Electrocardiogram and Heart Function

The Heart

• The heart is a muscular organ that functions as the body's circulatory pump.
• It is located in the mediastinum, near the midline, and is rotated and positioned on its side, with 2/3 of the heart on the left side of the chest.
• The base of the heart faces up and to the right, while the apex faces down, out, and to the left.

Chambers of the Heart

• The heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle.
• The wall of the left ventricle is thicker than that of the right ventricle.
• The function of the right side of the heart is to deliver deoxygenated blood from the body to the lungs.
• The function of the left side of the heart is to deliver oxygenated blood from the lungs to the body.

Heart Conduction System

• The heart conduction system is a network of specialized cardiac cells designed to start each heart contraction and coordinate the spread of excitation.
• The conduction system consists of the sinoatrial node (SA node or pacemaker), atrioventricular node (AV node), atrioventricular bundle (AV bundle or bundle of His), right and left branches, and Purkinje fibers.

Electrocardiogram (ECG)

• An ECG is a painless test that records the electric currents produced by the heart and converts them into waveforms that can be seen on a monitor or printed out on paper.
• The ECG machine is used to record the electrocardiogram.
• The waveforms created by the ECG can be divided into time segments to measure the rate of movement of the heart's electrical impulses.

ECG Leads

• ECG is recorded by placing metallic electrodes on the surface of the body in designated anatomical locations.
• The standard ECG has 12 leads, which are subdivided into bipolar leads (3) and unipolar leads (9).
• The 12 leads provide a comprehensive picture of the electrical activity of the heart.

Routine ECG Examination

• The routine ECG examination records from two planes: the frontal plane and the transverse plane.
• The frontal plane leads include standard bipolar limb leads and augmented unipolar leads.
• The transverse plane leads include precordial leads or chest leads, which are numbered V1-V6.

ECG Components

• The ECG is composed of:
  • P wave: the wave of atrial depolarization
  • QRS complex: the wave of ventricular depolarization
  • T wave: the wave of ventricular repolarization
  • Related intervals

Intervals and Waves

• P wave:
  • Is the wave of atrial depolarization
  • Normally has a smooth and rounded shape
  • Not more than 3 mm tall
  • Upright in leads I, II, and aVF
• PR interval:
  • Includes the P wave and the space up to the beginning of the QRS complex
  • Represents the time it takes the electrical impulse to travel from the SA node to the ventricles
  • Normally 0.12-0.20 seconds or 120-200 msec
• QRS complex:
  • Is the wave of ventricular depolarization
  • Should be at least 5 mm and not more than 20 mm tall
  • Normal QRS duration is 0.06-0.10 seconds
• ST segment:
  • Is the period of time between the end of ventricular depolarization and the beginning of ventricular repolarization
  • Should be isoelectric, or on the baseline
  • A deviation of the ST segment from the baseline may be indicative of coronary artery disease
• T wave:
  • Is the wave of ventricular repolarization
  • Normally deflects in the same direction as the QRS complex
  • Should be smooth and rounded

ECG Interpretation

• Heart rate: normal range is 60-100 bpm
• Rhythm: abnormal cardiac rhythms are called arrhythmias
• Duration of waves and intervals: should be within normal ranges

Calculating Heart Rate using ECG Paper

• Each small horizontal square is 0.04 seconds, and each large horizontal square is 0.2 seconds
• Normal duration of R-R interval is 0.8 seconds
• Heart rate = 1500 / # of small squares (R-R)