People with third degree heart block have an irregular heartbeat due to a total blockage of the electrical impulses that control the heart’s activity. This is a medical emergency.

The possible symptoms of third degree heart block include:

  • fatigue or tiredness
  • chest pain or pressure
  • shortness of breath
  • lightheadedness or dizziness
  • loss of consciousness

People with these symptoms should dial 911 or seek immediate emergency medical attention.

Read on to learn more about what a third degree heart block is, the types, how doctors diagnose it, the available treatments, and the outlook.

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Third degree heart block occurs when the electrical impulses that control the heartbeat cannot travel from the atria, or top chambers of the heart, to the ventricles, or bottom chambers.

The ventricles are the chambers that push the blood out of the heart and around the body. When the heart cannot pump as it should, it affects blood flow to the organs.

There are three degrees of heart block, with the third degree being the most serious. Doctors tell them apart by looking at an EKG to check the rhythm and electrical activity of a person’s heart.

The “P wave” on an EKG shows the doctor when the atria contract. The QRS complex, which is a combination of the Q, R, and S waves, shows healthcare teams when the ventricles contract.

First degree

In first degree heart block, all electrical impulses still travel from the atria to the ventricles, but they do so at a slower rate.

Sometimes, this can be due to a typical variation in electrical signals and may not cause any problems. At other times, it is the result of an underlying condition.

Second degree

In second degree heart block, some electrical impulses from the atria reach the ventricles and some do not. There are three subtypes of second degree heart block:

  • Second degree Mobitz I: In this subtype, the time between the P wave and the QRS complex gradually gets longer. The part of the heart that passes the electrical signal from the atria to the ventricles gradually fatigues until it cannot send an electrical impulse between the two sets of chambers. The heart then drops a beat. Despite this problem, people with second degree Mobitz I heart block often do not need any treatment.
  • Second degree Mobitz II: In this subtype, heart cells suddenly and unexpectedly fail to conduct an electrical impulse. Structural damage to the system that sends the electrical impulses, such as from a heart attack or fibrosis, can cause this type of heart block. A person with Mobitz II has unstable blood flow and has more chance of developing third degree heart block than someone with Mobitz I.
  • Second degree high grade: A second degree high grade heart block presents with two or more blocked P waves in a row. The difference between P waves to QRS complexes is greater, but some electrical impulses still make it through from the atria to the ventricles. This is what sets this form of the condition apart from third degree heart block.

Third degree

In third degree heart block, the rhythm of the P waves occurs completely independently of the rhythm of the QRS waves. The electrical impulses stop getting through from the atria to the ventricles. The heart may stop beating, and the person may die without treatment.

Third degree heart block may cause a person to feel:

  • dizzy or faint
  • fatigued and possibly confused
  • pressure or pain in their chest
  • short of breath

While some people do not get any symptoms, this is rare.

Third degree heart block can be due to:

  • a complication of a medical procedure, such as surgery or radiation therapy
  • heart diseases such as cardiomyopathy or coronary heart disease
  • the heart’s electrical pathways deteriorating with age
  • serious infections, such as diphtheria or Lyme disease
  • secondary cancer that has spread to the heart
  • a serious injury to the chest
  • electrolyte imbalance
  • a genetic heart condition

Certain medications may also cause third degree heart block, including:

  • blood pressure medications such as calcium channel blockers, beta-blockers, and clonidine
  • tricyclic antidepressants
  • digoxin, a heart medication

Other factors do not directly cause heart block but appear to elevate the risk. They include having diabetes and high blood pressure.

Doctors will look for several signs when diagnosing third degree heart block, but a person with the condition might be in a state of distress. Once the healthcare professionals help stabilize them, they may recommend performing an EKG.

An EKG can show if there is no relationship between the atrial and ventricular contractions. The rate of atrial contraction will also be faster than the rate of ventricular contraction.

A doctor will also look for signs of decreased blood flow and oxygen. They may also order blood tests to check:

  • electrolyte levels
  • glucose levels, which can be low if beta-blockers are causing the heart block
  • troponin levels to check for signs of a heart attack
  • digoxin levels to check for digoxin toxicity

However, because third degree heart block is an emergency, treatment may start before the results of these tests come back.

If the cause of heart block is unclear, a doctor may order further tests, such as a chest radiograph or blood test.

Sometimes, doctors detect genetic heart conditions before birth via an ultrasound scan. A slow heartbeat can indicate that the fetus has heart block.

A doctor may first try intravenous atropine, a medication that slows heart rate, to treat a third degree heart block. However, this often does not work in people with this condition.

Doctors may next administer dopamine and epinephrine. These neurotransmitters may temporarily support the heart but may not help regulate heart rate.

If that is the case, a cardiologist may begin transcutaneous pacing (TCP). This involves a doctor placing pads on the skin that conduct electrical signals, which temporarily restore a typical heartbeat. TCP can feel unpleasant, so a person may receive a sedative to make them drowsy during the treatment.

If electrical signals through the skin are not working well enough, a doctor may try transvenous pacing instead. This involves inserting a temporary pacemaker through a vein. Similar to TCP, pacemakers emit electrical signals to regulate the heartbeat.

If a person needs a pacemaker over the long term, a surgeon will later replace this with a permanent pacemaker. However, they do not work in all situations — it can depend on the underlying cause of the heart block. For example, people with drug toxicity may not respond to pacing.

In these cases, doctors will prioritize treating the underlying cause of the heart block.

If treatment for third degree heart block is successful, a person can go home once they have reached a stable condition.

Recovery at home may involve:

  • caring for wounds from pacemaker implantation
  • wearing an arm sling to keep the shoulder still, especially at night
  • avoiding driving for 2–3 weeks
  • taking antibiotics to prevent infection

If someone has a permanent pacemaker, they may need to avoid electromagnetic devices, such as MRI scans, for as long as they have the device. However, some newer pacemakers models are safe for MRIs. A pacemaker technician will need to check the devices before and after the procedure.

People will also benefit from frequent checkups to check the device’s battery life and function.

Temporary or permanent pacemakers carry a risk of infection, particularly in older adults. A person should speak with a doctor if they develop:

  • swelling or pain around the wound
  • sores or pus around the wound
  • fever
  • chills
  • nausea

A person’s chances of recovery from third degree heart block depend on the underlying cause and severity of their condition and the treatments they receive in hospital.

Doctors may be able to reverse complete heart block in some cases. For instance, they may be able to restore blood flow through the heart after a heart attack or administer antibiotics to treat an infection.

However, some causes of heart block have less favorable outlooks than others. For example, a person with a type of heart attack known as an anterior myocardial infarction (MI) is more likely to die from a complete heart block than someone with inferior MI, another type of heart attack.

Third degree heart block occurs when the electrical signal from the heart’s atria, or top chambers, cannot reach the ventricles, or bottom chambers. Without treatment, the heart may eventually stop.

Third degree heart block may result from an underlying genetic condition, heart disease, or toxicity from medication. Doctors can treat the condition with medication and various forms of electrical stimulation to regulate the heartbeat.