Electrocardiography Lesson 2: Performing an Electrocardiogram 49 Electrocardiography Lesson 2: Performing an Electrocardiogram

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.

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

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

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

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

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

ECG Tracing 8

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Einthoven’s Triangle 34

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

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

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

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

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

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

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

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

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

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

Questions? 45