An Electrocardiogram (ECG or EKG) is a non-invasive diagnostic tool that records the electrical signals of your heart. It helps healthcare providers quickly detect heart problems and monitor overall heart health. Understanding the basic components of this medical test can help understand its purpose and the lines on the graph.
The Heart’s Electrical Language
The heart, a muscular pump, relies on an intricate electrical conduction system to coordinate its contractions and pump blood throughout the body. This system begins with the sinoatrial (SA) node, often called the heart’s natural pacemaker, located in the upper right chamber (atrium) of the heart. The SA node generates electrical impulses at a regular rate, usually 60-100 times per minute at rest.
These electrical impulses spread across the atria, causing them to contract and push blood into the ventricles. The signal then travels to the atrioventricular (AV) node, which briefly delays the impulse. This delay ensures that the atria have fully emptied their blood into the ventricles before the ventricles begin to contract.
After this brief pause, the electrical signal continues down through the bundle of His and into the left and right bundle branches, distributing the impulse rapidly through Purkinje fibers to the ventricular muscle cells. This rapid transmission causes the ventricles to contract, pushing blood out to the lungs and the rest of the body. This entire sequence of electrical activity is what the ECG machine detects and translates into wave patterns.
Deciphering the ECG Tracing
An ECG tracing displays a series of waves and segments, each corresponding to a specific electrical event within the heart’s cycle. The initial upward bump on the tracing is the P wave, which represents atrial depolarization, the electrical activation of the atria as they contract. A normal P wave is upright in most leads and lasts 0.11 seconds or less.
Following the P wave is the PR interval, which measures the time from the start of atrial depolarization to the start of ventricular depolarization. This interval includes the brief delay at the AV node, ensuring proper ventricular filling. A PR interval usually ranges from 0.12 to 0.20 seconds.
The most prominent feature is the QRS complex, a sharp, tall deflection that signifies ventricular depolarization, the electrical activation that causes the ventricles to contract. The Q wave is the initial downward deflection, the R wave is the first upward deflection, and the S wave is the downward deflection following the R wave. The QRS complex normally lasts between 0.07 to 0.10 seconds, representing the rapid spread of electricity through the ventricles.
After the QRS complex, the ST segment is the flat line connecting the end of ventricular depolarization to the beginning of ventricular repolarization. This segment should be horizontal along the baseline. Finally, the T wave represents ventricular repolarization, the electrical recovery of the ventricles. A normal T wave is rounded and asymmetrical, often deflecting in the same direction as the QRS complex in most leads.
Characteristics of a Healthy ECG
A healthy ECG displays a consistent pattern, reflecting the heart’s regular and efficient electrical activity. The heart rate, measured by the frequency of the QRS complexes, is usually 60 to 100 beats per minute (bpm) at rest. The rhythm should be regular, meaning the R-R interval (time between heartbeats) is consistent.
Each QRS complex should be preceded by a P wave, indicating the impulse originated in the heart’s natural pacemaker. The P wave should be upright in leads I, II, and aVF, signifying normal atrial activation. The PR interval, from the start of the P wave to the beginning of the QRS complex, should measure between 0.12 and 0.20 seconds.
The QRS complex should appear narrow, lasting between 0.07 and 0.10 seconds, reflecting efficient electrical conduction through the ventricles. The ST segment should be flat or isoelectric, aligning with the baseline. The T wave should be rounded and in the same direction as the QRS complex in most leads, indicating normal ventricular repolarization. These characteristics define a normal sinus rhythm, the heart’s healthy electrical pattern.
When the ECG Signals Change
Deviations from a healthy ECG pattern can indicate underlying heart conditions. Changes in heart rate, such as below 60 bpm (bradycardia) or above 100 bpm (tachycardia), can indicate an electrical issue. Irregular rhythms, where the time between heartbeats is inconsistent, signal problems with the heart’s electrical system.
Alterations in the shape, duration, or direction of the P waves, QRS complexes, or T waves can be significant. For example, a prolonged QRS complex might suggest an issue with electricity spread through the ventricles. Changes in the ST segment, such as elevation or depression, can be associated with reduced blood flow to the heart muscle.
While understanding these changes is helpful, only a qualified healthcare professional can accurately interpret an ECG tracing. Interpreting patterns requires extensive training and clinical context. Self-diagnosis based on ECG readings is not recommended. Any concerns about heart health should always be discussed with a doctor.