BASIC ECG INTERPRETATION Electrophysiology The cardiac cell has 2 primary electrolytes: Sodium (Na+) and Potassium (K+). Resting cardiac cell (K+) is mostly on the inside, while (Na+) is mostly on the outside of the cell. Sodium has a stronger charge than potassium making the outside of the cell more positive that the inside. Positive and negative charges are balanced in a resting state which equals no electrical charges = polarized state. Cells are able to modify their membrane and pull sodium into the cell while potassium exits the cell. Resulting in the initiation of electrical flow = Sodium Pump. Polarization – The phase of readiness; the muscle is relaxed and the cardiac cells are ready to receive an electrical impulse Depolarization – The phase of contractions; the cardiac cells have transmitted an electrical impulse, causing the cardiac muscle to contract Repolarization – The recovery phase; the muscle has contracted and the cells are returning to a ready state. Conduction System 1. Sinoatrial (SA) Node 2. Intraatrial Pathway 3. Intranodal Pathway 4. Atrioventricular(AV) junction 5. Bundle of His 6. Left Bundle Branch 7. Right Bundle Branch 8. Purkinje Fibers Inherent Rates SA Node 60-100 BPM AV Junction 40-60 BPM Ventricle 20-40- BPM Nervous System Influence Sympathetic – Affects the atria and the ventricles Increases: Heart Rate Conduction Irritability Parasympathetic- Affects the atria Decreases: Heart Rate Conduction Irritability Irritability is when a site speeds up and takes over as the pacemaker. Measurements If electricity flows toward the (+) electrode, the pattern will be upright. If electricity flows toward the (-) electrode, the pattern will be downward. Graph Paper Measurements One large box = .20 sec One small box = .04 sec One small box vertical = 1 mm When properly calibrated a 1 Millivolt will produce a defection measuring 10 mm tall. Waves P Wave Usually the first wave form Originates in the SA node Stimulates the Right Atrium Reaches AV node Downslope of the P wave reflects stimulation of Left Atrium Normal Characteristics Smooth and rounded From beginning of P wave to beginning of QRS complex Includes P wave and PR segment Normal duration: 0.12 to 0.20 second No more than 2.5 mm in height No more than .11 second in duration (+) in Leads I,II, aVF and v2-v6 Abnormal Characteristics Tall, Pointed, wide, notched P wave may reflect COPD, CHF, valvular disease, atrial enlargement QRS Complex QRS Complex represents ventricular depolarization. Q Wave Always negative waveform, 1st negative wave after P wave. Depolarization of interventricular septum Normal Less than .04 sec in duration Less thatn1/3 the height of R wave Abnormal More than .04 sec More than 1/3 height of R wave R Wave 1st positive wave following P wave S Wave Negative waveform following R Wave RS – Depolarization of right and left ventricle If a QRS complex is entirely positive it is considered an R wave. If the QRS complex is entirely negative it is considered a QS wave. If there are 2 positive deflections in the same complex the 2nd is called R prime written R1 . If there are 2 negative deflections in the same complex following a R wave it is called S prime, written S1 QRS Duration = .08 - .12 second Abnormal QRS Complex QRS measuring .10-.12 second can be considered incomplete BBB T Wave Ventricular Repolarization Beginning of the T wave is the absolute Refractory time Normal • Asymmetric • Direction of T wave normally direction as the QRS wave that precedes it. • Usually 5 mm or less in height in the limb lead • Usually 10 mm or less in precordial leads • Usually .5 mm or more in height in leads I and II Abnormal • Opposite direction of the preceding QRS complex • Negative T wave suggests MI • Tall, pointed – Hyperkalemia • Low amplitude T wave – hypokalemic • Deep and inverted is associated with cerebral disease U Wave The U wave follows the T wave. It suggests repolarization of the purkinje fibers. It is usually round, small and less than 1.5 mm. Easily seen when the heart rate is slow. They are usually tallest in V2 and V3. They appear in the same direction as the T wave that precedes it. Tall U Wave Causes • Electrolyte imbalance – hypokalemia • Medications: quinidine, procainamide, amiodarone, digitalis • Hyperthyroidism • Central nervous system disease • Long QT syndrome Negative U Wave • Suggestive of organic Heart disease • Patients with ischemic heart disease Segments PR Segment • Represents activation of the AV node, Bundle of His, Bundle Branches and Purkinje Fibers. • Atrial repolarization occurs during this time. • PR Segment may be depressed in the presence of ventricular hypertrophy or chronic pulmonary disease. • Anything >0.8 mm is pathological ST Segment • Represents early part of repolarization of the right and left ventricles. • ST segment depression suggest ischemia • ST segment elevation suggest injury • Other causes of ST segment elevation may suggest pericarditis, ventricular aneurysm • ST segment elevation with concavity is usually benign. • ST segment elevation with coved is called an acute injury pattern. • Digitalis cases a depression (scoop) of the ST segment J Point The point where the QRS complexes and the ST segment meet is called the Junction (J Point). Look at the point where the end of the QRS complex makes a sudden sharp change in direction n. Intervals PRI • From beginning of P wave to beginning of QRS complex • Includes P wave and PR segment • Reflects depolarization of the right and the left atria. • Normally shortens as heart rate increases • Normally lengthens as heart rate decreases • Measures 0.12 – 0.20 second Causes of Abnormal PRI • Patients taking beta-blockers or calcium channel blockers • 1st degree AV block • Hypothyroidism • Digitalis toxicity • Less than 0.12 second, impulse originates in an ectopic pacemaker in the atria close to the AV node or in the AV junction. QT Interval • Represents total ventricular activity. Depolarization to repolarization. • Measured from beginning of QRS to end of T wave. • A QT interval that is more than half the R-R interval is considered prolonged • QT intervals may be caused by electrolyte disorders and medications • Generally not considered abnormal unless the QT interval corrected for the heart rate exceeds 0.44 second R-R and P-P Intervals • Used to determine the rate and regularity of a cardiac rhythm. • R-R intervals is used to determine ventricular rhythm regularity • P-P intervals are used to determine atria rhythm regularity. Analyzing a Rhythm Strip Assess the Rate Method 1: Six Second Method – Take a 6 second strip and multiply by 10 to find the number of complexes in a minute. Method 2: Large Boxes – Count the number of large boxes between two consecutive R waves (R-R interval) and divide into 300. Method 3: Small Boxes – Count the number of small boxes between two consecutive R waves and divide into 1500 Method 4: Sequence Method – Select an R wave that falls on a dark vertical line. Number the next 6 consecutive dark vertical lines as follows: 300, 150, 100, 75, 60, and 50. Note where the R wave falls in relation to the 6 dark vertical lines already marked. Assess Rhythm/Regularity Ventricular Rhythm – measure the distance between two consecutive R-R intervals. Atrial Rhythm – Measure the distance between two consecutive P-P intervals. If the variation between the shortest and longest R-R intervals (P-P intervals) is less than four small boxes (0.16) the rhythm is termed essentially regular. This is usually due to ectopic beats. If the intervals vary by more than 0.16 second, the rhythm is considered to be regularly irregular. Identify and Examine P Wave • Do all the P waves look the same in size, shape and position? • If no P wave is present the rhythm originated in the AV junction or the ventricles • If the P wave is positive the rhythm probably begins in the SA node. • If the P wave is negative or absent and the QRS is regular, the rhythm probably started in the AV junction Assess Intervals (Evaluate Conduction) PRI – Normal PR interval is 0.12 – 0.20 second QRS Duration – Normal is 0.08 – 0.12 second QT Interval – If the QT interval is less than half the R-R interval it is probable normal. Evaluate the overall Appearances of the Rhythm ST Segment – Determine the presence of elevation or depression. T Wave – Are the T waves upright and of normal height? The T wave following an abnormal QRS complex is usually opposite in direction of the QRS. Negative T wave would suggest ischemia. Tall, pointed T waves are commonly seen in hyperkalemia. Sinus Rhythms Rhythm that begins in the SA node has the following characteristics: A positive P wave before each QRS complex P waves that look alike A constant PR interval A regular atrial and ventricular rhythm An electrical impulse that beings in the SA node may be affected by the following Medications Diseases or conditions that cause the heart rate to speed up, slow down or beat irregularly Diseases or condition that delay or block the impulse from leaving the SA node Diseases or conditions that prevent an impulse from being generated in the SA node Characteristics of Sinus Rhythm: Rate 60-100 BPM Rhythm P-P interval regular, R-R interval regular P Wave Positive in lead II, on precedes each QRS complex, P waves look alike PR duration 0.12-0.20 second and constant from beat to beat QRS duration 0.12 second or less unless an intraventricular conduction delay exists Sinus Bradycardia Characteristics of Sinus Bradycardia: Rate Less than 60 BPM Rhythm P-P interval regular,