Heart Function and the ECG

The heart normally beats between 60 and 100 times per minute, with many normal variations. For example, athletes at rest have slower heart rates than most people. This rate is set by a small collection of specialized heart cells called the sinoatrial (SA) or sinus node.

Located in the right atrium, the sinus node is the heart's "natural pacemaker."

• It has "automaticity," meaning it discharges all by itself without control from the brain.
  
  
• Two events occur with each discharge: (1) both atria contract, and (2) an electrical impulse travels through the atria to reach another area of the heart called the atrioventricular (AV) node, which lies in the wall between the 2 ventricles.
  
  
• The AV node serves as a relay point to further propagate the electrical impulse.
  
  
• From the AV node, an electrical wave travels to both ventricles, causing them to contract and pump blood.
  
  
• The normal delay between the contraction of the atria and of the ventricles is 0.12 to 0.20 seconds. This delay is perfectly timed to account for the physical passage of the blood from the atrium to the ventricle. Intervals shorter or longer than this range indicate possible problems.

The ECG records the electrical activity that results when the heart muscle cells in the atria and ventricles contract.

• Atrial contractions (both right and left) show up as the P wave.
  
  
• Ventricular contractions (both right and left) show as a series of 3 waves, Q-R-S, known as the QRS complex.
  
  
• The third and last common wave in an ECG is the T wave. This reflects the electrical activity produced when the ventricles are recharging for the next contraction (repolarizing).
  
  
• Interestingly, the letters P, Q, R, S, and T are not abbreviations for any actual words but were chosen many years ago for their position in the middle of the alphabet.
  
  
• The electrical activity results in P, QRS, and T waves that have a myriad of sizes and shapes. When viewed from multiple anatomic-electric perspectives (that is, leads), these waves can show a wide range of abnormalities of both the electrical conduction system and the muscle tissue of the heart's 4 pumping chambers.