ECG Basics

Electrocardiograms produce recordings of the electrical conduction through the heart. To understand ECG, we should first review the basics.

Lets first review the pacemakers of the heart

SA Node-----Ist Pacemaker Keeps the heart rhythm between 70-100 bpm

AV Node------2nd Pacemaker. Heart Rhythm- 40-60 bpm

Purkinje Fibers –3rd Pacemaker. Heart Rhythm approx.20-30 bpm

ECG Terminology

DEPOLARIZATION (contraction) Na moves into the cell, K+ moves out=change in charge on cell membrane

REPOLARIZATION (Relaxation) Na leaves cell, electrical recovery as cell membrane facilitates more K+ coming into cell.

Understanding ECG paper

Horizontal axis reflects time in seconds. The vertical axis reflect speed/amplitude in mm/second

One small box on strip = 0.04 sec

One large box on strip = 0.20 sec/5 small boxes

An ECG recording is composed of numerous complexes. Each inclusive of individual waveforms-P, Q, R, S, T and the U wave.

P wave

Atrial depolarization/contraction, evidence SA node firing

P wave to R wave normal duration 0.12-0.20 sec (3-5 small boxes)

Should come before each QRS, a normal P wave is round and upright.

We should check each P wave-are they present? Does a QRS follow every P?

Are the P-P intervals equal?

Are P-R intervals prolonged?

QRS Wave

Q -downward deflection= septal depolarization (septum is dividing wall in heart)

R-large upward deflection= apical and left ventricular depolarization

S-downward deflection =lower segment of left ventricle depolarization

QRS normal duration-0.06-0.12 sec

Are all QRS same shape?

Is there a P before every QRS?

Are the R-R intervals equal/same?

ST Segment

Between the end of S wave to beginning of T wave =end of ventricular depolarization and beginning of ventricular repolarization. Time between contraction and relaxation of the ventricles. (Generally no net change in electrical activity, so ST segment generally flat)

T wave-repolarization of the right and left ventricles/relaxation

ST segment changes due to ischemic injury and fluctuations in Ca levels. T wave changes due to K imbalances as well.

Calculating a Rate

1. Regular Rhythm-300/# of boxes between R to R

Ex. 300/6 boxes between R waves=50 bpm

2. Count # of R waves between 3 harsh marks x 10= Rate

References

Lewis, S.L., Dirksen, S.R., Heitkemper, M.M., & Bucher, L. (2014). Medical-Surgical Nursing: Assessment and Management of Clinical Problems (9th ed.). St. Louis: Elsevier.

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