EARLIER TODAY, Mr. S, 67, was admitted to obtain I.V. Antibiotics to treat a far-ranging case of reduced extremity cellulitis secondary to a cut he continual fishing. Responding to the call bell, his nurse find him sit up in bed and also complaining that chest discomfort. The nurse takes his an important signs and performs a pains assessment, which includes documenting the onset, location, quality, intensity, duration, and any radiation of the discomfort. The nurse asks about associated signs and symptoms and also factors that aggravate or relax the pain. Adhering to facility protocol, the nurse administers supplemental oxygen in ~ 4 L/minute via sleep cannula and pages the physician on call, who orders stat serum cardiac biomarkers, a 12-lead ECG, and also sublingual nitroglycerin.

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A nurse who have the right to independently analyze a 12-lead ECG can anticipate and also prepare for any emergency treatment the patient might need. This post explains the basics the 12-lead ECG interpretation, concentrating on a normal ECG. Next month, the second part of this article will discuss ECG abnormalities.

 What"s happening in the heart

The heart"s inner conduction mechanism initiates each heartbeat and coordinates all parts of the love to contract at the proper time. A typical heartbeat is initiated in the sinoatrial (SA) node, a devoted group of cell in the appropriate atrium. (See Taking anatomy come heart.) The SA node depolarizes in ~ a rate of 60 to 100 times/minute, resulting in the atria come contract and also propel blood right into the ventricles.

 

Atrial depolarization produce the very first element ~ above the ECG waveform: the P wave. The an initial part that the cardiac cycle, the p wave appears as a small, semicircular bang (see Tracing a regular ECG waveform).

 

The wave of depolarization proceeds through the atria until it encounters the next important structure, the atrioventricular (AV) node, i m sorry receives the atrial impulse. After a short pause come let the ventricles fill, the AV node transmits the impulse to the ventricles via the bundle that His. A repertoire of cardiac conduction fibers, the bundle of His splits into the right and left bundle branches.

 

The bundle branches are high-speed conducting fibers that operation down the intraventricular septum and transmit the cardiac advertise to the Purkinje fibers, which type a complex network the mingles through ventricular myocardial cells. The duty of the Purkinje fibers is to swiftly stimulate ventricular muscle fibers, leading to the next significant event in the cardiac cycle: ventricular depolarization.

 

Ventricular depolarization generates the QRS complex, the electric equivalent the ventricular systole. (Remember that electrical task precedes mechanical activity, and also the ECG reflects only electric activity.) If friend palpate a carotid or radial pulse when looking at a cardiac monitor, you need to feel a pulse with each QRS facility on the monitor.

 

The QRS complex normally has a expression of 0.06 to 0.1 second. A duration of 0.12 2nd or an ext usually indicates an extensive ventricular conduction resulted in by a bundle-branch block. The QRS facility is variable in appearance and may have actually a various shape (morphology) in different patients or even look various in miscellaneous ECG leader in the same patient. The QRS facility may have one, two, or 3 wave components, relying on the lead and your patient"s clinical status.

 

The last significant wave component of the ECG is the T wave, which is larger than the p wave and also rounded or contempt peaked. Instantly following the QRS complex, it represents ventricular repolarization or a metabolic rest period between heartbeats. Depolarization and also repolarization are caused by the activity of cations, consisting of sodium, potassium, and calcium, throughout the myocardial cabinet membrane.

  
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Figure. Taking anatomy come heart

Besides the 3 waveforms, the normal ECG cardiac bicycle tracing has two necessary segments, or flat (isoelectric) parts of the tracing in between the waveforms: the PR interval and the ST segment.

 

The PR term is the duration from the start of the P tide to the start of the QRS complex. It consists of the P tide plus the brief isoelectric segment the terminates in ~ the begin of the QRS complex. The typical PR interval big 0.12 come 0.2 second; this represents the time from SA node depolarization come ventricular depolarization. If the PR term is less than 0.12 second, then the cardiac advertise didn"t monitor the normal conduction pathway. If the PR term is longer 보다 0.2 second, then a an illness process may be affect the cardiac conduction pathway, maintaining it native functioning properly.

  
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Figure. Tracing a common ECG waveform

The ST segment consists of the isoelectric line between the end of the QRS complex and the start of the T wave. The ST segment discover information about the heart"s oxygenation status. For example, myocardial ischemia (a temporary, reversible diminish in oxygenation) regularly results in one ST segment below the baseline the the ECG tracing. As soon as myocardial cells room injured (reversible physical damage from absence of oxygen), the ST segment often is elevated over the baseline. So ST-segment key is a vital indicator of myocardial infarction. Because that tips on how to usage the ECG to calculation heart rates and also more, check out Paper training.

 Catching the wave

Examination that a 12-lead ECG reveals that part QRS complexes have actually upward (positive) deflections and others have actually downward (negative) deflections. Here"s why.

 

Each ECG lead has actually a hopeful (or sensing) electrode and also a negative electrode, i beg your pardon acts as an anchor. The optimistic electrode looks towards its negative electrode and senses whether electric energy is being directed toward or far from the optimistic electrode.

 

When electrical energy is directed toward the optimistic monitoring electrode, the QRS complicated has an upward deflection. As soon as the electric energy is directed far from the hopeful monitoring electrode, the QRS complex has a downward deflection. The more directly aligned the direction of the electric energy with the positive electrode, the much more upright the complex. If the electrical energy viewpoints the positive monitoring electrode in ~ a slim angle, the complicated will still it is in upright, yet less upright than if the power were directly aligned through the confident electrode.

 

Energy showing up at a perpendicular edge to the confident electrode results in one of two people a waveform with little deflection (isoelectric) or equal quantities of confident and an adverse deflection.

 

As power is directed far from the hopeful electrode, the QRS complex becomes progressively much more negative. When energy flow is directed totally away indigenous the hopeful electrode, the QRS complex is deflected straight downward.

 Going through the flow: A look in ~ vectors

All cardiac cells room electrochemical, meaning they generate electrical energy during depolarization. This electric energy, dubbed a vector, has actually strength (measured in millivolts) and direction (measured in degrees from an arbitrarily zero allude called the electric axis). Every cardiac cell generates its very own microvector. The mathematical average of this microvectors is the median QRS vector or typical vector, which complies with the normal conduction pathway that the heart-downward and to the left. The average vector flows slightly come the left that the ventricular septum since the left ventricle has more and larger cardiac cells.

 

Generally, every person has actually a unique mean vector direction, i beg your pardon remains constant unless his cardiac standing changes. Because that example, left ventricular hypertrophy secondary to heart fail pulls the median vector even much more sharply come the left side.1 A patient who has a median vector in one abnormal direction is stated to have an axis deviation. (For details, watch Axis deviation: As straightforward as pie .)

 

The median vector is a depiction of the heart"s in its entirety electrical properties. A 12-lead ECG is the electrical record the the mean vector native 12 various monitoring website (leads) on the surface ar of the body. As as soon as you look at any kind of object, you need to see all the angle to get a finish picture.

 Looking at body leads

The first six leads of the 12-lead ECG come from 4 electrodes put on the patient"s arms and legs; the right lower leg electrode is the floor electrode. The limb leads record the average vector in the up-down and left-right direction along the body"s frontal plane. Due to the fact that they usage separate optimistic and negative electrodes, they"re dubbed bipolar or standard leads.

 

* Lead I place the hopeful electrode top top the left arm and also looks towards the an unfavorable electrode on the best arm for electrical energy. Since the mean vector travel from upper ideal to reduced left, power flows towards the hopeful electrode of lead I, resulting in an increase deflection that the QRS. And also because the median vector doesn"t flow directly toward command I yet approaches it at a somewhat wide angle, the increase deflection the the QRS complicated is moderate.

 

  * In lead II, the confident electrode is top top the left foot and also the negative electrode is top top the right arm. Since the typical vector flows straight at the positive lead II electrode, this lead usually has actually the most upright QRS complexes and also the most prominent P tide of the whole 12-lead ECG. That"s why command II is a favorite surveillance lead in plenty of intensive care and also telemetry units.

  
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  * Lead III puts the hopeful electrode ~ above the left foot and the an unfavorable one top top the left arm. The average vector flow approaches lead III bottom from the right, again producing an upward QRS deflection. Because the angle is narrower than the angle in between the typical vector and also lead I, the lead III QRS complex is much more upright 보다 the lead i QRS complex.

  
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 The second set of limb leads are called the augmented or unipolar leads and also use a solitary positive surveillance electrode. The negative electrode is one electrically calculated ar at the facility of the heart.2

  
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* Lead aVR is the only limb command on the ideal side the the body. Its confident monitoring electrode is situated on the best arm and also looks downward and to the left. The mean vector also flows downward and to the left, straight away from command aVR, bring about a negative deflection for every waveforms. In a normal ECG, aVR is the just limb lead through a downwardly deflected QRS.

 

  * Lead aVL location a positive electrode ~ above the left arm and looks to the right and downward toward the facility of the heart (in comparison to lead I, which looks strictly come the right). The median vector approaches aVL at a an extremely broad angle, creating the least upright QRS complicated among the body leads.

  
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  * Lead aVF has actually its confident monitoring lead on the left leg and looks directly up come the center of the chest. The mean vector viewpoints aVF at a reasonably direct angle, although no as directly as command II, so command aVF has very upright QRS complexes with prominent P waves. Leads II, III, and aVF every look upward at the oncoming median vector, so their waveforms share plenty of qualities, such as highly positive QRS complexes and also prominent ns waves. Because these leader look increase at the bottom or worse ventricular wall surface of the heart, they"re known as the worse leads.

  
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 Six chest leads sweet inThe 6 chest or precordial leader lie across the anterior chest and measure the typical vector in the horizontal plane.

 

* Lead V1 is situated at the right sternal border, 4th intercostal space, and also lies over the ideal ventricle and also septum.

 

* Lead V2 is in ~ the left side of the sternum, fourth intercostal space.

 

* Lead V3 is midway in between leads V2 and also V4.

 

* Lead V4 is in ~ the midclavicular heat in the 5th intercostal space.

 

* Lead V5 is in ~ the anterior axillary heat in the fifth intercostal space.

 

* Lead V6 is at the midaxillary line, 5th intercostal space, and also is positioned over the lateral wall of the left ventricle.

 

 The median vector in the horizontal airplane is influenced by the overwhelming strength of the left ventricle and also can be thought of as flowing toward the left side. Because the average vector flows away from command V1, this lead has actually a downward QRS deflection; the QRS is practically totally upright in leader V5 and also V6 because the mean vector flows straight at these leads. The QRS complex becomes progressively much more upright across the chest wall surface from V1 come V6, a adjust known as R-wave progression (see R-wave ups and downs).3 This is another characteristic that a normal ECG.

 Putting it all together

With this understanding of 12-lead ECGs in mind, Mr. S"s nurse examines his 12-lead ECG. His heart price is normal, through clear ns waves, QRS complexes, and T waves. The PR expression is 0.14 second, which drops within the common range. The QRS complicated should be much less than 0.12 second; Mr. S"s QRS complexes are 0.08 2nd wide. The T waves room upright and normal looking. Finally, the ST segment is level with the baseline (isoelectric).

 

Mr. S"s limb leads space all upright with the exemption of aVR, which is normal. Command II is the many upright and also aVL is the least upright. The chest leads show downward command V1 and upright leads V5 and also V6 v normal R-wave progression.

 

The nurse concludes that Mr. S has a typical 12-lead ECG, denote no electric abnormalities. However, he"s not out that the woods yet. Some species of myocardial ischemia aren"t noticeable on a typical 12-lead ECG, therefore the healthcare provider may think about following up through a cardiac tension test.4

 

Mr. S"s normal ECG, an unfavorable serum cardiac biomarkers, and benign patient background lead the medical team to dominion out a cardiac resource for his discomfort. He"s discharged residence the next day and scheduled for outpatient cardiac stress and anxiety testing.

 Paper training

Use the markings top top ECG document to calculate events within the cardiac cycle. The ECG paper is a net of huge and small blocks. Top top the horizontal axis, a large block is equal to 0.2 2nd and a tiny block is same to 0.04 second. The vertical axis represents voltage or electrical energy, through each upright millimeter (small block) being 0.1 millivolt of electrical energy. However, in practice, deflections are generally described in millimeters, no millivolts.

 

Count the variety of small squares and multiply by 0.04 to calculate the duration of any kind of event in the ECG tracing. A QRS complex that"s 2.5 tiny squares vast is 0.1 second. The ECG record can additionally be offered to calculate heart rates, using one of two methods. In the 6-second method, look because that the markings (usually short vertical lines) in ~ the top of the rhythm strip or ECG paper. These markings divide the ECG document into 3-second intervals. Count the variety of QRS complexes consisted of in 2 intervals (6 seconds) and also multiply by 10. This technique works for both regular and irregular heart rhythms.

 

In the division method, count the number of small squares between any two heartbeats. Use the same part in both QRS complexes-usually the height of the facility works best. Separating 1,500 by the variety of small squares provides the heart price in beats per minute. This method is accurate only with continuous heart rates since irregular love rhythms have a varying variety of small squares between any two QRS complexes.

 Axis deviation: As straightforward as pie (charts)

Combining assessment skills with an expertise of axis deviation can provide nurses a an ext detailed photo of a patient"s condition. The hexaxial reference system and also the quadrant technique can aid you visualize difficulties with cardiac conduction.

 Hexaxial reference system

The regular QRS complicated (or vector) represents the average electric signal that the love generates throughout depolarization. In ~ the heart, the typical vector normally flows indigenous upper appropriate to lower left. The specific direction the that circulation (called the electric axis) have the right to be supplied as an assessment device in the 12-lead ECG since an abnormal axis can give clues about what"s going not correct in the heart"s electrical system.

 

To measure up the electric axis, imagine all six limb leads presented simultaneously around a central point in a circle, which represents the heart (see the illustration in ~ left). In this hexaxial system, the leads division the circle right into equal 30-degree segments.

  
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Each lead can be assigned a number of degrees, and the median vector"s direction deserve to be offered in degrees. If the mean vector is aligned directly with command I, that is axis is 0 degrees. A average vector directed halfway in between leads II and aVF has actually an axis of 75 degrees. (Although a patient"s electrical axis can be manually calculated, all modern 12-lead ECG machines administer this information automatically.)

 

The normal electric axis the the love falls in between 0 and +90 degrees. Return this is a large range, it"s a numeric equivalent of the concept that the electrical conduction that the normal heart is best to left and also top come bottom.

 

A left axis deviation occurs once the electric axis of the love is in between 0 and also 90 degrees. A right axis deviation occurs when the electric axis is in the +90 to +180 degree range. A mean vector having an electric axis in ~ the selection of 90 to 180 levels is dubbed an indeterminate axis or extreme right axis deviation.

 Quadrant method

To approximate axis deviation utilizing the quadrant method, division the circle (which represents the patient"s heart) into four quadrants (see the illustration below). Just two ECG leader are compelled to make this assessment. Study leads I and also aVF. If lead ns is upright, then the vector is flowing ideal to left. If lead aVF is upright, the vector is directed optimal to bottom. If they"re both upright, the electrical axis must autumn into the reduced left or regular quadrant. This quadrant roughly matches the criteria because that normal electrical axis, describe a regular direction of electric conduction.

 

Left axis deviation occurs once lead i is upright and also lead aVF is down or negative. The electric axis is located in the upper ideal quadrant. The mean vector is abnormally command to the left side of the heart. A left axis deviation deserve to be caused by many different pathologic conditions. Part left bundle-branch blocks will create a left axis deviation since the cardiac vector flows abnormally native the right side that the heart to the left. Because the typical vector is not performed by infarcted tissue and flows away from it, one inferior-wall myocardial infarction will develop a left axis deviation (due to a an unfavorable QRS in command aVF). Many patients through pacemakers have actually a left axis deviation since the pacemaker leads room on the right side the the heart.

  
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Finally, part structural body transforms will create a left axis deviation. In advanced pregnancy, the enlarged uterus may occupy for this reason much space in the abdomen the the elevated diaphragm pushes the heart to a an ext horizontal or leftward-lying position, creating a left axis deviation. Similarly, morbidly obese patients or patients v ascites or an abdominal muscle tumor may have a left axis deviation because of the heart"s place in the chest.

 

A right axis deviation is evident when lead ns is an unfavorable and command aVF is upright. The mean vector is abnormally directed to the appropriate side that the heart. Causes of appropriate axis deviation include chronic obstructive pulmonary condition and appropriate ventricular hypertrophy. In both instances, enlargement of the appropriate cardiac chambers pulls the median vector to the right side. A right bundle-branch block reasons the typical vector to flow from left come right, resulting in right axis deviation. Children and tall, thin adult may have actually a non-pathologic appropriate axis deviation if the heart hangs under in a much more vertical position.

See more: Which Statement Best Explains How The Idea Of Manifest Destiny Lead To The Mexican-American War?

 

If both leader I and also aVF space negative, then the axis deviation is termed indeterminate axis or extreme appropriate axis deviation. The average vector is command upward and to the right. If you uncover an indeterminate axis deviation on her patient"s ECG, inspect the leads; not correct ECG lead placement is a common reason of this finding. Other causes are some types of pacemakers, abnormal cardiac rhythms such together ventricular tachycardia, congenital love disease, or dextrocardia (heart positioned top top the ideal side of the chest).