Understanding the Understanding the 12 12- lead ECG, part 1 lead ECG, part 1 2 The heart’s electrical conduction The heart’s electrical conduction Electrical activity Electrical activity • precedes mechanical activity precedes mechanical activity • coordinates all parts of the heart to coordinates all parts of the heart to contract at the same time contract at the same time • initiated in the initiated in the sinoatrial sinoatrial (SA) node (SA) node specialized group of cells in the right atrium specialized group of cells in the right atrium depolarizes at a rate of 60 to 100 times/minute depolarizes at a rate of 60 to 100 times/minute makes the atria contract and propel blood into the makes the atria contract and propel blood into the ventricles ventricles 3 Normal ECG waveform Normal ECG waveform 4 Atrial depolarization Atrial depolarization produces the P wave produces the P wave • first element on the ECG first element on the ECG • first part of the cardiac cycle first part of the cardiac cycle • appears as a small semicircular bump appears as a small semicircular bump atrioventricular atrioventricular (AV) node (AV) node • receives the atrial impulse and (after a brief pause to let receives the atrial impulse and (after a brief pause to let the ventricles fill) transmits it to the ventricles via the the ventricles fill) transmits it to the ventricles via the bundle of His bundle of His bundle of His bundle of His • collection of cardiac conduction fibers that splits into the collection of cardiac conduction fibers that splits into the right and left bundle branches right and left bundle branches 5 6
Transcript
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Understanding the Understanding the 1212--lead ECG, part 1lead ECG, part 1
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The heart’s electrical conductionThe heart’s electrical conduction
Electrical activityElectrical activity•• precedes mechanical activityprecedes mechanical activity•• coordinates all parts of the heart to coordinates all parts of the heart to
contract at the same timecontract at the same time•• initiated in the initiated in the sinoatrialsinoatrial (SA) node(SA) node
specialized group of cells in the right atriumspecialized group of cells in the right atriumdepolarizes at a rate of 60 to 100 times/minutedepolarizes at a rate of 60 to 100 times/minutemakes the atria contract and propel blood into the makes the atria contract and propel blood into the ventriclesventricles
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Normal ECG waveformNormal ECG waveform
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Atrial depolarizationAtrial depolarizationproduces the P waveproduces the P wave•• first element on the ECGfirst element on the ECG•• first part of the cardiac cyclefirst part of the cardiac cycle•• appears as a small semicircular bumpappears as a small semicircular bump
atrioventricularatrioventricular (AV) node (AV) node •• receives the atrial impulse and (after a brief pause to let receives the atrial impulse and (after a brief pause to let
the ventricles fill) transmits it to the ventricles via the the ventricles fill) transmits it to the ventricles via the bundle of Hisbundle of His
bundle of Hisbundle of His•• collection of cardiac conduction fibers that splits into the collection of cardiac conduction fibers that splits into the
right and left bundle branchesright and left bundle branches
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Bundle branchesBundle brancheshighhigh--speed conducting fibers that run down the speed conducting fibers that run down the intraventricular septumintraventricular septum
transmit cardiac impulse to the Purkinje fiberstransmit cardiac impulse to the Purkinje fibers•• complex network that mingles with ventricular complex network that mingles with ventricular
resulting in ventricular depolarizationresulting in ventricular depolarization
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Ventricular depolarizationVentricular depolarizationgenerates the QRS complexgenerates the QRS complex——the electrical the electrical equivalent of ventricular systoleequivalent of ventricular systole•• Remember that electrical activity Remember that electrical activity
precedes mechanical activity and ECG precedes mechanical activity and ECG shows only electrical activity.shows only electrical activity.
•• You should be able to palpate a pulse You should be able to palpate a pulse with each QRS complex on the ECG with each QRS complex on the ECG monitor.monitor.
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QRS complexQRS complex
QRS complexQRS complex•• normal duration is 0.06 to 0.1 secondnormal duration is 0.06 to 0.1 second•• duration greater than 0.12 seconds duration greater than 0.12 seconds
usually indicates prolonged ventricular usually indicates prolonged ventricular conduction caused by a bundle branch conduction caused by a bundle branch blockblock
•• can look different among different can look different among different patients or among leads in the same patients or among leads in the same patientpatient
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QRS complexQRS complex
Mill
ivol
ts
Milliseconds
0 200 400 600
-0.5
0
0.5
1.0
QRS
P
R
T
Q
S
The width of the QRS complex should not exceed 110 ms, less than 3 littlesquares
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T waveT waveis larger than the P waveis larger than the P wave
is rounded or slightly peakedis rounded or slightly peaked
represents ventricular repolarization or metabolic represents ventricular repolarization or metabolic rest periods between heartbeatsrest periods between heartbeats
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PR intervalPR intervalconsists of the period from the beginning of the consists of the period from the beginning of the P wave to the beginning of the QRS complexP wave to the beginning of the QRS complex
represents SA node depolarization to ventricular represents SA node depolarization to ventricular depolarizationdepolarization
has normal duration of 0.12 to 0.2 second; if has normal duration of 0.12 to 0.2 second; if longer than 0.2 second, a disease process may longer than 0.2 second, a disease process may be affecting the cardiac conduction pathwaybe affecting the cardiac conduction pathway
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PR intervalPR interval
PRinterval
Mill
ivol
ts
Milliseconds
0 200 400 600
-0.5
0
0.5
1.0
P
R
T
Q
S
PR interval should be 120 to 200 milliseconds or 3 to 5 little squares
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ST segmentST segmentconsists of the isoelectric line between the end of consists of the isoelectric line between the end of the QRS complex and the beginning of the the QRS complex and the beginning of the T waveT wave
reveals information about the heart’s oxygen reveals information about the heart’s oxygen statusstatus
when elevated, is a key indicator of myocardial when elevated, is a key indicator of myocardial infarction (MI)infarction (MI)
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Examining a 12Examining a 12--lead ECGlead ECGSome QRS complexes have upward spike, others Some QRS complexes have upward spike, others have downward deflections.have downward deflections.
Each lead has a positive electrode and a negative Each lead has a positive electrode and a negative electrode, which acts as an anchor.electrode, which acts as an anchor.
When energy is directed toward the positive When energy is directed toward the positive electrode, the QRS complex spikes up; when electrode, the QRS complex spikes up; when directed toward the negative electrode, the QRS directed toward the negative electrode, the QRS complex spikes down.complex spikes down.
Limb leads I to IIILimb leads I to IIIFirst six leads of the 12First six leads of the 12--lead ECGlead ECG•• come from four electrodes on the patient’s arms and come from four electrodes on the patient’s arms and
legslegs•• called bipolar or standard leadscalled bipolar or standard leads
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Lead ILead Iconsists of positive electrode on the left arm consists of positive electrode on the left arm looking toward the negative electrode on the looking toward the negative electrode on the right arm for electrical energyright arm for electrical energy
produces upward deflection of the QRSproduces upward deflection of the QRS
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Lead IILead IIconsists of positive electrode on the left foot, consists of positive electrode on the left foot, negative electrode on the right armnegative electrode on the right arm
shows most upright QRS complexes and most shows most upright QRS complexes and most prominent P wavesprominent P waves
is favorite monitoring lead in many ICUs and is favorite monitoring lead in many ICUs and telemetry unitstelemetry units
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Lead IIILead IIIconsists of positive electrode on the left foot, consists of positive electrode on the left foot, negative electrode on the left armnegative electrode on the left arm
Limb leads Limb leads aVRaVR to to aVFaVFNext set of leadsNext set of leads•• use single positive monitoring electrodeuse single positive monitoring electrode•• called augmented or called augmented or unipolarunipolar leadsleads
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Lead aVRLead aVRconsists of positive electrode on the right armconsists of positive electrode on the right arm----only limb lead on the right side of the bodyonly limb lead on the right side of the body
is the only lead with downward deflected QRS (in is the only lead with downward deflected QRS (in normal ECG)normal ECG)
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Lead aVLLead aVLconsists of positive electrode on the left arm, consists of positive electrode on the left arm, looks to the right and downlooks to the right and down
produces the least upright QRS among the limb produces the least upright QRS among the limb leadsleads
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Lead aVFLead aVFconsists of positive electrode on the left leg and consists of positive electrode on the left leg and looks straight up to the center of the chestlooks straight up to the center of the chest
has very upright QRS complexes with prominent has very upright QRS complexes with prominent P wavesP waves
known as inferior lead (along with leads II and known as inferior lead (along with leads II and III) because all look upwardIII) because all look upward
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Chest leads VChest leads V11 to Vto V66
Chest (or Chest (or precordialprecordial) leads:) leads:•• lie across anterior chestlie across anterior chest•• measure the mean vector in the horizontal measure the mean vector in the horizontal
planeplane
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Chest leadsChest leadsLead VLead V11
•• located at right located at right sternalsternal border, border, fourth fourth intercostalintercostal spacespace
•• lies above right ventricle and lies above right ventricle and septumseptum
Lead VLead V22•• located at the left side of sternum, located at the left side of sternum,
fourth fourth intercostalintercostal spacespace
Lead VLead V33•• located midway between leads Vlocated midway between leads V22
and Vand V44
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Chest leads Chest leads
Lead VLead V44•• located at the located at the midclavicularmidclavicular line, fifth line, fifth
intercostal spaceintercostal space
Lead VLead V55•• located at the anterior axillary line, located at the anterior axillary line,
fifth intercostal spacefifth intercostal space
Lead VLead V66•• located at the located at the midaxillarymidaxillary line, fifth line, fifth
intercostal space, above lateral wall intercostal space, above lateral wall of the left ventricleof the left ventricle
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Arrangement of Leads on the EKGArrangement of Leads on the EKG
Determining the Heart RateDetermining the Heart Rate
Rule of 300Rule of 300
10 Second Rule10 Second Rule
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Rule of 300Rule of 300
Take the number of “big boxes” Take the number of “big boxes” between neighboring QRS complexes, between neighboring QRS complexes, and divide this into 300. The result and divide this into 300. The result will be approximately equal to the ratewill be approximately equal to the rate
Although fast, this method only works Although fast, this method only works for regular rhythms.for regular rhythms.
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What is the heart rate?What is the heart rate?
(300 / 6) = 50 bpm
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What is the heart rate?What is the heart rate?
(300 / ~ 4) = ~ 75 bpm
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What is the heart rate?What is the heart rate?
(300 / 1.5) = 200 bpm
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The Rule of 300The Rule of 300It may be easiest to memorize the following It may be easiest to memorize the following
table:table:
505066
606055
757544
10010033
15015022
30030011
RateRate# of big # of big boxesboxes
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10 Second Rule10 Second Rule
As most EKGs record 10 seconds of rhythm As most EKGs record 10 seconds of rhythm per page, one can simply count the number per page, one can simply count the number of beats present on the EKG and multiply by of beats present on the EKG and multiply by 6 to get the number of beats per 60 6 to get the number of beats per 60 seconds.seconds.
This method works well for irregular This method works well for irregular rhythms.rhythms.
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What is the heart rate?What is the heart rate?
33 x 6 = 198 bpm 4444
The QRS AxisThe QRS Axis
The QRS axis represents the net The QRS axis represents the net overall direction of the heart’s overall direction of the heart’s electrical activity.electrical activity.
Abnormalities of axis can hint at:Abnormalities of axis can hint at:Ventricular enlargementVentricular enlargementConduction blocks (i.e. Conduction blocks (i.e.
hemiblockshemiblocks))
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The QRS AxisThe QRS Axis
By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°.
-30° to -90° is referred to as a left axis deviation (LAD)
+90° to +180° is referred to as a right axis deviation (RAD)
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Determining the AxisDetermining the Axis
The Quadrant ApproachThe Quadrant Approach
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Determining the AxisDetermining the Axis
Predominantly Positive
Predominantly Negative
Equiphasic
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The Quadrant ApproachThe Quadrant Approach1. Examine the QRS complex in leads I and 1. Examine the QRS complex in leads I and aVFaVF to to
determine if they are predominantly positive or determine if they are predominantly positive or predominantly negative. The combination should predominantly negative. The combination should place the axis into one of the 4 quadrants below.place the axis into one of the 4 quadrants below.
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The Quadrant ApproachThe Quadrant Approach2. In the event that LAD is present, examine lead II to 2. In the event that LAD is present, examine lead II to
determine if this deviation is pathologic. If the QRS determine if this deviation is pathologic. If the QRS in II is predominantly positive, the LAD is nonin II is predominantly positive, the LAD is non--pathologic (in other words, the axis is normal). If it pathologic (in other words, the axis is normal). If it is predominantly negative, it is pathologic. is predominantly negative, it is pathologic.
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Quadrant Approach: Example 1Quadrant Approach: Example 1
Negative in I, positive in aVF RAD
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Quadrant Approach: Example 2Quadrant Approach: Example 2
Positive in I, negative in aVF Predominantly positive in II
