ECG Leads and Basic Overview Flashcards
The resting ECG is normal in ___ -___% of patients who have CAD, but have not yet had an MI.
25-50%
What information does ECG monitoring give you?
-Cardiac Rate
-Cardiac Rhythm
-Oxygen Balance
What are indications for ECG monitoring?
-Diagnosis of arrhythmias, electrolyte imbalances & conduction defects
-Diagnosis of ischemia
-Pathologic Q Waves (previous MI)
Which leads are Einthoven’s Standard Limb Leads? (Bipolar leads)
I, II, and III
Which are Goldberger’s Augmented leads?
aVR, aVF, and aVL
Which are the Precordial (Chest) leads?
V1 - V6
How is Lead I constructed?
-By comparing the left arm (positive) to the right arm’s electrode
-Zero point is in the center of the lead
-Any current flowing left (toward the left arm’s electrode) will produce a positive deflection on the ECG, while any current flowing to the right produces a negative deflection.
-Gives us a very good view of what is going on from left to right in the heart, but a poor view of events moving up or down (perpendicular to the lead I axis).
What is different about the Augmented leads? (Unipolar)
Augmented signal from positive lead to get a better view of electrical signal.
-View the heart in the frontal plane.
-Has a single positive lead, and all others are averaged together as the negative lead.
-Augmented are unipolar leads with one lead serving as the positive and the other two leads serve as “indifferent” leads neither positive or negative. The two indifferent leads produce the vector for the augmented leads
What do the Precordial leads show? (Unipolar)
-Give an idea of the full functioning of the heart.
-Shows coronary views, frontal views, etc.
-Close to the heart, so don’t need augmentation
-Views the heart in the horizontal plane
-Has a single positive lead, and all others are averaged together as the negative lead.
What is positive/negative for Lead II?
-Left leg is positive
-Right arm is negative
What is positive/negative for Lead III?
-Left leg is positive
-Left arm is negative.
How do the augmented leads work?
-Devised as a way to increase the size of the signal (the heart’s electrical activity” and give us new views of the activity
-Use the same electrodes as you used for the standard limb leads
-For each of the augmented unipolar leads, two of the three electrodes we had used are tied together and brought to ground.
-The remaining electrode becomes the exploring or active lead.
-In the case of any unipolar lead, current flow heading towards the active (exploring) electrode produces a positive deflection, while current going away from the electrode produces a negative deflection.
What is the limitation associated with the augmented (unipolar) or standard limb leads?
-All look at the heart from the Coronal Plane
-In order to view the heart from another plane (the transverse or horizontal plane), the six chest leads came into use.
What is different about the Precordial/Chest Leads?
-Unipolar leads (so current moving towards the active electrode produces a positive deflection, away a negative deflection)
-Can get info about the anterior and posterior parts of the heart (as well as inferior and superior).
In the standard limb leads (I, II, and III), all waveforms should be?
Positively deflected (upright)
-Heart is depolarized from R to L
How does deflection vary within the Augmented Leads?
-aVR: all waveforms negative
-aVL: P and T are negative, but QRS is biphasic, (waveforms equally positive and negative)
-aVF: all waveforms are positive
How does deflection vary within the Precordial Leads?
-P and T positive
-QRS starts negative and ends positive.
-Important with Bundle Branch Blocks
-There should be normal R wave progression throughout precordial leads.
What leads are used in a 3 lead ECG?
-RA
-LA
-LLL
All are bipolar leads
How does a 5 lead ECG work?
-The augmented leads use the same leads as I,II, and III, but the negative is a composite.
How does a Standard 12 lead ECG work?
-Actually uses 10 cables
-Can see 12 views
What is a Derived 12 lead?
-Used in the OR
-Has 5 cables
-Depending on placement of leads, can get different views
-RA, RL, LA, LL, and V
-Moving the V lead gets you different views. Can get V1-V6.
Where should V1 be placed?
4th IC space right sternal border
Where should V2 be placed?
4th IC space left sternal border
Where should V3 be placed?
between V2 and V4
Where should V4 be placed?
Midclav at the 5th IC space
Where should V5 be placed?
horizontal to V4 on the anterior axillary line
Where should V6 be placed?
horizontal to V5 midax line
How should you determine what leads to monitor?
-Identify high risk patients first
-May choose alternative leads if patient is high risk
Which leads would detect supply ischemia/transmural injury?
II and V3
Which lead is best for rhythm assessment?
II
Best view of P wave
Best for enhancing diagnosis of dysrhythmias.
Which lead should you monitor if you anticipate seeing depression (Subendo injury or ischemia)?
V5
Monitoring V5 is __% sensitive for ischemia detection in patients with known CAD.
75%
Monitoring V4 is __% sensitive for ischemia detection in patients with known CAD.
60%
Monitoring both V4 and V5 together is ___% sensitive for detection of ischemia in patients with known CAD.
90%
Which lead is the best for RV ischemia?
V4
What complicates lead placement in the OR?
-Patient positioning
-Skin preparation: sweaty, oily, etc.
What is different about artifact in the OR?
-Cables used in the OR have better interference
-Interference in the OR is more significant than on regular ECG
What are the frequency limits for ECG cables in the OR?
-High frequency: 100 Hz (accurate display of rapid ECG events).
-Low frequency: 0.5 Hz (accurate representation of slow events like ST changes or T wave changes)
Ensure limits are set this way to filter out electromechanical interference from cautery, patient movement, anything like that.
What is the purpose of high frequency filters?
Reduce interference caused by electrical lights and cautery.
What is the purpose of low frequency filters?
Lead to a more stable baseline by reduced respiratory and body movement artifact.
What is the diagnostic mode?
-Not used as often
-Uses ST and T wave analysis to diagnose ischemia.
-Filter eliminates frequencies outside 0.05 - 100 Hz
-Often results in excessive baseline drift and artifact.
What is the Monitoring Mode?
-Filter out baseline drift and artifact.
-Filters out frequencies outside 0.5 - 40.0 Hz which removes most OR interference. (may lose a little bit of info on fast stuff)
-May introduce elevation and depression of ST and T waves.
-More filtered than diagnostic mode due to narrower pass width.
What factors affect the accuracy of interpretation of ECG?
-Skill of the provider
-Patient-electrode interference (skin prep)
-Electrodes (silver-chloride type; avoid needle electrodes)
-Leads (Need adequate insulation, motion artifact, don’t cross cables). Place leads so they are coming up towards head of the bed
-Filtering capabilities of the monitors: modern monitors have very good filtering, not a big issue
What does one small box represent?
0.04 sec (or 40 millisec) and 0.1 mV amplitude
What does one large box represent?
0.2 seconds and 0.5 mV amplitude
What is the normal duration of the QRS?
QRS 80-120 ms
What does the QRS correlate with?
Rapid depolarization of right and left ventricle
What is the normal duration of the PRI?
PRI 50-120ms
What does the PRI correlate with?
AV node function
What is the normal duration of the QT segment?
QT segment varies with heart rate
What does the T wave represent?
Repolarization
What is absolute and relative refractory period? And where do they fall on the EKG?
-The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period.
-The last half of the T wave is referred to as the relative refractory period (or vulnerable period).
What does the p wave represent?
Atrial depolarization.
P wave is 80 ms
What does the PR segment represent?
-Atrium to ventricle.
-AV node to Purkinje is flat because no contraction.
What is the J Point?
-Area where you start to measure ST segment.
-Looking at ischemia and dysrhythmias
What are U waves?
-Negative U waves may signify LAD occlusion, ischemia, AR/R, or increased afterload.
-Little help after the T Wave
