Cardiology: P-R Interval Insights

Electrophysiology (Abnormal Rhythms & Conduction Defects)

• A subject of study in medical science; covers abnormal heart rhythms and conduction defects.
• The study material is based from Guyton 14th edition, Block 1.3 chapter 13.
• Important to note: do not solely rely on lecture explanations; verify information with the textbook.

Case Study 1: Tachycardia

• A 20-year-old patient with high fever (104°F), nausea, and vomiting.
• Diagnosed with malaria.
• Patient experienced palpitation (feeling of heart beat) during fever.
• ECG taken due to palpitation.
• Lead II ECG reading.

Tachycardia Definition and Characteristics

• Tachycardia: fast heart rate, usually above 100 beats per minute in adults.
• Normal QRS complexes but the time interval between complexes was 150 beats per minute instead of the normal 72 beats per minute.
• ECG was normal aside from the reduction in the interval between these complexes

Causes of Tachycardia

• Increased body temperature (10 beats per minute increase per degree Fahrenheit)
• Stimulation of sympathetic nerves.
• Toxic heart conditions.

Case Study 2: Hypertension and Bradycardia

• A 40-year-old woman with hypertension.
• Prescribed beta-blockers to control blood pressure.
• Patient complains of tiredness (lethargy), shortness of breath (dyspnea), and excessive sweating during daily activities.
• ECG taken to assess cardiac function.
• Indicates Bradycardia.

Bradycardia Definition and Causes

• Bradycardia: slow heart rate (fewer than 60 beats per minute).
• Causes of bradycardia include:
  • Athletes.
  • Beta-blocker use.
  • Vagal stimulation.

Sinus Arrhythmia

• Heart rate increases and decreases by 5% during quiet respiration. (Heart rate more in inspiration than during deep expiration, from the lecture).
• Result of "spillover" of signals from the respiratory center into the adjacent vasomotor center that happens during respiratory cycles.

AV Nodal Block

• Impulses from the sinoatrial (SA) node to the atrioventricular (AV) node do not occur.
• AV nodal block is due to:
  • Inflammation.
  • Ischemia.
  • Scarring.
  • Extreme vagus nerve stimulation.

Type 1 Incomplete A-V Node Heart Block

• Transfer of impulses from the SA node to the AV node decreases.
• PR interval length increases above the normal range (0.20 seconds) to more than 2.0 seconds, is the defining factor of the block
• Most common in rheumatic fever.

Incomplete/Second Degree Atrioventricular Block

• Some, but not all, impulses from the SA node reach the AV node.
• PR intervals lengthen, followed by a dropped QRS complex.
• Two types: Mobitz I (Wenckebach block) and Mobitz II.
• No specific treatment is necessary for type 1.

Type 2 Atrioventricular Block

• A fixed ratio of non-conducted P waves to each QRS complex; (e.g., 2:1).
• This type commonly involves the bundle of His-Purkinje system.
• A pacemaker is frequently required to prevent further progression to complete heart block.

Complete Heart Block

• There is no conduction of impulses from the SA node to the AV node.
• Impulses are generated in the ventricles independent of the sinoatrial node.
• The ventricle rhythm is slower.
• This is a life threatening arrhythmia.

Stokes-Adams Syndrome

• Ventricles enter a period of inactivity.
• Loss of brain blood supply
• Symptoms include loss of consciousness, as a consequence of inadequate blood circulation to the brain.
• Patients recover when ventricular escape occurs.

Re-Entry Phenomena

• Impulse re-enters and causes repeated activity instead of a single impulse.
• This can be fatal.
• Some reasons of re-entry are:
  • Longer or abnormal pathway.
  • Decreased conduction velocity.
  • Shortened refractory period.

Ventricular Fibrillation

• Irregular, rapid contractions of ventricle.
• No effective pumping occurs.
• Fatal unless treated with defibrillation.

Atrial Fibrillation

• Irregular, rapid contractions of atria.
• Ventricular rate is usually faster than normal, typically between 125-150 beats per minute.
• Treatment is not necessary in all cases of atrial fibrillation.

Atrial Flutter

• Rapid, regular contractions of atria.
• The atrium beats more frequently than the ventricles resulting in only a fraction of the impulses passing through the AV node to the ventricles.
• Often manifests as a 2:1 or 3:1 ratio between atria and ventricle contractions.
• Commonly a result of a short refractory period for the heart.

Additional Topics

• Angina Pectoris: chest pain due to reduced blood supply to the heart
• Myocardial Infarction: heart attack, due to decreased oxygen supply from ischemia.
• Premature Contractions: heart contraction before expected contraction (can be atrial or ventricular).