ECG Basics and Function

The Electrocardiogram (ECG)

• The ECG is a recording of the electrical activity of the heart.
• It is an important diagnostic tool used to evaluate heart function and identify any abnormalities.
• The ECG is a graphical representation of the electrical activity of the heart, recorded using electrodes placed on the skin.

How the ECG Works

• The P wave: reflects the electrical activity of the atria as they depolarize, which is initiated by the sinoatrial (SA) node.
• The PR interval: represents the time it takes for the electrical impulse to travel from the SA node through the atria, the atrioventricular (AV) node, and to the ventricles.
• The QRS complex: represents ventricular depolarization, the electrical activation of the ventricles which begins at the interventricular septum, the wall between the ventricles. The Q wave represents the depolarization of the septum, the R wave represents the depolarization of the ventricles, and the S wave represents the depolarization of the posterior part of the left ventricle.
• The ST segment: represents the time when the ventricles are fully depolarized. It is the period between the end of ventricular depolarization and the start of ventricular repolarization.
• The T wave: reflects the electrical activity of the ventricles as they repolarize (return to their resting state).
• The QT interval: represents the time it takes for the ventricles to depolarize and repolarize.
• The U wave: reflects the repolarization of the papillary muscles and Purkinje fibers.

ECG Components

• Wave: A deflection from the isoelectric baseline.
• Segment: A region between two waves.
• Interval: A duration of time that includes one or more waves and a segment.

Reading the ECG

• P Wave: Amplitude of 0.05-0.25 mV, duration of 0.06-0.10 seconds, waveform is upright and slightly asymmetrical.
• PR Interval: Duration of 0.12-0.20 seconds.
• QRS Complex: Amplitude of 0.5-3.0 mV, duration of 0.06-0.10 seconds.
• ST Segment: Is the isoelectric line that follows the QRS complex.
• T Wave: Is a larger, slightly asymmetrical waveform that follows the ST segment, the J point is the start of the ST segment.
• QT Interval: Distance from the onset of the QRS complex to the end of the T wave, the normal duration is 0.36 to 0.44 seconds.
• U Wave: Small, upright wave following the T wave, but before the next P wave.

ECG Leads & the Electrical View of the Heart

• Limb Leads: Are bipolar or unipolar leads. The bipolar leads include I, II, and III. These record the flow of electrical impulses between two electrodes (one positive, one negative) and form a triangle called Einthoven's Triangle.
• Bipolar Leads: Record the difference in potential between a positive and a negative electrode.
• Unipolar Leads: Use only one positive electrode with a reference point calculated by the ECG machine. Unipolar leads include aVR, aVL, and aVF (augmented limb leads), and V1-V6 (chest leads).
• The 12-Lead EKG: Provides a three-dimensional view of the heart by using nine electrodes (plus a ground) placed on the extremities and chest wall.
  • Frontal Plane: View provided by the limb leads (I, II, III, aVR, aVL, and aVF), providing inferior, superior, and lateral views of the heart.
  • Horizontal Plane: View provided by the chest leads (V1, V2, V3, V4, V5, and V6), providing anterior and lateral views.

Performing an ECG:

• Electrodes: Are positioned to form leads.
• Standard 12-lead ECG: - Is typically performed with the person lying supine. - Includes three bipolar limb leads (I, II, III), three augmented voltage leads (aVR, aVL, aVF), and six chest leads (V1 – V6).

Artifact

• Artifact is any marking on the ECG tracing that is not a product of the heart’s electrical activity, Patient movement is one of many causes, and can mimic life-threatening dysrhythmias.

Summary

• The ECG is a record of the electrical activity of the heart.
• Each lead offers a different view.
• Impulses traveling towards a positive electrode are recorded on the ECG as upward deflections.
• Electrodes are positioned on the patient’s skin to pick up electrical activity of the heart.
• Any abnormalities in the heart’s rhythm or rate – are called dysrhythmias.

This quiz covers the essential aspects of the electrocardiogram (ECG), including its function, components, and how it reflects heart activity. You'll learn about the P wave, PR interval, and QRS complex, along with their significance in diagnosing heart conditions. Test your understanding of this crucial diagnostic tool used in cardiology.