Questions and Answers
What is the normal composition of an ECG?
What causes the P wave in an ECG?
During what phase does depolarization occur in cardiac cells?
What is the T wave in an ECG also known as?
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What happens to cardiac cells during depolarization?
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What causes repolarization in cardiac cells?
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What is the primary function of the SA node in the heart's conduction system?
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Which type of lead in an ECG uses two electrodes to measure differences in electrical potential?
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What does the lead aVF in augmented unipolar limb leads primarily measure?
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Which structure of the heart does the impulse reach after the AV node?
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What is the primary purpose of modifying the augmented unipolar limb leads?
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In the conduction system, where does the impulse travel after the intraatrial tracts?
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Where is lead V1 placed?
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What area does lead V5 represent in the heart?
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Which leads are located in the anterior portion of the heart?
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What is the placement location for lead V4?
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Which lead is positioned between V2 and V4?
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What does the P wave in the ECG represent?
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How many small squares correspond to 1 mV in amplitude on the ECG paper?
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What is the normal duration of the PR interval in seconds?
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What does the QRS wave represent in an ECG?
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In an ECG, what is the significance of the ST segment?
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What is the standard speed of the ECG paper?
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What direction does the vector for ventricular depolarization typically go?
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What is the maximum duration of the P wave in an ECG?
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What does the ST segment measure in the cardiac cycle?
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During which phase does systole occur in the cardiac cycle?
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Which of the following correctly describes diastole?
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What does the T wave represent in the cardiac cycle?
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What occurs first in the cardiac cycle as indicated by the P wave?
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Study Notes
Electrocardiography (ECG)
- Records the electrical activity of the heart.
- Normal ECG consists of three main components: P wave, QRS complex, and T wave.
- P wave represents atrial depolarization preceding atrial contraction.
- QRS complex indicates ventricular depolarization preceding ventricular contraction.
- Both P wave and QRS complex are classified as depolarization waves.
- T wave (repolarization wave) reflects the recovery phase of the ventricles after depolarization.
Cellular Electrophysiology
- Cardiomyocytes, or cardiac cells, are electrically polarized during the resting state.
- Depolarization, also known as activation, is a primary electrical event in the heart where cells lose internal negativity.
- Phase 0 of action potential occurs when sodium (Na+) ions enter the cell, making it more positive and propagating action potential to neighboring cardiomyocytes.
- Repolarization occurs as potassium (K+) ions move out of the cell, restoring the cell's negative charge.
- Overall, during depolarization, the cell experiences a shift toward a more positive interior.
Conduction System of the Heart
- Impulse generation begins at the SA node, also known as the sinus node.
- Impulse travels to the atria through intraatrial tracts, causing a brief atrial contraction.
- After the atria, the impulse moves through internodal pathways to the AV node.
- From the AV node, the signal travels down the Bundle of His and branches into the right and left bundle branches in the ventricles.
- Conduction continues to the Purkinje fibers, stimulating contraction of the ventricles.
ECG Leads
- ECG leads measure the magnitude and direction of the heart's electrical impulses.
- Three types of leads: Limb leads, Augmented limb leads, and Precordial leads.
Limb Leads
- Bipolar leads consist of 2 electrodes at different sites, measuring the voltage difference between them.
- Unipolar leads measure the electrical potential at one site compared to a reference electrode at zero potential.
- Color coding for limb electrodes:
- Right arm: Red
- Left arm: Yellow
- Left foot: Blue/Green
- Right foot: Black
Augmented Unipolar Limb Leads
- Lead aVR: Records potential between the right arm and the average potential of the left arm and left foot.
- Lead aVL: Measures potential in the left arm relative to the average potential of the right arm and left leg.
- Lead aVF: Assesses potential in the left foot with reference to the average of the left arm and right arm.
- Augmentation of leads increases potential recording by 50% for improved clarity in ECG interpretation.
Precordial Lead Placements
- Precordial leads, commonly used in adults, include V1 through V6.
- V1 is located in the fourth intercostal space at the right parasternal area.
- V2 is positioned in the fourth intercostal space at the left parasternal area.
- V3 is placed between V2 and V4, completing the anterior view.
- V4 is found in the inframammary area, aligning with the heart's anterior aspect.
- V5 is situated in the midclavicular area at the same level as V4.
- V6 is positioned in the anterior axillary line, aligned with V4 and V5.
- These lead placements focus on the anterior portion of the heart.
- V1 and V2 specifically monitor the septal area of the left ventricle.
- V3 and V4 assess the anterior area of the heart.
- V5 and V6 are associated with the lateral area of the heart.
ECG Paper
- ECG paper consists of horizontal and vertical lines.
- Horizontal lines indicate duration in seconds or milliseconds.
- Vertical lines measure amplitude in millivolts.
- Standard speed for ECG tracing is 25 mm/sec.
- Standard amplitude is set at 10 mm/mV.
- The smallest square on ECG paper represents 1 mV (amplitude) and 0.04 seconds (duration).
- Five small squares equal 0.2 seconds, while 25 small squares equal 1 second.
- One millivolt corresponds to approximately 10 small squares vertically.
Waves, Segments, and Intervals
- The PQRST complex represents one complete cardiac cycle.
- The P wave indicates atrial depolarization, occurring before contraction.
- The QRS wave indicates ventricular depolarization just prior to ventricular contraction.
- The Q wave is the first negative deflection after the P wave.
- The R wave is the first positive reflection following the Q wave.
- The S wave is the first negative reflection occurring after the R wave.
- The T wave represents the process of ventricular repolarization.
- The P wave should last no longer than 0.12 seconds and reach a maximum amplitude of 0.25 mV (2.5 small squares).
- The PR interval measures the duration from the start of atrial depolarization to the onset of ventricular depolarization (0.12 – 0.20 seconds).
- The QRS interval measures the duration of ventricular depolarization (0.08 – 0.10 seconds).
- The ST segment reflects the time from the end of ventricular depolarization to the beginning of ventricular repolarization.
- The T wave records the process of ventricular repolarization.
- The vector direction for ventricular depolarization (and septal depolarization) is from left to right.
- Ventricular depolarization predominantly occurs in the left ventricle, with a leftward and inferior direction.
Cardiac Cycle Components
- P wave: Indicates atrial depolarization, which occurs before the atrium fills with blood.
- QRS complex: Represents ventricular depolarization and occurs before the contraction of the ventricles.
- T wave: Reflects ventricular repolarization, marking the end of the cardiac cycle.
ST Segment
- Measures time from the end of ventricular depolarization to the beginning of ventricular repolarization.
- Crucial for diagnosing heart attacks or myocardial infarction.
Phases of the Cardiac Cycle
- Systole: Starts at the QRS complex and lasts until the later half of the P wave, representing ventricular contraction.
- Diastole: Extends from the T wave up to the start of the QRS complex, indicating the relaxation phase of the heart.
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Description
This quiz covers the fundamental concepts of electrocardiography, including the recording of electrical activity in the heart. Key components such as the P wave, QRS complex, and T wave will be explored, along with their roles in depolarization and repolarization. Test your knowledge on how these waves contribute to heart function.
