The Electrocardiogram 2 The Electrocardiogram Fast & Easy ECGs – A Self-Paced Learning Program Q I A

A tracing of the heart’s electrical activity Electrocardiogram A tracing of the heart’s electrical activity

Electrocardiograph Detects heart’s electrical activity through electrodes positioned on patient’s skin Instructional point: Lead wires transfer electrical activity back to ECG machine where it is displayed on an oscilloscope and/or printed onto graph paper. I

ECG Paper Grid layout on ECG paper consists of horizontal and vertical lines. Allows quick determination of duration and amplitude of waveforms, intervals and segments. I

ECG Paper Each small square = 0.04 sec in duration and 0.1 mV in amplitude Five small squares = One large box and 0.20 seconds in duration Horizontal measurements determine heart rate Instructional point: Vertical markings on top or bottom of ECG paper can be used to quickly approximate the heart rate. I

Isoelectric Line Flat line that occurs: When no electrical activity is occurring When impulses are too weak to be detected Used as a baseline to identify changing electrical movement

P Wave Produced by: Waveform is upright and round initiation of impulse in SA node depolarization of RA and LA Impulse passing through AV junction Waveform is upright and round

PR Segment Flat (isoelectric) line seen as impulse travels through His-Purkinje system

QRS Complex Follows PR segment and consists of: Q wave First negative deflection following PR segment R wave First positive deflection following Q wave or PR segment S wave First negative deflection that extends below the baseline following the R wave Instructional points: The S wave begins at the point where the negative deflection of the R wave reaches the baseline. I

ST Segment and T Wave ST segment is flat line that follows QRS complex T wave is larger, slightly asymmetrical waveform that follows ST segment Instructional point: The J point is the site where the S wave connects to the ST segment. I

QT Interval The distance from onset of QRS complex until end of T wave Measures time of ventricular depolarization and repolarization Has normal duration of 0.36 to 0.44 seconds Instructional points: The QT interval shouldn’t be greater than half the distance between consecutive R waves when the rhythm is regular. It varies according to age, sex, and heart rate, the faster the heart rate the shorter the QT interval. I

Typical ECG Upright, round P waves occurring at regular intervals at a rate of 60 to 100 beats per minute PR interval of normal duration (0.12 to 0.20 seconds) followed by a QRS complex of normal upright contour, duration (0.06 to 0.12 seconds) and configuration Flat ST segment followed by an upright, slightly asymmetrical T wave

Calibration or Registration Mark Helps ensure ECG machine is properly calibrated Serves as reference point on ECG tracing Instructional points: The calibration or registration mark should be included on every ECG tracing. Some ECG machines create this mark automatically while with others it must be placed manually. I

Artifact Markings on ECG tracing that are not a product of heart’s electrical activity Patient movement is among its many causes Can mimic life-threatening dysrhythmias Instructional point: Emphasize the need to assess the patient whenever any sort of chaotic electrical activity is seen on ECG. I

ECG Machines A variety of machines are available Some monitor from one to five different leads Others are capable of monitoring up to twelve or more leads simultaneously Instructional points: ECG machines used to provide continuous cardiac monitoring are often referred to as an ECG monitor. Many are equipped with a defibrillator and pacemaker. I

ECG Leads Each lead provides a different view of the heart: Electrodes are placed on chest, arms and legs Sites vary depending on which view of the heart's electrical activity is being assessed Instructional points: A lead is the view between one positive and one negative pole. Information obtained through just one lead is limited while multiple leads can help identify needed information. I

ECG Leads Direction an ECG waveform takes depends on whether electrical currents are traveling toward or away from a positive electrode Instructional note: A biphasic waveform is composed of both positive and negative deflections. I

ECG Leads Planes provide a cross-sectional view of heart Frontal plane Horizontal plane Instructional point: Frontal plane is a vertical cut through middle of heart while horizontal plane is a transverse cut through middle of heart. I

Limb Leads View the frontal plane Include leads I, II, III, aVR, aVL and aVF Provide inferior, superior, and lateral views of heart

Precordial Leads View horizontal plane and include leads V1, V2, V3, V4, V5, and V6 Provide inferior, superior, and lateral views of heart

Unipolar Leads Use one positive electrode and a reference point which is calculated by the ECG machine

Uses for Different Leads A given lead may be used to highlight: A specific part of ECG complex Electrical events of a specific cardiac cycle Conditions such as an enlargement of heart muscle or presence of ischemia, injury and infarction Instructional points: Leads I, II, III or MCL leads are typically used for continuous cardiac monitoring to identify the presence of and/or developing cardiac dysrhythmias). 12 leads are used to obtain specific diagnostic information. I

Bipolar Leads Record difference in electrical potential between a positive and negative electrode Uses a third electrode called a ground Include leads I, II and III Instructional point: Axes of these three bipolar leads forms a triangle around the heart referred to as Einthoven’s triangle. I

Limb Leads - Lead I Positive electrode - left arm (or under left clavicle) Negative electrode - right arm (or below right clavicle) Ground electrode - left leg (or left side of chest in midclavicular line just beneath last rib) Waveforms are positive

Limb Leads - Lead II Positive electrode - left leg (or on left side of chest in midclavicular line just beneath last rib) Negative electrode - right arm (or below right clavicle) Ground electrode - left arm (or below left clavicle) Waveforms are positive

Limb Leads - Lead III Positive electrode - left leg (or left side of the chest in midclavicular line just beneath last rib) Negative electrode - left arm (or below left clavicle) Ground electrode - right arm (or below right clavicle) Waveforms are positive or biphasic

Limb Leads - Augmented Leads Includes aVR, aVL and aVF Unipolar Enhanced by ECG machine because waveforms produced by these leads are normally small Instructional point: The ECG machine augments these leads by 50% to make them easier to see I

Limb Leads - Lead aVR Positive electrode placed on right arm Waveforms have negative deflection

Limb Leads - Lead aVL Positive electrode placed on left arm Waveforms have positive deflection

Limb Leads - Lead aVF Positive electrode located on left leg Waveforms have a positive deflection

Precordial Leads Includes leads V1, V2, V3, V4, V5 and V6 Positioned in order across the chest Unipolar Opposing pole is center of heart as calculated by ECG Instructional point: Provide information about horizontal plane of heart. I

Modified Chest Leads (MCL) MCL1 and MCL6 provide continuous cardiac monitoring Place positive electrode in same position as precordial leads V1 or V6 Instructional point: Modified chest leads are employed primarily to detect dysrhythmias. I

Summary Electrodes placed on patient’s skin detect heart’s electrical activity. Graphic record or tracing is called an electrocardiogram. Abnormalities in cardiac rate and/or rhythm are called dysrhythmias.

Summary ECG paper consists of horizontal and vertical lines that form a grid. Horizontal measurements used to determine heart rate and duration of various waveforms, segments and intervals. Vertically on ECG paper, distance between lines, or boxes, represents amplitude in millimeters (mm) or electrical voltage in millivolts (mV).

Summary Conduction of electrical impulse through the heart can be seen on ECG as P wave, PR interval, QRS complex, ST segment and T wave. Artifact is markings on ECG tracing that have no relationship to electrical activity of the heart.

Summary Each lead provides a different view of heart. Impulses traveling toward a positive electrode are recorded on ECG as upward deflections. Impulses traveling away from a positive electrode or toward a negative electrode are recorded as downward deflections.

Summary Limb leads are produced by placing electrodes on right arm (RA), left arm (LA) and left leg (LL). Include leads I, II, III, augmented vector right (aVR), augmented vector left (aVL) and augmented vector foot (aVF). Precordial leads include leads V1, V2, V3, V4, V5, and V6.