PRACTICAL ELECTROCARDIOGRAPHY

M. Catherine Hough Ph.D, RN Kathy Robinson RN, Ph.D, CCRN

Spring Semester 2002

Electrocardiogram

EKG or ECG

Characteristics of EKG Paper

• Horizontal Axis is a function of time

• Vertical Axis indicates amplitude ONLY in a calibrated recording

Calculating EKG Rate

Calculating EKG Rate

Components of the QRS Complex

P wave

• The P wave represents the depolarization of the atria (atrial depolarization)

• The P wave contour is usually smooth– entirely positive (Leads I, II, III, aVF, and V4 to V6) – negative (aVR) (monophasic) in all leads except V1

P wave

• The P wave duration is normally less than 0.12 sec.

• The P wave amplitude is normally less than 0.25 mV in all leads

• The P wave normally appears entirely upright

PR Interval

• The PR interval measures the time required for the impulse to travel from the atria myocardium adjacent to the SA node to the ventricular myocardium adjacent to the fibers of the Purkinke network (atrial and ventricular depolarization)

PR Interval

• The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.

• Normal PR interval duration range is from 0.12 sec - 0.20 sec

QRS Complex

• represents depolarization of the ventricles (ventricular depolarization)

• If the first deflection from the isoelectric line is negative it is a Q wave (not always present)

• The first positive deflection from the isoelectric line is an R wave

• The negative deflection following an R wave is an S wave

QRS Complex

• Normal QRS interval range is from 0.04 sec - 0.12 sec

• Measured from the first deflection from the isoelectric line to the J-point– (J-point is where the QRS complex ends and

the ST segment begins)

ST Segment

• represents the plateau (phase 2) of the action potential (ventricles in active state following depolarization, but NO electrical activity occurs at this time.

• Is normally isoelectric - no difference exists in electrical potential among the action potentials of the heart

• No current flow occurs because all cells are at zero potential

T wave

• represents phase 3 of the action potential, when the ventricles are being rapidly repolarized (ventricular repolarization)

• Is normally rounded, slightly asymmetric, and the same polarity as the QRS complex

• The effective refractory period is present during the beginning of the T wave.

QT interval

• represents the entire duration of ventricular depolarization and repolarization (ventricular refractory period is the time necessary for the ventricle to depolarize, then repolarize)

• The normal QT varies with age, gender, and heart rate

Cardiac Rhythms

Five Steps to Rhythm Interpretation

• Determine the rate• Identify the P Waves– are they present?– How do they relate to the QRS

• Identify the QRS Complex– How do they relate to the P Waves?

• Measure the P-R interval• Measure the QRS interval

Specialized Conduction System of

• SA Node (highest rate of automaticity)– Located on POSTERIOR surface of Rt. Atria

• Internodal atrial pathways– Anterior Tract (Bachman’s Tract)– Middle Tract (Wenchebach)– Posterior Tract (Thorel’s)

• Bachman’s BUNDLE - conducts impulse from SA node to Lt.. Atria• AV NODE

– Delays impulses from atria before they go to ventricle– Allows for ventricular filling

Intraventricular Conduction System

• Bundle of His• RBBB• LBBB• Purkinje System

Electrical Activity of the Heart

The above is borrowed from the web site: http://www.heartsite.com

Cardiac Properties

The heart normally possesses four intrinsic functional properties that are the result of electrical activity occurring @ the myocardial cell membrane

1. Automaticity - intrinsic to the pacemaker cells of the heart. These cells have the ability to discharge an electrical impulse automatically without & outside stimulusInherent rates of automaticity:a. SA node 60-100 bpmb. AV junction 40-60 bpmc. His-Purkinje system 20-40 bpm

Cardiac Properties

2. Excitability - refers to the ability of cells to become electrically charged in response to an outside electrical stimulus

• The degree of excitability is directly related to how much recovery time the heart has had before the next electrical stimulus is received

Cardiac Properties

3. Contractility - refers to the ability of cardiac muscle fibers to shorten or contract in response to an electrical stimulus

• This property is scientifically based on the Frank-Starling Law which claims:

• “To a point, the longer the muscle fiber is stretched, the greater its force of contraction. If however the muscle fiber is overstretched, the contraction will be weak”

Cardiac Properties4. Conductivity - refers to the propagation of an impulse from

cell-to-cell. Achieved by two methods:

• Interconnected muscle fibers with intercalated discs• Specialized conduction system

SA node

Normal Sinus or Regular Sinus Rhythm (NSR or RSR)

• P wave present and regular. • Atrial rate (P waves) between 60 and 100

beats/min • Each P wave is followed by a QRS complex

Sinus Bradycardia

• P wave present and regular• Atrial rate (P waves) < 60 beats/min• Each P wave is followed by a QRS complex• RX:– May require no treatment– Atropine IV– May require temporary pacemaker or permanent

pacemaker

Sinus Tachycardia

• P wave present and regular• Atrial rate (P waves) > 100 beats/min• Each P wave is followed by a QRS complex• RX:– treat underlying cause

Sinus Arrhythmia

• P wave present• P-P interval - phasic shortening then

lengthening of P-P interval, usually with respirations

• Impulse initiation by SA node• RX:– usually none

Common Arrhythmias Originating from Atria:

Premature Atrial Contractions (PAC)

• initiated by ectopic focus in the atria• premature P wave with a contour different from a

sinus P wave (location of ectopic focus determines its shape)

• QRS may or may not be normal• PAC is followed by a pause ~ equal to the sinus

cycle • (measured R to R)

Premature Atrial Contractions (PAC)

• associated with use of caffeine, stress, or use of tobacco

• may be a precursor to developing uncontrolled AF• RX:– may require no treatment– sedation– quinidine

Atrial Flutter

• rapid sawtooth P waves• ventricular rate regular• associated with CAD, pulmonary embolism, mitral

valve disease, and thoracic surgical procedures.

• atria depolarize at a rate of 250 to 350 beats/min

Atrial Flutter

• RX:– cardioversion– digitalis– ibutilide– IV diltiazem

Atrial Fibrillation

• rapid irregular P waves > 350/min• ventricular rate irregularly irregular• ventricular rate varies, may increase to greater

than 150 if untreated• if rate > 100 beats/min referred to as uncontrolled

AF• if rate < 100 beats/min referred to as controlled

AF

Atrial Fibrillation

• RX:– digitalis– cardioversion– quinidine– IV dilitiazem

Sick Sinus Syndrome (SSS)

• term to describe several disorders of the SA node• tachycardia-bradycardia syndrome is the most

common type of SSS• complication associated with SSS is CHF and CVA

resulting from thromboembolisms• RX:– stabilization of heart with perm pacemaker

Atrial Tachycardia

• rate 150 to 250 beats/min• P wave present but may be hidden • QRS is generally normal• ventricular rate is regular• RX:– usually none– prolonged episodes may require carotid sinus pressure,

vagal stimulation, verapmil, digitalis, or beta blocks

Impulse Conduction Deficits

A-V Blocks

First Degree AV Block

– PR interval prolonged - > 0.20 sec– RX:• usually none

– May warn of impaired conduction

Second Degree AV Block

Mobitz I (Wenckebach)

– PR interval progressively lengthens until a P wave is not followed by a QRS complex

– Ratio of P waves to QRS complexes varies, i.e. can be 5:1, 4:1, 3:1, or 2:1

– more often is a transient event – seen with patients post Inferior MI, Digitalis

toxicity, or postoperative– RX:• generally none unless symptoms occur because of

slow rate seen with 2:1 ratio

Mobitz II

• more serious that Mobitz I• less common than Mobitz I• characterized by nonconducted sinus impulses

despite constant PR intervals• usually the QRS are widened because of a BBB,

the dropped beat represents a form of intermittent blockage of both bundle branches

Mobitz II

• the defect is found in either the bundle branches or the bundle of HIS

• occur more frequently with patients with acute anterior septal wall MIs

• often progress to CHB• RX:– temporary pacemaker– possible need for perm pacer

Third Degree AV Block (Complete Heart Block)

• atria and ventricles beat independently• P waves have no relation to QRS• ventricular rate may be as low as 20-40 beats/min• RX:– temporary or permanent pacemakers– Isoproterenol to increase HR– Epinephreine if Isuprel ineffective

Bundle Branch Block (BBB)

• same as normal sinus except QRS complex is > 0.12 sec

• depending on site of defect is labeled RBBB or LBBB - the LBBB has two main devision the anterior and posterior fascicles

• seen with severe CAD, acute anterior wall MI, hypertensive pts

Bundle Branch Block (BBB)

• RX:– usually none – if sufficient blockage is present may require

perm pacer

Ventricular Arrhythmias

Premature Ventricular Contractions (PVCs)

• arise from an ectopic focus in the ventricles• wide bizarre QRS greater than 0.12 sec• no associated P wave• T wave is in the opposite direction from the main

QRS deflection– ~ 50% of PVCS are followed by a compensatory pause

(the interval from the beat preceding to the beat following the PVC is equal to two sinus cycles)

Premature Ventricular Contractions

• the remaining 50% PVCs result in retrograde conduction to the atria causing the next sinus P wave to be early and a compensatory pause occurs

• associated with CHF, digitalis toxicity, electrolyte imbalances, and excessive caffeine intake

• RX:– intravenous lidocaine (bolus and drip)– oral antiarrhythmic drugs:– Norpace, Quinidine, oxygen, KCL

Ventricular Tachycardia

• no P wave before QRS• QRS wide and bizarre• ventricular rate > 100 beats/min (usually 140-240)• RX:– lidocaine– procainamide– cardioversion

Ventricular Fibrillation

• chaotic electrical activity• no recognizable QRS complex• associated with MI, drug toxicity, electrocution,

freshwater drowning• no CO• absent pulse or respirations -- CARDIAC ARREST

Ventricular Fibrillation RX:

• initiate CPR• defibrillation• epinephrine• sodium bicarbonate• lidocaine, bretylium

Miscellaneous EKG Examples

• Pacemakers