What is the difference between tachycardia and bradycardia?

Tachycardia is a fast heart rate exceeding 100 bpm, while bradycardia is a slow heart rate below 60 bpm. Both are identified on an ECG by the frequency of the QRS complexes.

How does an ECG differ from a pulse measurement?

An ECG measures the electrical impulses that trigger the heart to beat, whereas a pulse measures the mechanical pressure wave of blood moving through arteries. An ECG can detect electrical issues that a pulse measurement cannot.

Why is the QRS complex much larger than the P wave?

The QRS complex represents ventricular depolarization. Since the ventricles have a much larger muscle mass than the atria to pump blood to the body, they generate a significantly stronger electrical signal.

What is a common misconception regarding the T wave?

A common error is thinking the T wave represents atrial repolarization. In reality, the T wave represents ventricular repolarization; atrial repolarization is hidden by the QRS complex.

What error occurs if you calculate heart rate using the P-P interval instead of the R-R interval in a heart with a conduction block?

In a heart block, the atria and ventricles may beat at different rates. Using the P-P interval would give the atrial rate, which might be normal, while the R-R interval would reveal the dangerously slow ventricular rate.

Why might a student fail to see a U wave on a standard ECG?

The U wave is very small and not always present or visible in all leads or individuals. It is often considered a normal variation rather than a requirement for a healthy trace.

Define the P wave in terms of electrical and mechanical activity.

Electrically, the P wave is atrial depolarization. Mechanically, it leads to atrial systole (contraction).

What does the term 'myogenic' mean in the context of the heart?

Myogenic means the heart muscle can generate its own electrical impulses to initiate contraction without needing a signal from the nervous system.

What is fibrillation?

Fibrillation is a condition where the heart's electrical activity becomes chaotic and uncoordinated, leading to an irregular rhythm where the heart 'quivers' rather than pumping effectively.

What is the formula for calculating heart rate from an ECG trace?

The formula is $HR = \frac{60}{\text{time for one cardiac cycle (R-R interval) in seconds}}$. This converts the duration of one beat into beats per minute.

Electrocardiograms (ECGs) | OCR AS-Level Biology

3. Exchange & Transport

Electrocardiograms (ECGs)

Summary

An electrocardiogram (ECG) is a diagnostic tool used to monitor and record the electrical activity of the heart over time. By detecting electrical impulses on the skin's surface, it provides a visual representation of the cardiac cycle, allowing for the assessment of heart rate, rhythm, and the coordination of atrial and ventricular contractions.

1. Definition & Core Concepts

Electrocardiography is the process of recording the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle's electrophysiologic pattern of depolarizing and repolarizing during each heartbeat.
The heart is myogenic, meaning it generates its own electrical impulses without external nerve stimulation. The ECG captures these internal signals as they spread through the heart's specialized conduction system.
A standard ECG trace consists of a series of waves, each corresponding to a specific electrical event in the cardiac cycle. These events precede the mechanical contraction (systole) and relaxation (diastole) of the heart chambers.

A standard ECG trace showing the P wave, QRS complex, and T wave on a grid.

2. The ECG Waveform Components

P Wave: This small upward deflection represents atrial depolarization. This electrical signal triggers the contraction of the atria (atrial systole), pushing blood into the ventricles.
QRS Complex: This large, sharp spike represents ventricular depolarization. It is significantly larger than the P wave because the ventricles have a much greater muscle mass than the atria. This signal triggers ventricular contraction (ventricular systole).
T Wave: This represents ventricular repolarization, which occurs as the ventricles relax (ventricular diastole) and prepare for the next cycle.
U Wave: Occasionally seen after the T wave, its origin is less certain but is often attributed to the repolarization of the Purkyne (Purkinje) fibers.

3. Diagnostic Interpretation of Rhythms

Tachycardia: Defined as a resting heart rate exceeding 100 beats per minute (bpm). On an ECG, this is visualized as the peaks (R-waves) being very close together.
Bradycardia: Defined as a resting heart rate below 60 bpm. This is common in highly trained athletes but can indicate issues in others; it is shown by widely spaced peaks.
Ectopic Heartbeat: An 'extra' beat that occurs earlier than expected in the rhythm, often followed by a brief pause before the next normal beat.
Fibrillation: A complete loss of rhythm where the heart's electrical activity becomes irregular and chaotic. This prevents coordinated contraction and is a medical emergency.

4. Methods: Calculating Heart Rate

5. Key Distinctions

6. Exam Strategy & Tips