Introduction to the Mechanical & Electrical Properties of the Heart A. Bornstein, MD, FACC Assistant Professor of Science Education Hofstra North Shore-LIJ School of Medicine Hempstead, NY

 Define the elements of the cardiac cycle  Define the relationship between electrical activity in the various regions of the heart and the electrocardiogram, discussing specifically the P wave, QRS complex, T wave, P-R interval, S-T segment, and the Q-T interval  Identify normal ECG wave forms  Define the term dipole, the characteristics that define a vector, and how dipoles generated by the heart produce the waveforms of the ECG  Understand the genesis of abnormal forms on the ECG Learning Objectives

The Cardiac Cycle: Putting it All Together

What happens at each of these points and why? What is the difference in these graphs between the left and right heart? 7

An electrocardiogram is a graphic representation of the propagation of electrical vectors in the heart An electrocardiogram is a graphic representation of the propagation of electrical vectors in the heart Vectors (as you remember from college physics) are representations of force/energy that have magnitude and direction Vectors (as you remember from college physics) are representations of force/energy that have magnitude and direction Definitions

Review of vectors, and how we graph them on an EKG

Graphing the Vectors Time Electrical force Towards lead Away from lead

Electrical Pathways of the Heart 1)SA node 2)AV node 3)Bundle of His 4)Left Bundle 5)Left posterior hemifascicle 6)Left anterior hemifascicle 7)Left ventricle 8)Interventricular septum 9)Right ventricle 10)Right bundle

With the Heart Removed

A Single Beat: PQRST The morphology will vary depending on the lead

The leads give the viewer an opportunity to trace the propagation of the impulses from multiple different axes The leads give the viewer an opportunity to trace the propagation of the impulses from multiple different axes Lead Systems

Lead Systems: Standard & Augmented (aV) Leads (Coronal Plane)

Lead Systems: Precordial Leads (Transverse Plane)

Electrocardiogram

How It Works

Because of the intercalated discs, myocardial muscle functions as an electrical syncytium so that even when the normal pathways are interrupted, the cardiac muscle can still be activated (albeit in an abnormal manner) Pacemaker cells (cells that initiate heart beat) are regular in nature. Since normal propagation of the impulses is down & to the left, lead II will generally be the best lead to look at to determine the atrial rhythm There is a delay in the normal EKG after the P wave which is the time it takes for the impulse to get through the AV node to the Bundle of His (called the A-H interval) and from the Bundle of His to the ventricles (called the H-V interval); this is called the PR interval A Few Important Points

The more rapidly you stimulate the AV node, the longer it takes to repolarize. This is called decremental conduction and it’s “REALLY IMPORTANT”!! The His Purkinje system is a specialized conduction system that allows for very efficient impulse conduction. Impulses that go down that pathway result in narrow complex QRS. There is a delay in the normal EKG after the QRS complex which is the time it takes for the ventricle to repolarize for the next beat. The commonly used interval extends from the beginning of the QRS to the end of the T wave and is called the QT interval. A Few Important Points (Continued)

Dissecting an EKG interpretation: Every interpretation should have the phrases below or a phrase to replace the respective colored phrase Normal sinus rhythm at 60, normal electrical axis, normal intervals, good R wave progression with no significant repolarization abnormalities Atrial rhythm & rate Axis Conduction (includes AV & interventricular) R wave progression (includes infarcts & hypertrophy) repolarization abnormalities (ischemia, infarction, hypertrophy, etc.)

EKG Schedule Where/WhenTopics Today 1. Sinus rhythms 2. Electrical axis Blackboard 1. Atrial enlargement 2. Ventricular enlargement/hypertrophy 3. Bundle branch blocks Next week 1. AV block 2. Supraventricular tachycardia 3. Atrial fibrillation 4. Atrial flutter 5. Ventricular tachycardia 6. Ventricular fibrillation 7. Torsade de pointes Forever Everything else

Calculating Heart Rate The scale of a standard EKG is: Time axis: 1 small box = 40 msec 1 big box = 5 small boxes = 200 msec Voltage axis: 2 big boxes = 10 small boxes = 1 mV Mnemonic: 300,150,100,75,60,50,43,38

PR interval: msec QRS segment: <120 msec QT interval: <440 msec Heart rate: Normal: BPM Normal: BPM Bradycardia: <60 BPM Bradycardia: <60 BPM Tachycardia: >100 BPM Tachycardia: >100 BPM Numbers to Remember

Normal sinus rhythm: atrial beats are coming from the SA node (located in the high right atrium and propagated down and to the left) at a rate between 60 and 100 beats a minute Normal sinus rhythm: atrial beats are coming from the SA node (located in the high right atrium and propagated down and to the left) at a rate between 60 and 100 beats a minute Sinus bradycardia: atrial beats are coming from the SA node at a rate less than 60 beats a minute Sinus bradycardia: atrial beats are coming from the SA node at a rate less than 60 beats a minute Sinus tachycardia: atrial beats are coming from the SA node at a rate greater than 100 Sinus tachycardia: atrial beats are coming from the SA node at a rate greater than 100 Sinus Rhythm

Normal ECG

From the collection of Dr. A Bornstein

Electrical Axis Normal electrical axis Left axis deviation (A) Right axis deviation (B) Indeterminate axis I++- II+- III (A) Left anterior hemifascicular block, inferior wall myocardial infarction (B) Left posterior hemifascicular block, pulmonary disease sometimes