Understanding EKG Basics

• EKG stands for electrocardiogram

Objectives for EKG Study

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

EKG Basic Principles

• An EKG or ECG provides a graphic representation of the heart's depolarization.
• It records the heart’s electrical activity.
• Electrical current moving toward the positive electrode causes positive deflection.
• Electrical current moving away from the positive electrode causes negative deflection.
• EKGs measure electrical activity, not function.

Basic EKG features

• EKG is practical, time-efficient, and non-invasive.
• EKGs can assess: Rate, Rhythm, Axis, Hypertrophy, and Infarction (ischemia, injury, and other problems).

Physiologic Properties of Myocardial Cells

• Automaticity refers to the intrinsic ability to contract without external stimuli.
• Rhythmicity refers to the intrinsic ability to contract in a rhythmic manner.
• Conductivity refers to the intrinsic ability to transmit impulses.
• Excitability refers to the ability to respond to an impulse.
• Contractility refers to the ability to respond with a pumping action.

Myocardial Cells States

• In the resting/polarized state, the myocardium has a negative charge inside the cell membrane (K+).
• During depolarization, myocytes become positive and contract, creating a wave through the myocardium; positive charge is carried by fast-moving Na+ ions.
• Repolarization (relaxation) involves myocardial cells returning to their resting negative charge inside.

Deflection Wave

• Myocardial cell depolarization and replolarization cause myocardial contraction and relaxation.
• Positive deflection is when the wave of depolarization moves toward the positive electrode.
• Negative deflection is when the wave of depolarization moves away from the positive electrode.
• Isoelectric deflection is when the wave of depolarization moves perpendicular to the positive electrode.

Cardiac Nervous System

• Sympathetic nervous system supplies both atria and ventricles.
• Norepinephrine is the mediator in the sympathetic nervous system.
• The sympathetic nervous system increases the rate of the SA node, rate of AV conduction, excitability, and force of contraction.
• Parasympathetic nervous system (Vagus Nerve) primarily supplies the atria.
• Acetylcholine is the mediator in the parasympathetic nervous system.
• The parasympathetic nervous system decreases the rate of the SA node, rate of AV conduction, and excitability.

Cardiac Conduction System Components

• Sino-Atrial node (SA)
• Atrio-ventricular node (AV)
• Bundle of His
• Left and Right Bundle Branches
• Purkinje Fibers
• The action potential of the cardiac conduction system is positive.

SA Node Characteristics

• The SA node is the heart’s dominant pacemaker.
• The SA node pacing activity is the Sinus Rhythm.
• The SA node emits a depolarization wave that spreads throughout both atria, which produces the P wave on an EKG.
• During atrial contraction, blood is forced through the valves, specifically the tricuspid valve on the right and the mitral valve on the left

AV Node Information

• The AV node is the only conduction path between the atria and ventricles.
• Depolarization slows within the AV node before being conducted to the ventricles.
• The slow depolarization allows blood to finish passing from atria to ventricles.
• Short flat baseline after each P wave (PR segment) is a representation of the slow depolarization.

Ventricular Conduction System

• Bundle of His
• Right and Left Bundle Branches
• Purkinje fibers
• After depolarization slows through the AV node, it conducts rapidly through the Bundle of His to the Right and Left Bundle Branches.
• The Purkinje Fibers have terminal filaments that carry depolarization of the myocytes to contract and is represented by the QRS complex on an EKG.

QRS Complex

• Q wave = 1st downward wave of the QRS complex which may or may not be present.
• R wave = 1st upward wave of the QRS complex.
• S wave = ANY downward wave preceded by an upward wave.

Ventricular Repolarization Phases

• Initial plateau after QRS complex refers to 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, starting with the QRS and continues until the end of the T wave.

Cardiac Conduction Route

• SA node -> internodal pathways (to left atrium via Bachman’s bundle; to AV node via ant/middle/post internodal pathways) -> AV node -> Bundle of His -> Right and Left bundle branch (RBB is thinner; LBB has three divisions or fascicles – anterior, septal and posterior) -> Purkinje fibers located in the ventricles.

EKG Electrode Information

• Surface sensors used to detect the heart’s electrical activity.
• Electrodes can be positive or negative.

Telemetry vs Hardwire

• Telemetry or Hardwire are frequently used in Acute care/ICU settings.
• Telemetry or Hardwire are easier to maintain and provides accurate data.
• One lead is viewed on a "RHYTHM STRIP"
• 12 lead EKG is used for more specific diagnostics.
• 5 colored wires are used.
• White clouds over Green grass
• Black smoke over Red fire
• Brown: center of chest (ground)

Key EKG Terms

• Automaticity foci: areas of the heart that have automaticity
• Waves: P, QRS complex, T
• Segments: PR, ST
• Intervals: PR, QRS, QT, RR, ST

EKG waves

• Each impulse represents different stages of myocardial stimulation.
• P wave: Atrial depolarization
• QRS complex: Ventricular depolarization
• T wave: Follows ST segment and marks where ventricular repolarization occurs and is complete at the end of the wave.
• The PR segment is an isoelectric line after the P wave.
• The PR segment is also known as the atrial kick.
• The ST segment starts at the end of the QRS complex and ends at the T wave
• Elevation/Depression of the ST segment can be used to identify problems with the heart.
• PR interval: Atrial depolarization
• .ST interval: Full ventricular repolarization.
• QT interval: duration of ventricular systole (and repolarization)
• RR interval: Use to calculate the rate is it regular?

Intrinsic Rates

• SA Node: Typically controls the heart's rate and rhythm (SINUS RHYTHM).
• Inherent rate of the SA node is between 60-80 bpm, and is controlled with atrial automaticity focus, taking over the atria.
• AV Node (Junctional Foci): May become the pacemaker if the SA node isn’t working, or is diseased, or in emergent situations
• Ventricular Foci (Purkinje Fibers): May become the pacemaker if the SA or AV node aren’t working, or are diseased, or in emergent situations.

Three Things to Consider When Reading an EKG

• Is there a P wave for every QRS?
• Is the rhythm regular?
• What is the rate?
• These three will tell you if the rhythm is a sinus rhythm and what type
• Bradycardia
• Normal
• Tachycardia

EKG Paper Characteristics

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

EKG Small and Large Boxes Measurements

• 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 Measurements

• 1 small box's amplitude is 0.10mV.
• 1mm = 0.10mV
• 1 large box's amplitude is 0.50mV.
• 5mm = 0.50mV
• Standard Calibration: 10mm (1cm)= 1 mV
• 2 large boxes

P Wave on ECG

• Represents atrial depolarization—SA node function with the repolarization that is lost in the QRS complex.
• Duration is between 0.08 and 0.10 sec

PR interval on ECG

• Measured from the beginning of the P wave to the beginning of the QRS complex
• Represents the time for the impulse to travel from the atria to the ventricles and represents the delay at the A-V node
• Normally 0.12 – 0.20 seconds (less than 1 large square)
• Usually isoelectric (PR segment)

QRS Complex on ECG

• 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: usually not >0.03 sec or is usually

ST Segment on ECG

• The J-point = where QRS joins the ST segment, where the ST segment originates and ends at the beginning of the T wave, usually isoelectric.
• Changes in elevation or depression are important markers to determine such things as Ischemia, injury, and infarction

T Wave on ECG

• Represents the final, “rapid” phase of ventricular repolarization.
• Second ½ of the T wave is called the relative refractory period.
• Symmetry, skewedness, slope, and amplitude often describes it
• Wave inversion in certain leads (negative T waves) can be a sign of ischemia

QT Interval on ECG

• Total time of ventricular systole.
• Is a good indicator of repolarization.
• Changes with heart rate since depolarization & repolarization occurs faster with a faster HR
• Normal QT is less than ½ RR interval at normal rates (~0.42 sec)

EKG Measurements

• The P waves should be regular
• PR Interval and QRS complex should be measured

Additional EKG Information

• Changes in waveforms and intervals should be measured
• Measurements should be compared to normal values in order to indicate rhythm and conduction abnormalities, as well as presence of hypertrophy

12-Lead EKG

• Total of 12 leads: 6 Limb leads and 6 Precordial (chest) leads
• Limb leads look at the heart in the frontal plane.
• Leads aVR, aVL, and aVF look from three separate directions, while Leads I, II, and III are summation of potential differences between limb leads
• Chest leads look at the heart across the transverse plane, "in front of the heart”
• Limb Leads: Bipolar and Unipolar/Augmented

Bipolar Limb Leads

• Lead I which depicts high lateral wall of the heart.
• Lead II depicts inferior surface of the heart.
• Lead III depicts inferior surface of heart.

Unipolar/Augmented Limb Leads

• Lead aVR- looks at nothing in the left ventricle
• Lead aVL- looks at high lateral wall of the heart
• Lead aVF- looks at inferior wall of the heart

Precordial Leads

• Precordial Leads are unipolar and all are POSITIVE
• V1 through V6 view the horizontal plane of the heart
• Lead V1 the 4th intercostal space @ right sternal border.
• Lead V2 is the 4th intercostal space @ left sternal border.
• Lead V3 is equal distance b/t V2 & V4.
• Lead V4 is the 5th intercostal space @ left midclavicular line.
• Lead V5 is the 5th intercostal space @ anterior axillary line.
• Lead V6 the 5th intercostal space @ mid axillary line.
• When observing leads V1-6, you will see gradual changes in all of the waves as the position of the positive electrode changes, with the QRS in V1 will be negative and will progress to positive by V6.
• Precordial Leads

Reading an EKG Strip for Axis

• Occurs in frontal plane is related to Limb, and is called the Axis Deviation, while, while Axis on horizontal is called Axis Rotation

Cardiac Axis and Vectors

• Axis refers to the direction of depolarization as it passes through the heart
• A vector (arrow) is used to demonstrate the direction of depolarization as well as the magnitude of depolarization
• Depolarization that moves away from a (+) electrode, will have a (-) deflection
• Depolarization that moves towards a (+) electrode, will have a (+) deflection
• Both ventricles depolarize within 0.08 seconds
• During a single QRS interval, sequential, instantaneous vectors are generated, and the larger/greater the muscle mass, the bigger the arrow will be

P Wave Vectors

• Represents normal depolarization of both atria
• Usually points downward to the left side of the patient

Mean QRS Vector

• Mean QRS vector = the sum of all the smaller vectors of ventricular depolarization
• Origin of mean QRS vector = AV Node
• Depolarization vectors of left ventricle are larger than those of a right ventricle, MEAN QRS VECTOR points down and toward the LEFT (between 0 - 90°)

Clinical Correlation: Mean QRS Vector

• Clinical Correlation: mean/ mean Vector, if the axis of the mean vector can assist you can identify between diagnosis, if there is hypertrophy or infarction.
• Hypertrophy: axis moves toward the hypertrophied region
• Infarction: axis moves away from infarcted region
• In obesity and pregnancy-the axis shifts left and upwards (likely due to upward and possibly more horizontal displacement of the heart)

Understanding Key Factors when Reading an EKG

• Will tell you if you have a “sinus rhythm” and what type by the rate
• Is there a P for every QRS
• Is the rhythm regular
• What is the rate
• Bradycardia
• Normal
• Tachycardia

Reading an EKG Strip Includes:

• Check for P wave for every QRS
• Identify the R wave
• Mark the distance between the R waves (RR Interval)
• Assess if the distance is consistent

Calculating the Rate (Countdown method/ Triplicate method) by Counting Boxes

• 300-150-100-75-60-50" Sometimes called “triplets” or triplicate method, and the x-axis represents time.
• Big boxes=0.2 sec
• Small boxes=0.04 sec
• 5 Small boxes=1 Big box" There are 300 Big boxes in 1 minute
• 60/0.2=300" Starting point for counting boxes

Measuring the Rate by Using Six Second Method

• Useful if it is for slow rates and Irregular Rates –
• 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

To Measure The PR Interval

• Begin measuring from the beginning of the P wave to the beginning of the QRS complex
• P-R interval of:Normal values = 0.12 - .20 sec

QRS Complex: Measurement on EKG

• Measure QRS complex's activity
• The Definition of activity in the ventriculars repolarization
• To define normal value on EKG= is 0.06-.10 sec