Electrocardiography (ECG/EKG) Basics

• Electrocardiography is the registration of electrical events in the heart using recording electrodes placed on the skin's surface.
• It represents the sum of action potentials that occur simultaneously in many individual cells.
• ECGs or EKGs help identify cardiac rhythm disturbances such as atrial fibrillation.
• They help detect inadequate coronary artery blood flow as seen in myocardial infarction.
• ECGs are useful for heart disease and pre-operative screening.

Basic Description of ECG

• The P wave reflects atrial depolarization
• The QRS complex corresponds to ventricular depolarization.
• The T wave represents ventricular repolarization.

Intervals

• The PQ (PR) interval measures from the start of the P wave to the beginning of the Q wave, indicating AV conduction time which is normally 0.12-0.2 seconds.
• The QRS interval measures from the start of the Q wave to the end of the S wave and represents the duration of ventricular depolarization, which is normally 0.08-0.11 seconds.
• The QT interval measures from the beginning of the Q wave to the end of the T wave, indicating the electrical systole of the heart, approximately 0.4 seconds.

Segments

• Segments are isoelectric lines between two successive waves.
• The PQ (PR) segment measures from the end of the P wave to the beginning of the Q wave, during which the atria are depolarized and depolarization spreads over the bundle of His, bundle branches, and Purkinje network.
• The ST segment measures from the end of the S wave to the beginning of the T wave, during which the ventricles are depolarized and in the plateau phase of the action potential.

Intracellular and Extracellular Recordings

• Intracellular and extracellular recordings demonstrate the electrical activity of individual cells versus a summated representation.

Working Cardiomyocyte Action Potential

• Working cardiomyocyte action potentials display an ionic basis for ventricular function.

ECG as Summed Activity

• The ECG is the summed action potential recorded extracellularly, reflecting the overall electrical activity of the heart.
• Cardiac excitation and ECG components relate to the action potentials in the SA node, atria, AV node, Purkinje fibers, and ventricles.

Propagation of Electrical Activity

• The P wave correlates with atrial depolarization.
• The QRS complex correlates with ventricular depolarization.
• The T wave correlates with ventricular repolarization.

ECG Registration

• ECG registration uses electrodes that can be active (exploring) or indifferent (reference, zero).
• Leads refer to a combination of two electrodes.
• Lead types include bipolar (two active electrodes), unipolar (one active and one indifferent electrode), as well as pseudounipolar leads.

Bipolar Leads

• Bipolar leads have positive and negative electrodes.
• The lead axis is an imaginary connection between both electrodes, and depolarization toward the positive electrode yields a positive deflection on the ECG.
• The magnitude of the wave depends on the projection of the cardiac electric field onto the lead axis.

Unipolar Leads

• Unipolar leads use an active electrode, considered the positive pole, and an indifferent electrode, considered the negative pole.
• The axis of the lead connects both electrodes and depolarization heading towards the active electrode results in a positive deflection on the ECG.
• The magnitude of the wave depends on the projection of the cardiac electric field onto the lead axis.

Depolarization and Repolarization

• Depolarization toward a positive electrode produces a positive deflection.
• Depolarization away from a positive electrode produces a negative deflection.
• Repolarization toward a positive electrode produces a negative deflection.
• Repolarization away from a positive electrode produces a positive deflection.
• Depolarization and repolarization have the same direction in the atria, but opposite directions in the ventricles.

Standard ECG

• A standard ECG uses three bipolar limb leads which includes leads I, II and III
• Three unipolar augmented limb leads, including aVR, aVL, and aVF.
• There are also six unipolar chest leads as well as V1 to V6.
• A grounding electrode is placed on the right leg.
• In the equipment, a calibration signal is set, so 1 mV = 10 mm.
• The registration speed is 25 mm/s such that:
  • One small 1 mm x 1 mm block represents 40 ms time and 0.1 mV amplitude.
  • One large 5 mm x 5 mm box represents 0.2 seconds (200 ms) time and 0.5 mV amplitude.

Bipolar Limb Placement

• Standard limb leads (I, II, III) are bipolar. --Right wrist is Red -- Left wrist is Yellow -- Left leg is green
• These leads form Einthoven's triangle.

Augmented Limb leads

• Standard ECGs use augmented limb leads, namely aVR, aVL, aVF.

Chest Leads (V1-V6)

• Chest leads (V1 - V6) positions are:
• • V1: the 4th right intercostal space near the sternum.
• • V2: the 4th left intercostal space near the sternum.
• • V3: midway between V2 and V4.
• • V4: the 5th left intercostal space in the mid-clavicular line.
• • V5: the 5th left intercostal space in the anterior axillary line.
• • V6: the 5th left intercostal space in the mid-axillary line.

Electrical Heart Axis

• A normal electrical axis falls between -30° to +90°
• The electrical axis is determined by:
  • the heart's position
  • ventricular muscle mass
  • propagation of depolarization.
• Axis deviation in lead I and lead III can indicate issues.
  • A normal is positive in lead I and positive in lead III.
    • Left axis deviation = Positive in lead I and negative in lead III
    • Right axis deviation = negative in lead I and positive in lead III

ECG Analysis

• Analysis occurs via : - Heartbeat (regular) regular, irregular - Heart rate (60 – 80/min) - Rhythm (sinus) - Electrical axis (-30 to +90°) - Description of waves, intervals and segments on ECG.
• Normal sinus rhythm has a visible P wave in front of the QRS complex and has a normal heart rate
• Third-degree atrioventricular (AV) block results from a defect in the cardiac conduction system which leads to complete dissociation of the atria and ventricles.

Waves and description of their segments

• atrial depolarization and is positive in most leads but can be positive or negative in lead III and usually biphasic in lead V₁.
  • Usually negative in lead aVR and lasts approximately 0.1 s.

PQ Interval

• The duration of PQ atrial one is normally 0.12 – 0.2 s long and is inversely proportional to the heart rate

QRS complex

• Produced by the ventricular depolarization with normal duration of 0.06 - 0.11 s

• R wave always positive

• Q and S waves always negative

• Q wave precedes R wave whereas S wave follows R wave

• The sequence of the ventricular depolarization is as follows: interventricular septum (from the left to the right) – cardiac apex - free ventricular walls – atrioventricular groove; from endocardium to epicardium

• Three waves are visible Q, R and S

Q wave

• duration should be less than 0.03 s
• amplitude maximum 1/4 of R wave amplitude (in the same lead).
• parameters outside these ranges - pathological Q

R wave

• produced by free ventricular walls depolarization
• time interval from the onset of Q wave to the peak of R wave - ventricular activation time - depolarization of the whole ventricular walls.
  • measured in V₁ - the activation of the right ventricle (normally up to 0.03 s) -- measured in V6 - the left ventricular activation (up to 0.04 s).

ST segment

• should be at the same level as the beginning of Q wave (i.e. on the isoelectric line).
• ST segment above the baseline - ST elevation (tolerated 0.1 mV in the limb leads; 0.2 mV in the chest leads).
• shift of ST segment below the baseline - ST depression (tolerated 0.1 mV).

T wave

• polarity of T wave corresponds to the polarity of the QRS
• negative T wave - in leads aVR, III, V₁ and V2

U wave

• small positive wave sometimes visible after T wave
• young people and sportsmen
• hypokalemia
• probably results from the delayed repolarization of Purkyne fibers or some areas of the ventricular myocardium.

Ions and their relationship to ECG

• Hyperkalemia slows down depolarization and shortens repolarization.
  • PQ interval and QRS complex are longer.
  • accelerated repolarization - QT interval shortening,
  • T waves - tall and spiky
• Hypokalemia accelerates depolarization and slows down repolarization.
  • prolongs QT interval -- the flattened T waves are followed by the positive U wave

Calcium

• Hypercalcemia accelerates the ventricular repolarization
  • shortening of QT interval caused by shortening of ST segment

Hypocalcemia

• slows down the ventricular repolarization
  • QT interval is longer due to prolonged ST segment