Understanding Electrocardiograms (EKG/ECG)

EKG (Electrocardiogram)

• This is often abbreviated to EKG or ECG.

Treat or Not to Treat?

• A premature ventricular contraction is displayed

Objectives of Studying EKGs

• Review the basic principles and conduction system of the heart.
• Learn to identify the components of a rhythm strip.
• Establish the basics for reading a rhythm strip/EKG.
• Recognise Limb and Precordial/Chest Leads.
• Establish how leads provide different views of the heart.
• Learn calculating heart rate (HR) from an EKG or rhythm strip using the countdown/triplicate method, or the 6-second method.

EKG Basic Principles

• An EKG is a graphic representation of the depolarization of the heart and records the heart's electrical activity.
• Electrical current moving toward the positive electrode causes positive deflection on the EKG.
• Electrical current moving away from the positive electrode causes negative deflection.
• EKGs only measure electrical activity.

EKG Basic Applications

• EKGs area practical, time efficient, non-invasive test.
• EKGs allow determining: Rate, Rhythm, Axis, Hypertrophy, and Infarction (ischemia, injury, and other problems).

Physiologic Properties of Myocardial Cells

• Automaticity allows intrinsic ability to contract in the absence of external stimuli.
• Rhythmicity allows ability to contract in a rhythmic manner.
• Conductivity allows ability to transmit impulses.
• Excitability allows ability to respond to an impulse.
• Contractility allows ability to respond by pumping.

Resting/Polarized State

• The myocardium has a negative charge on the inside of the cell membrane (K+).

Depolarization of Myocardium

• Myocytes (inside of myocardial cells) become positive and contract with movement via wave throughout the myocardium.
• Positive charge is carried by fast moving Na+ ions.

Repolarization

• Myocardial cells return to a resting negative charge inside.

Myocardial Cell Depolarization and Repolarization

• This depolarization and repolarization induces myocardial contraction (Systole) and relaxation (Diastole).

Positive Deflection

• This shows a wave of depolarization moves toward a (+) electrode.

Negative Deflection

• This shows a wave of depolarization moves away from (+) electrode.

Isoelectric Deflection

• This shows a wave of depolarization moves perpendicular to the (+) electrode.

Cardiac Nervous System Review: Sympathetic System

• Supplies both the atria and ventricles of the heart.
• Norepinephrine is the mediator.
• Increases the rate of the SA node.
• Increases the rate of AV conduction.
• Increases excitability.
• Increases the force of contraction.

Cardiac Nervous System Review: Parasympathetic (Vagus Nerve)

• Primarily supplies the atria.
• Acetylcholine is the mediator.
• Decreases the rate of the SA node.
• Decreases the rate of AV conduction.
• Decreases excitability.

Cardiac Conduction System Composition

• This cardiac system is composed of the following elements: the Sino-Atrial (SA) node, Atrio-ventricular (AV) node, Bundle of His, Left and Right Bundle Branches, and Purkinjé Fibers.
• This system is positive in action.

SA Node Function

• Functions as the heart's dominant pacemaker.
• Pacing activity corresponds to Sinus Rhythm, meaning the SA node is in charge.
• A depolarization wave emitted spreads throughout both atria producing a P wave on the EKG.
• During atrial contraction, blood is forced through the atrioventricular valves (Tricuspid valve on the right and Mitral valve on the left).

AV Node Function

• AV node serves as the only conduction path between the atria and the ventricles.
• Depolarization slows within the AV node before impulses are conducted to the ventricles, which allows blood to finish passing from atria to ventricles.
• This is represented by a short, flat baseline after each P wave of the EKG also known as the PR segment.

Ventricular Conduction System

• Depolarization slows through AV node which then conducts rapidly through the Bundle of His to Right and Left Bundle Branches.
• Purkinje Fibers have “terminal filaments” that carry depolarization of the myocytes of the ventricles causing them to contract.
• The action of the Purkinje Fibers is represented by the QRS complex on EKG.

QRS Complex

• Q wave = the first downward wave of the QRS complex, may or may not be present.
• R wave = the first upward wave of the QRS complex.
• S wave = Any downward wave PRECEDED by an upward wave.

Ventricular Repolarization

• Initial plateau after QRS complex = ST segment which represents the initial phase of ventricular repolarization.
• T wave = represents the final “rapid” phase of ventricular repolarization.
• Ventricular systole spans depolarization AND repolarization of the ventricles, beginning with the QRS and continuing until the end of the T wave.

Cardiac Conduction Review

• SA node to internodal pathways (to the left atrium via Bachman's bundle and to the AV node via anterior/middle/posterior internodal pathways).
• AV node to the Bundle of His, then to the Right and Left bundle branches.
• Impulses travel through Purkinje fibers (located in the ventricular walls) to myocardial cells.

EKG Electrodes

• Surface sensors are used to detect the heart's electrical activity.
• These can be either positive or negative.

12 Lead EKG

• This assesses the electrical activity of the heart.

Telemetry or Hardwire

• Telemetry can be used in acute care and ICU settings
• It is easy to maintain as it provides accurate data
• It displays 1 lead on a "RHYTHM STRIP"
• Typically a 12 lead EKG is used for more specific diagnostics.
• There are 5 colored wires: White, Green, Black, Red, and Brown.

EKG Wire Colours

• White clouds over Green grass
• Black smoke over Red fire
• Brown wire sits in the center of the chest as ground.

Useful Terms

• Automaticity foci refers to areas of the heart that have automaticity
• Waves are P, QRS complex, and T.
• Segments are Straight line connecting 2 waves, PR and ST.
• Intervals Encompass 1 wave + 1 line are PR, QRS, QT, RR, and ST.

EKG Impulses

• Each impulse represents a different stage of myocardial stimulation.

EKG: Waves

• P wave: Atrial depolarization
• QRS complex: Ventricular depolarization
• T wave: Follows ST segment. Ventricular repolarization occurs here and is complete at the end of the T wave.

EKG: Segments

• PR segment: Isoelectric line after P wave, and is also known as atrial kick.
• ST segment: Starts at the end of the QRS complex and ends at the T wave; Elevation/Depression of this segment helps to identify problems with the heart.

EKG: Intervals

• PR interval: Atrial depolarization
• ST interval: Full ventricular repolarization
• QT interval: duration of ventricular systole (and repolarization)
• RR interval: Used to calculate is the rate. Check to determine if it is regular.

EKG: Intrinsic Rates

• SA Node: Intrinsic rate of 60-100 bpm which normally controls the heart's rate and rhythm to produce Sinus Rhythm.
• If SA node is not working, the Atrial Automaticity Focus takes over with a rate of 60-80 bpm.
• AV Node (Junctional Foci): Intrinsic rate of 40-60 bpm, AV node becomes the pacemaker if SA node isn't working or is diseased/ in emergent situations.
• Ventricular Foci (Purkinje Fibers): Intrinsic rate of 20-40 bpm, Purkinje Fibers becomes the pacemaker if SA or AV node aren't working or are diseased/ in emergent situations.

Reading EKGs: Key Questions

• Is there a P wave for every QRS?
• Is the rhythm regular?
• What is the rate, and does it imply a Sinus Rhythm?

Reading EKGs: Possible Rhythms Based on Rate

• Bradycardia
• Normal
• Tachycardia

EKG Paper

• Paper speed: 25mm/sec
• The smallest divisions on paper are 1mm high and 1 mm wide.
• “Heavy” lines are 5 small squares each and equal 5mm.
• Standard paper has vertical marks on top or bottom edge of paper to mark 3 seconds of time.

EKG: Time (Horizontal Axis)

• 1 small box = 0.04 sec
• 1mm = 0.04 sec
• 1 large box = 0.20 sec
• 5mm = 0.20 sec
• 5 large boxes = 1 sec

EKG: Voltage (Vertical Axis)

• 1 small box = 0.10mV, 1mm = 0.10 mV
• A large box = 0.50 mV. 5mm = 0.50 mV
• Standard Calibration shows 10mm (1cm)= 1 mV

EKG: Voltages

• The millimeters of height/depth of waves AND elevation/depression of baseline segments is typically assessed.

EKG Complex: P wave

• This represents atrial depolarization-SA node function.
• Repolarization (of atria) occurs but the signal is lost in the QRS complex.
• Duration is between 0.08 and 0.10 sec.

EKG Complex: PR Interval

• Measured from the beginning of the P wave to the beginning of the QRS complex, this segment represents the time for the impulse to travel from the atria to the ventricles
• Represents the A-V node delay; also, it's typically .12 -0.20 seconds (less than 1 large square.
• Note this segment is usually isoelectric (PR segment).

EKG Complex: QRS Complex

• It represents ventricular depolarization
• Duration or width is < 0.12 seconds (less than 3 small squares)
• Normally 0.06 -0.10 seconds
• If the QRS complex duration is prolonged, conduction is impaired within the ventricles
• Q wave is usually not >0.03 sec or is usually <25% of the amplitude of the R wave.

ST segment

• An isoelectric portion that occurs at the point where the QRS joins.
• This segment segment originates at the J point and ends at the beginning of the T wave.
• It is based on elevation or depression, and it defines include ischemia, injury and infarction.

EKG Complex: T wave

• This represents the final, “rapid” phase of ventricular repolarization,
• The second half of the T wave is called the relative refractory period
• Typically described by its symmetry, skewedness, slope, and amplitude
• Wave inversion in certain leads (negative T waves) can be a sign of ischemia and more.

EKG Complex: QT interval

• This represents the total time of ventricular systole and is a good indicator of repolarization during systole.
• It Varies with heart rate.
• QT interval measurements are often corrected for rate and are called QTc values.
• Normal QT is when it is < ½ RR interval at normal rates (~0.42 sec).

EKG Complex: How to

• Draw 1-2 complexes
• Identify the waves: P, QRS and T.

EKG Leads

• Total of 12 leads
• 6 Limb leads that look at the heart in the frontal plane
• There are 3 Bipolar (use 2 electrodes to create the "lead"): Leads I,II, & III
• There are 3 Unipolar/Augmented: aVF, aVR, aVL with the letters reflecting where the “+” electrode is located (F = foot, R = right arm, L = left arm)
• Leads aVR, aVL and aVF look from three separate directions.
• Leads I, II and III are summation of potential differences between limb leads
• 6 Precordial leads assess the heart across the transverse plane in "in front of the heart" with labels V1-V6.

Limb Leads: Functions

• Depict high lateral wall of heart.
• Depict inferior heart surface.
• Einthoven's triangle defines the placement.

Limb Leads: More functions

• Lead aVR: Looks at nothing in the left ventricle.
• Lead aVL: Looks at high lateral wall of the heart.
• Lead aVF: Looks at the inferior wall of the heart.

Limb Leads: Hexaxial and Semicircle Reference Systems

• Einthoven's Triangle defines the formation of a triaxial system.
• Pushing allows all leads to the center so that a hexaxial reference system is revealed.

Limb Leads and AV Node

• Each limb lead records from a different angle, providing a different view of the same cardiac activity.
• This configuration depends on positive-negative charge positioning causing tracin differences in each lead and each location.
• A wave of depolarization is what causes these progressive positive charges through the myocardial cells.

Precordial Leads: Attributes

• Unipolar, with each lead showing a positive reaction.
• V1 through V6 view the horizontal plane of the heart.

Precordial Leads: Positions

• Lead V1 is positioned a the 4th intercostal space at the right sternal border.
• Lead V2 is positioned at the 4th intercostal space at the left sternal border.
• Lead V3 is positioned and an equal distance between V2 & V4.
• Lead V4 is positioned at the 5th intercostal space at the left midclavicular line.
• Lead V5 is positioned at the 5th intercostal space at the anterior axillary line.
• Lead V6 is positioned at the 5th intercostal space at the mid axillary line.

Observing Leads V1-6

• When observing leads V1-6, the waves change gradually as the position of the positive electrode changes.
• The QRS in V1 will be negative and will progress to positive by V6.

Cardiac Axis and Vectors

• Axis in the frontal plane relates to Limb leads and is referred to as AXIS DEVIATION.
• Axis in the horizontal plane relates to Precordial leads and is referred to as AXIS ROTATION.

Cardiac Vectors

• Reflects normal depolarization of both atria.
• It usually points downward to the left side of the patient.
• Provides the sum of all the smaller vectors of ventricular depolarization.
• Origin of mean QRS vector = AV Node
• Because depolarization vectors of left ventricle are larger than those of right ventricle, MEAN QRS VECTOR points down and toward the LEFT (between 0 - 90°)
• Varies in individuals by size and age.

Mean QRS Vector Descriptors

• This is described in degrees within a circle drawn in the frontal plane over the patient's chest with the limb leads allowing determination of the axis.
• Can assist with diagnosis of hypertrophy of the ticker muscle or in cases of infarction when not vecto intact.

Calculating the Rate

• In both situations, consider the 3 key factors to reading an EKG: are QRS complexes correlated to P waves? 2: Is there standard rhythmicity?
• Consider if a sinus rhythm correlates to bradycardia, a normal state or tachycardia.

Reading an EKG: Rhythmicity factors

• Find p waves for every EKG.
• Assess the distance between each R wave.

Reading an EKG: Rate

• May be calculated using the R waves.

Calculating the Rate by Counting Boxes: Method

• The method uses calculation based on 300-150-100-75-60-50 which is sometimes called "triplets" or triplicate method.
• X axis represents time where big boxes=0.2 sec, small boxes=0.04 sec and there are five small boxes for every one big box.
• There are 300 Big boxes in 1 minute, meaning 60/0.2=300.
• Start measuring from any point and assess deviations from expected normal distances.

Measuring Rate: Counting Boxes

• Each grouping of small boxes will represent different amounts.
• If between 300 and 150, each small box = 30.
• However if between 100 and 150, 50/5 equals 10.
• For between 100 and 75, each Box equals 5.

Rate: 6 Second Method

• Count the number of complete complexes found between two sets of 3 second marks (equals 6 seconds) on the ECG strip and multiply by 10
• This calculates slow and irregular rhythms.

Measuring Wave Forms Using an EKG Strip

• Assess the value between P and R during the P-R interval period.
• Definition: Measured from the beginning of the P wave to the beginning of the QRS complex.
• The range should stay between .12 and .20sec
• AsessQRS complex.
• Definition: Ventricular depolarization.
• The normal value shoudl be 0.06 to .10 sec.

EKG Review: Questions to Ask

• Is there a P for every QRS.
• Are the P waves regular
• If there is rhythm regularity.
• Calculate the PR and QRS intervals for presence.
• Clinical correlation can find rhythm and conduction abnormalities, as well as presence of hypertrophy.