1. Electrocardiography Lesson 2: Performing an Electrocardiogram49
Electrocardiography
Lesson 2:
Performing an Electrocardiogram

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? 4

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 5

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 6

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 7

8. ECG Tracing 8

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 9

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 10

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 11

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

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) 13

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

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 15

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

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 17

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 18

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 19

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

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 21

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 22

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 23

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 24

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 25

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) 26

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 27

28. Heart Rate 6-second Method You can estimate the heart rate from an ECG
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 28

29. Heart Rate Count-off Method
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 29

30. Heart Rate Exact Calculation
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 30

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 31

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) 32

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 33

34. Einthoven’s Triangle 34

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 35

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 36

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 37

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 38

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

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 40

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 41

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 42

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 43

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

45. Questions? 45