Understanding EKG Waveforms: A Comprehensive Guide, Papers of Physiology

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Lab Activity 24

EKG

Portland Community College

BI 232

Reference: Dubin, Dale. Rapid Interpretation of EKG’s

th^. 6edition.

Tampa: Cover Publishing Company, 2000.

Graph Paper 1 second equals25 little boxes or5 big boxes

1 second

P Wave •^ Indicates atrial depolarization, or contraction ofthe atrium. •^ Normal duration is not longer than 0.11 seconds(less than 3 small squares) •^ Amplitude (height) is no more than 3 mm •^ Dysfunctions of the sinoatrial node result in theobservance of abnormalities in the P-wave; (i.e.,longer, wider or absent)

PR Segment^ •^

Measured from the end of the P wave to thebeginning of the QRS complex • This pause is caused by the slowdepolarization within the AV node.

QRS Complex

-^ Indicates ventricular depolarization, throughthe Bundle Branches and Purkinje fibers.(Starts the contraction of the ventricles) •^ Normally not longer than .10 seconds induration

ST Segment^ •^

Indicates early ventricular repolarization;the plateau phase • Represents the ventricles in an activecontraction state but with no electricalactivity occurring. • The S-T segment is measured from the endof the QRS complex to the beginning of theT-wave

ST Interval •^ ST interval=T wave + ST segment •^ Represents the complete repolarizationphase of the ventricle (plateau phase andrapid phase)

QT Interval •^ Represents the duration of ventricularsystole (depolarization and repolarization). •^ General rule: duration is less than half thepreceding R-R interval

Rate •^ When examining an EKG, you shoulddetermine the rate first •^ The time required to record 5 large boxes willbe one full second (0.20 X 5 = 1.0 second). •^ Thus, if a

QRS complex occurs with each

large box

, then the R-R interval will be 0.

second, and the rate of the rhythm is

beats/minute

(i.e., 5 beats occur each second

X 60 seconds/minute = 300/minute).

Rate: 300-150-100-75-60-50 • R-R interval is 1 large boxes

, rate =

300 (

300 ÷ 1)

-^ R-R interval is 2 large boxes

, rate =

150 (

300 ÷ 2)

-^ R-R interval is 3 large boxes

, rate =

100

(300 ÷ 3)

-^ R-R interval is 4 large boxes

, rate =

75

(300 ÷ 4)

-^ R-R interval is 5 large boxes

, rate =

60

(300 ÷ 5)

-^ R-R interval is 6 large boxes

, rate =

50

(300 ÷ 6)

-^ If the R-R interval is between boxes, you just estimate or divide1500 by the number of small boxes per R-R interval.

Determining Rate

-^ Find an R wave on a thick line, then start counting Start here: It is on a thick line

The next R wave is 2.5 large boxes away

2 boxes=150 and 3 boxes = 100So 2.5 boxes is about 120 beats/minute

Axis •^ Axis

refers to the average direction ofthe movement ofdepolarization, whichspreads throughoutthe heart to stimulatethe myocardium tocontract.

Vectors

-^ A vector

is the average direction of all of thepositive charges as theytravel through themyocardium • Since the left ventricle isthicker, its vectors arebigger (which contributesto the average beingtoward the left)

Average Vector

Influences on Vector Direction •^ Anything that influences theoverall amount of chargeflowing through themyocardium will change theaverage direction the thecharge is flowing •^ Infarction would not have avector associated with it sothe average vector wouldpoint somewhat away fromthat area

Average Vector