EKG Interpretation Lecture #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-wave 2AV NODE is depolarized; AV nodal delay = 0.10 sec 3. Depolarization is conducted via the Bundle of His 4. Depolarization is conducted further into the ventricles via the Left & Right Bundle Branches 5. Septal Depolarization = Q-wave 6. “Early” Ventricular Depolarization (depolarization of the apex) = R-wave 7. “Late” Ventricular Depolarization (depolarization of the ventricular myocardium) = S-wave 8.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

irregular Determining the rate for irregular 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 interval c. QRS interval d. 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 < sec (approximately 3 small boxes) 2.PR interval = beginning of the P-wave to the beginning of QRS. Normal = – sec 3.QRS interval = from the first deflection to return to the baseline. Normal < sec 4. QT interval = beginning of the QRS to the END of the T-wave. Normal < sec

Interpreting EKG 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 the area of electrical activity

0 -90 I aVF If aVF is positive, the red zone reveals the area of 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 the green zone encompasses the area of electrical activity _ Lead I

0 -90 I aVF If aVF is positive, the red zone reveals the area of electrical activity _ aVF

0 -90 I aVF If we superimpose these onto one another, we find the axis to be between 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!