VECTORIAL ANALYSIS OF ECG

Transcription

1 LEARNING OBJECTIVES At the end of the lecture student should VECTORIAL ANALYSIS OF ECG 1. Understand about vector and the principles apllied in plotting electrical vector of heart 2. Understand the relationship between vectors generated in the heart and EKG waves 3. Be able to assign appropriate vectors to depolarizing and repolarizing signals at different angles to the recording electrodes 4. Understand the concepts of instantaneous vectors and net vectors 5. Be able to dissect a vector loop into component instantaneous vectors 6. Be able to reconstruct a vector loop from individual traces 7. Understand how the hexaxial reference system is generated and be able to identify the individual axes At the end of the lecture the student should be able to know : About the rate of ecg tracing About how to calculate the rate from ECG tracing About ruleof 300 About sinus rhythm About sinus brady and tachycardia About overdrive supression INTRODUCTION TO VECTOR

2 During each cardiac cycle current flows in a particular direction The heart is a functional syncytium Depolarization (or repolarization) of one cell will depolarize (or repolarize) adjacent cells Many dipoles will be formed The electrical fields generated by summating dipoles are represented by vectors Any change in the direction of impulse transmission causes the abnormal electrical potentials around the heart which alters the shapes of waves in the ECG THEORETICAL CONCEPTS OF VECTOR ANALYSIS A wave of depolarization or repolarization moving Towards the positive electrode of a lead will cause a positive (upward) deflection away from the positive electrode of a lead will cause a negative (downward) deflection perpendicular to a lead will cause no net deflection. AXIS OF LIMB LEADS Each lead have two electrodes direction from ve to +ve electrode is called axis of the lead Axis of lead 1 is zero (0) degree because electrode lies in the horizontal direction with +ve to left side (left arm) Axis of lead II is degree III degree avr degree avl degree

3 avf degree EINTHOVEIN S TRIANGLE Triangle around the area of heard Apices represent the positive electrodesof bipolar limb leads Einthovein s law: Sum of voltage in any 2 leads is equal to voltage in 3 rd lead HEXAXIAL GRID To work with vectors in the frontal plane, a combination of the three standard limb leads and the three augmented limb leads is often used The common way to represent this is as a hexaxial grid Hexaxial = 6 axes, 1 for each lead VECTOR Arrow that shows the direction of electrical potentials generated by elect flow Arrowhead in the positive direction Length proportional to voltage of potential Direction of vector is denoted in degrees e g. When vector is horizontal and is directed to left side the vector is said to extend in the direction of 0 degree When extends straight down wards it is equal to 90 degrees MEAN QRS VECTOR

4 Normally direction of vector during spread of depolarization wave through ventricle called QRS vector is about +59 degree VECTOR ANALYSIS OF POTENTIALS RECORDED IN DIFFERENT LEADS When we use vector and axis together we will be able to determine the potentials recorded in ECG of each lead If we know the direction and amplitude of instantaneous mean vector of heart, by using axis of limb leads we can draw vectors and know about potential and direction of deflection in each limb lead VECTOR ANALYSIS - FROM THEORY TO APPLICATION: Determining the Mean QRS Axis from EKG Traces

5 Most often used to determine the direction of depolarization (or repolarization) of the atria and ventricles Most common use of vector analysis is to determine the mean axis of depolarization of the ventricles Referred to by several additional names (mean QRS axis, or often simply mean axis) By determining the mean axis of ventricular depolarization information is obtained about the conduction pathway, anatomical abnormalities. Stepwise Determination of Mean Axis Using the Triaxial Method Using the following tracing

6 STEP 3 Draw perpendiculars from the apices of two net vectors of two lead STEP 4 The point of intersection of these two perpendicular line represent the apex of mean QRS vector The point of intersection of lead I and III axes represent the negative end of mean QRS vector Mean QRS vector is drawn The length determines the magnitude of average potential generated by ventricles during depolarization Direction represent mean electrical axis calculating AXIS FROM LEAD I AND avf Using Lead I and AVF: Lead I: +10 mm AVF: mm Use 2 divisions per mm to keep vector arrows on scale. Draw perpendiculars from heads of arrows. Draw head of mean arrow at intersection of perpendiculars. the use of AVF and Standard Limb Lead I is a common shortcut

7 CAUSES OF AXIS DEVIATION Common Causes of Left Axis Deviation: a. Left ventricular hypertrophy. b. Pregnancy. c. Obesity. d. Infarct in right ventricle. Common Causes of Right Axis Deviation: a. Right ventricular hypertrophy. b. Infarct in left ventricle. c. Slight right axis deviation may be normal for children and very tall, thin adults. VECTOR ANALYSIS OF THE WAVE OF DEPOLARIZATION IN THE VENTRICLES At time zero (T0),before depolarization starts, the ventricles are completely polarized the cells are all at their resting membrane potential no voltage differences between points there is no vector at this time. At T1 the septum begins to depolarize from the patient s left to right. the sum of all the electrical activity in the ventricles can berepresented by a short vector, oriented as indicated. VECTOR ANALYSIS OF THE WAVE OF DEPOLARIZATION IN THE VENTRICLES

8 At T2 the septum, the apex, most of the right ventricle, and part of the left ventricle are depolarized magnitude of the vector is large directed along the axis of the heart from base to apex At T3 the sum of the electrical activity is directed towards the left smaller in magnitude than at T2. Except for posterior basilar portion of the ventricle, all ventricle is depolarized at this time. VECTOR ANALYSIS OF THE WAVE OF DEPOLARIZATION IN THE VENTRICLES At T4 the remaining portion of the ventricle depolarizes. represented by a small vector directed towards the base of the left ventricle At T5 the ventricles are completely depolarized there is no voltage difference between different locations no vector. VECTORCARDIOGRAM The sum of the instantaneous vectors leads to the waveforms we record as an vectorcardiogram RATE AND RHYTHM OF ECG RATE

9 REVISION OF ECG PAPER CALIBRATION 1 inch = 1 second Each inch is divided by dark black lines into 5 big square Each big square= 1/5=0.2 sec Each big square is further divided into 5 small square 1 small square = 0.2/5= 0.04 sec 1smallsquare = 0.04 sec =1mm 1 second =25 smallsquare 60second =1min =25 X 60=1500 small square =300 big square DETERMINING THE HEART RATE RULE OF 300 Take the number of big boxes between neighboring QRS complexes divide this into 300 The result will be approximately equal to the rate this method only works for regular rhythms. # of big boxes Rate

10 RYTHM SINUS RYTHM Normal rhythm of heart Cardiac impulse originated in SA node, atria depolarize Represented by P wave Travel down to AV node AV nodal delay Represented by PR interval

11 Impulse travel down to purkinje fibers Ventricles depolarize Represented by QRS complex Then repolarize Represented by T wave Again SA node send another impulse and cycle repeats Sinus node discharge these impulse at a pace of /min SINUS RYTHM Rhythm that originated by SA node on ecg, P wave followed by QRS complex QRS complex followed by P impulses per min SINUS BRADYCARDIA Sinus rhythm Originated in SA node P wave followedby QRS complex Rate slower than 60/ min

12 SINUS TACHYCARDIA Sinus rhythm Originated in SA node P wave followed by QRS complex Rate more than 100/min SINUS ARRYTHMIA Normal physiological mechanism Minimal variation in pace of SA node with respiration Minimal increase in heart rate with inspiration Inspiration- activated sympathetic stimulation of SA node Minimal decrease in heart rate with expiration Expiration activated parasympathetic stimulation of SA node

13 AUTOMATICITY FOCI Potential pace makers in heart that have the ability to pace whenever SA node fails to do so Pace at their own inherent rate Atrial automaticity foci in atrial conductionsystem Rate is /min Junctional automaticity foci Middle and distal portion of AV beats/min Ventricular automaticity foci In bundleof Hisand bundlebranches 40 20beats/min OVERDRIVE SUPPRESSION Heart s fail safe pacing mechanism Functional characteristic of all automaticity foci Rapid pace making activity suppresses any other slower pace making activity Normally SA node has the highest pacing rate--- overdrive suppress all other foci If one focus fail the next in line, no longer over suppressed and

14 become the pace maker overdrive-suppressing all other foci REFERENCES Guyton and Hall text book of physiology THANKS