Cardiac Resynchronization Therapy

CRT or Cardiac Resynchronization Therapy, is a device used in patients with heart failure who are refractory to medical management. Heart failure can be caused by Dilated Cardiomyopathy or an electrically dyssynchronous ventricle. CRT is like a bridge between LVAD. The last treatment is a cardiac transplant.

What is Dyssynchrony?

Dessynchrony means that the synchrony between two ventricles is lost. Impaired mechanical coordination is due to impaired electrical activation of cardiac muscle. Diagnosis of synchrony is made by doing an ECG. A normal ECG will show a QRS complex duration of 70-100 msec, but during Dyssynchrony, there is a wide QRS > 120 msec.

Normal Conduction without Desynchrony 

When two ventricles contract simultaneously – The duration of the QRS complex is 70-100 msec.

Dyssynchrony (Bundle Branch Block BBB)

There is a Dyssynchrony in the right and left ventricles. Wide QRS complexes are also seen in patients with bundle branch blocks. In RBBB – the Left ventricle will contract first, followed by the right ventricle. In LBBB – The right ventricle will contract first, followed by the left ventricle. 

What are the modes of Dyssynchrony?

• Segmental: Only a part of LV or RV is not contracting with other parts of cardiac muscles.
• Global: Dyssynchrony among along all segments of ventricle myocardium.

What is the Effect of Dyssynchrony?

• Intraventricular Dyssynchrony: Dyssynchrony between walls of same ventricle.
• Interventricular Dyssynchrony: Dyssynchrony Between 2 ventricles.
• Effect of Dyssynchrony: 
  • Decreasing systolic function
    • Cardiac output decreases 
    • Decrease in mechanical efficiency of contractility – Reduced ionotropicity
    • Fatigue
• Mechanical Dyssynchrony: 
  • It results in an increase in the wall stress.
  • Hence, there is worsening of functional MR.

How is The Synchrony is Achieved with Cardiac Resynchronization Therapy (CRT)?

In CRT - 3 leads are used. These 3 leads are placed in 

• Right Atrium
• Right Ventricle apex
• Another lead passed through coronary sinus into lateral wall of left ventricle. This is usually a quadripolar lead.

By placing these leads, synchrony between the right and left ventricles is achieved. This results in increased cardiac output and decreased fatiguability. 

Lead which is placed in lateral wall of left ventricle pass through coronary sinus. So, there can be a high chance of rupture of coronary sinus which can cause complications.

Who are the Candidates of Cardiac Resynchronization Therapy (CRT)?

Placed in patients with DCMP.

• QRS complex should be wide > 120 msec.
• Refractory to Medical management
• Wide QRS >150 msec + LBBB
• There should be Dyssynchrony,
  • With ejection fraction <35%
  • Patient having Class 2 or class 3 symptoms.
  • Expected survival - 1 year.

What is the Mechanism of Cardiac Resynchronization Therapy (CRT)

• Intra ventricular Synchrony
  • Increase the ejection fraction.
  • Increase dP/dT i.e., The force of rise of LV contraction will increase.
    • So, End Systolic volume decreases and subsequently end diastolic volume also decreases.
• Atrioventricular Synchrony
  • Increase in LV Diastolic Filling
  • Increase of Cardiac Output
• Interventricular Synchrony
  • Increase in RV stroke volume.
• CRT reduces cardiac remodeling

What are the Benefits of Cardiac Resynchronization Therapy(CRT)?

• Improvement in exercise tolerance.
• Decrease in Dyspnea – Pulmonary edema decreases due to increase in ejection fraction.
• Reversal of LV remodeling
  • Improvement in EF.
  • Decrease in LV size.
  • Reduction in functional MR, TR.
• Prognosis of CRT
  • Reduce hospitalization in DCMP patients.
  • Reduce Mortality in DCMP patients.

What are the Complications of Cardiac Resynchronization Therapy (CRT)?

• Dissection of Coronary Sinus – Have no significance
  • Remove the sheath from Coronary sinus
• Coronary Sinus Perforation or branch rupture
  • Pericardial effusion can happen.
  • Usually, coronary sinus perforation go asymptomatic because Venous circulation is a low pressure area.
• Inability to calculate the CS.
  • In 5% of patients, the coronary sinus cannot be cannulated or branches are not present for LV lead placement. Then, the LV lead has to be placed epicardially.
• Lead Dislodgement – Seen in 5-7%
  • It is less common because leads that are used have improved fixation mechanisms.

Frequently Asked Questions:

Q: When two ventricles contract simultaneously, what is the duration of the QRS complex?

Answer: 70-100 msec.

Q: What is cardiac resynchronization therapy (CRT)?

Answer: Cardiac Resynchronization Therapy or CRT is a device used in patients with heart failure who are refractory to medical management.

Q: What is Dyssynchrony?

Answer: Dyssynchrony means that the synchrony between two ventricles is lost.

Also Read: High-Yield Image Based Question on ECG

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