Electrocardiogram Study Guide

Leading Cause of Death:
- Heart disease is the foremost cause of death in the United States.
- Major contributor to disability.
Economic Impact:
- Estimated cost of coronary heart disease:
- 2007: $151.6 billion (direct and indirect costs).
- 2010: $108.9 billion.
Mortality Statistics:
- Heart disease accounts for 25% of all deaths in the U.S.
- Approximately 55-60% of these deaths are linked to coronary artery disease.

Definition:
- An electrocardiogram (EKG) is a graphic representation of the heart's electrical activity.
- Provides a detailed record of cardiac electrical activity, which includes insights into the heart's movement, function, and structure.
Uses:
- Widely employed for assessment and diagnosis of heart problems.

Cardiac Generator: The source of electrical impulses in the heart.
Cardiac Electrical Field: Represents the distribution of electrical activity across the heart.
Activation Sequence: Refers to how electrical signals activate the heart muscle.
Body Surface Potentials: Electrical signals detected at the body surface.
Transmission Factors: How signals are transmitted from the heart to the skin.
Visual Display: Output representation of the ECG via screens or paper.
Instrumentation:
- Leads: Various electrodes placed on the body to record heart activity, including standard limb leads (I, II, III) and augmented leads (aVR, aVL, aVF).
- Amplifiers: Systems that enhance signal strength.
- Computer Processors: Analyze and interpret data.
- Data Storage: Archives ECG recordings for future reference.
- ECG Interpretation: Methodology for analyzing ECG results.

Axis:
- Represents the average electrical signal of the heart, indicating its relative position within the chest cavity.
Rhythm:
- Determines if the heart rhythm is regular or irregular, including the presence of ectopic beats, their form, and origination within the heart.
Rate:
- Identification of heart rate conditions:
- Normal Sinus Rhythm.
- Bradycardia (abnormally slow heart rate).
- Tachycardia (abnormally fast heart rate).
- Presence of any heart block.
Enlargement:
- Detection of atrial or ventricular hypertrophy (enlargement of heart chambers).
Ischemia, Injury, and Infarct:
- Identification of tissue damage or inadequate blood supply to heart muscle.

Start of Action Potential:
- Generated at the Sinoatrial (SA) Node: Initiates atrial activation.
- Stimulus spreads across the atrial surface via cell-to-cell contact, reaching the Atrioventricular (AV) Node.
AV Node Delay:
- 100-msec Delay occurs at the AV node allowing for complete atrial contraction before ventricular stimulation.
Impulse Propagation:
- After the AV node, impulse travels through the AV Bundle, along Bundle Branches to Purkinje Fibers.
- This leads to contraction of the ventricles as the impulse is distributed throughout the ventricular myocardium.
Ventricular Contraction:
- Initiates shortly after impulse distribution, leading to effective heart function.

P Wave:
- Represents the beginning of atrial contraction when the SA node fires.
QRS Complex:
- Composed of three waves representing the excitation of ventricles.
T Wave:
- Indicates repolarization of the ventricles, returning to resting state.
Sequential Activation Phases:
- Atrial excitation and contraction occur, followed by a delay at the AV node; completion of ventricular excitation and relaxation.

Post ventricular contraction, there is a transition to relaxation (dilation) to prepare for the next cycle, depicted by the T-wave in the EKG.
U Wave:
- A small positive wave may follow the T-wave, often associated with repolarization of the papillary muscles or Purkinje fibers.
- Prominent or inverted U waves indicate conditions such as hypokalemia, hypercalcemia, thyrotoxicosis, or reactions to digitalis and epinephrine treatment.

Limb Leads:
- Lead I: Measures potential difference between right arm and left arm.
- Lead II: Measures between right arm and left leg.
- Lead III: Measures between left arm and left leg.
Augmented Leads:
- aVR: Potential difference between heart and right arm.
- aVL: Heart versus left arm.
- aVF: Heart versus left leg/foot.
Precordial Leads:
- Leads V1 to V6 positioned across the chest to give a comprehensive view of the heart's electrical activity.

Limb Lead Locations:
- Right Arm (RA), Left Arm (LA), Right Leg (RL), Left Leg (LL).
Precordial Lead Locations:
1. V1: 4th intercostal space at right sternal border.
2. V2: 4th intercostal space at left sternal border.
3. V3: Midway between V2 and V4.
4. V4: 5th intercostal space in the left midclavicular line.
5. V5: Anterior axillary line in the 5th intercostal space.
6. V6: Mid axillary line in the 5th intercostal space.

General Measurement:
- 10 mm is equivalent to 1 mV in ECG readings.
Wave Durations:
- P wave: 0.08 - 0.10 seconds.
- QRS complex: 0.06 - 0.10 seconds.
- P-R interval: 0.12 - 0.20 seconds.
- QT interval duration should be below 0.44 seconds, with specific adjustments for heart rate using the formula: $QT_1 = rac{QT}{ ext{sqrt}(RR)}$
Complex & Interval Relationships:
- PR Interval: Duration from the beginning of the P wave to the start of the QRS complex, indicating the AV node and conduction system functionality; impaired conduction correlates with prolonged intervals.
- QRS Complex: Indicates ventricular depolarization duration. Norm ranges: 0.04 sec to 0.12 sec.
- Isoelectric Line: Represents the baseline voltage of the ECG, indicating periods of no electrical activity.
- J-Point: Connection point between QRS and ST wave.
- ST Segment: Usually isoelectric; ends ventricular depolarization and starts repolarization; deviations may indicate ischemia or hyperkalemia.
- QT Interval: Extends from the onset of QRS to end of T wave; reflects time for ventricular depolarization to repolarization, which may be influenced by drugs, electrolyte balance, and ischemia.

Explanation of Wave Polarities:
- Wave directionality depends on the electrical stimulation's pathway: moving from positive electrode to negative generates a negative wave, while the reverse creates a positive wave.