5.2 Principles of Electrocardiography
4 min read•august 7, 2024
Electrocardiography is the cornerstone of cardiac diagnostics, measuring the heart's electrical activity. This section dives into the principles behind ECG, from the cardiac cycle's electrical phases to the intricate conduction system that orchestrates each heartbeat.
We'll explore various ECG lead systems, including Einthoven's triangle and precordial leads, which provide different perspectives on cardiac function. Understanding these concepts is crucial for interpreting ECGs and diagnosing heart conditions accurately.
Cardiac Electrical Activity
Phases of the Cardiac Cycle
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- Cardiac cycle consists of the sequence of electrical and mechanical events that occur during one heartbeat
- represents atrial depolarization, which triggers atrial contraction (atrial systole)
- corresponds to ventricular depolarization, initiating ventricular contraction (ventricular systole)
- Q wave is the first negative deflection
- R wave is the first positive deflection
- S wave is the negative deflection following the R wave
- represents ventricular repolarization, allowing ventricles to relax (ventricular diastole)
- PR interval measures the time from the start of atrial depolarization to the start of ventricular depolarization (normally 120-200 ms)
- QT interval measures the time from the start of ventricular depolarization to the end of ventricular repolarization (normally < 440 ms)
Conduction System of the Heart
- Electrical activity originates in the sinoatrial (SA) node, the heart's natural pacemaker located in the right atrium
- Atrioventricular (AV) node delays the electrical signal, allowing time for atrial contraction before ventricular contraction begins
- His bundle, bundle branches, and Purkinje fibers rapidly conduct the electrical signal throughout the ventricles, ensuring coordinated contraction
- Abnormalities in the conduction system can lead to arrhythmias (abnormal heart rhythms) such as heart blocks or bundle branch blocks
ECG Lead Systems
Einthoven's Triangle and Limb Leads
- Einthoven's triangle is an imaginary equilateral triangle formed by the two shoulders and pubic region, representing the standard limb leads (I, II, III)
- Lead I measures the potential difference between the right arm (-) and left arm (+)
- Lead II measures the potential difference between the right arm (-) and left leg (+)
- Lead III measures the potential difference between the left arm (-) and left leg (+)
- Augmented limb leads (aVR, aVL, aVF) provide additional perspectives by comparing each limb to the average of the other two
- aVR (augmented vector right) has the positive electrode on the right arm
- aVL (augmented vector left) has the positive electrode on the left arm
- aVF (augmented vector foot) has the positive electrode on the left leg
Precordial Leads and Vectorcardiography
- Precordial (chest) leads (V1-V6) are unipolar leads placed at specific points on the chest to provide a more detailed view of the heart's electrical activity
- V1 and V2 are placed at the 4th intercostal space on the right and left sternal border, respectively
- V4 is placed at the 5th intercostal space in the midclavicular line
- V3 is placed halfway between V2 and V4
- V5 is placed at the same level as V4 in the anterior axillary line
- V6 is placed at the same level as V4 in the midaxillary line
- Vectorcardiography uses the concept of a vector to represent the magnitude and direction of the heart's electrical activity in three dimensions
- Frontal plane vectors are derived from limb leads and represent the heart's electrical activity from a vertical perspective
- Transverse plane vectors are derived from precordial leads and represent the heart's electrical activity from a horizontal perspective
Cardiac Axis
- Cardiac axis refers to the average direction of the heart's electrical activity in the frontal plane
- Normal cardiac axis ranges from -30° to +90°, with 0° being a vertical line passing through the center of the heart
- Left axis deviation (LAD) occurs when the cardiac axis is between -30° and -90°, often associated with left ventricular hypertrophy or bundle branch blocks
- Right axis deviation (RAD) occurs when the cardiac axis is between +90° and +180°, often associated with right ventricular hypertrophy or pulmonary embolism
ECG Analysis
Heart Rate and Rhythm
- is calculated by dividing 60 by the time (in seconds) between two consecutive R waves (HR = 60 / R-R interval)
- Normal heart rate (normocardic) ranges from 60 to 100 beats per minute (bpm)
- Bradycardia is a heart rate below 60 bpm
- Tachycardia is a heart rate above 100 bpm
- Rhythm refers to the regularity and origin of the heart's electrical activity
- Normal sinus rhythm (NSR) originates from the SA node and has a regular R-R interval
- Sinus arrhythmia is a normal variation in heart rate during respiration (rate increases during inspiration and decreases during expiration)
- is an irregular rhythm characterized by the absence of P waves and an irregularly irregular R-R interval
- is a life-threatening rhythm characterized by three or more consecutive ventricular beats at a rate greater than 100 bpm
Heart Rate Variability
- Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between consecutive heartbeats
- HRV is influenced by the autonomic nervous system, with increased parasympathetic activity leading to higher HRV and increased sympathetic activity leading to lower HRV
- Reduced HRV is associated with an increased risk of cardiovascular events, such as myocardial infarction and heart failure
- Time-domain analysis of HRV involves statistical calculations of the R-R intervals, such as the standard deviation of N-N intervals (SDNN) or the root mean square of successive differences (RMSSD)
- Frequency-domain analysis of HRV involves spectral analysis of the R-R intervals, separating HRV into low-frequency (LF) and high-frequency (HF) components
- LF component (0.04-0.15 Hz) is influenced by both sympathetic and parasympathetic activity
- HF component (0.15-0.4 Hz) primarily reflects parasympathetic activity and is associated with respiratory sinus arrhythmia
Key Terms to Review (19)
Action Potential: An action potential is a rapid and temporary change in the electrical charge across a cell membrane, primarily occurring in neurons and muscle cells. This event is crucial for the transmission of signals within the nervous system and the contraction of muscles, playing a vital role in bioelectric signaling. The generation and propagation of action potentials are essential for understanding how electrical impulses are used in various physiological functions, including the electrocardiographic signals recorded in the heart.
Amplitude: Amplitude refers to the maximum extent of a vibration or oscillation, measured from the position of equilibrium. In the context of biomedical signals, it is an essential characteristic that indicates the strength or intensity of the signal, affecting how data is interpreted and processed. A higher amplitude in a biomedical signal generally represents a more significant physiological event or change, making it crucial for analyzing various biomedical phenomena.
Arrhythmia Detection: Arrhythmia detection refers to the identification and analysis of irregular heartbeats through various methods, particularly using electrocardiograms (ECGs). This process involves the interpretation of electrical signals generated by the heart, allowing for the recognition of abnormal rhythms that could indicate underlying health issues. By utilizing advanced instrumentation and signal processing techniques, healthcare professionals can effectively monitor cardiac health and make informed decisions about treatment options.
Atrial Fibrillation: Atrial fibrillation is a common cardiac arrhythmia characterized by rapid and irregular beating of the atria, leading to inefficient blood flow and increased risk of stroke. This condition results from chaotic electrical signals in the atria, which disrupt the normal rhythmic contractions of the heart, often causing palpitations, fatigue, and other symptoms. Understanding atrial fibrillation is crucial for interpreting ECGs accurately, as its presence alters the typical waveform patterns observed in electrocardiography.
Cardiac monitoring: Cardiac monitoring refers to the continuous observation and assessment of the electrical and mechanical activity of the heart, primarily through the use of specialized equipment that can detect and display heart rhythms. This process is crucial in identifying any abnormalities or changes in heart function, allowing for timely medical intervention. Cardiac monitoring plays a key role in various clinical settings, particularly during surgeries, in intensive care units, and for patients with known heart conditions.
ECG Leads: ECG leads are the electrical connections between the body and the electrocardiogram (ECG) machine, which record the heart's electrical activity from various angles. Each lead provides unique information about different parts of the heart, allowing healthcare professionals to diagnose and monitor cardiac conditions. Understanding ECG leads is crucial for interpreting heart rhythms and detecting abnormalities that may indicate underlying heart disease.
Electrode placement: Electrode placement refers to the specific positioning of electrodes on the skin or body to ensure optimal signal acquisition or stimulation during medical procedures. Proper electrode placement is crucial as it directly influences the effectiveness and accuracy of electrotherapy and electrocardiography, impacting patient outcomes and diagnostic accuracy.
Electrolyte imbalances: Electrolyte imbalances refer to the abnormal levels of electrolytes in the body, which are essential minerals that carry an electric charge and play a critical role in various physiological functions. These imbalances can affect the heart's electrical activity, influencing the results of an electrocardiogram (ECG) and leading to potential cardiac issues. Proper electrolyte levels are vital for maintaining normal heart rhythm and muscle function, making understanding these imbalances crucial for assessing cardiovascular health.
Frequency response: Frequency response refers to the measure of a system's output spectrum in response to an input signal of varying frequencies. It essentially shows how different frequencies are amplified or attenuated by a system, helping to understand its behavior in processing signals. This concept is crucial when analyzing how systems filter signals and how they perform in various applications, such as in signal processing and medical diagnostics.
Heart rate: Heart rate refers to the number of times the heart beats in one minute, typically measured in beats per minute (bpm). It is a crucial physiological parameter that reflects the overall health of the cardiovascular system and can indicate the body's response to various physiological states such as exercise, stress, or rest. Monitoring heart rate plays a significant role in assessing cardiac function and is a vital component of biomedical instrumentation and electrocardiography.
Holter monitor: A Holter monitor is a portable electrocardiography device that continuously records the heart's electrical activity for an extended period, typically 24 to 48 hours. It helps in diagnosing irregular heartbeats, assessing the effectiveness of medications, and understanding the heart's behavior during daily activities. By providing a detailed account of heart rhythms over time, it offers valuable insights that standard ECGs might miss.
Myocardial depolarization: Myocardial depolarization refers to the process by which heart muscle cells (myocytes) become less negatively charged, leading to the initiation of an electrical impulse that triggers heart contractions. This process is essential for coordinating the heartbeat and involves the movement of ions, particularly sodium and calcium, across cell membranes, affecting how the heart pumps blood effectively.
P wave: The P wave is the first deflection seen in an electrocardiogram (ECG), representing atrial depolarization. This electrical activity occurs when the sinoatrial node triggers the heart's atria to contract, pushing blood into the ventricles. The P wave is crucial for understanding heart rhythms and diagnosing potential issues in cardiac function, as it provides insights into atrial health and electrical conduction pathways.
Paul Zoll: Paul Zoll was an American cardiologist known for his pioneering work in electrocardiography and the development of cardiac pacing. His contributions significantly advanced the understanding and treatment of heart rhythm disorders, influencing modern cardiology practices through the introduction of the first practical external cardiac pacemaker.
QRS Complex: The QRS complex is a key feature of an electrocardiogram (ECG) representing the rapid depolarization of the ventricles, which leads to ventricular contraction. This component is essential for understanding the heart's electrical activity, as it indicates how well the ventricles are functioning and how they are responding to electrical signals from the atria. The morphology, duration, and amplitude of the QRS complex can provide insights into various cardiac conditions and help guide diagnostic decisions.
Stress Testing: Stress testing is a method used to evaluate the cardiovascular system's response to increased physical exertion, typically through exercise or pharmacological means. This process helps identify potential heart issues by monitoring changes in heart rate, blood pressure, and ECG signals during stress. The information gathered from stress testing can aid in diagnosing conditions like coronary artery disease, assessing exercise capacity, and determining appropriate treatments.
T Wave: The T wave represents the repolarization of the ventricles in the heart during the cardiac cycle, appearing as a distinct upward deflection on an electrocardiogram (ECG). It plays a crucial role in the ECG waveform, providing important information about the heart's electrical activity and helping to identify various cardiac conditions.
Ventricular tachycardia: Ventricular tachycardia (VT) is a type of fast heart rhythm that originates from the ventricles, causing them to beat rapidly, usually at a rate of more than 100 beats per minute. This condition can lead to insufficient blood flow to the body and may progress to more severe arrhythmias, such as ventricular fibrillation. VT is critical in the interpretation of electrocardiograms (ECGs) because it significantly affects the heart's electrical activity and overall functionality.
Willem Einthoven: Willem Einthoven was a Dutch physician and physiologist known for his pioneering work in electrocardiography, particularly the invention of the electrocardiogram (ECG). His contributions revolutionized the way heart activity is monitored, leading to better diagnosis and treatment of cardiovascular conditions. Einthoven's work laid the groundwork for understanding electrical activity in the heart, and his innovations in this field have had a lasting impact on cardiology and medical instrumentation.