Basics of EKG Interpretation

EKG

a recording of the electrical activity of the heart over a period of time

-allows clinicians to assess cardiac rhythm, conduction abnormalities, ischemia

Myocardial cells

-found in the muscular layers of the atria and ventricles

-contain myofibrils

Pacemaker cells

-found in the electrical conduction system responsible for generation of electrical impulses

-automaticity

Automaticity

ability of pacemaker cells to spontaneously generate their own electrical impulses

-specific to pacemaker cells

Excitability

The ability of cardiac cells to respond to electrical impulses

-characteristic shared by both cell groups

Conductivity

ability of cardiac cells to conduct the impulse to adjacent cardiac cells

both cell groups

Contractility

ability of cardiac cells to shorten and cause muscle contraction in response to electrical stimulation

-specific to myocardial cells

Electrolyte

A Charged impulse that helps generate electrical impulses in the heart

Sodium: primary ion outside cell, initiates depolarization by entering cells

Potassium: primary ion inside cell, regulated repolarization

Calcium: essential for contraction, triggers acti-myosin

Magnesium: stabilizes cardiac membranes and assists with regualtion

Depolarization

when a cardiac cell in stimulated by an electrical impulse, the membrane becomes permeable to ion exchange

-potassium begins to leave the cell, sodium enters

-muscle contraction follows depolarization

Repolarization

recovery from depolarization to polarized state

-cardiac cell cannot respond to stimuli until it has been repolarized

Cardiac Action Potential

Phase 0: depolarization

-sodium influx into cell

Phase 1: early repolarization

-sodium channels close and some potassium exits

Phase 2: Plateau

calcium moves into cell

potassium continues to leave cell

Phase 3: repolarization

-calcium channels close, potassium continues to move out

Phase 4: resting

-sodium-potassium pump actively moves sodium out and potassium in, restoring resting state

SA node

located in the upper right atrium near the inlet of the superior vena cava

-primary pacemaker of heart

-60-100 impulses per min

AV node

-located in lower right atrium

-relays electrfical impulse from atria to ventricles

-slows conduction to allow time for atria to contract

-secondary pacemaker (40-60 impulses per min)

-block rapid atrial impulses from being conducted into ventricles

Bundle branches and purkinje fibers

can serve as secondary pacemaker

firing rate 30-40 impulses per min

PR interval

the time from the onset of atrial depolarization to ventricular depolarization

PR segment

represents normal delay in the AV node

QT interval

total time for the ventricles to depolarize and repolarize

ST segment

represents ventricular repolarization

P wave

-normal P wave= SA node functioning properly

-represents atrial depolarization

-followed by atrial contraction

Duration: less than 0.12 seconds

P wave Abnormalities

Peaked: right atrial enlargement

Notched or wide: left atrial enlargement

Absent: Afib, SA node dysfunction

Inverted: impulse originated at AV node

Flutter: Aflutter

QRS complex

-second segment, normal menas normal ventricular conduction

-represents ventricular depolarization

-Pathological Q-wave represents old MI

Duration: 0.06-0.1 seconds

QRS complex abnormalities

wide: bundle branch block, Vtach, htperkalemia

Low voltage: pericardial effusion, obesity

Pathological Q: old MI

Delta wave: wolf-parkinson-white syndrome

Twave

ventricular repolarization

-reflects recovery phase

-0.1-.25 seconds

T wave abnormalities

Tall/peaked: hyperkalemia

Flat/flow: hypokalemia

Inverted: Ischemia, LVH, pericarditis

Biphasic: ischemia, repolarization abnormality

Cardiac Cycle

1 hearbeat (1 PQRST segment)

-represents sequence of systole and diastole

Isoelectric line

flat segment between cardiac cycles

Above isoelectric line: positive deflection

Bellow: negative deflection

Above and bellow: biphasic deflection

Waveform flow

Current towards positive pole: positive deflection

Current towards negative pole: negative deflection

Size of wave: depends on how much voltage is generated during depolarization

Refractory Period

time during cardiac cycle when a cell may or may not respond to an electrical stimulus

Absolute refractory period

period when cells cannot respond to an electrical stimulus, lasts from depolarization to most of repolarization

Relative refractory period

period during ventricular repolarization when repolarization is almost complete, and cells may respond to strong stimulus