ECG CONVENTIONS AND INTERVALS

Transcription

1 1 ECG Waveforms and Intervals ECG waveforms labeled alphabetically P wave== represents atrial depolarization QRS complex=ventricular depolarization ST-T-U complex (ST segment, T wave, and U wave)== V repolarization. J point = junction between end of QRS complex -beginning ST Segment Atrial repolarization =too low in amplitude to be detected, may become apparent in acute pericarditis and atrial infarction ECG CONVENTIONS AND INTERVALS Depolarisation towards electrode: positive deflection Depolarisation away from electrode: negative deflection Sensitivity: 10 mm = 1 mv Paper speed: 25 mm per second Each large (5 mm) square = 0.2 s Each small (1 mm) square = 0.04 s HR= 1500/R-R interval (mm) (i.e. 300 No. of large sq.between beats) 1

2 2 Origin of ECG waves & segments 1-Normal Complexes (waves) 2-Normal Intervals 3-Normal Segments and Junctions P wave Origin=produced by atrial depolarization Length(duration)= 0.12 second long or less Amplitude(height)(voltage)=2.5mm Direction= leftward and inferiorly in the frontal plane normal P wave -positive in lead II and negative in lead avr. normal P wave in lead V1 =biphasic with a positive component reflecting right atrial depolarization, followed by a small (<1 mm2) negative component reflecting left atrial depolarization. 2

3 3 Abnormalities of P wave Absent (ventricular or supraventricular arrhythmias) Relation of P wave to QRS complex Long P wave = Right atrial enlargement or hypertrophy, or interatrial conduction delay (conduction system disease. Notched=left Atrial enlargement (MS) Retrograde P waves (negative in lead II, positive in lead avr) (activation of atria from ectopic pacemaker in lower part of either atrium or in AV junction region) PR interval measures Atrio-Ventricular conduction time= 1- time required for atrial depolarization 2- normal conduction delay in AV node (approx sec.) 3- impulse propagation through His bundle and bundle branches to the onset of ventricular depolarization. Normal PR interval= second related to heart rate and to prevailing autonomic tone 3

4 4 QRS Complex QRS-T waveforms of surface ECG correspond in general way with =different phases of simultaneously obtained ventricular action potentials (intracellular recordings from single myocardial fibers Rapid upstroke (phase 0) of AP = onset of QRS. Plateau (phase 2) corresponds to = isoelectric ST segment Active repolarization (phase 3) = inscription of T wave. Nomenclature of QRS complex (ventricular depolarization). Q (q) wave =initial negative deflection = onset of ventricular depolarization; R (r) wave =first positive deflection = ventricular depolarization S (s) wave =negative deflection = ventricular depolarization that follows first positive (R) wave. QS wave =negative deflection that does not rise above baseline. R' (r') wave = second positive deflection and follows an S wave; s wave= negative deflection that follows r is termed; If s wave does not follow initial R wave==> second positive deflection ==> R'(r') wave.qrs complex ==> Rr (rr') complex. 4

5 5 Capital letters (Q, R, S) =waves greater than 5 mm; lowercase letters (q, r, s) =waves less than 5 mm. QRS Phases of vent.depolarization 1-Depolarisation of interventricular septum =moves from left to right==>small initial negative deflection in lead V6 (Q wave) and initial positive deflection in lead V1 (R wave). 2-activation of body of LV, =large positive deflection or R wave in V6 (with reciprocal changes in V1). 3-final phase of depolarisation =RV ==>produces small negative deflection or S wave in V6. Appearance of QRS Complex during Normal V. dep. Right precordial lead (V1) =biphasic depolarization process = small positive deflection (septal r wave) followed by a larger negative deflection (S wave). Left precordial lead-v6= same sequence with a small negative deflection (septal q wave) followed by a relatively tall positive deflection (R wave). Intermediate leads =relative increase in R-wave amplitude (normal R-wave progression) and decrease in S-wave amplitude progressing across chest from right to left. Precordial lead where R and S waves are of approximately equal amplitude ==transition zone (usually V3 or V 5

6 6 Abnormalities of QRS complex 1-Wide QRS=bundle branch blo 2-High(tall) QRS = ventricular hypertrophy 3-Deep Q wave= pathological Q=M infarction 4-Progression of R w ave 0 6

7 7 ST segment J junction= point at which the QRS complex ends and t ST segment begins; cannot be easily discernible during rapid heart rates and hyperkalemia. can be depressed or elevated relative to the isoelectric baseline. ST segment = begins at J point and ends at onset of T wave. usually isoelectric - may vary from -0.5 to +2 mm in precordial leads; considered elevated or depressed compared with portion of baseline between end of T wave and beginning of P wave (TP segment). ST-segment abnormalities are important diagnostically in acute myocardial ischemia and infarction and pericarditis Abnormalities of ST segment ST elevation ST depression 7

8 8 T wave = deflection produced by ventricular repolarization. Normally= mean T-wave vector oriented roughly concordant with mean QRS vector (within about 45 in the frontal pla Abnormalities of T wave T-wave inversion = myocardial ischaemia or infarction, and electrolyte disturbances Hyperacute T-acute MI.hyperkalaemia 8

9 9 U Wave Normal U wave = small, rounded deflection ( 1 mm) that follows T wave and usually has same polarity as T wave. (usually positive) deflection preceding subsequent P wave; (??repolarization of intraventricular (Purkinje) conduction system Abnormities of U wave 1-Abnormal increase in U-wave amplitude drugs (dofetilide, amiodarone, sotalol, quinidine, procainamide, disopyramide) or 9

10 10 hypokalemia 2- Very prominent U waves = marker of increased susceptibility to torsades de pointes (type of ventricular tachycardia). 3-Negative U waves, best seen in leads V4-6, acute myocardial ischemia (insensitive but relatively specific markers of left anterior descending coronary artery disease) left ventricular hypertrophy 4- some circumstances(hypokalemia and hypomagnesemia)= U wave represent an oscillatory membrane potential, called afterdepolarization. Depolarisation towards electrode: positive deflection Depolarisation away from electrode: negative deflection Sensitivity: 10 mm = 1 mv Paper speed: 25 mm per second Each large (5 mm) square = 0.2 s Each small (1 mm) square = 0.04 s HR=Regular 1500/R-R interval (mm) (i.e. 300 No. of large sq.between How to read a 12-lead ECG: examination sequence Rate &Rhythm strip (lead II) =rate and rhythm Cardiac axis =leads I and lead 11(normal +ve) P-wave shape =Tall P waves /notched P waves PR interval Normal = secs.prolongation//short QRS duration> 0.12 secs = BBB 10

11 11 amplitude=large LVH Q waves= previous MI ST segment = elevation = MI, pericarditis or LVaneurysm depression = Ml isch. or infar) T wave= inversion = M ischaemia or infarction, electr.dist. U-wave =hypokalemia and hypomagnesemia Very prominent = increased susceptibility to torsades de pointes Negative U waves, ( V4-6) = in acute Mischemia +LVH QT interval =onset of Q to the end of T=Normal < 0.42 sec. QT Varies with HR, must be corrected(q-tc).max.=0.42(men),0.43 sec. (women) prolongation =congenital long QT syndrome, low K+, Mg2+ or Ca2+, drugs HEART RATE Regular- 1500/No.of sm.sq. between 2 R or 300/No.of large Irregular- No.of R in one min.=60 sec. or 3 sec(15 s sq.)x20 11

12 12 Cardiac axis average vector of ventricular depolarisation= frontal cardiac axis. vector at right angles to a lead= depolarisation in that lead is equally negative and positive (isoelectric). QRS complex=isoelectric in avl--negative in avr most strongly positive in lead II main vector or axis of depolarisation =60. normal cardiac axis lies between -30 and +90 QRS pattern in extremity leads - vary depending on electrical axis of QRS, (mean orientation of QRS vector) Normally=QRS axis ranges from 30 to axis more negative than 30 =left axis deviation, axis more positive than +100 right axis deviation. Left axis deviation normal variant left ventricular hypertrophy, block in ant. fascicle of left bundle system (left anterior fascicular block or hemiblock), inferior myocardial infarction. Right axis deviation 12

13 13 Normal variant (particularly in children and young adults) Spurious finding (reversal of left and right arm electrodes) Right ventricular overload (acute or chronic), Infarction of lateral wall of left ventricle Dextrocardia, Left pneumothorax Left posterior fascicular block. 13