ECG in Hypothyroidism – Dr. Akif Baig

Description

ECG in Hypothyroidism

Hypothyroidism, particularly in its more severe forms, can have significant effects on the cardiovascular system, leading to various electrocardiogram (ECG) changes. These changes are primarily due to the decreased metabolic rate and the effects on cardiac function that result from thyroid hormone deficiency.

Common ECG Changes in Hypothyroidism

1. Sinus Bradycardia:
   • One of the most common findings in hypothyroidism.
   • The heart rate slows due to decreased metabolic demand and reduced sympathetic nervous system activity.
   • Typical heart rates may range from 50 to 60 beats per minute or lower.
2. Low Voltage QRS Complexes:
   • Low voltage on the ECG is often seen, particularly in the limb leads.
   • It results from increased soft tissue or fluid around the heart, such as pericardial effusion, which is common in severe hypothyroidism (myxedema).
   • Defined as QRS complexes <5 mm in limb leads and <10 mm in precordial leads.
3. Prolonged QT Interval:
   • Hypothyroidism can prolong the repolarization phase of the heart, leading to a prolonged QT interval.
   • This can predispose patients to ventricular arrhythmias, although they are rare in hypothyroidism.
   • QTc (corrected QT) is typically longer than 440 milliseconds.
4. Flattened or Inverted T Waves:
   • T-wave changes, such as flattening or inversion, are common and may reflect the altered repolarization that occurs with hypothyroidism.
   • This is often seen in the precordial leads (V1-V6).
5. Bundle Branch Blocks:
   • Hypothyroidism can occasionally be associated with conduction abnormalities, including right bundle branch block (RBBB) or left bundle branch block (LBBB).
   • These are rare and typically seen in severe cases.
6. Prolonged PR Interval (First-Degree AV Block):
   • Delayed conduction through the AV node may result in a prolonged PR interval (>200 milliseconds), known as first-degree AV block.
   • This is typically benign but can reflect slowed cardiac conduction due to the hypothyroid state.
7. Pericardial Effusion-Related Findings:
   • In cases of significant pericardial effusion, you may also see electrical alternans (variation in QRS amplitude between beats) due to the heart swinging in the pericardial sac.
   • This is a sign of larger effusions and can lead to tamponade if untreated.

Example of Typical ECG Changes in Hypothyroidism:

Pathophysiology Behind ECG Changes

• Decreased Thyroid Hormone: Leads to reduced metabolic rate and reduced sympathetic tone, which slows the heart rate.
• Myxedema (Severe Hypothyroidism): Causes fluid retention and pericardial effusion, contributing to low voltage QRS and other ECG changes.
• Altered Ion Channel Function: Changes in calcium and potassium channel function, as well as action potential duration, contribute to the prolongation of the QT interval and the altered repolarization seen in T-wave changes.

Clinical Relevance

• Sinus bradycardia and other ECG abnormalities typically resolve with thyroid hormone replacement therapy.
• The presence of pericardial effusion and severe bradycardia in hypothyroidism is a sign of myxedema and may require urgent treatment, especially in cases of myxedema coma.