ECG Techniques and Recognition

Standard ECG leads

- 12 leads are used: - 3 standard leads - 3 augmented leads - 6 precordial leads - the standard and augmented leads view the heart from a frontal plane and the precordial leads view the heart from a horizontal plane - the standard leads are bipolar and contain one positive and one negative electrode - Augmented leads are unipolar and contain one positive electrode - Precordial leads are also unipolar with only one electrode

P wave

- 1st phase of the cardiac cycle is seen as the P wave and corresponds to the wave of depolarization that spreads from the SA node to the atria, representing activation of the atria. - Normal length is 0.08- 0.1 seconds -Appearance is smooth and round - The first segment of the P wave represents activation or contraction of the right atrium, followed by activation of the left atrium. - Activation of the AV node is represented by the middle of the P wave. - Absent P waves may suggest certain arrhythmias.

Sinus Tachycardia

- Greater than 100 BPM

Voltage

- Voltage is measured as the height of each wave in mm - ECG graph paper is composed of small boxes, each one mm in height - 5 one mm boxes make up larger 5 mm boxes and are marked w/ thicker grid lines. - Each 1 mm in height = 0.1 milliVolts in electrical potential - The reference point is always the isoelectric line or baseline. - Waves are measured from the isoelectric line to the top or bottom point of the wave. - Measuring the voltage of the waves on an ECG is important in examining abnormalities in the cardiac cycle

Unipolar ECG Leads

- 3 augmented ECG leads are unipolar leads - They contain only one positive electrode and are placed on the right arm, left arm, and left leg - They are termed : - aVR: augmented voltage of the right arm - aVL: augmented voltage of the left arm - aVF: augmented voltage of the left foot - Electrodes record the electrical potential at one point in reference to the other 2 leads - The augmented leads view the heart's activity in a frontal place

Calculating the QRS axis

- 6 leads are most useful in deterring the QRS axis: I, II, III, aVR, aVL, and aVF - First, assess these six leads and determine which lead shows the most positive or most negative voltage in the QRS complex - If the R wave shows the most positive voltage, the measurement points directly to the axis - If the Q or S wave is the most negative waveform, it points directly away from the QRS axis - If a QRS complex exhibits equal positive and negative voltages, then the QRS axis is at a right angle to the complex ( either -90 or +90)

Precordial ECG leads

- 6 precordial leads are used in ECG measurement - the leads are unipolar and have only one positive electrode - the leads are placed across the chest horizontally. This is what allows the heart's activity to be viewed in a horizontal plane. - Leads V1 and V@ are placed over the right ventricle @ the 4th intercostal space, on the right and left sides of the sternum - Lead v4 is placed @ the 5th intercostal space on the midclavicular line - Lead V3 lies in between V2 and V4 - Leads V5 and V6 are placed over the left ventricle at the 5th intercostal space at the anterior axillary line and the midaxillary line, respectively

Normal Sinus Rhythm (NSR)

- 60-100 BPM -Normal heart rhythm begins: - SA node to atrial depolarization - P wave = depolarization of the atria - QRS interval = depolarization of the ventricle - ST segment & T wave = repolarization of the ventricle

Bundle Branch Block

- A block in the BB requires that the electrical impulse from a ventricle be used to activate the opposite, blocked, ventricle - The ventricles, therefore, are not activated at the same time, but in succession - This prolongs the ventricular activation phase of the cardiac cycle, thus widening the QRS complex on the ECG -The ST segment slopes into the T wave - It is important to note that the enlargement of either ventricle may produce a similar ECG tracing to a complete bundle branch block

ST segment and T wave alterations

- A normal elevation of the ST segment is seen in leads V1, V2, and V3 - ST-segment elevation that is convex or straight upwards suggests myocardial infarction - ST segment depression is always an abnormal finding. This may be caused by hypothermia, electrolyte abnormalities, or ischemic heart disease. -The T wave is always upright in leads I, II, V3, V4, V5 and V6 and inverted in aVR - A tall, peaked T wave indicates hyperkalemia or a myocardial infarction - An abnormally inverted T wave indicates cerebral or cardiac disease, conduction block, or ventricular hypertrophy

Myocardial ischemia

- the lack of oxygen delivery to an organ due to the obstruction or constriction of blood vessels - In myocardial ischemia, the left ventricle does not receive enough oxygenated blood. - Atherosclerosis is a common cause of myocardial ischemia. The fatty plaque in the coronary arteries obstructs blood supply - If ischemia persists and the heart continues to receive inadequate blood supply, injury or infarction can occur in the heart muscle - On an ECG, myocardial ischemia will have inverted T waves or St segment depression, - Inverted T waves are normal in the aVR lead and can also be a normal finding in leads III and V1 -In many pts with heart disease, different areas of the heart muscle may be in different stages of the ischemia -injury infarction progression, making ECG interpretation difficult

Ventricular arrhythmias

- An arrhythmia occurs when there is an abnormal variation in the regular pattern of heartbeats - May originate from the SA node, the heart's pacemaker, or the electrical conduction system that sends impulses throughout the heart's muscle cells - A variety of arrhythmias occur in the ventricles: - premature ventricular contractions ( PVC'S) - ventricular tachycardia - ventricular fibrillation - Premature ventricular contractions are early depolarizations of the ventricle and are often perceived as palpitations - Ventricular tachycardia is a series of premature ventricular contractions - If the series is prolonged, pts may become symptomatic, but many pts never experience symptoms - Ventricular fibrillation is life-threatening - The irregular contraction of the ventricles interferes with the delivery of blood to the body - Ventricular tachycardia may be seen on the ECG as deformed P waves, or a narrow QRS complex - Ventricular tachycardia that changes its axis over time is called Torsades de Pointes - In ventricular fibrillation, no QRS complex is seen on the ECG

Cardiac Cycle and ECG Relationship

- Each heartbeat is seen as 3 major waves or complexes on an ECG - the heart rhythm begins at the SA node and lead to atrial depolarization - the P wave represents depolarization of the atria - QRS interval represents depolarization of the ventricle - ST segment and T wave = repolarization of the ventricle - A small U wave may follow the T wave and represents further repolarization of the ventricle

Atrial hypertrophy

- Enlargement of either the right or left atria, or both - Hypertrophy may be caused by an increase in the volume of blood present in the atria or an increase in the resistance to blood flow out of the atria - An increase in blood volume causes dilation of the atria while an increase in resistance to blood flow causes a thickening of the muscle of the walls of the chamber - Since atrial depolarization begins in the SA node, the P wave is the diagnostic waveform for evaluating atrial hypertrophy - Depolarization of the right atria occurs first in the cardiac cycle and is seen as an abnormal beginning of the P wave - Right atrial hypertrophy exhibits a positive inflection at the beginning of the P wave - Activation of the left atria occurs later in the cardiac cycle and is apparent at the end of the P wave - Left atrial hypertrophy is seen as a negative deflection in the middle or latter part of the P wave.

Evaluating an ECG tracing

- First, calculate the length of the PR and QRS intervals. - If the length of the segments is abnormally long or short, heart block or bundle branch block may be present. - Determine the heart rate and rhythm and calculate the QRS axis - Examine the tracing for atrial or ventricular enlargement - Evaluate the QRS axis to diagnose conduction abnormalities - Examine the tracing to see if myocardial ischemia, injury or infarction is present - Evaluate any other abnormalities seen on the ECG tracing - Interpret this info by examining the pt hx, risk factors, medications, or medical problems, or other testing interferences

QRS Complex

- together indicates depolarization of both right and left ventricles. -Normal QRS = 0.06- 0.1 seconds in duration - Indicates that ventricular depolarization occurs very rapidly -a long QRS interval suggests impaired conduction in the ventricles - the Q wave precedes the R wave on an ECG and is a negative wave. -R wave is a positive wave -S wave is a negative wave seen after the R wave.

Failure of a cardiac pacemaker

- If the artificial pacemaker fails to sense the heart's own rhythm and generates additional impulses, a fatal arrhythmia can occur - In this case, a vertical spike is seen in the ECG tracing in the QRS complex or during the absolute or relative refractory period after the T wave - If the pacemaker fails to capture a contraction, the output of the pacemaker is too low to cause a depolarization of the ventricle - No contraction is produced and the heart fails to produce a heartbeat - The ECG tracing will produce an irregular rate, due to lack of depolarization - Small pacemaker spikes may still be seen

AV dissociation

-Atrioventricular (AV) dissociation occurs when the atria and the ventricles do not beat in unison - An accelerated junctional rhythm is a type of AV dissociation caused by increased automaticity of the AV node - On an ECG, the QRS complex will be normal, but there is no P wave present - In other cases, a normal P wave may be seen, but the PR interval will vary in length

SA block

-Sinoatrial block (SA) is the inability of an electrical impulse to be generated by the SA node - On an ECG, no P wave is seen during SA block - This type of sinus arrhythmia may occur intermittently and cause the pt no problems or symptoms

QT interval

-represents the entire action potential, the time for both ventricular repolarization and depolarization - Normal QT interval = 0.2- 0.4 seconds - AT increased heart rates, the interval shortens - Long QT intervals may be indicative of arrhythmias

Ventricular hypertrophy

- Left ventricular hypertrophy= enlargement of the wall of the left ventricle - Normally the wall of the left ventricle is 3x as thick as the wall of the right ventricle. - When the left ventricle is further enlarged, abnormalities will be viewed in leads I, aVL, V5, and V6. - These leads will record taller than usual R waves, as the increase in muscle mass increases the voltage passing through the ventricle - Right ventricular hypertrophy is the enlargement of the right ventricle -If the right ventricle is still thinner than the left ventricle, a change in ECG tracings may be undetectable - If right ventricular enlargement increases leads V1 and V2 will produce tall R waves as the voltage passing through the thickened ventricles increases

Myocardial injury

- MI occurs after a prolonged period of myocardial ischemia - Ischemia= lack of sufficient oxygen delivery to the heart due to an occluded blood vessel - MI & ischemia are both reversible if diagnosed early in its progression - In an MI: - an elevated ST segment is present - in many pts with heart disease, different areas of the heart muscle may be in different stages of the ischemia-injury-infarction progression, making ECG interpretation difficult

Myocardial infarction

- MI= death of heart cells due to the inability of the circulatory system to deliver enough oxygen to the myocardial tissue - Several changes are seen on the ECG that are useful in diagnosing an MI: - A ST segment elevation occurs in the electrode over the area of injury - The QRS complex deviates from its axis, away from the area of injury - The T wave is inverted in the electrode over the injured area - The abnormal ST segment should return to baseline, but the T wave may remain inverted - If the St segment is still elevated and the T wave is inverted, the MI is most likely acute - If however, the ST segment has returned to baseline, but the T wave remains inverted, it is impossible to determine the age of the infarction. - If the St segment is at baseline and the T wave is upright, the infarction is most likely old.

Q wave

- Negative wave marking the beginning of the QRS complex - Normal duration is small and will depend on which lead is used to measure the wave in an ECG - May often be less than 0.03 seconds - An enlarged Q wave suggests abnormal conduction in the ventricles or a MI

Time

- Normal ECG paper speed = 25 mm per second - Time on an ECG is measured horizontally, with each small 1 mm square of the graph paper corresponding to 0.04 seconds - Each large square composed of 5 one mm squares and marked with thicker grid lines, corresponds to 0.20 seconds - 5 large squares = one second in duration - The measurement of time on an ECG is important in determining heart rate and the length of various intervals in the cardiac cycle

Atrial arrhythmias

- Occurs when there is an abnormal variation in the regular pattern of heartbeats - This may originate from the SA node, the heart's pacemaker, or the electrical conduction system that sends impulses throughout the heart's muscle cells - An arrhythmia arising from the atria is termed atrial flutter or atrial fibrillation - The atria contract at rapid, irregular intervals - This irregular contraction interferes with the atria's ability to empty blood into the ventricles, which pump blood to the rest of the body - On an ECG, atrial flutter is seen with F waves at rate of 200 - 350 BPM in a sawtooth pattern - Atrial fibrillation = the rate will increase beyond 350 BPM and P waves will be absent, since there is no complete depolarization of the atria

PR interval

- PR interval represents the period from the beginning of the atrial depolarization to the beginning of ventricular depolarization - This also includes the activation of the bundle of HIS and bundle branches - Measured from the beginning of the P wave to the beginning of the QRS complex -Normal PR interval is 0.12- 0.20 seconds - If PR interval is shorter than 0.12 seconds = accelerated conduction - If PR interval is greater than 0.20 seconds= first degree AV block - PR interval may vary w/ heart rate, decreasing in length as heart rate increases - PR interval increases w/ age (short @ childhood & lengthning into adulthood)

AV & ventricular cardiac pacemakers

- Pacemaker artifacts, or impulses generated by an artificial pacemaker in the heart, are seen on an ECG tracing as vertical lines extending above or below the baseline - The amplitude of the vertical line will vary with each lead, and may not be seen in all leads - An AV pacemaker is a device that generates an impulse to the atria and ventricles, leading to contraction - In pts with AV pacemakers, an atrial spike precedes the P wave and a ventricular spike precedes a wide QRS complex - A ventricular pacemaker stimulates contraction in the ventricles, but not the atria - In pts w/ ventricular pacemakers, one spike is seen before the QRS complex on the ECG

QRS Axis

- QRS axis is the measurement of the direction of electrical activity in the heart - The QRS interval may change due to a conduction abnormality or movement of the heart - In these cases, the electrical activity is said to move away from the damaged or diseased area of the heart. - A normal QRS axis = -30 degrees to +90 degrees when viewed from the hexaxial system - A deviation of the QRS axis greater than +90 degrees is a right deviation and usually caused by hypertrophy of the right ventricle - Right ventricle hypertrophy is common in people with pulmonary disorders - A left deviation occurs when the QRS axis appears more than - 30 degrees to the left -Left deviation indicates left ventricular hypertrophy, often caused by hypertension, conduction abnormalities, or aortic stenosis

R wave and S wave

- R wave is a positive wave and is the predominant portion of the QRS complex - It may/ may not be preceded by a Q wave - If a second positive wave is seen following the R wave, it is termed R' wave - S wave is a negative wave that follows the R wave in the QRS complex - Together with the Q wave, the R and S waves represent ventricular depolarization

Normal and abnormal ST segments

- ST segment follows the QRS interval and corresponds to the period in which the ventricle is completely depolarized. - Measured from the end of the QRS complex to the beginning of the T wave - J point is the junction of the QRS complex and the ST segment - This point is usually a 90-degree angle with the S wave - A normal ST segment does not vary more than 1 mm above or below the isoelectric line - The ST segment is at the same baseline level as the PR segment - The ST segment exhibits an abnormal baseline when there is inadequate perfusion to the myocardium, leading to tissue damage - A depressed ST segment indicates ventricular ischemia, while an elevated ST segment may indicate hypoxia or lack of adequate oxygen perfusion

Hexaxial Reference System

- Series of lines used to describe the heart in a frontal plane, that represent the standard limb leads used in ECG measurement - Six ECG leads are placed around the heart and the heart is visualized as a circle. - The circle is divided into segments of 30 degrees each, starting w/ 0 degrees @ Lead I and proceeding in a clockwise direction - Clockwise from 0 degrees, there are six segments, comprising 180 degrees, and are numbered in a positive category - Proceeding counterclockwise from 0 degrees, the six segments are numbers in a negative category at 30-degree increments. - The normal axis = 0 degrees to +90 degrees - The left axis = 0 degees to -90 degrees - The right axis = +90 degrees to 180 degees - The extreme right of the left axis = -90 degrees to 180 degrees.

Sinus arrhythmias

- Sinus arrhythmia = any sinus rhythm that exhibits an irregular rate - Sinus arrhythmias are considered normal ECG findings in children and young adults - Respiratory sinus arrhythmia indicates normal changes in the heart's rhythm that occur with each respiratory cycle - Sinus rhythm increases with inhalation and decreases with exaltation - sinus arrhythmias are normal variations seen on an ECG and are not clinically important - A variation of more than 0.1 seconds in PP or PR intervals may be measured with a sinus arrhythmia

T wave

- T wave is a positive wave following the S wave & represents ventricular repolarization or recovery of the ventricle. - It is the final major wave of the cardiac cycle - It is longer than the QRS complex, indicating that repolarization takes longer than depolarization during the cardiac cycle - T wave is smooth and round, similar in shape to the P wave - Abnormal T waves may indicate heart disease or electrolyte imbalances - The T wave may be followed by a small U wave that still represents the final repolarization of the ventricles.

ST segment

- The ST segment follows the QRS interval and corresponds to the period in which the ventricle is completely depolarized - Measured from end of the QRS complex to the T wave - This corresponds to the plateau phase of the action potential - The J point is the junction of the QRS complex and the ST segment - This point is usually a 90-degree angle with the S wave - The St segment may be depressed or elevated and can be used for dx ventricular ischemia or hypoxia

Measuring heart rate

- Usually, there is the same number of P waves and QRS complexes and they occur at regular intervals - To calculate heart rate, simply count the number of horizontal squares on the graph paper between recurring waves or cycles. - Counting the number of cardiac cycles in 6 seconds and multiply this number by 10 may also determine heart rate - If the heart rate is less than 100 BPM, only the large squares on the ECG paper need to be counted to determine time - If the heart rate is greater than 100 BPM, it is best to consider the smaller squares when measuring the time interval

Wolff-Parkinson White syndrome

- WPW syndrome is known as a pre-excitation abnormality in which the electrical impulse from the atria reaches the ventricle too soon - An extra pathway is present that conducts electrical impulses from the atria to the ventricles - This is seen on an ECG as a series of three abnormalities: - a short PR interval - a long upstroke on the QRS complex - tachyarrhythmia - Pts w/ this syndrome may experience a rapid heart rate, dizziness, chest pain, or fainting. Some pts feel no symptoms

Artifacts in ECG recordings

- artifact = interference or influences that do not originate from the heart muscle - Common physiologic factors include muscle movements or spasms, sweat, and respiratory influences - Common external causes include loose connection of an electrode, the presence of alternating current from equipment, and movement in the environment

Sinus tachycardia

- heart rate greater than 100 BPM - May originate from a pathologic condition of the heart's pacemaker or may originate from activation of the sympathetic nervous system - Sympathic nervous system stimulation may increase the heart rate to its max of near 200 BPM - The commonly accepted formula for determining maximum heart rate is 220 BPM minus age - Rarely does sympathetic activation increases the heart rate to more than 160 BPM - Sinus tachycardia exhibits one P wave to each QRS complex, but the PR interval is shorter than in average sinus rhythm

Sinus Bradycardia

- heart rate slower than 60 BPM - This slowing originates from the sinus node - Other bradycardia may originate at places below the sinus node - Bradycardia is often normal in well-trained athletes, but may also be seen when there is a decrease automaticity of the sinus mode, either a pacemaker failure, physiological slowing of the sinus rate, or activation of the parasympathetic nervous system.

Sinus Bradycardia

- less than 60 BPM

Bipolar ECG leads

- the 3 standard leads used in ECG measurement are bipolar leads - They contain 1 positive and 1 negative electrode - The ECG measures the difference in electrical potential recorded by each electrode -Lead I is placed on the right arm (negative electrode) & the left arm ( positive electrode) - Lead II is placed on the right arm (negative) and the left leg (positive) -Lead III is placed on the left arm (negative) and the left leg (positive) - Einthoven's Law describes their placement and mathematical relationship to one another - The standard leads view the heart's activity in a frontal plane