Instructions
This worksheet is designed to test your understanding of electrocardiograms (ECGs) and common cardiac arrhythmias. Read each section carefully and answer the questions to the best of your ability. The goal is to connect the electrical events of the heart with their graphical representation on an ECG and their resulting physiological effects.
Section 1: The Anatomy of an ECG Waveform
An ECG trace graphically represents the heart's electrical activity. Each part of the wave corresponds to a specific event in the cardiac cycle.
1. Fill in the blanks below to describe the primary components of a standard ECG trace.
- The P wave represents the depolarization of the ____________.
- The QRS complex represents the depolarization of the ____________.
- The T wave represents the repolarization of the ____________.
2. Match the electrical event with its corresponding mechanical action in the heart.
| Electrical Event | Mechanical Action |
|---|---|
| 1. Atrial Depolarization (P wave) | A. Ventricular Relaxation (Diastole) |
| 2. Ventricular Depolarization (QRS complex) | B. Atrial Contraction (Atrial Systole) |
| 3. Ventricular Repolarization (T wave) | C. Ventricular Contraction (Ventricular Systole) |
Your answers: 1-____, 2-____, 3-____
Section 2: Measuring Time - Key Intervals
The duration of intervals on an ECG provides crucial information about the speed of electrical conduction through the heart.
Complete the table below.
| Interval | What It Represents | Normal Duration Range |
|---|---|---|
| PR Interval | ||
| QRS Duration | ||
| QT Interval |
Section 3: The Heart's Electrical Axis
The overall direction of the heart's electrical activity can be visualized as a vector. Einthoven's triangle is a foundational concept for understanding this.
Answer the following questions in 1-2 sentences.
- What is the mean ventricular vector (also known as the mean electrical axis)?
- What is Einthoven's triangle and what do its points represent?
Section 4: Identifying Common Arrhythmias
An arrhythmia is any deviation from the normal sinus rhythm of the heart. Below are descriptions of simplified ECG traces.
Part A: Match the arrhythmia with its definition.
- 1. Sinus Bradycardia _____
- 2. Atrial Fibrillation (AFib) _____
- 3. Sinus Tachycardia _____
- 4. Ventricular Fibrillation (VFib) _____
- A normal sinus rhythm with a resting heart rate greater than 100 bpm.
- A chaotic quivering of the atria, resulting in an irregularly irregular rhythm with no discernible P waves.
- A life-threatening, chaotic rhythm from the ventricles causing no effective cardiac output. The ECG shows no organized waves.
- A normal sinus rhythm with a resting heart rate less than 60 bpm.
Part B: Analyze the following ECG trace descriptions and identify the arrhythmia.
Use the arrhythmia names from Part A.
-
Trace 1:
- Rhythm: Regular
- Heart Rate: 135 bpm
- P waves: Present and normal, one for every QRS
- Intervals: PR and QRS are within normal limits
-
Trace 2:
- Rhythm: Irregularly irregular
- Heart Rate: Varies, approx. 120 bpm
- P waves: Absent, replaced by a chaotic fibrillatory baseline
- Intervals: QRS is narrow, but R-R interval is variable
-
Trace 3:
- Rhythm: Chaotic and disorganized
- Heart Rate: Indeterminable, >300 bpm
- P waves: Absent
- Intervals: No identifiable QRS complexes or T waves; just a wavy, erratic line
-
Trace 4:
- Rhythm: Regular
- Heart Rate: 48 bpm
- P waves: Present and normal, one for every QRS
- Intervals: PR and QRS are within normal limits
Answer Key
Section 1: The Anatomy of an ECG Waveform
1. Fill in the blanks:
- The P wave represents the depolarization of the atria.
- The QRS complex represents the depolarization of the ventricles.
- The T wave represents the repolarization of the ventricles.
2. Matching: 1-B, 2-C, 3-A
Section 2: Measuring Time - Key Intervals
| Interval | What It Represents | Normal Duration Range |
|---|---|---|
| PR Interval | Time from the start of atrial depolarization to the start of ventricular depolarization (includes AV node delay). | 0.12 – 0.20 seconds |
| QRS Duration | The time taken for ventricular depolarization. | < 0.12 seconds |
| QT Interval | The total time for ventricular depolarization and repolarization (the full ventricular action potential). | ~0.40 – 0.44 seconds (highly rate-dependent) |
Section 3: The Heart's Electrical Axis
- What is the mean ventricular vector?
(Model Answer) The mean ventricular vector is the sum of all individual electrical signals generated by ventricular depolarization, representing the overall direction of electrical movement in the ventricles, which is normally downwards and to the left. - What is Einthoven's triangle?
(Model Answer) Einthoven's triangle is an imaginary equilateral triangle on the torso used in electrocardiography. Its points represent the standard limb leads (right arm, left arm, and left leg) and it provides a framework for measuring the heart's electrical axis.
Section 4: Identifying Common Arrhythmias
Part A: Matching
- 1. Sinus Bradycardia D
- 2. Atrial Fibrillation (AFib) B
- 3. Sinus Tachycardia A
- 4. Ventricular Fibrillation (VFib) C
Part B: Analysis and Diagnosis
- Diagnosis: Sinus Tachycardia
- Diagnosis: Atrial Fibrillation (AFib)
- Diagnosis: Ventricular Fibrillation (VFib)
- Diagnosis: Sinus Bradycardia