How is the cardiac cycle mapped to a normal ECG trace, and what information can be gathered from it?

Sourav PanNovember 9, 2024

Answered

The cardiac cycle is a series of events that occur during a single heartbeat, including both electrical and mechanical changes in the heart. These events can be mapped to a normal electrocardiogram (ECG) trace, providing valuable information about heart function. Here’s how the cardiac cycle relates to the ECG and what can be interpreted from it.

Mapping the Cardiac Cycle to the ECG Trace

1. P Wave:
   • The P wave represents atrial depolarization, which occurs when the sinoatrial (SA) node initiates an electrical impulse that spreads through the atria, causing them to contract. This phase corresponds to atrial systole, where blood is pushed from the atria into the ventricles.
2. PR Interval:
   • The PR interval is the time between the onset of the P wave and the beginning of the QRS complex. It reflects the time taken for the electrical impulse to travel from the atria through the atrioventricular (AV) node and into the ventricles. This delay allows for complete filling of the ventricles before they contract.
3. QRS Complex:
   • The QRS complex represents ventricular depolarization, which triggers ventricular contraction (systole). This phase is critical as it indicates that blood is being ejected from the ventricles into the aorta and pulmonary artery. The QRS complex also coincides with atrial repolarization, although this repolarization is not visible on the ECG due to its overshadowing by the larger ventricular depolarization.
4. ST Segment:
   • The ST segment follows the QRS complex and represents the period between ventricular depolarization and repolarization. It reflects early ventricular contraction and is crucial for assessing myocardial ischemia or infarction.
5. T Wave:
   • The T wave signifies ventricular repolarization, during which the ventricles recover electrically and prepare for the next contraction. This phase corresponds to ventricular diastole, where the heart muscle relaxes, allowing blood to fill back into the ventricles.
6. QT Interval:
   • The QT interval measures the time from the beginning of ventricular depolarization (start of QRS) to the end of ventricular repolarization (end of T wave). It reflects both ventricular systole and diastole and is important for assessing potential arrhythmias.

Information Gathered from ECG Interpretation

1. Heart Rate:
   • The frequency of P waves or R waves can be used to calculate heart rate, helping identify bradycardia (slow heart rate) or tachycardia (fast heart rate).
2. Rhythm Analysis:
   • The regularity of P waves and QRS complexes allows clinicians to determine if there are any arrhythmias, such as atrial fibrillation or flutter.
3. Conduction Delays:
   • Prolonged PR intervals may indicate first-degree AV block, while variations in QRS duration can suggest bundle branch blocks or other conduction abnormalities.
4. Myocardial Ischemia/Infarction:
   • Changes in ST segment elevation or depression can indicate ischemic conditions or myocardial infarction, guiding further diagnostic and therapeutic interventions.
5. Ventricular Hypertrophy:
   • Abnormalities in voltage criteria within QRS complexes can suggest left or right ventricular hypertrophy, often due to chronic hypertension or valvular disease.
6. Electrolyte Imbalances:
   • Certain changes in T waves or QT intervals can signal electrolyte disturbances such as hyperkalemia or hypocalcemia.