Introduction_to_the_ECG_2023.V4.pptx

Introduction to the ECG Oliver W. Hayes, DO, MPH Cardiovascular System I October 17, 2022 8:00 – 8:50 AM Objectives Review basic principles regarding ECGs which include: • Distinguish between monitor and ECG; • Explain how the heart’s electrical current is generated; • Describe current propagation...

Introduction to the ECG Oliver W. Hayes, DO, MPH Cardiovascular System I October 17, 2022 8:00 – 8:50 AM Objectives Review basic principles regarding ECGs which include: • Distinguish between monitor and ECG; • Explain how the heart’s electrical current is generated; • Describe current propagation through four heart chambers; • Identify heart’s electrical activity on ECG; • Explain how ECG machine detects and records these waves; • Discuss that ECG looks at heart from 12 different perspectives, providing a three-dimensional electrical map; • Recognize and understand lines and waves on 12-lead ECG; and, • Complete exercises to practice learning about ECGs. Research by Willem Einthoven, MD • Brilliant Dutch physician and scientist who developed ECG • “Now we can record a heart’s abnormal electrical activity . . . and compare it to the normal.” • ElectroKardioGram • EKG or ECG ECG and Monitor • ECG is inscribed on ruled paper (originally a strip of paper now a sheet of paper). • Permanent record • 12 Leads • Cardiac Monitor provides continuous readout of cardiac activity often with assessment of a patient’s condition. • Wall mounted in CCU or portable • Only 1 or 2 leads Basic Principles • ECG records electrical impulses that stimulate heart to contract. • Normally, electrical stimulation begins at SA node in right atrium. • Cardiac pumping cycle is controlled by heart’s own electrical system, capable of working without input from CNS. SA Node Basic Principles • Heart cells are charged (polarized) in resting state and when stimulated, cells depolarize and contract. • Resting myocyte is polarized, cell interior is negatively charged. • When stimulated, interior of myocyte changes from negative to positively. • Electrical stimulation of myocardial cells called depolarization and causes contraction. Basic Principles • Thus, a progressive wave of stimulation (depolarization) passes through heart causing contraction. • Depolarization is an advancing wave of positive charges. • Electrical stimulus (depolarization) causes progressive contraction of myocardial cells as a wave of positive charges advances. • After the heart depolarization, it Blue Arrow represents normal overall repolarizes. direction (vector or axis) of depolarization, that is from right side downward toward left, which is called Basic Principles • Depolarization and repolarization of various regions of the heart causes waves are recorded on ECG. • Stimulation (depolarization) charges myocyte cells interior positively. • During repolarization myocytes regain negative charge within cell. • P wave – atrial depolarization • QRS waves – ventricular depolarization • Atrial repolarization is not seen but occurs during the QRS time period. • ST Segment and T wave (and U wave) – ventricular repolarization Basic Principles • Depolarization is an advancing wave of positive charges. • Wave of stimulation moving towards a skin sensor (lead) produces an upward ECG deflection. • Upward deflection on ECG due to a depolarization stimulus moving toward skin sensor. Cell Skin Sensor Cel l ECG Tracing Basic Principles • Heart’s electrical activity can be picked up by sensors on skin and recorded as an ECG. • Here are shown precordial sensors (or leads) on patient’s chest wall. • Depolarization/repolarization are electrical activities, but only depolarization associated with contraction. Basic Principles • Wave of depolarization begins at SA node sweeping through the atria and is picked up by skin sensors. • Atrial stimulation is recorded on an ECG as a P wave. • There are conduction bundles between SA node and AV node/L atrium. Basic Principles • When the depolarization impulse reaches AV node, there is 0.1 second delay. • This 0.1 second pause allows blood to pass through AV valves into ventricles due to atrial contraction. Basic Principles • After a pause, AV node receives impulse from atria. • Stimulus passes from AV node down His bundle to R/L bundle branches. • As stimulus passes away from AV node, ventricular depolarization is initiated. Blue Arrow represents the normal general direction of heart depolarization, that is from right side downward toward left, which is called axis of depolarization. Basic Principles • Impulse travels from AV node to His bundle R/L Bundle Branches ending in Purkinje Fibers. • QRS occurs on ECG as stimulus passes through conduction system and myocardial cells of ventricle. • QRS complex (or some variation seen on the left) represents ventricular depolarization. • Waves of Ventricular Depolarization • Ventricular depolarization causes the QRS waves to occur; taken together the waves are call the QRS complex. • Q wave is 1st downward wave of QRS complex. • R wave is 1st upward deflection of QRS complex. • Any downward wave PRECEDED by an upward wave is a S wave. • Entire QRS complex represents ventricular depolarization. Q wave Q, R, and S waves together are called the QRS complex Basic Principles • Depending on which lead is recording the QRS complex, it looks different. • Name each numbered waves in the picture: 1._____ 2._____ 3._____ 4._____ Q wave, R wave, S wave, QS wave (this is a little unfair, because there is no upward wave, cannot determine whether 4 is a Q wave with small S wave. Therefore, it is called a QS wave, but it will be considered a Q wave later in Basic Principles • Variation of the QRS complex with cardiac pathology • 1st positive deflection in complex is an R wave with negative deflection after an R wave is S wave. 2nd positive deflection after S wave, if there is one, is called R’ (pronounced R prime) wave. • There is no Q wave here. • This would be written rSR’ meaning that first r wave is smaller than second R wave. rSR’ Basic Principles • Following QRS, a horizontal baseline known as ST segment which is followed by broad T wave. • Physiologically, this is time ventricles are contracting and emptying. • Electrocardiography, ST segment connects QRS complex the T wave • Normally it is level with baseline. U The blue line does not appear on an ECG, I have placed it there to show the baseline. Basic Principles • Following QRS, a horizontal baseline known as ST segment which is followed by broad T wave. • ST segment elevation or depression is usually a sign of serious pathology. Basic Principles • There are nuances in ST segment interpretation. • Normal Variant "Early Repolarization" (usually concave upwards, ending with symmetrical, large, upright T waves). • This is a normal variant. • In the lower image the ST segment elevation is convex upwards and represents pathology. Basic Principles • T wave represents final/rapid phase of ventricular repolarization. • During repolarization ventricular myocytes recover their negative charge. • Ventricular repolarization begins after QRS and persists until end of T wave. T wave represents the final rapid phase of ventricular repolarization. Basic Principles • T wave represents final/rapid phase of ventricular repolarization. • During repolarization ventricular myocytes recover their negative charge. • Ventricular repolarization begins after QRS and persists until end of T wave. T wave represents the final rapid phase of ventricular repolarization. Basic Principles • Ventricular systole = beginning of QRS to end of T wave and is measured as QT interval. • QT interval represents duration of ventricular contraction and is measured from beginning of QRS to end of T wave. • QTc is corrected interval. • The QT interval has normal duration. But some folks have a long QT interval syndrome, which is a risk for sudden cardiac death. Systole or contraction Basic Principles • Physiologically, cardiac cycle is atrial systole, followed by ventricular systole, rest, then repeated (until you die). • Atrial depolarization is represented by P wave. • Ventricular depolarization is represented by QRS complex, and ventricular repolarization is the ST segment and T wave. Ventricular Atrial Contractio Contraction n Recording the ECG Recording the ECG On this ECG, there is information which has not been put in yet; name of patient, date of ECT, as well as other information. Although ECG are similar, sometimes laid out a little differently; here the lead ID are above the actual wave patterns Recording the ECG • ECG recorded on graph paper, smallest square = 1 mm, red arrow • Smallest squares 1 millimeter long and 1 millimeter high. • Between heavy red lines, five small squares. • Time axis is horizontal; moves left to right. • Each small box = 40 milliseconds or 0.04 seconds • Each large box is 200 milliseconds or 0.2 second • Vertical axis measures number of millivolts from ECG signal. • Each small box = 0.1 mv Recording the ECG • Wave height and depth measured from baseline in mm; and this amplitude represents a measure of voltage. • R wave is 4 mm high or + .4 millivolts • S wave is 6 mm deep or – .6 millivolts { 4 mm high } 6 mm deep Recording ECG • Measurement of horizontal axis, determines duration of any part of cardiac cycle on ECG. • Beginning of P wave beginning of the R wave (or QRS complex) is four small squares = .16 of a second. • Duration of the RS complex is two small boxes or 0.08 seconds. • PR interval = 0.16 sec or 160 millisec • QRS duration = 0.08 sec or 80 millisec • These time amounts have clinical significance when long or short. • Prolonged or shorten PR interval or widened QRS complex (bundle branch block) PR Interval QRS Duration Recording ECG • As a stimulus is moving towards a sensor, deflection is upward. • As a stimulus is moving away from a sensor, deflection is downward. Stimulus Directio n Recording ECG • Heart depolarizes from inside towards the outside. • Heart is three dimensional, therefore some stimuli are moving away from sensors and some stimuli are moving towards sensors. • Therefore, a sensors on chest wall measuring combination of stimulus of anterior and posterior walls of with depolarization moving in opposite directions. Direction of Depolarization Recording ECG • Standard ECG has 12 separate sensors (or leads). • Standard ECG has six limb leads recorded using arm and leg sensors. • Also, standard ECG has leads obtained from sensors on chest wall. • Limb leads and chest leads provide 12 different electrical views of the heart. Recording ECG • Limb leads are from sensors (electrodes) placed on right arm, left arm and left leg. • By placing sensors on right arm, left arm and left leg, you can record limb leads. • Placement of these electrode leads is same as by Einthoven. Recording ECG • Limb leads are bipolar using two separate sensors. • Each limb lead consists of a pair of electrodes; one positive and one negative, therefore bipolar. • Horizontal lead I, left arm is positive, right arm is negative. • Lead III, left arm is Recording ECG • The triangle has a center, and each lead is pushed towards center point (moving from Figure A to Figure B). • By pushing leads I, II, and III to triangle center, intersecting reference lines are formed. • Although three leads are moved to center of triangle, same angles relative to each other are maintained. Recording ECG • Another standard lead is AVF. • AVF uses left foot electrode as positive. • In AVF, both right and left arm electrodes are channeled into a common ground that has a negative charge. • A stands for augmented, in that signal is amplified, sometimes written aVL Recording ECG • Remaining augmented limb leads are AVR and AVL. • For AVR lead, right arm electrode is positive, and two other electrodes are negative. • To obtain AVL lead, left arm electrode is positive; and the other two electrodes are negative. Recording ECG • Augmented limb leads AVF, AVL, and AVR, intersect at different angles then bipolar limb leads. • Augmented limb leads intersect at 60° angles, but angles differ from bipolar limb leads. • Start at lead 1 with zero degrees and work around. • The diagram plots out the limb lead position. This diagram must be memorized. 15 0 Intersect Recording ECG By having multiple pictures of the car, you can evaluate it better, similar with the ECG, with having various limb leads, you get more information about the heart. Recording ECG • Having six limb leads is like getting different camera views of heart’s electrical activity. • ECG records same cardiac electrical activity in each lead but from a different viewpoint. Recording ECG • Importance of positive sensor is emphasized by conventional grouping of limb leads. • AVL and I look at left lateral aspect of heart’s electrical activity. • II, III, and AVF look at the inferior aspect of heart’s electrical activity. • This is all in frontal plane (or top to bottom plane). I and AVL are “lateral” lead II, III, and AVF are “inferior” Recording ECG • Chest (precordial) leads in horizontal plane (front to back) oriented through AV node. • Leads V1- V6 like spokes in a wheel, center is AV node. • V1 and V2 are chest leads and provide a view of interventricular septum. • V3 and V4 are chest leads provide a view of anterior wall of the heart. • V5 and V6 are chest leads and provide a view left side of heart. Recording ECG • V1-V2 (“septal leads”): primarily observes the ventricular septum, but may occasionally display ECG changes originating from right ventricle. • Note that none of leads in 12-lead ECG are adequate to detect vectors of the right ventricle. • V3-V4 (“anterior leads”): observes the anterior wall of the left ventricle. • V5-V6 (“anterolateral leads”): observes the lateral wall of the left ventricle. Recording ECG • Normally progressive changes in ECG waves moving V1 to V6 • V1 QRS complex is mainly negative (downward) because signal is combination of right ventricle towards and left ventricle away. • V6 QRS complex is mainly positive (upward). • Normally R wave grows taller and S wave disappears moving from V1 to V6 because the left ventricle is the thickness part of the heart and generates more electrical signal. In this tracing of the precordial leads, R wave is growing and S wave is shrinking from V1 to V6. Why Poor R Wave Progression This ECG shows low vertical amplitude (voltage – less mv) of QRS and poor R wave progression (it does not get larger going from V1 to V6 secondary to Recording ECG • The six limb leads are all in the frontal plane (top to bottom), which can be visualized on the patient’s chest. • The six chest leads lie in the horizontal plane. Limb lead s I II III AVR AVL AVF Chest Leads V1 V2 V3 V4 V5 V6 Recording ECG ECG Interpretation – CV II • Knowing basic principles of electrocardiograms, we will in CV II move onto interpretation of normal and abnormal ECGs • Rate – determining the ventricular rate • Rhythm – determining if the rhythm is normal or abnormal. • Axis – determining the heart’s overall axis of depolarization • Hypertrophy – determining if any of heart chambers are enlarged • Infarction – determining if there is evidence of an evolving, new, or old myocardial infarction is present • Miscellaneous – reviewing ECG findings to assist with effects of lungs, electrolytes, medications, artificial pacemakers, or transplant