
In heart block, this signal may be:
- Delayed
- Partially blocked
- Completely blocked
The result depends on the degree of block. A mild block may only show a long PR interval. A severe block may cause the atria and ventricles to beat independently.
Why Heart Blocks Matter
Heart blocks matter because they can reduce the heart’s ability to pump blood. When ventricular beats are delayed or dropped, blood flow to the brain and body may fall.
This can cause:
- Dizziness
- Fatigue
- Weakness
- Shortness of breath
- Chest pain
- Hypotension
- Syncope
- Confusion
- Weak pulse
Some blocks are stable. Others can progress to complete heart block. That is why ECG interpretation must always be connected with patient assessment.
Quick ECG Review Before Heart Blocks
Before reading a heart block rhythm, review three ECG parts.
| ECG Part | Meaning | Why It Matters in Heart Block |
|---|---|---|
| P wave | Atrial depolarization | Shows atrial impulse |
| PR interval | Time from atrial impulse to ventricular response | Main measurement for AV block |
| QRS complex | Ventricular depolarization | Shows ventricular response |
Normal PR Interval
A normal PR interval is usually:
0.12 to 0.20 seconds
This equals 3 to 5 small boxes on ECG paper.
A PR interval longer than 0.20 seconds is prolonged. This is the key finding in first-degree heart block.
Types of Heart Blocks
Heart blocks are commonly divided into three major degrees.
| Type | Main Problem | ECG Clue |
| First-degree AV block | Slow AV conduction | PR interval longer than 0.20 sec |
| Second-degree type I | PR gets longer until QRS drops | Longer, longer, longer, drop |
| Second-degree type II | Some P waves fail to conduct | Constant PR with dropped QRS |
| Third-degree AV block | No atrial-ventricular communication | P waves and QRS complexes are independent |
First-Degree Heart Block
First-degree heart block is the mildest AV block. The electrical impulse moves from the atria to the ventricles, but it moves slower than normal.
No QRS complexes are dropped. Every P wave is followed by a QRS complex. The main abnormal finding is a prolonged PR interval.
ECG Features of First-Degree Heart Block
| ECG Feature | Finding |
| Rate | Usually normal, but may be slow |
| Rhythm | Regular |
| P wave | Upright and before every QRS |
| PR interval | Prolonged, more than 0.20 seconds |
| QRS complex | Usually normal |
Simple Memory Tip
If the R is far from P, then you have a first degree.
This means the distance from the P wave to the QRS complex is too long.
Causes of First-Degree Heart Block
Common causes include:
- Older age
- Coronary artery disease
- Electrolyte imbalance
- Myocardial ischemia
- Increased vagal tone
- Beta blockers
- Calcium channel blockers
- Digoxin
- Some antiarrhythmic medicines
In some patients, first-degree AV block may be a benign finding. It still needs monitoring, especially if new or medication-related.
Symptoms of First-Degree Heart Block
First-degree heart block is usually asymptomatic.
Some patients may have no symptoms at all. It may be found during routine ECG monitoring. Symptoms are more likely if the heart rate is slow or if another cardiac condition is present.
Treatment of First-Degree Heart Block
First-degree heart block usually does not need active treatment.
Management may include:
- Monitor ECG rhythm
- Review medications
- Check electrolytes
- Assess for ischemia if clinically suspected
- Watch for progression to higher-degree block
If the patient is stable and asymptomatic, observation is often enough.
Second-Degree Heart Block
Second-degree heart block means some impulses from the atria do not reach the ventricles. This causes one or more QRS complexes to be dropped.
There are two major types:
- Second-degree type I, also called Wenckebach
- Second-degree type II, also called Mobitz II
These two rhythms are different. Type I is often less dangerous. Type II is more serious because it can progress to third-degree heart block.
Second-Degree Type I Heart Block
Second-degree type I heart block, or Wenckebach, occurs when the PR interval gradually gets longer until one QRS complex is dropped.
After the dropped QRS, the cycle starts again.
ECG Features of Second-Degree Type I
| ECG Feature | Finding |
| Rate | Often normal or slow |
| Rhythm | Regularly irregular |
| P wave | Usually normal |
| PR interval | Gradually prolongs |
| QRS complex | Drops in a repeating pattern |
Simple Memory Tip
Longer, longer, longer, drop. Now you have a Wenckebach.
This means the PR interval gets longer with each beat until the ventricle fails to respond.
What Happens in Wenckebach?
The AV node becomes progressively slower at conducting each impulse. Eventually, one atrial impulse fails to pass through. This creates a P wave without a QRS complex.
Then the AV node resets, and the pattern repeats.
Causes of Second-Degree Type I
Common causes include:
- Increased vagal tone
- Inferior myocardial infarction
- Rheumatic fever
- Myocarditis
- Beta blockers
- Calcium channel blockers
- Digoxin
- Other AV-node-slowing medicines
Wenckebach can be temporary and may occur during sleep or high vagal tone.
Symptoms of Second-Degree Type I
Some patients are asymptomatic.
Possible symptoms include:
- Dizziness
- Shortness of breath
- Weakness
- Chest pain
- Altered mental status
- Fatigue
Symptoms depend on how many beats are dropped and whether cardiac output falls.
Treatment of Second-Degree Type I
Treatment depends on symptoms.
If asymptomatic:
- Monitor rhythm
- Review medications
- Check vital signs
- Assess for reversible causes
If symptomatic:
- Notify the provider
- Check vital signs
- Obtain ECG
- Give oxygen if needed
- Check labs
- Prepare for atropine or pacing if ordered and clinically indicated
Second-Degree Type II Heart Block
Second-degree type II heart block, or Mobitz II, is more serious than type I. In this rhythm, the PR interval stays constant, but some QRS complexes are randomly or intermittently dropped.
The problem is often below the AV node, in the His-Purkinje system. This makes it more likely to progress to complete heart block.
ECG Features of Second-Degree Type II
| ECG Feature | Finding |
| Rate | Often normal or slow |
| Rhythm | Irregular |
| P wave | Normal, marching through |
| PR interval | Constant when conducted |
| QRS complex | Randomly dropped |
Simple Memory Tip
If some P waves do not get through, then you have Mobitz II.
This means P waves continue to appear, but some do not produce a QRS complex.
What Happens in Mobitz II?
The atria continue to fire. The P waves appear at regular intervals. Some impulses reach the ventricles, while others fail without warning.
Unlike Wenckebach, the PR interval does not gradually lengthen. It stays the same before conducted beats.
Causes of Second-Degree Type II
Common causes include:
- Coronary artery disease
- Myocardial infarction
- Cardiomyopathy
- Fibrosis of conduction tissue
- Beta blockers
- Calcium channel blockers
- Digoxin
- Post-cardiac surgery conduction injury
Mobitz II is treated seriously because it can progress to third-degree heart block.
Symptoms of Second-Degree Type II
Symptoms may include:
- Dizziness
- Weakness
- Syncope
- Fatigue
- Shortness of breath
- Chest pain
- Hypotension
A patient may be stable at first, then worsen quickly if more QRS complexes are dropped.
Treatment of Second-Degree Type II
Treatment depends on symptoms and stability.
If asymptomatic:
- Notify cardiology or provider
- Review medications
- Monitor closely
- Prepare for pacing if progression occurs
If symptomatic:
- Notify the provider urgently
- Prepare for temporary pacing
- Monitor blood pressure and perfusion
- Anticipate permanent pacemaker evaluation
- Avoid delaying escalation
Mobitz II is not a rhythm to ignore.
Third-Degree Heart Block
Third-degree heart block, also called complete heart block, is the most severe AV block.
In this rhythm, there is no communication between the atria and ventricles. The atria follow one pacemaker, usually the SA node. The ventricles follow a separate escape pacemaker.
The result is AV dissociation.
ECG Features of Third-Degree Heart Block
| ECG Feature | Finding |
| Rate | Usually less than 60 bpm |
| Rhythm | Regular atrial rhythm and regular ventricular rhythm |
| P wave | Independent from QRS |
| PR interval | Variable |
| QRS complex | Independent from P waves |
Simple Memory Tip
If P waves and QRS complexes do not agree, then you have third degree.
The P waves march through at their own rate. The QRS complexes also march through at their own rate. They do not work together.
What Happens in Complete Heart Block?
The atria continue to receive impulses from the SA node. The ventricles do not receive those impulses. A backup pacemaker below the block takes over to keep the ventricles beating.
This backup rhythm is often slow. If the escape rhythm is too slow, cardiac output falls.
Causes of Third-Degree Heart Block
Common causes include:
- Myocardial infarction
- Coronary artery disease
- Cardiomyopathy
- Digoxin toxicity
- Severe conduction system disease
- Cardiac surgery
- Myocarditis
- Electrolyte imbalance
- Congenital heart block
Third-degree heart block is a medical emergency when the patient has poor perfusion.
Symptoms of Third-Degree Heart Block
Symptoms occur due to low cardiac output.
Common findings include:
- Hypotension
- Chest pain
- Weakness
- Pale skin
- Weak pulse
- Dizziness
- Syncope
- Shortness of breath
- Confusion
- Signs of shock
The heart may not pump enough blood to support the brain and body.
Treatment of Third-Degree Heart Block
Third-degree heart block requires urgent assessment.
Treatment may include:
- Notify provider immediately
- Oxygen if needed
- IV access
- Continuous ECG monitoring
- Atropine in selected cases
- Temporary pacing
- Permanent pacemaker
- Treat reversible causes
A pacemaker is often needed because the atria and ventricles are not communicating.
Heart Block Comparison Table
| Feature | 1st Degree | 2nd Degree Type I | 2nd Degree Type II | 3rd Degree |
| Main issue | Slow AV conduction | PR lengthens until QRS drops | Some impulses fail suddenly | Complete AV dissociation |
| Rhythm | Regular | Regularly irregular | Irregular | Atrial and ventricular rhythms regular but separate |
| PR interval | Prolonged | Gradually longer | Constant when conducted | Variable |
| P wave | Before every QRS | Usually normal | Marching through | Independent |
| QRS drops? | No | Yes, repeating pattern | Yes, sudden drops | No relationship |
| Severity | Mild | Usually less severe | Serious | Most severe |
| Pacemaker likely? | Usually no | Sometimes | Often | Usually |
Heart Block Symptoms by Severity
| Severity | Possible Symptoms |
| Mild block | Often no symptoms |
| Moderate block | Dizziness, fatigue, weakness |
| Serious block | Syncope, hypotension, chest pain |
| Complete block | Low cardiac output, shock, altered mental status |
Symptoms matter more than the ECG label alone. A stable rhythm may need monitoring. An unstable patient needs urgent action.
Nursing Priorities for Heart Blocks
When a heart block appears on the monitor, assess the patient first.
First Nursing Actions
- Check the patient.
- Assess airway, breathing, and circulation.
- Check pulse and blood pressure.
- Assess chest pain and shortness of breath.
- Check mental status.
- Confirm lead placement.
- Obtain a 12-lead ECG if ordered or per protocol.
- Notify the provider if symptomatic or new.
What to Monitor
Monitor:
- Heart rate
- Blood pressure
- Oxygen saturation
- Level of consciousness
- Chest pain
- Skin color
- Pulse strength
- Urine output
- ECG rhythm changes
- Frequency of dropped beats
Medication Review
Many medications slow AV conduction.
Review:
- Beta blockers
- Calcium channel blockers
- Digoxin
- Amiodarone
- Other antiarrhythmics
- Sedatives
- Opioids if bradycardia is present
Medication-related blocks may improve after dose adjustment or drug discontinuation, but only under provider direction.
Labs and Reversible Causes
Heart blocks can be worsened by metabolic and cardiac problems.
Useful labs may include:
- Potassium
- Magnesium
- Calcium
- Troponin
- Digoxin level
- Kidney function
- Thyroid tests
- Arterial blood gas if critically ill
Correcting the cause can improve conduction in some patients.
When Heart Blocks Become an Emergency
Heart block becomes urgent when the patient shows poor perfusion.
Emergency warning signs include:
- Hypotension
- Chest pain
- Syncope
- New confusion
- Severe dizziness
- Shortness of breath
- Weak pulse
- Pale or cool skin
- Signs of shock
- Worsening bradycardia
Mobitz II and third-degree heart block need close attention because they can become unstable.
Atropine, Pacing and Pacemakers
Atropine
Atropine may increase heart rate in some symptomatic bradycardias. It works better when the block is at the AV node level.
It may be less effective in advanced infranodal blocks, such as Mobitz II or some third-degree blocks. Do not delay pacing or expert care when the patient is unstable.
Temporary Pacing
Temporary pacing may be used when the heart rate is too slow and the patient has poor perfusion.
It may be transcutaneous or transvenous depending on the clinical setting.
Permanent Pacemaker
A permanent pacemaker may be needed for serious or persistent conduction problems.
It is commonly considered for:
- Symptomatic Mobitz II
- High-grade AV block
- Third-degree AV block
- Recurrent syncope due to conduction disease
- Persistent dangerous bradycardia
Common Student Mistakes
Avoid these errors:
- Calling every long PR interval a dangerous rhythm
- Missing dropped QRS complexes
- Confusing Wenckebach with Mobitz II
- Forgetting Mobitz II has a constant PR interval
- Forgetting third-degree block has AV dissociation
- Treating the ECG strip without checking the patient
- Ignoring medication causes
- Delaying provider notification in symptomatic patients
- Assuming an asymptomatic patient will always stay stable
Easy Memory Tips for Heart Blocks
Use these rhythm clues:
- First degree: PR is longer than 0.20 seconds
- Wenckebach: longer, longer, longer, drop
- Mobitz II: PR stays the same, QRS drops
- Third degree: P waves and QRS complexes do not match
- Danger rhythm: Mobitz II and third-degree block
- First action: assess the patient first
FAQs
1. What is a heart block?
A heart block is a delay or blockage in the electrical signal moving from the atria to the ventricles. It usually involves the AV node or the conduction system below it. The ECG changes depend on how much conduction is delayed or blocked. Some heart blocks are mild, while others can reduce cardiac output and become emergencies.
2. What is first-degree heart block?
First-degree heart block is delayed conduction through the AV node. The ECG shows a PR interval longer than 0.20 seconds. Every P wave is still followed by a QRS complex. It is often asymptomatic and usually requires monitoring rather than emergency treatment.
3. What is Wenckebach?
Wenckebach is second-degree type I AV block. The PR interval gradually gets longer until one QRS complex is dropped. After the dropped beat, the cycle starts again. It is often remembered as “longer, longer, longer, drop.”
4. What is Mobitz II heart block?
Mobitz II is second-degree type II AV block. The PR interval stays constant, but some QRS complexes are suddenly dropped. It is more serious than Wenckebach because it can progress to complete heart block. Symptomatic Mobitz II often requires urgent provider notification and pacing evaluation.
5. What is third-degree heart block?
Third-degree heart block is complete AV block. The atria and ventricles beat independently because electrical signals do not pass from the atria to the ventricles. The ECG shows P waves and QRS complexes with no consistent relationship. It can cause low cardiac output and often requires pacing.
6. Which heart block is most dangerous?
Third-degree heart block is usually the most dangerous because there is complete loss of communication between atria and ventricles. Mobitz II is also serious because it can progress to third-degree block. First-degree block is usually the mildest. Patient symptoms and vital signs determine urgency.
7. How do you tell Wenckebach from Mobitz II?
In Wenckebach, the PR interval gets progressively longer before a QRS drops. In Mobitz II, the PR interval remains constant before conducted beats, but QRS complexes drop suddenly. Wenckebach often has a repeating pattern. Mobitz II is less predictable and more serious.
8. What symptoms can heart blocks cause?
Heart blocks can cause dizziness, weakness, fatigue, shortness of breath, chest pain, low blood pressure, syncope, and confusion. Mild blocks may cause no symptoms. Severe blocks can reduce cardiac output and cause shock. Symptoms require quick assessment and escalation.
9. What causes heart block?
Heart block can be caused by older age, coronary artery disease, myocardial infarction, cardiomyopathy, electrolyte imbalance, digoxin toxicity, and medications that slow AV conduction. Beta blockers and calcium channel blockers can contribute in some patients. Cardiac surgery and inflammatory heart disease can also affect conduction. Treatment depends on the cause and severity.
10. What is the first nursing action for heart block?
The first nursing action is to assess the patient, not only the monitor strip. Check pulse, blood pressure, oxygen saturation, breathing, chest pain, mental status, and signs of poor perfusion. Confirm lead placement and obtain further ECG data if needed. Notify the provider quickly if the patient is symptomatic or the block is new, Mobitz II, or third degree.
