ELECTROCARDIOGRAM NCM 118: CRITICAL CARE NURSING (LECTURE) BS Nursing | SEM 1 2022 ELECTROCARDIOGRAM - - ● AV node - located above the tricuspid valve anterior to the coronary sinus and at the base of interatrial septum ● Function is to slow the electrical impulse between the atria to contract and empty their content into the ventricles It also acts as a secondary pacemaker in case SA node fails Intrinsic rate is 40-60 beats/min Is a graphic representation of the electrical forces produced within the heart, necessary components in the assessment of cardiovascular status It shows the heart’s electrical activity CARDIAC CONDUCTION SYSTEM - Is the pathway of specialized cells that are capable of transmitting the electrical impulse of the heart - This pathway allows the 4 chambers of the heart to function in order to supply blood to the different parts of the body ● ● PROPERTIES OF THE HEART MUSCLE 1. Excitability - is the ability of a cell or tissue to depolarize in response to a stimuli 2. Conductivity - is the ability of cardiac cells to transmit a stimulus from cell to cell ● ● ● ● ● Bundle of His - a continuation of the AV node, which allows the impulse to enter into the ventricles It is located at across the interventricular septum Bundle of His divides into left and right bundle branches Left bundle has two fascicle Right bundle has one fascicle 1 ELECTROCARDIOGRAM NCM 118: CRITICAL CARE NURSING (LECTURE) BS Nursing | SEM 1 2022 ● ● - - - - - The 12-leads ECG provides information on the heart’s axis and the size of the cardiac chambers The electrical currents passing the heart are subsequently conducted to the body surface. These currents can be detected by electrodes and then measured when they reach the surface STANDARD LIMB LEADS Are designated by roman numerals I, II, III, consists of electrodes applied to the R arm, L arm, and L leg, the R leg acts only as the grounding electrodes The limb leads are termed bipolar leads because they register the electrical potential differences between two anatomic sites Lead 1 records the difference between the RA and LA potentials. The LA electrode is positive Lead II records the difference between the RA and LL potentials. The LL electrodes are positive Lead III records the differences between the LA and LL potentials. The LL is positive - - This connection nullifies any potential variation at the negative terminal The electrical potential variation is recorded only by the positive electrode, so it is term unipolar leads PRECORDIAL UNIPOLAR LEADS - There are six precordial or chest leads designated by the symbols V1 through V6 - V1, V2, V3, V4, V5, V6 ELECTROCARDIOGRAPHIC PAPER - The electrocardiographic tracing is recorded on the graph paper that passes by a heated pen at the speed of 25mm/sec - The graph paper is divided into millimeter squares. The millimeter squares are grouped and divided into larger squares by thick lines occurring every fifth squares - Horizontally, each millimeter square represents 0.04 second of time lapsed. Each thick line denotes the passage of 0.20 second - Fifteen hundred small or 300 large squares represent 1 minute AUGMENTED UNIPOLAR LIMB LEADS - They are designated by the abbreviated forms aVR, aVL, and aVF - A = represent augmented - V = represent unipolar - R = right arm - L = left arm - F = left leg become the positive electrode - The negative terminal is formed by electrically joining the remaining two limb electrodes 2 ELECTROCARDIOGRAM NCM 118: CRITICAL CARE NURSING (LECTURE) BS Nursing | SEM 1 2022 lead, whereas infarcted myocardium could produce no voltage or low voltage wave 3 METHODS TO MEASURE HEART RATE 1. Measure the interval between consecutive QRS complexes, determine the number of small squares, and divide 1500 by that number 2. Measure the interval between consecutive QRS complexes, determine the number of large squares and divide 300 by that number - Used only when the heart rate rhythm is regular, same with no. 1 3. Determine the number of R-R intervals in 6 seconds stip and multiple by 10 - This method is helpful when the heart rhythm is irregular WAVES, COMPLEXES, AND INTERVALS - The waves recorded electrocardiographically have been designated by the letters in the alphabet letters P, Q, R, S, and T wave - The baseline of the ECG tracing is known as the isoelectrical line - Waves are deflections, either above, they are positive, or below, they are negative, the isoelectrical line The direction of the deflection is determined by the following: 1. The direction in which the electrical flows 2. The distance between the source and the impulse and the exploring electrode 3. The site of the electrode ● - - - - Vertically, each small square is 1mm in height and represents 0.1mV of voltage Each large squares represents 5mm or 0.5mV The ECG machine is calibrated before use so that 1mV produces a 10mm deflection The voltage amplitude of a wave or complex in a given lead indirectly indicates the electrical activity of the muscle below the exploring or positive electrode Example: Hypertrophied myocardium will produce abnormally high voltage in some ● ● ● P Wave represents the depolarization of the atria, and the impulse originates in the SA node, Normal P wave is gently rounded, upright and precedes each QRS complex, does not exceed 2 to 3mm in amplitude and is 0.11 second or less in duration It is normally positive in leads I, II, aVF, V4 to V6 The interval is measured from the beginning of the P wave to the beginning of the QRS complex. Normally the PR interval measures from 0.12 to 0.20 second PR interval greater than the 0.20 second indicates that the impulse is delayed as it passes through the AV node 3 ELECTROCARDIOGRAM NCM 118: CRITICAL CARE NURSING (LECTURE) BS Nursing | SEM 1 2022 - - QRS Complex QRS complex represents depolarization of the ventricles and thus is often the most significant portion of the ECG The full duration of the QRS complex is measured from the first deflection from the isoelectric line. The J-point is where the QRS complex ends and the ST segment begins. The normal QRS complex is 0.05 to 0.10 second ● ● - - - - ● ● Q Wave The first deflection from the isoelectric line is negative, it is Q wave A small Q wave of less than 0.04 sec duration is normal finding in lead I, II, III, aVF and V4-V6. The full R Wave The first positive deflection from the isoelectric line is an R wave S Wave The negative deflection following the R wave is an S wave A widened QRS complex can occur when impulse conduction to one ventricle is slowed or when the impulse originates in the ventricles QRS complexes of varying size may indicate the occurrence of ectopic or aberrantly conducted impulse Normally there may be slight elevation not greater than 1mm or subtle depression no greater than 0.5 mm Abnormal elevation or depression of the ST segment can occur as a result of derangement in the action potential CAUSES OF ELEVATION OR DEPRESSION OF ST - - - - - - Myocardial muscle injury, conduction disturbances Pre-excitation Hypertrophy, tachycardia and Digitalis effect T Wave T wave represents phase 3 of the action potential, when the ventricles are being rapidly repolarized It is usually rounded, slightly asymmetric of the same polarity as QRS complex The height of the T wave should not exceed 5mm in a limb lead or 10mm in a precordial lead The effective refractory period is present during the beginning of the T wave Causes of T wave abnormality ischemia, ventricular hypertrophy, quinidine therapy, digitalis therapy, 4 ELECTROCARDIOGRAM NCM 118: CRITICAL CARE NURSING (LECTURE) BS Nursing | SEM 1 2022 acid-base hyperkalemia extrasystole - - - - - - - ● ● and imbalance, ventricular QT Wave QT interval is measured from the beginning of the QRS complex to the end of the T wave It represents the entire duration of ventricular depolarization & repolarization The QT interval should be less than the half of the preceding R-R interval, normally is 0.36 to 0.44 second A prolonged QT interval indicates a prolonged relative refractory period due to certain medication or congenital diseases A shortened QT interval may be caused by hyperkalemia or digoxin toxicity U Wave U wave is small, sometimes seen after the T wave It is usually deflects in the same direction as the T wave and is best seen in lead V3 The U wave is affected by drugs and some condition, but is known for its prominence in hypokalemia ECG - measures the heart’s electrical activity and the data are graphed in waveforms It is used to identify myocardial ischemia and infarction, rhythm and conduction disturbances, chamber enlargement, electrolyte imbalances, and drug toxicity RHYTHM STRIP INTERPRETATION REQUIRES THE FOLLOWING: 1. 2. 3. 4. 5. 6. 7. 8. Determine the rhythm Determine the rate Evaluate the P wave Determine the duration of PR interval Determine the duration of QRS complex Evaluate the T wave Determine the duration of QT interval Evaluate other components NURSING ALERT ● ● ● ● Review the patient’s record and assess for a need of ECG Assess patient’s cardiac status, HR, and BP Keep patient away from object that might cause electrical interference Inspect patient’s chest for irritation, breakdown or excessive hair that may interfere with electrode placement EQUIPMENT 1. 2. 3. 4. 5. 6. 7. ECG machine Recording paper Disposable pre-gelled electrodes Adhesive remover swab 4x4 gauze pads Soap and water if necessary KY jelly or any water based jelly 5
