Introduction to Clinical Electrocardiography Gari Clifford, PhD Andrew Reisner, MD Roger Mark, MD...

Introduction to Clinical Electrocardiography

Gari Clifford, PhDAndrew Reisner, MDRoger Mark, MD PhD

Electrocardiography

The heart is an electrical organ, and its activity can be measured non-invasively

Wealth of information related to: The electrical patterns proper The geometry of the heart tissue The metabolic state of the heart

Standard tool used in a wide-range of medical evaluations

A heart• Blood circulates, passing near every cell in the body, driven by this pump

• …actually, two pumps…

• Atria = turbochargers

• Myocardium = muscle

• Mechanical systole

• Electrical systole

To understand the ECG:

Electrophysiology of a single cell How a wave of electrical current

propagates through myocardium Specific structures of the heart

through which the electrical wave travels

How that leads to a measurable signal on the surface of the body

Part I: A little electrophysiology

Once upon a time, there was a cell:

ATPaseATPase

timetime

-90-90

Resting comfortably

a myocyte

timetime

Depolarizing trigger

timetime

Na channels

open, briefly

timetime

In: Na+

Mysterycurrent

timetime

In: Na+

Ca++ is in balancewith K+ out

timetime

In: Na+

Excitation/Contraction Coupling:Ca++ causes the Troponin Complex

(C, I & T) to release inhibitionof Actin & Myosin

timetime

In: Na+

Ca++ in; K+ out

More K+ out;Ca++ flow halts

timetime

In: Na+

In: Ca++; Out: K+

Out: K+

Sodium channels reset

timetime

In: Na+

Higher resting potentialFew sodium channels reset

Slower upstroke

timetime

a pacemaker cell

Slow current of Na+ in;note the resting potential

is less negative in apacemaker cell

-55-55

timetime

a pacemaker cell

Threshold voltage

-40-40

timetime

Ca++ flows in

timetime

. . . and K+ flows out

timetime

. . . and when it is negativeagain, a few Na+

channels open

How a wave of electrical current propagates through myocardium

Typically, an impulse originating anywhere in the myocardium will propagate throughout the heart

Cells communicate electrically via “gap junctions”

Behaves as a “syncytium” Think of the “wave” at a football

The dipole field due to current flow in a myocardial cell at the advancing front of depolarization.

Vm is the transmembrane potential.

Cardiac Electrical Activity

Important specific structures

Sino-atrial node = pacemaker (usually) Atria After electrical excitation: contraction Atrioventricular node (a tactical pause) Ventricular conducting fibers (freeways) Ventricular myocardium (surface roads) After electrical excitation: contraction

The Idealized Spherical Torso with the Centrally Located Cardiac Source (Simple dipole model)

Excitation of the Heart

Cardiac Electrical Activity

Recording the surface ECG

Clinical Lead Placement

Einthoven Limb Leads:

Precordial leads

12 Lead ECG

The temporal pattern of the heart vector combined with the geometry of the standard frontal plane limb leads.

Normal features of the electrocardiogram.

Normal sinus rhythm

What has changed?

Sinus bradycardia

timetime

Neurohumeral factors

Vagal stimulation makes the resting potential

MORE NEGATIVE. . .

timetime

Neurohumeral factors

. . . and the pacemakercurrent SLOWER. . .

timetime

. . . and raise the THRESHOLD

timetime

Catecholamines make the resting potential MORE EXCITED. . .

timetime

. . . and speed the PACEMAKER CURRENT. . .

timetime

. . . and lower the THRESHOLD FOR

DISCHARGE. . .

timetime

Vagal Stimulation:

timetime

Adrenergic Stim.

Sinus arrhythmia

Atrial premature contractions (see arrowheads)

Usually just a spark; rarely sufficient for an explosion

“Leakiness” leads to pacemaker-like current

Early after-depolarization Late after-depolarization

What’s going on here?

Wave-front Trajectory in a Ventricular Premature Contraction.

Is this the same thing?

What’s going on here?

Non-sustained ventricular tachycardia (3 episodes)

Slow Refractory

Quick Refractory

KeyWords: Heterogeneous, Circus, Self-Perpetuating

Side “A” Side “B”

No Longer Refractory

INCREASEDRefractory

Ventricular Fibrillation

Heart attack

Hyperkalemia

Understanding the ECG: A Cautionary Note

Basic cell electrophysiology, wavefront propagation model, dipole model:

Powerful, but incomplete There will always be electrophysiologic

phenomena which will not conform with these explanatory models

Examples: metabolic disturbances anti-arrhythmic medications need for 12-lead ECG to record a 3-D phenomenon

Questions?

Introduction to Clinical Electrocardiography Gari Clifford, PhD Andrew Reisner, MD Roger Mark, MD...

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