EKG BasicsEKG Basics

OutlineOutline

1.1. Review of the conduction systemReview of the conduction system

2.2. EKG waveforms and intervalsEKG waveforms and intervals

3.3. EKG leadsEKG leads

4.4. Determining heart rateDetermining heart rate

5.5. Determining QRS axisDetermining QRS axis

The Normal Conduction SystemThe Normal Conduction System

What is an EKG?What is an EKG?

The electrocardiogram (EKG) is a The electrocardiogram (EKG) is a representation of the electrical events of the representation of the electrical events of the cardiac cycle.cardiac cycle.

Each event has a distinctive waveform, the Each event has a distinctive waveform, the study of which can lead to greater insight study of which can lead to greater insight into a patient’s cardiac pathophysiology.into a patient’s cardiac pathophysiology.

What types of pathology can we What types of pathology can we identify and study from EKGs?identify and study from EKGs?

ArrhythmiasArrhythmiasMyocardial ischemia and infarctionMyocardial ischemia and infarctionPericarditisPericarditisChamber hypertrophyChamber hypertrophyElectrolyte disturbances (i.e. Electrolyte disturbances (i.e. hyperkalemia, hypokalemia)hyperkalemia, hypokalemia)Drug toxicity (i.e. digoxin and drugs which Drug toxicity (i.e. digoxin and drugs which prolong the QT interval)prolong the QT interval)

Waveforms and IntervalsWaveforms and Intervals

EKG LeadsEKG Leads

Leads are electrodes which measure the Leads are electrodes which measure the difference in electrical potential between difference in electrical potential between either:either:

1. Two different points on the body (bipolar leads)1. Two different points on the body (bipolar leads)

2. One point on the body and a virtual reference point 2. One point on the body and a virtual reference point with zero electrical potential, located in the center of with zero electrical potential, located in the center of the heart (unipolar leads)the heart (unipolar leads)

EKG LeadsEKG Leads

The standard EKG has 12 leads:The standard EKG has 12 leads: 3 Standard Limb Leads

3 Augmented Limb Leads

6 Precordial Leads

The axis of a particular lead represents the viewpoint from The axis of a particular lead represents the viewpoint from which it looks at the heart.which it looks at the heart.

Standard Limb LeadsStandard Limb Leads

Standard Limb LeadsStandard Limb Leads

Augmented Limb LeadsAugmented Limb Leads

All Limb LeadsAll Limb Leads

Precordial LeadsPrecordial Leads

Adapted from: www.numed.co.uk/electrodepl.html

Precordial LeadsPrecordial Leads

Summary of LeadsSummary of Leads

Limb LeadsLimb Leads Precordial LeadsPrecordial Leads

BipolarBipolar I, II, IIII, II, III(standard limb leads)(standard limb leads)

--

UnipolarUnipolar aVR, aVL, aVF aVR, aVL, aVF (augmented limb leads)(augmented limb leads)

VV11-V-V66

Arrangement of Leads on the EKGArrangement of Leads on the EKG

Anatomic GroupsAnatomic Groups(Septum)(Septum)

Anatomic GroupsAnatomic Groups(Anterior Wall)(Anterior Wall)

Anatomic GroupsAnatomic Groups(Lateral Wall)(Lateral Wall)

Anatomic GroupsAnatomic Groups(Inferior Wall)(Inferior Wall)

Anatomic GroupsAnatomic Groups(Summary)(Summary)

Determining the Heart RateDetermining the Heart Rate

Rule of 300Rule of 300

10 Second Rule10 Second Rule

Rule of 300Rule of 300

Take the number of “big boxes” between Take the number of “big boxes” between neighboring QRS complexes, and divide this neighboring QRS complexes, and divide this into 300. The result will be approximately into 300. The result will be approximately equal to the rateequal to the rate

Although fast, this method only works for Although fast, this method only works for regular rhythms.regular rhythms.

What is the heart rate?What is the heart rate?

(300 / 6) = 50 bpm

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What is the heart rate?What is the heart rate?

(300 / ~ 4) = ~ 75 bpm

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What is the heart rate?What is the heart rate?

(300 / 1.5) = 200 bpm

The Rule of 300The Rule of 300

It may be easiest to memorize the following table:It may be easiest to memorize the following table:

# of big # of big boxesboxes

RateRate

11 300300

22 150150

33 100100

44 7575

55 6060

66 5050

10 Second Rule10 Second Rule

As most EKGs record 10 seconds of rhythm per As most EKGs record 10 seconds of rhythm per page, one can simply count the number of beats page, one can simply count the number of beats present on the EKG and multiply by 6 to get the present on the EKG and multiply by 6 to get the number of beats per 60 seconds.number of beats per 60 seconds.

This method works well for irregular rhythms.This method works well for irregular rhythms.

What is the heart rate?What is the heart rate?

33 x 6 = 198 bpm

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The QRS AxisThe QRS Axis

The QRS axis represents the net overall The QRS axis represents the net overall direction of the heart’s electrical activity.direction of the heart’s electrical activity.

Abnormalities of axis can hint at:Abnormalities of axis can hint at:

Ventricular enlargementVentricular enlargement

Conduction blocks (i.e. hemiblocks)Conduction blocks (i.e. hemiblocks)

The QRS AxisThe QRS Axis

By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°.

-30° to -90° is referred to as a left axis deviation (LAD)

+90° to +180° is referred to as a right axis deviation (RAD)

Determining the AxisDetermining the Axis

The Quadrant ApproachThe Quadrant Approach

The Equiphasic ApproachThe Equiphasic Approach

Determining the AxisDetermining the Axis

Predominantly Positive

Predominantly Negative

Equiphasic

The Quadrant ApproachThe Quadrant Approach1. Examine the QRS complex in leads I and aVF to determine 1. Examine the QRS complex in leads I and aVF to determine

if they are predominantly positive or predominantly if they are predominantly positive or predominantly negative. The combination should place the axis into one negative. The combination should place the axis into one of the 4 quadrants below.of the 4 quadrants below.

The Quadrant ApproachThe Quadrant Approach2. In the event that LAD is present, examine lead II to 2. In the event that LAD is present, examine lead II to

determine if this deviation is pathologic. If the QRS in II is determine if this deviation is pathologic. If the QRS in II is predominantly positive, the LAD is non-pathologic (in other predominantly positive, the LAD is non-pathologic (in other words, the axis is normal). If it is predominantly negative, it words, the axis is normal). If it is predominantly negative, it is pathologic. is pathologic.

Quadrant Approach: Example 1Quadrant Approach: Example 1

Negative in I, positive in aVF RAD

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Quadrant Approach: Example 2Quadrant Approach: Example 2

Positive in I, negative in aVF Predominantly positive in II

Normal Axis (non-pathologic LAD)

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The Equiphasic ApproachThe Equiphasic Approach

1. Determine which lead contains the most equiphasic QRS 1. Determine which lead contains the most equiphasic QRS complex. The fact that the QRS complex in this lead is complex. The fact that the QRS complex in this lead is equally positive and negative indicates that the net equally positive and negative indicates that the net electrical vector (i.e. overall QRS axis) is perpendicular electrical vector (i.e. overall QRS axis) is perpendicular to the axis of this particular lead.to the axis of this particular lead.

2. Examine the QRS complex in whichever lead lies 90° 2. Examine the QRS complex in whichever lead lies 90° away from the lead identified in step 1. If the QRS away from the lead identified in step 1. If the QRS complex in this second lead is predominantly positive, complex in this second lead is predominantly positive, than the axis of this lead is approximately the same as than the axis of this lead is approximately the same as the net QRS axis. If the QRS complex is predominantly the net QRS axis. If the QRS complex is predominantly negative, than the net QRS axis lies 180° from the axis negative, than the net QRS axis lies 180° from the axis of this lead.of this lead.

Equiphasic Approach: Example 1Equiphasic Approach: Example 1

Equiphasic in aVF Predominantly positive in I QRS axis ≈ 0°

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Equiphasic Approach: Example 2Equiphasic Approach: Example 2

Equiphasic in II Predominantly negative in aVL QRS axis ≈ +150°

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