For more information about the use of the Propaq monitor, refer to the Propaq Directions For Use.

Transcription

1 Clinical Support 8500 S.W. Creekside Pl. Beaverton, OR U.S.A. Telephone: Toll Free: ELECTROCARDIOGRAPHY Introduction This article provides a basic introduction to the physiology of the human heart and the clinical information provided by electrocardiography (), with reference to monitoring with a Propaq vital signs monitor. For more information about the use of the Propaq monitor, refer to the Propaq Directions For Use. Monitoring HR/PR with the Propaq Monitor When monitoring a patient using leads, SpO2, and/or CO 2, the rate that is displayed is a true heart rate. If monitoring a patient using NIBP only, what is actually being displayed is the patient s pulse rate. This may be important when assessing your patient s cardiac status because a patient s heart rate and pulse rate may vary if there is any cardiac compromise. On the Propaq monitor you can set the HR/PR tone loudness to LOW, MEDIUM, HIGH, or OFF. This does not affect the tone of the alarm if a patient exceeds an alarm limit setting. Anatomical Structure of the Heart The main function of the heart is to pump blood throughout the body to deliver the oxygen and nutrient demands of the body s tissues as well as to remove carbon dioxide (a byproduct of metabolism). The heart is approximately the size of a clenched fist. The heart is positioned in the mediastinum, near the midline. The heart is rotated and positioned on its side.

2 2/3 of the heart is on the left side of the chest. The base of the heart faces up and to the right. The apex faces down, out, and to the left. The apex actually comes into contact with the chest wall at the 5 th intercostal space in the mid-clavicular line. This is the PMI or Point of Maximum Intensity. It s easiest to hear the heart at this area. Of course, people are of all different shapes and sizes. The position of the heart in the chest will vary slightly with age, weight, and physical conditions. Four Chambers of the Heart There are four chambers of the heart the right atrium and right ventricle, and the left atrium and left ventricle. The wall of the left ventricle is quite a bit thicker than that of the right ventricle. Actually, the wall of the right atrium is approximately 3-5 mm thick, and the right ventricle is about 2-6 mm thick. The wall of the left atrium is also about 2-6 mm, slightly thicker than the wall of the right atrium, and the left ventricle is the largest muscle mass of all at mm thick. The wall thickness directly affects the pressure in each of the chambers of the heart. The function of the right side of the heart is to deliver deoxygenated blood from the body to the lungs. The function of the left side of the heart is to deliver oxygenated blood from the lungs to the body. Systole and Diastole The two phases of the cardiac cycle are Systole and Diastole. Both the right and left ventricles go through each phase at the same time. In Systole, the ventricles are full of blood and begin to contract. The mitral and tricuspid valves (between the atria and ventricles) close. Blood is ejected through the pulmonic and aortic valves out to the lungs (RV) and the body (LV). The aortic and pulmonic valves then close. 2

3 During Diastole, blood flows into the atria and then through the now open mitral and tricuspid valves into the ventricles. The ventricles refill, and the cycle repeats. NOTE Atrial systole occurs during ventricular diastole, and atrial diastole during ventricular systole. Cardiac Conduction System In order for the heart muscle to contract, an electrical impulse is necessary. The electrical conduction system of the heart is known as the Cardiac Conduction System. SA Node The SA node is often referred to as the Pacemaker of the heart. In a normal heart, the SA node generates the electrical impulse and sets the pace of the heart. The intrinsic rate of a rhythm that begins at the SA node is 60 to 100 beats per minute. Once generated, the impulse spreads out along tiny nerve fibers called internodal tracts and stimulates the atrial muscle. AV Node The AV node is located on the floor of the Right Atrium. It can be thought of as a gateway to the ventricles. The AV node delays the electrical impulse just long enough to allow the atria to contract and blood to enter the ventricles. The intrinsic rate of the AV node is slower than that of the SA node, approximately bpm. 3

4 Bundle of His The Bundle of His is a thick bundle of nerve fibers that carries the electrical impulse very rapidly down the interventricular septum from the AV node. The bundle branches out to the right and left, terminating in tiny fibers called Purkinje fibers. These fibers bring the electrical impulse to the individual heart muscle cells, leading to ventricular stimulation or depolarization. Depolarization can be simply thought of as the electrical stimulation of the heart muscle cells. The resting heart is polarized. Charges are balanced in and out of the cell, and no electricity is flowing. The cell at rest is negatively charged. When a stimulus begins, positive ions enter the cell, changing the charge to positive. This depolarization spreads from cell to cell, causing the heart muscle fibers to shorten. The shortening of the heart muscle fibers causes contraction of the heart muscle as a whole. Repolarization is the return of the heart muscle cells to the polarized, or resting state. The positive ions are pumped out of the cells, the cells return to their normal shape, and the heart muscle relaxes. Tracings Each wave and interval appears on the display or printout as a result of a particular electrical function of the heart. The isoelectric line, also referred to as the baseline, is simply the point from which each of the waves of the departs. 4

5 P Wave The P wave is the wave of atrial depolarization. As the atria depolarize, the P wave shows up on the. In a patient with normal physiology and with the SA node acting as the pacemaker of the heart, the P wave has these characteristics: Smooth and rounded <= 3 mm tall upright in leads I, II, avf The P wave at right fulfills the criteria. PR Interval The next component is the PR interval, which includes the P wave and the space up until the beginning of the QRS complex. The PR interval represents the time it takes the electrical impulse to travel from the SA node to the ventricles. By the end of the PR interval, the atria are beginning to repolarize and the ventricles are beginning to depolarize or become electrically stimulated. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The normal PR interval duration is 0.12 to 0.20 seconds or ms. QRS Complex The QRS complex is the wave of ventricular depolarization. We generally call the wave of ventricular depolarization a QRS complex even if not all of the components (the Q, the R, and the S) are present. Technically, the Q wave is the first downward stroke. An R wave is the first positive stroke, and an S wave is a negative stroke that follows a positive upstroke. The QRS should be at least 5 mm and not more than 20 mm tall. The width of the QRS is measured from the beginning of the Q wave to the end of the S. Normal QRS duration is 0.06 to 0.10 seconds, and does not exceed 0.12 seconds. As discussed earlier, the left ventricular muscle is quite a bit larger than that of the right ventricle. Because there are more muscle cells to depolarize, the electrical charge of 5

6 the left ventricle is significantly greater than that of the right. Therefore, most of what we see on as the QRS complex is LEFT ventricular depolarization. ST Segment The next segment is the ST segment. The ST segment begins at the J point. The J point is the point at which the QRS complex ends and the ST segment begins. Measure the ST segment duration from the J point up to the beginning of the T wave. The ST segment indicates the period of time between the end of ventricular depolarization and the beginning of ventricular repolarization. Generally the ST segment is ISOELECTRIC, or on the baseline. A deviation of the ST segment from the baseline (either a depression or elevation) may be indicative of myocardial ischemia. T Wave The T wave is the wave of ventricular repolarization. The T wave usually deflects in the same direction as the QRS complex, and should be smooth and rounded. The period from the beginning of the T wave to nearly the end is called the relative refractory period. At this time, the ventricles are vulnerable. A stronger than normal stimulus could trigger depolarization. If an R wave (ventricular depolarization) should occur during this time, a potentially fatal arrhythmia could result. Summary and Review The P wave is the wave of atrial depolarization. The PR interval signifies the amount of time it takes the electrical impulse to travel from the SA node to the ventricles. The QRS complex begins to show up as ventricular depolarization begins and the atria repolarize. The QRS is complete when the ventricles are fully depolarized. The ST segment occurs on between the end of ventricular depolarization and the beginning of ventricular repolarization. The T wave begins as the ventricles start to repolarize and is finally complete when the ventricles have returned to their resting state. 6

7 Electrocardiogram The electrocardiogram, also called an or EKG, is a graph of the electrical activity of the heart over time. We have been discussing the waveforms you find plotted on the. When reading the, be aware of a few basic principles: The standard paper speed is 25mm/ second. This is also called Sweep Speed. The vertical lines on the measure time. The space between two small vertical lines (one small box) is 0.04 seconds or 4 ms. The space between two larger lines (5 small boxes or 1 large box) is 0.20 seconds or 20 ms. The horizontal lines on the measure voltage. The space between two small horizontal lines (one small box) is 1 mm or 0.1 mv. The space between two larger horizontal lines (5 small boxes) is 5 mm or 0.5 mv. Perhaps the best way to measure time and voltage on the is with calipers, but you can also use a ruler or a piece of paper. We can tell something about the direction the electricity in the heart is flowing by looking at the. If the electrical flow of the heart is TOWARDS a positive electrode, we will see a positive deflection on the. If the electrical flow is AWAY FROM the positive electrode, then the wave produced on the will be negative. 7

8 Remember that depolarization is a wave of POSITIVE charges flowing through the heart muscle. If that wave of depolarization is flowing towards a POSITIVE electrode, then the result will be a POSITIVE upstroke on the. 8

9 Understanding Leads Everyone has heard of 3-lead or 5-lead monitoring, or ordered a 12-lead. But what exactly is a LEAD? Leads, by definition, are positive and negative electrodes attached to a recorder and used to detect electrical activity of the heart. A simple way to think of a Lead is as a picture of the electrical activity of the heart. Imagine that a camera is positioned at the location of the positive electrode in each of the leads we will discuss. From each individual angle, a unique view of the heart can be captured. 3 Lead The 3 lead is one of the most common. Leads I, II, and III are also known as the Limb Leads. To obtain a 3-lead, electrodes are placed on the Right and Left arms and on the left leg. Lead I looks from RA to LA Lead II looks from RA to LL Lead III looks from LA to LL 9

10 Augmented Leads The Augmented leads are the Other Limb Leads. With the augmented leads, the two negative electrodes are combined to form a central negative reference point. These leads offer a mixed view, or a single view between two of the views offered by the standard limb leads. For example, in lead AVF, our positive electrode is on the LL, and the central negative reference point is between the LA and RA leads. This offers a view between Leads II and III. The imaginary camera is still at the Left Leg, but it s positioned at a different angle. Together with the standard Limb Leads, there are now six intersecting views of the heart. V Leads The V leads are also called the Chest leads. These six leads offer HORIZONTAL views of the heart. This time, the camera is positioned on the chest wall, taking pictures through the chest and the heart itself. Think of the V leads as spokes of a HORIZONTAL wheel, with the AV node being the hub of the wheel. The negative end of each lead is a point somewhere on the patient s back. 10

11 V1 & V2 are positioned over the Right Ventricle, V3 & V4 over the septum between the ventricles, and V5 and V6 over the left ventricle. Remember the rule of electrical flow. If the electrical flow of the heart is TOWARDS a positive electrode, there is a positive deflection on the. Remember that the electrical charge of the Left Ventricle is greater than that of the right. Therefore, there is a mostly negative deflection in V1 (over the Right Ventricle) most of the electricity is going down and to the left, away from V1. As the leads get closer to the Left Ventricle, the of a normal heart in a normal rhythm will demonstrate R wave progression or progressively more positive QRS complexes. 5 Lead With 5-lead monitoring, common in many hospitals, only one V lead is used. The most leads used for routine monitoring are 5 leads. The common placement of the leads is as follows. Proper placement of the V leads is very important if the V leads are going to be used for diagnostic purposes. Proper chest wall placement of the V leads are shown below. 11

12 Place the V1 lead just to the right of the sternum in the 4 th intercostal space. Place V2 just to the LEFT of the sternum in the 4 th intercostal space. Place V4 in the left midclavicular line in the 5 th intercostal space. Place V3 between V2 and V4. Place V5 in the anterior axillary line in the 5 th intercostal space. Place V6 in the mid-axillary line in the 5 th intercostal space. Lead Skin Preparation Good lead preparation is very important also. The can tell us many things, but artifact can hinder the accuracy of the. To avoid artifact, be sure to prepare the skin properly. Before applying electrodes, skin should be free of hair, clean, and dry. For best results, attach electrodes to the leads before placing the leads on the patient. Electrodes should have plenty of gel and should be replaced if they become soiled or wet. Place electrodes as close as possible to the recommended areas, but make an effort to keep them out of the way of areas of large muscle movement. Flat bony surfaces are the best location. 12

13 Rhythm Analysis Normal sinus rhythm is the rhythm that most of you are probably in right now. The rhythm is regular. Sinus tachycardia is really a fast normal sinus rhythm. The SA node still generates the impulse, but it will be generated at a higher rate. The rhythm should still be regular. However, the very high rate can cause strain on the heart, especially in a patient with CAD. Possible causes include caffeine, stress, nicotine, alcohol, pain, fever, congestive heart failure, hypovolemia, hyperthyroidism, dig toxicity, and some medications. 13

14 Sinus bradycardia is a slow sinus rhythm. Again, this rhythm is initiated by the SA node. The rhythm is regular. Sinus bradycardia can be normal in some people, especially the very athletic. It can also be caused by sedation, increased intracranial pressure, medications such as beta blockers, vagal stimulation as with straining or vomiting, hypothyroidism, and hyperkalemia. Treatment is based on symptoms. Some of the symptoms may include decreased urine output, dizziness, weakness, and hypotension. Treatment may include administration of Atropine or Dopamine, or placement of an external pacemaker. NOTE For additional lessons on Rhythm analysis, contact Welch Allyn Protocol Clinical Support to find out about our AACN-approved CEU offering: Interpretation and Basic Arrhythmia Analysis. 14

15 References 1. Barash, P., Cullen, B., and Stoelting, R. (1992). Clinical Anesthesia. 2 nd Edition. J.B. Lippincott Company. Philadelphia, PA. 2. Guyton, A. (1991). Textbook of Medical Physiology. W.B. Saunders Co. Philadelphia, PA. 3. Loeb, S. (1993). Monitoring Clinical Functions. Advanced Skills. Springhouse Corp. Springhouse, Pennsylvania. 4. Thomas, C. Taber s Cyclopedic Medical Dictionary. 16 th Edition. F. A. Davis Co. Philadelphia, PA Dubin D. Rapid Interpretation of EKG s. Tampa, FL: Cover; Flynn JM, Bruce NP. Introduction to Critical Care Skills. St. Louis, MO: Mosby; Grauer K, Cavallaro D. Arrhythmia Interpretation: ACLS Preparation and Clinical Approach. St. Louis, MO: Mosby; Lipman B, Cascio T. Assessment and Interpretation. Philadelphia, PA: FA Davis Co.; Huszar RJ. Basic Dysrhythmias: Interpretation and Management. St.Louis, MO: Mosby; Marler CA. Introduction to. Dallas, TX: Parkland Health & Hospital Systems; Marriott H, Conover M. Advanced Concepts in Arrhythmia. St. Louis, MO: Mosby, Metzgar ED, Polfus PM. A Study and Learning Tool: Health Assessment (Second Edition). Springhouse PA: Springhouse Corporation; Smith- Huddleston S, Ferguson SG. Critical Care and Emergency Nursing: 2 nd Edition. Springhouse, PA: Springhouse Corporation; Cummins, RO. Textbook of Advanced Cardiac Life Support. American Heart Association, Walraven G. Basic Arrhythmias (3rd. Edition). Englewood Cliffs, NY: Brady;