Understanding Cardiac Electrical Activity

Electrical Activity of the Heart

• The heart’s function is triggered by electrical activity that leads to muscle contractions.
• This electrical activity starts at the SA node which generates impulses that travel to the AV node.

Atrial and Ventricular Contraction

• Atrial depolarization occurs first - this is when the atria contract after depolarizing.
• After a brief delay to allow blood to fill the ventricles, the electrical signal moves down to the ventricles for contraction, resulting in ventricular depolarization.

Graphical Representation

• The phases of heart activity can be depicted using ECG graphs. Each waveform corresponds to specific electrical events:
  • P wave: Atrial depolarization
  • QRS complex: Ventricular depolarization
  • T wave: Ventricular repolarization

Heart Cycle Phases

• Resting Phase: Both atria and ventricles are in a state of relaxation before the next cycle (diastole).
• SA node activation leads to repetitive cycles of contraction mediated by electrical impulses.

Delays and Heart Sounds

• AV node causes a delay that ensures the ventricles fill adequately before they contract.
• Normal conduction ensures the heart rhythm maintains the necessary flow without issues.

Measurement of Electrical Activity

• An ECG measures electrical activity changes, primarily focused on the heart's action potential.
• Different leads (such as lead II) give various perspectives of the heart’s electrical activity.

Calculating Heart Rate from ECG

• ECG strips indicate time intervals between beats:
  • Each large square represents 0.2 seconds, while each small square represents 0.04 seconds.
• Heart rate can be calculated by counting the squares between R-R intervals and applying a formula or using a quick rule (i.e., 300, 150, 100, 75, 60) based on location on the graph.

Abnormal Heart Rhythms

• Bradycardia: A heart rate slower than 60 beats per minute; may indicate efficiency in athletes.
• Tachycardia: Elevated heart rate due to compensatory mechanisms for inadequate filling.
• Various abnormalities may occur:
  • Sinus Arrhythmias: Presence of P, QRS, and T waves signals normal sinus rhythm.
  • Atrial Fibrillation: Atrial contraction is irregular, risking clot formations and strokes.
  • Ventricular Tachycardia: Rapid heartbeat causing inefficiency in blood flow.

Heart Block Types

• First-degree block: Prolonged PR interval; still possesses a sinus rhythm but with delayed conduction.
• Second-degree block: Not all signals pass through from the SA node; certain beats may be dropped (no consistent rhythm).
• Importance of monitoring these blocks to determine underlying health issues.

Summary of Waves and Related Activity

• The conducting system of the heart is responsible for rhythmically contracting and relaxing, reflected in the P, QRS, and T waves on an ECG. Each segment reflects electrical activity with functional significance in the cardiac cycle.

Understanding Heart Health

• Regular monitoring and understanding of these electrical properties are critical for identifying cardiovascular health and potential maladies.