EKG EKG InterpretationInterpretation

Lecture #1Lecture #1

Current Flow & Lead Axis• Critical Learning Points:

– If the electrical current from the heart is moving toward an electrode placed on the chest, the EKG complex will be positive (upward deflection)

– If the electrical current of the heart is moving away from the electrode, the EKG complex will be negative (downward deflection)

– The sum of all the electrical activities of the heart is consistent with the electrical activity of the left ventricle and is therefore directed downward and to the left of the body, approximately at 60°

EKG leads and their Axis

• 12 leads of the EKG:

– 6 in the vertical plane• I, II, III, aVR, aVF, & aVL

– 6 in the horizontal plane• V1 – V6

Cardiac Depolarization

1. SA NODE initiates Atrial Depolarization = P-wave2 AV NODE is depolarized; AV nodal delay = 0.10 sec

3. Depolarization is conducted via the Bundle of His4. Depolarization is conducted further into the ventricles

via the Left & Right Bundle Branches5. Septal Depolarization = Q-wave6. “Early” Ventricular Depolarization (depolarization

of the apex) = R-wave7. “Late” Ventricular Depolarization (depolarization

of the ventricular myocardium) = S-wave8. Ventricular Repolarization = T-wave

Vertical Axis = Voltage

• Vertical axis represents voltage on the EKG

• One small box (1 mm) represents 0.10 mV

• A tall vertical axis therefore can reflect increased voltage and/or hypertrophy

• A short vertical axis reflects decreased voltage– example: obesity, COPD, cardiac tamponade

Horizontal Axis = Time

• 1 small (1 mm) box = 0.04 seconds (40 ms)

• 1 large (5 mm) box = 0.20 seconds (200 ms)

• 5 large (5 mm) boxes = 1 second (1000 ms)

• 15 large (5 mm) boxes = 3 seconds and is marked

on EKG paper

Approach to EKG Interpretation

• ALWAYS, ALWAYS, ALWAYS:

1. Rate

2. Rhythm (includes analysis of intervals)

3. Axis

4. Hypertrophy

5. Ischemia, Injury, or Infarct

Determining the Heart Rate

• Count # of large boxes between 2 successive R-waves:

– 1 box = 300 bpm– 2 boxes = 150 bpm– 3 boxes = 100 bpm– 4 boxes = 75 bpm– 5 boxes = 60 bpm– 6 boxes = 50 bpm– 7 boxes = 43 bpm– 8 boxes = 37 bpm

l

Determining the rate forirregularirregular rhythms

• If the R-R Interval is irregular:

– Count the number of QRS complexes in a 10 sec span (that is on the entire EKG) and multiply it by 6! {or no. of QRS complexes in a 6 sec span multiplied by 10}

1. First ALWAYS determine whether the rhythm is Sinus or Non-Sinus (SVT vs. VT)!

2. Determine the Intervals:

a. P-wave duration

b. PR intervalc. QRS intervald. QT interval

Determining the Cardiac Rhythm

1. Every QRS is preceded by a P-wave

2. P-waves appear normal, that is they are of sinus node origin:

A. Normal Morphology:

1. P-wave duration < 0.12 sec (< 3 boxes)

2. P-wave height < 2.5 mm

B. Normal Axis – upright P-waves in lead II

Sinus Rhythm…or Not!

EKG Intervals

1. P-wave < 0.110 sec (approximately 3 small boxes)

2. PR interval = beginning of the P-wave to the

beginning of QRS. Normal = 0.120 – 0.200 sec

3. QRS interval = from the first deflection to return to the baseline. Normal < 0.120 sec

4. QT interval = beginning of the QRS to the END of the T-wave. Normal < 0.450 sec

InterpretingInterpretingEKG AxisEKG Axis

Axis Basics

• The axis measures the direction of the overall electrical activity of the heart

• Hypertrophy of either ventricle can displace the axis:– Left Ventricular Hypertrophy (LVH) can

result in Left Axis Deviation (LAD)– Right ventricular Hypertrophy (RVH) can

result in Right Axis Deviation (RAD)– Similarly Bundle Branch Blocks can distort

the axis

Approach to EKG Axis

• The Limb Leads are used to determine the axis

• Normal axis usually results in a + QRS complex (majority of the complex is above the baseline) in leads I, II, III & aVF

• A normal axis falls between 0° & +90°

• However, many authorities consider a normal axis between –30° & +105°

0

-90

I

aVF

+_

Lead I If lead I is positive, the green zone reveals thearea of electrical activity

0

-90

I

aVF

If aVF is positive, the red zone reveals the areaof electrical activity

–

+

aVF

0

-90

I

aVF If we superimpose these onto one another we find the axis to be between 0° & +90°

+90

EKG with a Normal Axis.

LAD

• May be caused by: HTN, aortic valvular disease & cardiomyopathies

• Once you have determined that the axis lies between 0° & –90°, it is clinically useful to decided if it is between –30° & –90°, as this is true LAD

0

-90

I

aVF

+_

If lead I is positive then the blue zone is the area of electrical activity

+90

Lead I

0

-90

I

aVF

+

_If aVF is negative, the green zone is the area of electrical activity

+90

aVF

0

-90

I

aVF

If we superimpose these onto one another we find the axis to be between 0° & –90°

+90

True LAD

• So again, if lead I is positive and aVF is negative, we suspect LAD

• To diagnose true LAD, we examine lead II:

– If lead II is positive, axis = 0° to –30°– If lead II is negative , axis = –30° to –90°

EKG with true LAD

RAD

• Right axis deviation is usually secondary to an enlarged right ventricle or pulmonary disease

• Some of the ethiology include: pulmonary HTN, COPD or acute PE

0

-90

I

aVF

If lead I is negative thegreen zone encompassesthe area of electrical activity

+90

180

+_Lead I

0

-90

I

aVF

If aVF is positive, the red zone reveals the areaof electrical activity

+90

180

+

_aVF

0

-90

I

aVFIf we superimpose these onto one another, we find the axis to be between 90° & 180°

+90

180

Extreme RAD

• If lead I is negative AND aVF is also negative – extreme RAD

• Clue: If aVR is positive = extreme RAD• This is seen with rare situations such as VT or

Pacemakers• In general we should never have an axis over

here!

EKG with Right Axis Deviation

EKG Axis Summary• look at lead I and aVF:

– if both are positive = Normal axis

– If I is positive but aVF is negative = probable LAD• If between 0° & –30° = upper limits of normal• If between –30° & –90° = True LAD (check lead II to

determine)

– If I is negative = RAD (axis > 90°)…you can be more specific and decide:

• If between +90 & +120 = upper limits of normal• If between +120 to +180 = True RAD

– When both I & aVF are negative, axis is between -90 and +/-180 – this is Extreme RAD

• Lead I = left thumb and aVF = right thumb

– If both I & aVF are up = Normal Axis

– If I is up but aVF is down = LAD

– If I is down but aVF is up = RAD

– If both I & aVF are down = Extreme RAD

Finally: EKG Axis for Dummies!