Problem 9 - Biofluid Mechanics: An Introduction to Fluid Mechanics, Macrocirculation, and Microcirculation

Question

A patient has an ECG, and the physician finds that there are two \(\mathrm{P}\) waves, followed by the \(\mathrm{QRS}\) complex and then the T wave. What is a possible reason behind this?

Step-by-Step Solution

Answer

The finding suggests a second-degree AV block.

Step 1: Identify ECG components

The components of an ECG signal include the P wave, QRS complex, and T wave. The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization.

Step 2: Understand the occurrence order in a normal ECG

In a normal ECG, the order of the waves is: P wave, followed by the QRS complex, and then the T wave. Normally, a single P wave precedes each QRS complex.

Step 3: Interpret the abnormal finding

The observation of two P waves before a QRS complex indicates that the atria are depolarizing twice before the ventricles depolarize. This is abnormal and suggests an issue with the conduction system.

Step 4: Consider possible medical conditions

A possible condition that leads to multiple P waves before a QRS complex is called a 'second-degree atrioventricular (AV) block'. In this condition, the electrical signals from the atria do not always reach the ventricles.

Key Concepts

ECG Components

ECG, or electrocardiogram, is a test that measures the electrical activity of the heart. It helps in diagnosing various heart conditions. Let's break down its components to better understand how it works.

Every ECG has several specific wave patterns:

• The **P wave**: This is the first part of the ECG and it represents atrial depolarization. In simple terms, it shows that the upper chambers of the heart (atria) are turning on electrically to prepare for a contraction.
• The **QRS complex**: This follows the P wave and indicates ventricular depolarization. This means the lower chambers of the heart (ventricles) are activated electrically, preparing them for a strong contraction to pump blood out.
• The **T wave**: After the QRS complex, the T wave appears. It represents ventricular repolarization, where the ventricles recover and prepare for the next contraction cycle.

Understanding these components helps in identifying normal and abnormal heart activities.

Heart Electrical Conduction

Our heart's rhythmic beating is controlled by a complex electrical system. This ensures that the heart beats efficiently and pumps blood to all parts of the body.

The conduction system includes various key components:

• The **sinoatrial (SA) node**: Known as the heart's natural pacemaker, it initiates the electrical impulse that sets the pace for the heart rate.
• The **atrioventricular (AV) node**: This node acts like a gatekeeper, slowing down the electrical signal from the atria before it passes into the ventricles. This delay ensures the atria contract before the ventricles, allowing efficient blood flow.
• The **bundle of His** and **Purkinje fibers**: These structures facilitate the rapid transmission of the electrical impulse through the ventricles, leading to a coordinated contraction that efficiently pumps blood out of the heart.

This coordinated firing of electrical signals ensures sequential contractions, vital for maintaining a steady and efficient heart rhythm.

Second-degree AV Block

A second-degree AV block occurs when some of the electrical signals from the atria do not reach the ventricles. This can lead to irregular heart rhythms, where sometimes the ventricles do not receive the signal to contract.

In an ECG, this condition can be identified by the presence of more P waves than QRS complexes. Essentially, this implies that while the atria are activated, the ventricles are not always able to respond due to the blockage of signals.

There are types of second-degree AV blocks that we should be aware of:

• **Mobitz Type I (Wenckebach)**: The electrical signals get progressively delayed with each heartbeat until one is blocked completely. This may result in symptoms like dizziness or shortness of breath, but it is often less dangerous.
• **Mobitz Type II**: There are occasional dropped beats where the signal fails to pass entirely through the AV node without prior warning. This type tends to be more serious and may necessitate medical intervention, such as a pacemaker.

Understanding AV blocks is crucial for diagnosing potential issues and deciding on the right treatment options.