077 - EKGs - EKG Basics

The electrocardiogram (EKG or ECG) is a test that detects and visually represents the electrical activity of the heart over a period of time. It records the heart's rhythm, the size of the heart chambers, and the presence of any damage to the heart muscle. The EKG plots electrical activity (y-axis) versus time (x-axis).

1. Normal Electrical Conduction Pathway

Under normal conditions, electricity moves through the heart in a specific pathway:

  • SA Node (Sinoatrial Node): Located on the right atrial free wall, it is the heart's natural pacemaker. It depolarizes and sends a wave of depolarization.

  • Atrial Myocytes: The wave spreads to myocytes in the left and right atria, causing atrial depolarization.

  • AV Node (Atrioventricular Node): Depolarization funnels through this node.

  • His Bundle (Bundle of His): Electricity passes from the AV node to the His bundle.

  • Bundle Branches: Continues to the left and right bundle branches.

  • Purkinje Fibers: Finally reaches the Purkinje fibers, embedded in the myocytes of the left and right ventricles. When Purkinje fibers depolarize, they lead to ventricular depolarization.

2. EKG Waveforms and Intervals

Electrical events in the heart generate either a positive (upward) or negative (downward) deflection on the EKG.

2.1 EKG Waves

  • P Wave: Represents atrial depolarization. It is the first event in the cardiac cycle.

  • QRS Complex: Represents ventricular depolarization. It is the largest section on the EKG.

    • Can include:

      • Q wave: Negative deflection.

      • R wave: Positive deflection.

      • S wave: Another negative deflection.

  • T Wave: Represents repolarization of the ventricle.

  • Atrial Repolarization: Not visible because it occurs at the same time as the QRS complex and is embedded within it.

  • U Wave: A deflection that comes after the T wave. Its origin is unclear, potentially representing repolarization of Purkinje fibers. Can be normal but prominent in hypokalemia.

2.2 EKG Intervals

  • PR Interval: Time from the start of the P wave to the start of the QRS complex.

    • Normal: 120-200 ms (less than one big box).

    • Prolonged: >200 ms, e.g., First-degree AV block.

    • Short: E.g., Wolff-Parkinson-White (WPW) syndrome due to a bypass tract.

  • QRS Interval: Time from the start to the end of the QRS complex.

    • Normal: <120 ms (less than three small boxes).

    • Prolonged: E.g., Bundle branch blocks.

  • QT Interval: Time from the start of the QRS complex to the end of the T wave. Varies with heart rate, requiring correction for accurate assessment.

    • Rule of Thumb: The T wave should end before you are halfway between two QRS complexes.

    • Short QT: Relatively few causes, but importantly: hypercalcemia.

      • Calcium is responsible for Phase 2 (plateau phase) of the myocyte action potential. Less calcium drives force means this phase is drawn out, shortening the QT interval.

    • Prolonged QT: More common causes include:

      • Hypocalcemia (opposite of short QT causes).

      • Certain drugs (e.g., antiarrhythmics, levofloxacin, Haldol).

      • Long QT Syndrome.

2.3 Long QT Syndrome

  • Torsades de Pointes: A special form of ventricular tachycardia (twisting of the points) that is a feared outcome of QT prolongation, leading to cardiac arrest. Can also be caused by hypokalemia and hypomagnesemia (not related to QT prolongation in these cases).

  • Congenital Long QT Syndrome: A rare genetic disorder caused by abnormal potassium and sodium channels in the heart, leading to a prolonged QT interval. Often presented with a family history of sudden death or recurrent seizures (syncopal episodes).

    • Dravet and Lang Nielsen Syndrome: A variant associated with congenital deafness.

  • Acquired Long QT Syndrome: A patient acquires a prolonged QT interval due to drug intake. Common culprits include many antiarrhythmic drugs, levofloxacin (fluoroquinolone antibiotic), and haloperidol (antipsychotic). Patients with congenital long QT syndrome must avoid these drugs.

3. EKG Leads: Vantage Points

A standard EKG has 12 leads, each viewing the heart's electrical activity (summation vector of forces) from a different vantage point.

  • Deflection Rule: Electrical activity moving towards a lead generates a positive (upward) deflection. Electrical activity moving away from a lead generates a negative (downward) deflection.

  • Lead Placement and View:

    • Lead AVR: Views from the right side of the heart; typically shows a downward (negative) QRS complex as electricity moves away.

    • Leads I and AVL: View from the left side of the heart; typically show an upward (positive) QRS complex as electricity moves towards them.

    • Leads II, III, and AVF: View from the inferior portion of the heart.

    • Leads V1-V6: View across the heart, with V1 towards the right and V6 towards the left. V3 is often biphasic (partly upward, partly downward) as it is in the middle.

4. Cardiac Pacemakers

  • SA Node: The dominant pacemaker (normal heart rate: 60-100 bpm).

  • AV Node: Can take over if the SA node fails (rate: 40-60 bpm).

  • His Bundle, Bundle Branches, Purkinje Fibers: Can also act as pacemakers, but at slower rates (approx. 25-40 bpm).

5. Conduction Velocity

  • Slowest: In the AV node, which allows ventricles more time to fill with blood, optimizing the heart's pumping efficiency.

  • Fastest: In the Purkinje fibers.

  • Intermediate: In atrial and ventricular myocytes.

6. Determining Heart Rate from EKG

Heart rate is determined by the time between two QRS complexes (each representing a heartbeat).

  • Box Measurement:

    • Small box: 40 milliseconds (ms).

    • Big box: 200 ms (five small boxes).

  • Quick Method: Identify a QRS complex on a dark line. Count the number of big boxes until the next QRS complex:

    • 1 big box = 300 bpm

    • 2 big boxes = 150 bpm

    • 3 big boxes = 100 bpm

    • Formula: Heart Rate = 300 / ( ext{Number of big boxes between QRS complexes})

    • A normal heart rate (60-100 bpm) usually corresponds to 3 to 5 big boxes between QRS complexes.

7. QRS Axis Determination

The QRS axis represents the summation vector of all electrical forces moving through the heart during ventricular depolarization.

  • Normal QRS Axis: Between -30^ extrm{o} and +90^ extrm{o} (usually in the bottom right or slightly into the top left quadrant of the cardiac axis grid).

  • Left Axis Deviation (LAD): Between -30^ extrm{o} and -90^ extrm{o}. Causes include Left Bundle Branch Block (LBBB) or ventricular rhythms like ventricular tachycardia.

  • Right Axis Deviation (RAD): Between +90^ extrm{o} and +180^ extrm{o}. Causes include Right Bundle Branch Block (RBBB) or right ventricular hypertrophy.

  • Quick Method for Axis Determination:

    1. Check Lead AVR: Should always be negative. If upright, suspect limb lead reversal.

    2. Check Leads I and II:

      • If both are positive, the axis is normal (0^ extrm{o} to +90^ extrm{o}).

    3. If not both positive:

      • Left Axis Deviation: Look at Lead II. If Lead II is negative (or partly upward/downward, which can be physiologic left axis).

      • Right Axis Deviation: Look at Lead I. If Lead I is negative.