Exercise 31 Conduction System Of The Heart And Electrocardiography

Ventricular contraction, the synchronized symphony of heart muscle cells, relies on a highly specialized internal electrical conduction system. Understanding this intricate network, as well as the tool used to visualize its activity—electrocardiography (ECG)—is fundamental to grasping cardiac physiology and pathology. Let's delve into the fascinating world of the heart's conduction system and its graphical representation through ECG.

The Heart's Intrinsic Conduction System: A Hierarchical Network

The heart possesses an inherent ability to generate its own rhythmic electrical impulses and transmit them throughout the myocardium. This remarkable characteristic, known as automaticity, is orchestrated by a dedicated conduction system comprised of specialized cardiac muscle cells. These cells, while contractile to some extent, primarily function to rapidly conduct electrical signals, ensuring coordinated contraction of the atria and ventricles.

The conduction system follows a precise hierarchical order:

1. Sinoatrial (SA) Node: The heart's natural pacemaker, located in the right atrium near the superior vena cava. It initiates electrical impulses at a rate of 60-100 beats per minute under normal conditions.
2. Internodal Pathways: Specialized tracts within the atria that rapidly conduct the impulse from the SA node to the atrioventricular (AV) node. While their existence as distinct anatomical structures is debated, they represent functional pathways for rapid atrial depolarization.
3. Atrioventricular (AV) Node: Located in the interatrial septum, this node acts as a gatekeeper, briefly delaying the impulse before it enters the ventricles. This delay allows the atria to fully contract and empty their contents into the ventricles before ventricular contraction begins.
4. Bundle of His (AV Bundle): A bundle of specialized fibers that originates in the AV node and travels through the fibrous skeleton separating the atria and ventricles. It is the only electrical connection between the atria and ventricles.
5. Right and Left Bundle Branches: The Bundle of His divides into two branches, the right bundle branch (RBB) and the left bundle branch (LBB), which travel down the interventricular septum toward the apex of the heart.
6. Purkinje Fibers: A network of rapidly conducting fibers that extend from the bundle branches and spread throughout the ventricular myocardium. They ensure rapid and coordinated depolarization of the ventricular muscle cells, leading to efficient ventricular contraction.

Action Potentials in the Conduction System: A Deep Dive

The electrical activity of the heart, including the conduction system, is based on changes in membrane potential due to the movement of ions across the cell membrane. These changes generate action potentials, which are the fundamental electrical signals that drive cardiac contraction. However, the action potentials of the conduction system differ significantly from those of contractile cardiac muscle cells.

• SA Node Action Potentials: SA node cells exhibit automaticity because they have an unstable resting membrane potential. This instability is due to a unique "funny current" (If) carried by sodium ions, which slowly depolarizes the cell membrane. When the membrane potential reaches a threshold, a rapid influx of calcium ions triggers an action potential. The action potential repolarizes due to potassium efflux. This spontaneous depolarization-repolarization cycle continues rhythmically, setting the heart rate.

• AV Node Action Potentials: AV node cells also exhibit automaticity, but at a slower rate than the SA node. Their action potentials are similar to those of the SA node, relying on calcium influx for depolarization and potassium efflux for repolarization. The slower conduction velocity through the AV node is due to the smaller size of the cells and fewer gap junctions, which contributes to the AV nodal delay.

• Purkinje Fiber Action Potentials: Purkinje fibers have the fastest conduction velocity in the heart. Their action potentials are characterized by a rapid upstroke due to sodium influx, a brief plateau phase due to calcium influx, and a repolarization phase due to potassium efflux. Although Purkinje fibers possess automaticity, it is typically suppressed by the faster rate of the SA node. However, if the SA node fails, Purkinje fibers can act as a backup pacemaker.

Electrocardiography (ECG): A Window into the Heart's Electrical Activity

Electrocardiography (ECG or EKG) is a non-invasive diagnostic tool that records the electrical activity of the heart from the surface of the body. By placing electrodes on the limbs and chest, the ECG captures the changes in electrical potential that occur during each cardiac cycle. The resulting recording, called an electrocardiogram, provides valuable information about the heart's rhythm, conduction, and structural integrity.

ECG Waves, Intervals, and Segments: Deciphering the Code

A typical ECG tracing consists of several distinct waves, intervals, and segments, each corresponding to a specific electrical event in the heart:

• P Wave: Represents atrial depolarization, the electrical activation of the atria that leads to atrial contraction.
• QRS Complex: Represents ventricular depolarization, the electrical activation of the ventricles that leads to ventricular contraction. It is a complex wave consisting of three deflections: the Q wave (a negative deflection), the R wave (a positive deflection), and the S wave (a negative deflection following the R wave).
• T Wave: Represents ventricular repolarization, the return of the ventricles to their resting electrical state.
• PR Interval: Measures the time from the beginning of atrial depolarization (P wave) to the beginning of ventricular depolarization (QRS complex). It reflects the time it takes for the impulse to travel from the SA node through the atria, AV node, Bundle of His, and Purkinje fibers.
• QT Interval: Measures the time from the beginning of ventricular depolarization (QRS complex) to the end of ventricular repolarization (T wave). It represents the total duration of ventricular electrical activity.
• ST Segment: The segment between the end of the QRS complex and the beginning of the T wave. It represents the period when the ventricles are depolarized.

ECG Leads: Different Perspectives on the Heart

An ECG typically uses 12 leads, each providing a different "view" of the heart's electrical activity. These leads are strategically placed on the limbs and chest to capture electrical signals from different angles.

• Limb Leads: Six leads placed on the limbs (right arm, left arm, right leg, left leg) that provide information about the electrical activity in the frontal plane. They include:
  • Leads I, II, and III (Bipolar Limb Leads): Measure the potential difference between two limbs.
  • Leads aVR, aVL, and aVF (Augmented Limb Leads): Measure the potential difference between one limb and the average of the other two limbs.
• Chest Leads (Precordial Leads): Six leads placed on the chest that provide information about the electrical activity in the horizontal plane. They are labeled V1 through V6.

By analyzing the ECG tracing in different leads, clinicians can determine the location and extent of cardiac abnormalities.

Interpreting the ECG: A Systematic Approach

Interpreting an ECG requires a systematic approach to ensure that all important aspects of the recording are evaluated. A common approach involves the following steps:

1. Assess the Rate: Determine the heart rate by measuring the distance between consecutive R waves (R-R interval).
2. Assess the Rhythm: Determine if the rhythm is regular or irregular. Look for a consistent pattern of P waves and QRS complexes.
3. Evaluate the P Wave: Check for the presence, morphology, and axis of the P wave. It should be upright in leads I, II, and aVF and inverted in lead aVR.
4. Measure the PR Interval: Determine if the PR interval is within the normal range (0.12-0.20 seconds).
5. Evaluate the QRS Complex: Check for the morphology, duration, and amplitude of the QRS complex. It should be narrow (less than 0.12 seconds) in normal conduction.
6. Analyze the ST Segment: Look for ST segment elevation or depression, which can indicate myocardial ischemia or infarction.
7. Evaluate the T Wave: Check for the morphology, amplitude, and polarity of the T wave. It should be upright in most leads and inverted in lead aVR.
8. Measure the QT Interval: Determine if the QT interval is within the normal range, corrected for heart rate (QTc).

Common ECG Abnormalities: Clues to Underlying Cardiac Conditions

The ECG can reveal a wide range of cardiac abnormalities, including:

• Arrhythmias: Irregular heart rhythms, such as atrial fibrillation, atrial flutter, ventricular tachycardia, and ventricular fibrillation.
• Conduction Blocks: Delays or interruptions in the conduction of electrical impulses, such as AV block, bundle branch block, and hemiblock.
• Myocardial Ischemia and Infarction: Reduced blood flow to the heart muscle, leading to ST segment elevation or depression, T wave inversion, and Q wave formation.
• Hypertrophy: Enlargement of the heart chambers, such as left ventricular hypertrophy and right ventricular hypertrophy.
• Electrolyte Imbalances: Abnormal levels of electrolytes, such as potassium and calcium, can affect the ECG waveform.
• Drug Effects: Certain medications can prolong the QT interval or cause other ECG changes.

The Interplay of Exercise and the Conduction System

Exercise has a profound impact on the heart's conduction system. During physical activity, the sympathetic nervous system is activated, leading to an increase in heart rate and contractility. This is mediated by the release of catecholamines, such as epinephrine and norepinephrine, which bind to beta-adrenergic receptors on cardiac cells.

• Increased SA Node Firing Rate: Catecholamines increase the rate of spontaneous depolarization in SA node cells, leading to a faster heart rate.
• Enhanced AV Node Conduction: Catecholamines increase the conduction velocity through the AV node, shortening the PR interval.
• Increased Ventricular Contractility: Catecholamines increase the force of ventricular contraction, leading to a greater stroke volume.
• ECG Changes During Exercise: The ECG during exercise typically shows an increase in heart rate, a shortening of the PR interval and QT interval, and an increase in the amplitude of the QRS complex. In some individuals, ST segment depression may occur due to increased myocardial oxygen demand.

Exercise ECG (Stress Test): Unveiling Hidden Cardiac Issues

An exercise ECG, also known as a stress test, is a diagnostic procedure used to evaluate the heart's response to exercise. It involves recording the ECG while the patient exercises on a treadmill or stationary bike. The test is used to detect myocardial ischemia, arrhythmias, and other cardiac abnormalities that may not be apparent at rest.

• Indications for Exercise ECG: Chest pain, shortness of breath, known or suspected coronary artery disease, evaluation of arrhythmias, assessment of exercise capacity.
• Interpretation of Exercise ECG: The ECG is monitored for changes such as ST segment depression, T wave inversion, and arrhythmias. The patient's blood pressure, heart rate, and symptoms are also closely monitored.
• Positive Exercise ECG: A positive test result indicates the presence of myocardial ischemia or other significant cardiac abnormalities. Further testing, such as coronary angiography, may be needed.

Conduction System Pathologies and Their ECG Manifestations

Dysfunction in the heart's conduction system can lead to a variety of cardiac arrhythmias and conduction blocks, each with its own characteristic ECG pattern.

• Sick Sinus Syndrome (SSS): A group of arrhythmias caused by SA node dysfunction, including sinus bradycardia, sinus arrest, and tachy-brady syndrome (alternating periods of slow and fast heart rates). The ECG may show prolonged pauses, absent P waves, and alternating periods of bradycardia and tachycardia.

• Atrioventricular (AV) Blocks: Delays or interruptions in the conduction of electrical impulses from the atria to the ventricles. AV blocks are classified into three degrees:

  • First-degree AV block: Prolonged PR interval (greater than 0.20 seconds).
  • Second-degree AV block: Some P waves are not followed by QRS complexes. There are two types: Mobitz type I (Wenckebach) and Mobitz type II.
  • Third-degree AV block (Complete Heart Block): No relationship between P waves and QRS complexes. The atria and ventricles beat independently.

• Bundle Branch Blocks (BBB): Delays or interruptions in the conduction of electrical impulses through the right or left bundle branch. The ECG shows a widened QRS complex (greater than 0.12 seconds) and characteristic changes in the QRS morphology in specific leads.

  • Right Bundle Branch Block (RBBB): RSR' pattern in leads V1-V3.
  • Left Bundle Branch Block (LBBB): Broad, notched R wave in leads I, aVL, V5, and V6.

• Wolff-Parkinson-White (WPW) Syndrome: A congenital condition characterized by an accessory pathway (Bundle of Kent) that bypasses the AV node, leading to pre-excitation of the ventricles. The ECG shows a short PR interval, a delta wave (slurred upstroke of the QRS complex), and a widened QRS complex.

Conclusion: The Symphony of the Heart, Visualized

The heart's conduction system is a remarkable network that ensures coordinated and efficient cardiac contraction. Understanding its anatomy, physiology, and pathologies is crucial for healthcare professionals. Electrocardiography (ECG) provides a non-invasive window into the heart's electrical activity, allowing clinicians to diagnose a wide range of cardiac conditions. By carefully analyzing the ECG waves, intervals, and segments, and by understanding the effects of exercise and various pathologies on the conduction system, we can gain valuable insights into the health and function of this vital organ. The ECG remains an indispensable tool in modern cardiology, enabling early detection, accurate diagnosis, and effective management of heart disease.