Electrocardiography Lesson 2: Performing an Electrocardiogram

1. Electrocardiography Lesson 2: Performing an Electrocardiogram 49

2. Lesson Objectives Upon completion of this lesson, students should be able to … • Define and spell the terms for this chapter. • Explain the significance of each ECG wave. • Maintain and operate electrocardiogram equipment. • Identify by name and function the controls on an electrocardiograph machine.

3. Lesson Objectives • Name the standard 12 leads and the locations of their sensors. • State the cause and correction of artifacts.

4. Critical Thinking Question • What is the medical assistant’s role in ECGs?

5. The Electrocardiogram (ECG) • Electrical charges created by the conduction system can be sensed throughout the body • These charges can be picked up by placing sensors in specific areas of the skin • Sensors transmit the electrical charge to a computer for amplification of the signal

6. The Electrocardiogram (ECG) • Amplified signal is recorded on graph paper • A flat line means that no electrical signal is sensed • A deflection above or below the flat line means that there is an electrical signal of some kind • A positive (up) or negative (down) deflection is called a wave • The height of the wave reflects strength of the electrical signal

7. The ECG • A normal cardiac cycle is one series of PQRST waves. • P – initial atrial depolarization (change in electrical activity) • QRS complex – ventrical depolarization • T – repolarization (return to the resting electrical state) • Then it all starts over again

8. ECG Tracing

9. Time and the Cardiac Cycle • The P wave represents the impulse that originated in the SA node and spread through the atria (called atrial depolarization) • A normal looking P wave reflects a normally-functioning SA node • The P-R interval is the time from the beginning of P to the beginning of QRS

10. Time and the Cardiac Cycle • This time is between 0.12 and 0.20 seconds (2-5 small boxes on the EKG graph paper) • A deviation could represent an abnormality in the electrical system or structure of the heart

11. Time and the Cardiac Cycle • A P-R Interval that is too short means that the impulse has reached the ventricles through a shorter than normal pathway • A P-R Interval that is too long means that there is a conduction delay in the AV node possibly

12. Critical Thinking Question • In the previous two examples, how many small boxes are between the P and R?

13. Time and the Cardiac Cycle • The QRS complex represents the time necessary for the impulse to travel through the bundle of His, the bundle branches, and the Purkinje fibers to complete ventricular contraction • This is ventricular depolarization • This usually takes less than 0.12 second (3 small ECG boxes)

14. Critical Thinking Question • In this example, how many small boxes are between the Q and S?

15. Time and the Cardiac Cycle • The ST segment and the T wave represent repolarization of the ventricles • The ST segment is normally flat (on the baseline) or only slightly elevated • The T wave represents a part of recovery of the ventricles after contraction • The QRS complex and T wave typically point in the same direction • Anything else may indicate a problem in the heart or its electrical system

16. Critical Thinking Question • Is this a normal ST segment? Why?

17. ECG Machines • All ECG machines are calibrated to align with the international standard • The paper moves at a speed of 25 mm/second • Given the same amount of electrical energy, the recording stylus also moves the same distance • 1 mV of electricity input will cause the stylus to deflect 10 mm • This allows uniform recordings worldwide

18. ECG Machines • Older models are manual; newer models complete many of the procedures automatically via a computer • During patient preparation, all ten sensors are placed on the patient • The computer then switches from lead to lead in rapid succession

19. ECG Machines • Patient data is entered into the machine (name, date of birth, diagnoses, height, weight, age, bp, medications, and other information) • You can override the automatic controls, if desired

20. Critical Thinking Question • What can you do to help the patient relax during the preparation and testing? Why is relaxing important?

21. ECG Machines • Each lead is recorded in a separate channel or pathway for the signal • Many machines record more than one channel (typically 3) at once • Some machines will print out a statement about the status of the heart • Some even can fax the report to a physician’s office • Computerized ECGs should still be carefully examined to make sure that a clear ECG is made before disconnecting the sensors

22. The ECG Control Panel • Main power switch: Allows for a warm-up as specified by the manufacturer before using • Record switch: This switch moves the paper at the “run 25” speed

23. The ECG Control Panel • Lead selector: This determines from which sensors (electrodes) the machine will record: • Standard (limb) leads: Record from two electrodes placed on all extremities • Augmented leads: Record from the midpoint between two limb electrodes to a third limb sensor • Chest leads: Record from various positions on the thorax

24. The ECG Control Panel • Sensitivity control: Allows the operator to increase or decrease the recording size to enlarge or shrink the deflections to fit on the paper

25. The ECG Control Panel • Standard button: Allows verification of calibration to the international standard • Stylus control: Centers the recording in the middle of the page or the center of each channel by moving the stylus • Stylus heat control: Increases or decreases heat and adjusts for the sharpest tracing • Marker: Indicates, by a code, which lead is being recorded

26. The ECG Paper • Paper is pressure sensitive and must be handled carefully • If this paper is exposed to light for long periods, the markings will fade with time • Many newer models use an ink cartridge to supply the stylus and provide a longer-lasting printout • Paper records both time (horizontally) and voltage (vertically)

27. Time Markers on ECG Paper • Time markers (3-second markers) are printed on the ECG paper • Located at the top of single-channel paper and between channels in multi-channel paper • Small squares with a light line and larger squares of a darker line • Small squares: 1 mm by 1 mm square = 0.1 mV in height and .04 second time in the width • Large squares: 5 mm by 5mm square = 0.5 mV of voltage in the height and 0.20 second time in the width

28. Heart Rate • You can estimate the heart rate from an ECG 6-second Method • Begin at one 3-second marker and go to the right for two additional markers, for a total of 6 seconds • Count the number of QRS complexes between the first and third markers, and add a zero • This is estimated ventricular rate per minute

29. Heart Rate • You can estimate the heart rate from an ECG Count-off Method • Locate a QRS complex close to a 5-mm line • Move to the next deflection at the right or the left • Count how many 5-mm lines intersect the tracing before the next QRS complex • Count off at each 5-mm line saying “zero, 300, 150, 100, 75, 60, 50” • Stop counting when you reach the next QRS complex

30. Heart Rate • You can estimate the heart rate from an ECG Exact Calculation • Paper moves at a standard speed of 25 mm/second = 1500 mm/minute • Count the millimeter boxes between two QRS complexes • Divide the number into 1500 • Example: if there are 20 mm between two QRS complexes, 1500 divided by 20 = 75 beats per minute

31. Rhythm • The regularity of the occurrence of heartbeats • Ventricular rhythm is determined by measuring the distance between QRS complexes • There should be a consistent space between complexes • Atrial rhythm is determined by measuring the distance between P waves • There should be a consistent space between waves

32. Sensor Placement • The ECG machine records the cardiac cycle through sensors placed on the patient’s bare skin • Sensors are placed: • Over the fleshy part of the inner aspect of both lower legs (RL and LL) • Over both upper arms or both forearms, avoiding the bony prominences (RA and LA) • Chest sensors (V) are placed in 6 locations (V1 – V6)

33. ECG Leads • Each lead will record from a specific combination of sensors • By recording from different combinations of sensors, the electrical activity of the heart is seen from different angles

34. Einthoven’s Triangle

35. Patient Preparation for an ECG • Explain to the patient the equipment and procedure as well as what you will expect the patient to do • The surroundings should be pleasant and the table wide enough for adequate support • Patients will need to be bare to the waist so privacy should be provided for disrobing

36. Patient Preparation for an ECG • Position patient in the supine or semi-Fowler’s position • Jewelry, particularly metal jewelry, must be removed so that it does not interfere with the electrical current of the ECG • Prepare the skin where the sensors will be applied

37. Preparation of Equipment for an ECG • ECG machine that has been calibrated and is in good working order • Good supply of paper • Supply of electrolyte or conduction cream, gel, or pads

38. The ECG Recording • Normally, the ECG recording is made in sensitivity 1 = 10mm deflection per 1 mV of electricity • If size is doubled, it is sensitivity 2 • If size is cut in half, it is sensitivity 1/2

39. Satisfactory Tracings • Accurate • Readable • Clear • Travels down the center of the page • Has a consistently horizontal baseline

40. Artifacts • Electrical activity from a source other than the heart that the sensors detect • Can impair accurate interpretation of the tracings • Causes include: • Somatic tremors • Wandering baseline and baseline shifts • 60 cycle or AC interference • Erratic stylus

41. Mounting an ECG • Select the best part of the recording for that lead • Cut and trim it • Place it in the appropriate area of the folder • Double-check to ensure you have read the lead’s international code correctly • Repeat the process until all 12 leads have been properly mounted

42. Normal Sinus Rhythm • Has three distinct waves • P wave • T wave • QRS complex between the P and T waves where the Q is a downward deflection, the R is an upward deflection, and the S is a downward deflection following an R • The beats come at regular intervals • Within the lead being recorded, each cardiac cycle appears the same as previous ones

43. Abnormalities Caused by Cardiac Pathology • Atrial fibrillation • Atrial flutter • AV heart block • Myocardial infarction (MI) • Paroxysmal atrial tachycardia (PAT) • Premature atrial contractions (PACs) • Premature ventricular contractions (PVCs) • Sinus arrhythmia

44. Abnormalities Caused by Cardiac Pathology • Sinus bradycardia • Sinus tachycardia • Ventricular fibrillation • Ventricular tachycardia

45. Questions?