Electrocardiography – Normal 6 Faisal I. Mohammed, MD, PhD

Objectives Recognize the normal ECG tracing Calculate the heart rate Determine the rhythm Calculate the length of intervals and determine the segments deflections Draw the Hexagonal axis of the ECG Find the mean electrical axis of QRS (Ventricular depolarization)

Principles of Vectorial Analysis of EKG’s The current in the heart flows from the area of depolarization to the polarized areas, and the electrical potential generated can be represented by a vector, with the arrowhead pointing in the positive direction. The length of the vector is proportional to the voltage of the potential. The generated potential at any instance can be represented by an instantaneous mean vector. The normal mean QRS vector is 60o (-30◦ - 110◦)

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Mean Vector Through the Partially Depolarized Heart + _ _ _ _ + + _ _ _ + _ _ + _ _ _ + _ _ _ _ _ + _ + _ _ + _ _ _ _ + _ _ _ + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ +

Einthoven’s triangle and law  +   + +

Axes of the Three Bipolar and Augmented Leads Principles of Vectorial Analysis of EKG’s (cont’d) Axes of the Three Bipolar and Augmented Leads + _ 60o 120o 0o I III II + _ aVF 90o + _ -30o aVL + _ 210o aVR

Axes of the Unipolar Limb Leads + + aVL aVR - aVF +

Principles of Vectorial Analysis of EKG’s (cont’d) The axis of lead I is zero degrees because the electrodes lie in the horizontal direction on each of the arms. The axis of lead II is +60 degrees because the right arm connects to the torso in the top right corner, and left leg connects to the torso in the bottom left corner. The axis of lead III is 120 degrees.

Principles of Vectorial Analysis of EKG’s (cont’d)

Principles of Vectorial Analysis of EKG’s (cont’d) In figure B, the depolarization vector is large because half of the ventricle is depolarized. Lead II should be largest voltage when compared to I and III when the mean vector is 60o. In figure C, left side is slower to depolarize. In figure D, the last part to depolarize is near the left base of the heart which gives a negative vector (S wave). Q wave is present if the left side of the septum depolarizes first.

The T Wave (Ventricular Repolarization) First area to repolarize is near the apex of the heart. Last areas, in general, to depolarize are the first to repolarize. Repolarized areas will have a + charge first; therefore, a + net vector occurs and a positive T wave

Atrial Depolarization (P-Wave) and Atrial Repolarization (Atrial T Wave) Atrial depolarization begins at sinus node and spreads toward A-V node. This should give a + vector in leads I, II, and III. Atrial repolarization can’t be seen because it is masked by QRS complex. Atrial depolarization is slower than in ventricles, so first area to depolarize is also the first to repolarize. This gives a negative atrial repolarization wave in leads I, II, and III

Vectorcardiogram This traces vectors throughout cardiac cycle. When half of the ventricle is depolarized, vector is largest. Note zero reference point, number 5, is point of full depolarization.

Determining Mean Electrical Axis Use 2 different leads Measure the sum of the height and the negative depth of the QRS complex Measure that vaule in mm onto the axis of the lead and draw perpendicular lines The intersection is at the angle of the mean axis.

Plot of the Mean Electrical Axis of the Heart from Two Electrocardiographic Leads III _ -60o I _ + I I II 180o 0o 60o III + 120o III

90◦ 180◦ 0◦ +90◦ aVF SEVERE RIGHT  OR LEFT AXIS DEVIATION OF QRS From 180 to 360 (-90) 90◦ LEFT AXIS DEVIATION OF QRS Lead I  Lead I + 180◦ 0◦ NORMAL MEAN ELECTRICAL AXIS OF QRS From 0 to +90 RIGHT AXIS DEVIATION OF QRS From +90 to + 180 +90◦ aVF +

Heart Rate Calculation R-R interval = 0.83 sec Heart rate = (60 sec)/(0.83 sec) = 72 beats/min min beat

ECG Calculations

ECG Calculations

Determine regularity Regular Look at the R-R distances (using a caliper or markings on a pen or paper). Regular (are they equidistant apart)? Occasionally irregular? Regularly irregular? Irregularly irregular? Interpretation? Regular

Atrial repolarization ECG Deflection Waves Atrial repolarization (Pacemaker) As you will be required to draw and label electrocardiogram (ECG) deflection waves and describe what each indicates, let us consider a typical Lead II ECG. An ECG is a recording of the he deflection waves caused by depolarization of the heart. * When the SA node * (the pacemaker) * depolarizes * the wave of depolarization that sweeps through the atria is recorded as the P wave * on the ECG. * The P wave indicates depolarization of the atria. * The QRS complex * is caused by depolarization of the ventricles. * Hidden in the QRS complex * is the repolarization of the atria since that occurs while the ventricles are depolarizing. * The T wave * represents repolarization of the ventricles. *

60 seconds ÷ 0.8 seconds = resting heart rate of 75 beats/minute ECG Deflection Waves 60 seconds ÷ 0.8 seconds = resting heart rate of 75 beats/minute On the ECG, the PQ interval * is the time required for the wave of depolarization to spread through the conduction system from the beginning of atrial excitation to the beginning of ventricular depolarization. * Note that the PQ interval normally takes approximately 0.2 seconds in a resting heart. * The QT interval * represents the period from the beginning of ventricular depolarization through ventricular repolarization. Since the ventricles are so much larger than the atria, this normally takes approximately 0.4 seconds in a resting heart. * Another 0.2 seconds typically occurs before the next T wave. Since this cycle requires approximately 0.8 seconds to complete, 60 seconds divided by 0.8 gives us a resting heart rate * of approximately 75 beats/ minute. A PQ interval longer than 0.2 seconds is indicative of a 1st degree heart block, * meaning that something is delaying the spread of the wave of depolarization through the conduction system. * 1st Degree Heart Block = P-Q interval longer than 0.2 seconds.

ECG Deflection Wave Irregularities Enlarged QRS = Hypertrophy of ventricles Since you may be required to indicate conditions which may alter ECG waves, lets consider several of these. * An enlarged QRS * * is indicative of hypertrophy (enlargement) of the ventricles. * * *

ECG Deflection Wave Irregularities Prolonged QT Interval = A prolonged QT interval * * is indicative of repolarization abnormalities * which increase susceptibility to various ventricular arrhythmias. * Repolarization abnormalities increase chances of ventricular arrhythmias.

ECG Deflection Wave Irregularities Elevated T wave : Hyperkalemia You may recall from our previous study of electrolytes, * that an elevated T wave * is indicative of hyperkalemia, a condition which if not corrected may become life threatening. *

ECG Deflection Wave Irregularities Flat T wave : Hypokalemia or ischemia Likewise, a flat T wave * * is indicative of hypokalemia or ischemia. * *

Thank You