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Heart and ECG

Heart and ECG

Cardiac Conduction System

• The heart has 2 basic functional parts:
• Conduction system
• They are thin "wires" that conduct impulses throughout the heart
• Impulses spontaneously originate in the SA node
• they spread through the heart and activate the working myocardium
• Working myocardium
• It is the heart muscle, which contracts upon stimulation by an impulse
• This results in atrial systole, followed by ventricular systole
  
  
• ECG device has electrodes that are attached to the body
• ECG detects the spread of electrical impulses through the heart
• ECG does not record the mechanical work of the heart (systole and diastole)

Main Heart Vector

• The impulse activates the atria from the SA node
• This results in the formation of the P wave
• The impulse then travels to the ventricles via the AV node
• And begins to gradually activate the ventricular myocardium
• The more massive the myocardium is
• The more massive the cardiac vector will be
  
  
• First, the thin ventricular septum is activated
• Creating a small septal vector (V_S)
• Next, the massive left ventricle is activated
• Resulting in the large main vector (V_M)
• Simultaneously, the thin right ventricle is also activated
• Creating a small right ventricular vector
• which does not influence the direction of the main vector
• Finally, the base of the left ventricle is activated
• Creating a small terminal vector (V_T)

ECG and the Cardiac Vector

• The ventricles activate sequentially:
1. Ventricular septum (V_S - septal vector)
2. Left and right ventricles (V_M - main vector)
3. Base of the left ventricle (V_T - terminal vector)
• The QRS complex looks different in each lead
• Because each lead "views" the vectors from a different angle
• If the vector points
• Toward the lead - a positive wave is recorded
• Away from the lead - a negative wave is recorded
  
  
• The images show ventricular depolarization (QRS complex)

Limb Leads

• "View" the vectors in the frontal plane

Chest Leads

• "View" the vectors in the horizontal plane

What Does an ECG Diagnose?

• The ECG records heart vectors (their frequency and magnitude)
• Based on the ECG, the following can be determined:
• Heart Rate
• Calculated using QRS complexes
• Tachycardia (rate > 100/min)
• Bradycardia (rate < 50/min)
• Heart Rhythm
• Determined by the origin of the impulses that control the heart
• Sinus rhythm (impulses originate in the SA node)
• Junctional rhythm (impulses originate in the AV node)
• Ventricular rhythm (impulses originate in the ventricles)
• Orientation of the Heart
• Vector analysis can determine the heart's orientation in the horizontal and frontal planes
• Transitional Zone (provides information on the horizontal orientation of the heart)
• Main Cardiac Axis (provides information on the frontal orientation of the heart)
• Thickness of Cardiac Chambers
• Thicker myocardium creates larger vectors and larger waves on the ECG
• P pulmonale (Right atrial hypertrophy)
• P mitrale (Left atrial hypertrophy)
• Left ventricular hypertrophy
• Right ventricular hypertrophy
• Conduction Disorders in the Conduction System
• Occurs when the conduction system is interrupted or slowed
• AV blocks (disturbances in conduction through the AV node)
• Bundle branch blocks (disturbances in conduction through the bundle branches)
• Arrhythmias (Rhythm Disorders)
• Occurs when impulses spread through the heart via abnormal pathways
• Supraventricular tachycardia (impulse originates above the ventricles)
• Ventricular tachycardia (impulse originates in the ventricles)
• Drugs and Electrolytes
• Drugs and electrolytes affect the action potential and consequently the ECG waveform
• Electrolyte imbalances
• Drugs and ECG
• Myocardial Infarction and Ischemic Injury
• In cases of stenosis (narrowing) or occlusion (blockage) of an artery, ischemia or even necrosis (infarction) can occur
• Both ischemia and infarction alter heart vectors
• Acute Coronary Syndrome