An Approach to EKGs

By: Siraj Mithoowani & Richa Parashar 2012Medical Education Interest Group

The EKG Lecture Series

• What these lectures are:– An introduction to EKG interpretation– Focused on clinically relevant material– A safe forum to participate, practice and make

mistakes

• What these lectures aren’t:– A comprehensive course in electrophysiology

Schedule

• Week One: Introduction, Cardiac Physiology and Electrophysiology

• Week Two: Approach to EKG Intepretation, Rate, Rhythm• Week Three: Axis and Hypertrophy• Week Three: Tachyarrhythmias• Week Four: Bradyarrhythmias, Conduction Abnormalities• Week Five: Ischemic Changes & Electrolyte Abnormalities• Week Six: Review and Q&A

• Check the medportal calendar/forums/facebook for updates!

Recommended resources

• Dubin’s Rapid Interpretation of EKGs• Lilly, EKG Chapter• ECG Wave Maven (Harvard)

(http://ecg.bidmc.harvard.edu)• ECG Made Simple (UofT)

(http://www.ecgmadesimple.com/)• BMJ ABCs of EKGs (on Medportal IM Clerkship page)• Our Medportal forum posts• Your tutors, preceptors, residents...

Agenda

• Why study EKG interpretation?• What does the EKG tell you?• Basic cardiac anatomy/physiology• Introduction to EKGs• Lead placement• Physiology of the waveform (P, QRS, T)

Why study EKG interpretation?

• They’re everywhere!• Relevant to all specialties– (Yes, even Psychiatry)

• The earlier you learn, the earlier you start practicing in real life

• Look like a superstar (superclerk?) on the wards

What does the EKG tell you?

• Ischemia/infarction• Conduction

– Heart block, bundle branch block...• Rhythm

– Atrial fibrillation, ventricular tachycardia...• Anatomy

– Left ventricular hypertrophy...• Electrolytes

– Hyperkalemia, hypocalcemia...

• Remember, the EKG is a non-invasive diagnostic test that can be performed in minutes

Anatomy and blood flowSVC/IVC

Right Atrium

Right Ventricle

Pulmonary Artery

LUNGS

Pulmonary Vein

Left Atrium

Left Ventricle

Aorta

Illustration: NIAAA/Chung, M.K., and Rich, M.W. Introduction to the cardiovascular system. Alcohol Health and Research World 14(4):269–276, 1990.

The cardiac cycle

Illustration: Guyton's: Textbook of Medical Physiology, 10th Edition. 2000. Chapter 9 . Page 99.

Blood Supply

The conduction system

Illustration: University of the West of England, Bristol. http://hsc.uwe.ac.uk/rcp/cs-heart-inside.aspx

SA node (60 – 100 bpm)

AV node (40 – 60 bpm)

Bundle of His

Left Bundle Branch (2) and Right Bundle Branch

Purkinje Fibers

Myocardial cells (20 – 40 bpm)(endocardium outwards)

Ye olde action potential

Illustration: Wikipedia (Cardiac action potential). http://en.wikipedia.org/wiki/Cardiac_action_potential.

At last... the EKG

EKG: University of Nebraska, Department of Internal Medicine. http://www.unmc.edu/intmed/geriatrics/ekgs.htm.

EKG paper

Amplitude: 1 small box = 1 mm = 0.1 mVTime: 1 small box = 40 ms = 0.04 seconds

1 large box = 200 ms = 0.2 secondsLength of EKG paper = 10 seconds

Illustration: Wikipedia (ECG). http://upload.wikimedia.org/wikipedia/commons/9/96/ECG_Paper_v2.svg.

EKG Leads

• Electrodes placed on the skin that pick up the current

• “View” the heart from different directions• Current towards the electrodes, or

repolarization going away = positive deflection on the EKG

• Current going away the electrodes = negative deflection

Lead Axes

• 12 leads in a standard EKG• Imagine the body sectioned two ways:– Frontal – represents the zones of the limb leads– Transverse – represents the zones of the

precordial leads• Limb leads: I, II, II, avR, avL, avF• Precordial leads: V1 to V6

Precordial leads: V1 to V6

Limb leads: I, II, II, avR, avL, avF

Inferior: II, III, AVF

Anterior: V1, V2, V3, V4

Lateral: I, AVL, V5, V6

The Typical Wave Form

P wave

• Represents atrial depolarization– Starting at the sinus node

• The first and second halves represent right and left atrial depolarization respectively

• Atrial repolarization is usually buried in the QRS complex

• <0.12 seconds (3 small squares)

P-R Interval

• From the beginning of the P wave to the first deflection after

• Represents conduction through the AV node (delay) and the His-Purkinje system.

• Anatomically, this delay allows for ___ ?

• Normal length: 0.12 to 0.20 s (3 to 5 small squares)

QRS Complex

• Represents ventricular depolarization– Septal depolarization from left to right– Ventricular depolarization (dominated by the left

ventricle)– Endo- to epicardium

• Q waves: marker for infarcts• Width of the QRS complex• Normal width: </= 0.12 s (3 small boxes)

QRS Complex

ST Segment

• Represents the isoelectric period between depolarization and repolarization

• Elevation/depression can be a marker for ischemia

T wave

• Represents ventricular repolarization• Normally upright (inverted in avR and III)

QT Interval

• Represents the whole of ventricular depolarization and repolarization

• Long QT predisposes patient to lethal arrhythmias

• Changes with heart rate • QTc

U wave

• ???• May represent repolarization of the His-

Purkinje system• Present in hypokalemia• Can be confused for T waves

TP segment

BASELINE

Putting it all together…whew!

Next week…

• Begin reading EKGs!• Rate, rhythm, axis, intervals• Email: richa.parashar@medportal.ca,

siraj.mithoowani@medportal.ca• FB page and medportal