ACLS Chapter 3 Rhythm Review Instructor Lesson Plan to Accompany ACLS Study Guide 3e

Transcription

1 ACLS Chapter 3 Rhythm Review Lesson Plan Required reading before this lesson: ACLS Study Guide 3e Textbook Chapter 3 Materials needed: Multimedia projector, computer, ACLS Chapter 3 Recommended minimum time to complete: 60 min PowerPoint presentation Lesson Objectives 1. Name the primary branches of the right and left coronary arteries. 2. Describe the two types of myocardial cells and the function of each. 3. Describe the significance of each waveform in the cardiac cycle. 4. Describe the normal duration of the PR interval and QRS complex. 5. Describe at least two methods of determining heart rate. 6. Name the primary and escape pacemakers of the heart and the normal rates of each. 7. Define the absolute and relative refractory periods and their location in the cardiac cycle. 8. Describe the ECG characteristics of narrow-qrs tachycardias. 9. Describe the ECG characteristics of wide-qrs tachycardias. 10. Describe differentiation of right and left bundle branch block using lead V 1 or MCL Describe the ECG characteristics of irregular tachycardias. 12. Describe the ECG characteristics of sinus bradycardia, junctional escape rhythm, and ventricular escape rhythm. 13. Describe the ECG characteristics of first, second, and third-degree atrioventricular (AV) blocks. 14. Name and describe four dysrhythmias that may be observed during cardiac arrest. 15. Describe the appearance of the waveform on the ECG produced as a result of atrial pacing and ventricular pacing.

2 ACLS Chapter 3 Rhythm Review Page 2 Lesson Outline 1 Rhythm Recognition 2-4 Objectives 5 Anatomy Review 6 Right Coronary Artery Supplies inferior wall of the left ventricle Supplies SA and AV nodes in most people A common cause of myocardial infarction is a blocked coronary artery. When viewing the patient s 12-lead ECG, an understanding of the coronary artery anatomy makes it possible to predict which coronary artery is blocked. The right coronary artery (RCA) originates from the right side of the aorta. It travels along the groove between the right atrium and right ventricle. Blockage of the RCA can result in inferior wall MI and/or disturbances in AV nodal conduction. 7 Left Coronary Artery (LCA) Two main branches Left anterior descending Left circumflex

3 ACLS Chapter 3 Rhythm Review Page 3 The left coronary artery (LCA) originates from the left side of the aorta. The first part of the LCA is called the left main coronary artery. It is about the width of a soda straw and less than an inch long. Blockage of the left main coronary artery has been referred to as the widow maker because of its association with sudden death. The left main coronary artery supplies oxygen-rich blood to its two primary branches: the left anterior descending (LAD) artery and the left circumflex artery (LCx). These vessels are slightly smaller than the left main coronary artery. The major branches of the LAD are the septal and diagonal arteries. Blockage of the septal branch of the LAD can result in a septal MI. Blockage of the diagonal branch of the LAD can result in an anterior wall MI. Blockage of the LAD can result in pump failure and/or intraventricular conduction delays. The left circumflex (LCx) coronary artery circles around the left side of the heart. It is embedded in the epicardium on the back of the heart. Blockage of the LCx can result in a lateral wall MI. In some patients, the circumflex artery may also supply the inferior portion of the left ventricle. A posterior wall MI may occur because of blockage of the right coronary artery or the left circumflex artery. 8 Types of Cardiac Cells Myocardial cells Working or mechanical cells Contain contractile filaments Pacemaker cells Specialized cells of electrical conduction system Responsible for spontaneous generation and conduction of electrical impulses The heart has two main types of cells myocardial cells and pacemaker cells. Myocardial cells are also called working or mechanical cells. They contain contractile filaments. When electrically stimulated, these filaments slide together and the myocardial cell contracts. They do not normally spontaneously generate electrical impulses, depending on pacemaker cells for this function. Pacemaker cells are specialized cells of the electrical conduction system. They are responsible for the spontaneous generation and conduction of electrical impulses.

4 ACLS Chapter 3 Rhythm Review Page 4 9 Properties of Cardiac Cells Automaticity Excitability Conductivity Contractility Automaticity. Ability of pacemaker cells to spontaneously initiate an electrical impulse without being stimulated from another source (such as a nerve). SA node, AV junction, and Purkinje fibers normally possess this characteristic. Excitability (or irritability). Ability of cardiac muscle cells to respond to an external stimulus, such as that from a chemical, mechanical, or electrical source. All cardiac cells possess this characteristic. Conductivity. Ability of a cardiac cell to receive an electrical stimulus and conduct that impulse to the next cardiac cell. All cardiac cells possess this characteristic. Contractility. Ability of cardiac cells to shorten, causing cardiac muscle contraction in response to an electrical stimulus. Contractility can be enhanced with certain medications, such as digitalis, dopamine, and epinephrine. 10 Cardiac Action Potential Cell membranes contain membrane channels (pores) Cell membranes contain membrane channels. These channels are pores through which specific ions or other small, water-soluble molecules can cross the cell membrane from outside to inside. 11 Cardiac Action Potential Action potential of a ventricular muscle cell Electrical impulses are the result of the rapid flow of charged ions back and forth across the cell membrane. The cardiac action potential is an illustration of these events in a single cardiac cell during polarization, depolarization, and repolarization. The stimulus that changes the gradient across the cell membrane may be electrical, mechanical, or chemical. 12 Polarization (Ready State) Polarization is the resting state during which no electrical activity occurs in the heart. It is also called the resting membrane potential or ready state because the cells are waiting to respond to a stimulus. When a cardiac muscle cell is polarized, the inside of the cell is more negative than the outside. 13 Depolarization = Stimulation

5 ACLS Chapter 3 Rhythm Review Page 5 Before the heart s working cells can contract and pump blood, they must first be electrically stimulated. When a cardiac muscle cell is stimulated, the cell is said to depolarize. During depolarization, changes in the cell membrane allow sodium (Na + ) ions to rush into the cell through fast Na + membrane channels. Calcium (Ca ++ ) moves slowly into the cell through calcium channels. Movement of these charged particles causes the inside of the cell to become more positive. When the cell depolarizes, cardiac contraction begins. Depolarization proceeds from the innermost layer of the heart (endocardium) to the outermost layer (epicardium). Depolarization is not the same as contraction. Depolarization is an electrical event expected to result in contraction (a mechanical event). It is possible to view organized electrical activity on the cardiac monitor, yet assessment of the patient reveals no palpable pulse. This clinical situation is termed pulseless electrical activity (PEA). 14 Repolarization = Recovery After the cell depolarizes, changes occur again in the cell membrane. Changes in the cell membrane cause the fast sodium channels to close. This stops the rapid flow of Na + into the cell. Calcium channels close and potassium rapidly flows out of the cell. Active transport via the sodium-potassium pump begins restoring potassium to the inside of the cell and sodium to the outside of the cell. The cell returns to its negative state due to the outflow of potassium. This recovery stage is called repolarization. The cell gradually becomes more sensitive to external stimuli until its original sensitivity has been restored. The cell can then be stimulated again if another electrical impulse arrives at the cell membrane. Repolarization proceeds from the epicardium to the endocardium. 15 The Conduction System Sinoatrial (SA) node AV junction Bundle of His Right and left bundle branches Purkinje fibers Normally, the pacemaker cells with the fastest rate control the heart at any given time. Because it fires more quickly than other pacemaker sites in the heart, the sinoatrial node (also called the SA node or sinus ) is normally the heart s primary pacemaker. Built-in firing rate = 60 to 100 bpm.

6 ACLS Chapter 3 Rhythm Review Page 6 16 The Conduction System AV junction Area of specialized conduction tissue Provides electrical links between atrium and ventricle Built-in rate: 40 to 60 bpm Escape (back up) pacemakers include the atrioventricular (AV) junction and ventricles. The AV junction is the AV node and the nonbranching portion of the bundle of His. The pacemaker cells in the AV junction are located near the nonbranching portion of the bundle of His. This area consists of specialized conduction tissue that provides the electrical links between the atrium and ventricle. When the AV junction is bypassed by an abnormal pathway, the abnormal route is called an accessory pathway. If the SA node fails to produce an impulse at its normal rate, or stops functioning entirely, pacemaker cells in the AV junction will usually assume the role of the heart s pacemaker (but at a slower rate). Built-in firing rate = 40 to 60 bpm. 17 The Conduction System Purkinje fibers Receive impulse from bundle branches Relay it to ventricular myocardium Built-in rate: 20 to 40 bpm If the SA node and AV junction fail, an escape pacemaker below it (the bundle branches and the Purkinje fibers) may take over at an even slower rate. Built-in firing rate = 20 to 40 bpm. 18 The Electrocardiogram (ECG) Records electrical voltages (potentials) generated by depolarization of heart muscle Can provide information about: Orientation of heart in chest Conduction disturbances Electrical effects of drugs and electrolytes Mass of cardiac muscle Presence of ischemic damage An ECG records the electrical activity of a large mass of atrial and ventricular cells as waveforms and complexes. The ECG does not provide information about the mechanical (contractile) condition of the myocardium. To assess the effectiveness of the heart s mechanical activity, assess the patient s pulse and blood pressure.

7 ACLS Chapter 3 Rhythm Review Page 7 19 Electrodes Applied at specific locations on the patient's chest wall and extremities An electrode is a paper, plastic, or metal device that contains conductive gel and is applied to the patient s skin. An ECG cable is a wire that attaches to the electrode and conducts current back to the cardiac monitor. Electrodes are applied at specific places on the patient s chest and limbs in combinations of two, three, four, or five to view the heart s electrical activity from different angles and planes. 20 Leads Think of the positive electrode as an eye Position of the positive electrode determines portion of the heart seen by each lead The word lead is used in two ways: 1) the position of the positive electrode on the patient s body and 2) the actual ECG record (tracing) obtained. For example, V 1 position represents the proper location of the positive electrode on the patient s chest. Lead V 1 refers to the tracing obtained from that position. Each lead records the average current flow at a specific time in a part of the heart. Leads II and MCL 1 are commonly used for continuous ECG monitoring. Moving the lead selector on the ECG machine allows us to make any of the electrodes positive or negative. The position of the positive electrode on the body determines the portion of the heart seen by each lead. 21 Standard Limb Leads Leads I, II, and III Right arm electrode is always negative Left leg electrode is always positive Leads I, II, and III make up the standard limb leads. If an electrode is placed on the right arm, left arm, and left leg, three leads are formed. An imaginary line joining the positive and negative electrodes of a lead is called the axis of the lead. 22 Summary of Standard Limb Leads Lead Positive Negative Heart Electrode Electrode Surface Viewed Lead I Left arm Right arm Lateral Lead II Left leg Right arm Inferior Lead III Left leg Left arm Inferior

8 ACLS Chapter 3 Rhythm Review Page 8 23 Augmented Limb Leads Leads avr, avl, avf A = augmented V = voltage R = right arm L = left arm F = foot (usually left leg) Leads avr, avl, and avf are augmented limb leads. The electrical potential produced by the augmented leads is normally relatively small. The ECG machine augments (magnifies) the amplitude of the electrical potentials detected at each extremity by approximately 50% over those recorded at the bipolar leads. The a in avr, avl, and avf refers to augmented. The V refers to voltage. The R refers to right arm, the L to left arm, and the F to left foot (leg). The position of the positive electrode corresponds to the last letter in each of these leads. The positive electrode in avr is located on the right arm, avl has a positive electrode at the left arm, and avf has a positive electrode positioned on the left leg. While leads avr, avl, and avf have a distinct positive pole, they do not have a distinct negative pole. Since they have only one true pole, they are referred to as unipolar leads. In place of a single negative pole these leads have multiple negative poles, creating a negative field (central terminal), of which the heart is at the center. Theoretically, this makes the heart the negative electrode. The augmented voltage leads are not the only unipolar leads in the standard 12-lead ECG. The chest leads are also unipolar. 24 Summary of Augmented Leads Lead Positive Electrode Heart Surface Viewed Lead avr Right arm None Lead avl Left arm Lateral Lead avf Left leg Inferior 25 Chest Leads The six chest leads view the heart in the horizontal plane Identified as V 1, V 2, V 3, V 4, V 5, and V 6

9 ACLS Chapter 3 Rhythm Review Page 9 The six chest leads (also known as precordial or V leads) are unipolar leads that view the heart in the horizontal plane. The chest leads are identified as V 1, V 2, V 3, V 4, V 5, and V 6. Each electrode placed in a V position is a positive electrode. The wave of ventricular depolarization normally moves from right to left. In the right chest leads (V 1 and V 2 ), the QRS deflection is predominantly negative (moving away from the positive chest electrode). As the chest electrode is placed further left, the wave of depolarization is moving toward the positive electrode. Thus the QRS deflection recorded as the electrode is moved to the left becomes progressively more positive. Because their location varies, do not use the nipples as landmarks for chest electrode placement. 26 Summary of Chest Leads Positive Electrode Heart Surface Lead Position Viewed Lead V 1 Right side of sternum, 4 th intercostal space Septum Lead V 2 Left side of sternum, 4 th intercostal space Septum Lead V 3 Midway between V 2 and V 4 Anterior Lead V 4 Left midclavicular line, 5 th intercostal space Anterior Lead V 5 Left anterior axillary line at same level as V 4 Lateral Lead V 6 Left midaxillary line at same level as V 4 Lateral 27 Right Chest Leads Used to view the right ventricle Placement identical to standard chest leads except on right side of chest Other chest leads that are not part of a standard 12-lead ECG may be used to view specific surfaces of the heart. When a right ventricular myocardial infarction is suspected, right chest leads are used. Placement of right chest leads is identical to placement of the standard chest leads except it is done on the right side of the chest. Obtain a standard 12-lead first. The cables for the standard chest leads are then moved to the electrodes for the additional leads. If time does not permit obtaining all of the right chest leads, the lead of choice is V 4 R. 28 Posterior Chest Leads Used to view posterior surface of heart

10 ACLS Chapter 3 Rhythm Review Page 10 The leads corresponding to the posterior wall of the left ventricle are V 7, V 8, and V 9. These three leads are positioned horizontally level with V 4. Lead V 7 is placed at the posterior axillary line. Lead V 8 is placed at the angle of the scapula (posterior scapular line) and lead V 9 is placed over the left border of the spine. Any chest lead cable can be moved to obtain the right and/or posterior leads. However, once these leads are printed, the correct lead must be handwritten onto the ECG to indicate the origin of the tracing 29 Modified Chest Leads MCL 1 Variation of chest lead V 1 Negative electrode below left clavicle toward left shoulder Positive electrode right of sternum in 4th intercostal space Views ventricular septum The modified chest leads (MCL) are bipolar chest leads that are variations of the unipolar chest leads. Each modified chest lead consists of a positive and negative electrode applied to a specific location on the chest. Accurate placement of the positive electrode is important. Modified chest leads are useful in detecting bundle branch blocks, differentiating right and left premature beats, and differentiating supraventricular tachycardia (SVT) from ventricular tachycardia (VT). Lead MCL 1 is a variation of the chest lead V 1 and views the ventricular septum. The negative electrode is placed below the left clavicle toward the left shoulder, and the positive electrode is placed to the right of the sternum in the fourth intercostal space. In this lead the positive electrode is in a position to the right of the left ventricle. Because the primary wave of depolarization is directed toward the left ventricle, the QRS complex recorded in this lead will normally appear negative. Leads MCL 1 and V 1 are similar but not identical. In V 1, the negative electrode is calculated by the ECG machine at the center of the heart. In MCL 1, the negative electrode is located just below the left clavicle (Phalen 2006). 30 MCL 6 Variation of chest lead V 6 Negative electrode below left clavicle toward left shoulder Positive electrode 5th intercostal space, left midaxillary line Views low lateral wall of left ventricle Lead MCL 6 is a variation of the chest lead V 6 and views the low lateral wall of the left ventricle. The negative electrode is placed below the left clavicle toward the left shoulder and the positive electrode is placed at the fifth intercostal space, left midaxillary line.

11 ACLS Chapter 3 Rhythm Review Page ECG Paper Smallest squares are 1 mm wide and 1 mm high 5 small squares between the heavier black lines 25 small squares within each large square ECG paper is graph paper made up of small and large boxes. The smallest boxes are one millimeter wide and one millimeter high. The horizontal axis of the paper corresponds with time. Time is stated in seconds. ECG paper normally records at a constant speed of 25 mm/sec. Thus, each horizontal unit (1-mm box) represents 0.04 sec (25 mm/sec 0.04 sec = 1 mm). 32 Horizontal Axis = Time Width of each small box = 0.04 sec Large box width (5 small boxes) = 0.20 sec 5 large boxes = 1 second 15 large boxes = 3 seconds 30 large boxes = 6 seconds The lines between every five boxes on the paper are heavier. There are five small boxes in each large box. Since each large box is the width of five small boxes, a large box represents 0.20 second. Five large boxes, each consisting of five small boxes, represent 1 second. Fifteen large boxes equal an interval of 3 seconds. Thirty large boxes represent 6 seconds. 33 Vertical Axis = Voltage/Amplitude Voltage may appear as a positive or negative value. Size or amplitude of a waveform is measured in millivolts or millimeters. 34 Waveforms & Complexes Review 35 Segments & Intervals Review 36 ST-Segment Normally isoelectric (flat) in the limb leads The point at which the QRS complex and the ST-segment meet = J point or junction

12 ACLS Chapter 3 Rhythm Review Page 12 The ST-segment is portion of the ECG tracing between the QRS complex and the T wave. It represents the early part of repolarization of the right and left ventricles. The ST-segment begins with the end of the QRS complex (S wave) and ends with the onset of the T wave. In the limb leads, the normal ST-segment is isoelectric (flat) but may normally be slightly elevated or depressed (usually by less than 1 mm). In the chest leads, ST-segment deviation may vary from -0.5 to + 2 mm. The point where the QRS complex and the ST-segment meet is called the junction or J-point. 37 ST-Segment ST-segment depression may reflect myocardial ischemia or hypokalemia. ST-segment elevation may represent a normal variant, myocardial injury, pericarditis, or ventricular aneurysm. ST-segment elevation in the shape of a smiley face (upward concavity) is usually benign, particularly when it occurs in an otherwise healthy, asymptomatic patient. The appearance of coved ( frowny face) ST-segment elevation is called an acute injury pattern. 38 ST-Segment Other possible shapes of ST-segment elevation that may be seen with acute MI In a patient experiencing an acute coronary syndrome, myocardial injury refers to myocardial tissue that has been cut off from or experienced a severe reduction in its blood and oxygen supply. The tissue is not yet dead and may be salvageable if the blocked vessel can be quickly opened, restoring blood flow and oxygen to the injured area. ST-segment elevation provides the strongest ECG evidence for the early recognition of myocardial infarction (Phalen 2006). 39 TP-Segment

13 ACLS Chapter 3 Rhythm Review Page 13 The TP-segment is the portion of the ECG tracing between the end of the T wave and the beginning of the following P wave. When the heart rate is within normal limits, the TP-segment is usually isoelectric. With rapid heart rates, the TP-segment is often unrecognizable because the P wave encroaches on the preceding T wave. When assessing for ST-segment displacement, first locate the J point. Next use the TP-segment and the PR segment to estimate the position of the isoelectric line. Then compare the level of the ST-segment to the isoelectric line. While some deviation of the ST-segment from the isoelectric line can be a normal finding, the following findings are considered significant if they are seen in two or more leads facing the same anatomic area of the heart (also known as contiguous leads): o ST-segment depression of more than ½ mm (suggests myocardial ischemia) o ST-segment elevation (suggests myocardial injury) More than 1 mm in the limb leads More than 2 mm in the chest leads 40 Refractory Periods Refractoriness is the extent to which a cell is able to respond to a stimulus. The absolute refractory period (also known as the effective refractory period) corresponds with the onset of the QRS complex to the peak of the T wave. During this period, the myocardial cell will not respond to further stimulation, no matter how strong the stimulus. The relative refractory period (also known as the vulnerable period) corresponds with the downslope of the T wave. During this period, some cardiac cells have repolarized to their threshold potential and can be stimulated to respond (depolarize) if subjected to a stronger than normal stimulus. After the relative refractory period is a supernormal period during which a weaker than normal stimulus can cause depolarization of cardiac cells. It corresponds with the end of the T wave. It is possible for cardiac dysrhythmias to develop during this period. 41 Rate Measurement Six-Second Method

14 ACLS Chapter 3 Rhythm Review Page 14 Most ECG paper in use today is printed with 1-second or 3-second markers on the top or bottom of the paper. To determine the ventricular rate, count the number of complete QRS complexes within a period of 6 seconds and multiply that number by 10 to find the number of complexes in 1 minute. This method may be used for regular and irregular rhythms and is the simplest, quickest, and most commonly used method of rate measurement. 5 large boxes = 1 second 15 large boxes = 3 seconds 30 large boxes = 6 seconds 42 Large Box Method To determine the ventricular rate, count the number of large boxes between two consecutive R waves (R-R interval) and divide into 300. To determine the atrial rate, count the number of large boxes between two consecutive P waves (P-P interval) and divide into 300. This method is best used if the rhythm is regular; however, it may be used if the rhythm is irregular and a rate range (slowest and fastest rate) is given. 43 Small Box Method Each 1-mm box on the graph paper represents 0.04 second. There are 1500 boxes in 1 minute. 60 seconds/minute divided by 0.04 seconds/box = 1500 boxes/minute. To calculate the ventricular rate, count the number of small boxes between two consecutive R waves and divide into To determine the atrial rate, count the number of small boxes between two consecutive P waves and divide into This method is time-consuming but accurate. If the rhythm is irregular, a rate range should be given. 44 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 next R wave falls in relation to the 6 dark vertical lines already marked this is the heart rate.

15 ACLS Chapter 3 Rhythm Review Page Six Steps In Analyzing A Rhythm Strip 1. Assess the rate. 2. Assess rhythm/regularity. 3. Identify and examine P waves. 4. Assess intervals (evaluate conduction PR interval, QRS duration, QT interval). 5. Evaluate overall appearance of the rhythm (ST-segment elevation/depression, T wave inversion). 6. Interpret rhythm and evaluate clinical significance. 46 Rhythm Recognition 47 Sinus Rhythm Rate Rhythm P waves PR interval QRS Within normal limits for age; in adults, 60 to 100 bpm Regular Uniform in appearance, positive (upright) in lead II, one precedes each QRS complex Within normal limits for age and constant from beat to beat; in adults, 0.12 to 0.20 sec 0.10 sec or less unless an intraventricular conduction delay exists 48 Sinus Arrhythmia Rate Rhythm P waves PR interval QRS Usually 60 to 100 bpm, but may be slower or faster Irregular, phasic with respiration; heart rate increases gradually during inspiration (R-R intervals shorten) and decreases with expiration (R-R intervals lengthen) Uniform in appearance, positive (upright) in lead II, one precedes each QRS complex sec and constant from beat to beat 0.10 sec or less unless an intraventricular conduction delay exists