Block 3: 10 Electrocardiogram
How can heart rate be calculated with some precision from a ECG?
(60 min/sec) / (Number of small boxes in RR interval * 0.04 sec) = X bpm Ex: (60 min/sec) / (20 boxes * 0.04 sec/box) = 75 bpm
How long should the PR interval (beginning of P to beginning of QRS) last?
0.12-0.2 sec
Describe the Net Zero Lead Method for determining the MAE
1. Look at all lead's ECGs and determine which lead has a QRS complex that sums to or approximately to net 0 2. Decide which lead is exactly perpendicular to it and look at that one's ECG 3. Decide if this lead is predominately + or - net deflected: if + MAE is that lead's + vector, if - it is the - vector
Describe the Quick and Dirty Approximation for determining the MAE
1. Look at lead I and aVF only 2. Decide for each if lead I and aVF are net + or - 3. Based on + or - can decide which quadrant: If Lead 1 + and aVF + then MAE is normal (quad IV) If Lead 1 -, aVF - then RAD (quad III) IF Lead 1 +, aVF - then LAD (quad 1)
How long should the QRS interval (begin Q to end of S) last?
<0.1 sec
In which plane of the heart do the augmented limb leads record electrical activity?
A frontal plane
In which plane of the heart do the standard limb leads record electrical activity?
A frontal plane
VOCAB: Lead
A lead is a connection between two poles that measures the difference between them (so not just one of the electrodes, but the connection between two - thus a 10 electrode set up is called a 12 lead)
Which is usually more troubling health-wise a Mobitz I or Mobitz II block?
A mobitz II block, which can lead to cardiac arrest or pacemaker implantation
Describe what causes a AV block 1 (first degree or partial).
A partial block of the AV node, all impulses conduced but are delayed to ventricles
In which plane of the heart do the Precordial leads record electrical activity?
A transverse plane
On an EKG does a wave of depolarization moving toward a positive electrode produce an upward or downward deflection?
A wave of depolarization moving toward a + electrode produces an upward deflection
VOCAB: Einthoven's triangle
An equilateral triangle formed by the the standard limb leads (I, II, III) 60-60-60 degrees Used to estimate MEA in the frontal plane called
Where does the AP begin on the heart?
At the sinoatrial node
The P-wave on an ECG triggers which mechanical event?
Atrial depolarization and thus contraction
Which type of leads in a standard 12 lead EKG have both only a positive electrode (unipolar) that generates a very small wave on the EKG that must be augmented using fictional a midway point between the other limbs?
Augmented Limb Leads aVL, aVL, aVF
MOCK EXAM: Which of the following would be most likely to occur if the AV node was damaged by ischemia? A. The ability to intrinsically increase heart rate during stress (such as exercise) would be impaired. B. Conduction of the depolarizing electrical signal from the atria to the ventricles would be delayed or blocked. C. Spreading of the depolarizing electrical signal from the septum to the ventricular walls would be impaired. D. Cardiac contractility would be impaired. E. Cardiac contractility would be increased
B. Conduction of the depolarizing electrical signal from the atria to the ventricles would be delayed or blocked.
MOCK EXAM: What lead primarily measures forces moving from the head to the feet? A. Lead I B. aVF C. V1 D. V6 E. aVR
B. aVF
CARDIOVASCULAR QUIZ 1: If repolarization occurs in the opposite direction of depolarization, why is the T wave upright in most leads of the electrocardiogram?
Because depolarization and repolarization occur in opposite directions, but the change in potential... SEE QUIZ FOR EXPANDED ANSWER
MOCK EXAM: If two successive QRS complexes are 0.8 seconds apart: A. Heart rate is 48 bpm. B. Heart rate is 60 bpm. C. Heart rate is 75 bpm. D. Heart rate is 80 bpm E. Heart rate is 86 bpm.
C. Heart rate is 75 bpm. If QRS complexes are 0.8 seconds apart, then each heartbeat lasts 0.8 seconds. There are 60 seconds in a minute, so (1bt/0.80 sec)(60sec/min) = 75 bpm.
Describe the Semi-Quantitative Method for determining the MAE
Calculated from the net direction of QRS complexes of all six limbs = lots of math
Describe what causes a AV block 3 (third degree).
Complete AV node or Bundle of HIS block, blocking impulses from ventricles entirely Ventricles begin beating at their own slower rhythm that does not match atrial rhythm
How can heart rate be estimated quickly from an ECG?
Count number of large boxes in RR interval 1 box = 300 bpm 2 boxes = 150 bpm 3 boxes = 100 bpm 4 boxes = 75 bpm 5 boxes = 60 bpm 6 boxes = 50 bpm 7 boxes = 43 bpm
What does a EKG produced by normal sinus tachycardia look like?
Every P wave is followed by a QRS and T wave Heart rate is > 100 bpm (all normal, just fast)
What does a EKG produced by normal sinus rhythum look like?
Every P wave is followed by a QRS and T wave Intervals are normal lengths Heart rate is 60-100 bpm
What does a EKG produced by normal sinus bradycardia look like?
Every P wave is followed by a QRS and T wave Intervals are normal lengths Heart rate is <60 bpm (all normal, just slow)
T/F QUIZ: The augmented limb leads are bipolar leads in the frontal plane.
False, augmented limb leads are UNIpolar in the frontal plane
T/F QUIZ: Cardiac myocytes are electrically and mechanically coupled to adjacent cells by t-tubules.
False, cardio myocytes coupled to adjacent cells by t-tubles
T/F QUIZ: Heart rate cannot be determined from the electrocardiogram recording.
False, heart rate can be determined from an ECG
T/F QUIZ: In a heart displaying normal sinus rhythm, two P waves fall between each QRS complex on the electrocardiogram recording (regardless of the lead being
False, one P-wave to one QRS is normal
T/F QUIZ: The P wave of the ECG corresponds to ventricular repolarization.
False, the P-wave corresponds to atrial depolarization
T/F QUIZ: The wave of cardiac depolarization originates from the AV node.
False, waves originate at the SA node
VOCAB: ECG T-wave
Final wave on ECG, represents repolarization of the ventricles (+ deflection even though repolarization because now moving bottom right to top left)
CARDIOVASCULAR QUIZ 1: How is first degree AV block identified on the ECG?
First degree AV block is identified on the ECG by an increase in the PR interval. In first degree block the duration of the PR interval is steady from beat to beat, but it is prolonged beyond 0.2 seconds.
VOCAB: ECG P-wave
First depolarization (+) on EKG, corresponding to signal moving through atria Length reflects time required for atrial depolarization
What does a EKG produced by first degree AV block look like?
Has a sinus rhythm, but all PR intervals >0.2 sec (P-wave is atrial depolarization, PR segment is time between atria and ventricular depolarization - which is delayed due to AV block)
When does a reentrant current occur and what is it?
In some conditions the AP conduction to a portion of the cardiac tissue is so slow that it arrives after that portion has already repolarized, causing rexcitement of tissue as this 'reentrant current' goes around and around the heart causing non-rhthymic contraction/relaxation
VOCAB: ECG QRS interval
Interval on ECG containing downward Q, upward R, and downward S waves Length reflects time of ventricular depolarization Usually <0.1 sec
VOCAB: ECG PR Interval (also called the atrioventricular (AV) conduction time)
Interval on ECG containing entire P wave and interval between end of P and start of Q Normally 0.122-0.2 secs
VOCAB: ECG QT interval
Interval on ECG containing entire QRS deflectoins, ST segment, and T wave (start ot Q to end of T) Length reflects time for ventricles to depolarize and repolarize Usually <half the time of the RR intervale
VOCAB: ECG RR interval
Interval on the ECG running from the peak of on R to the top of the next R wave Length represents one complete cardiac cycle
Which type of MAE change is commonly seen with left ventricular hypertrophy
LAD
Describe an MAE with Left-axis deviation
LAD MAEs point 0* to -90* (top right quadrant on axis [I])
How long should the QT interval (Q to end of T) last?
Less than half of one RR interval
Given the standard layout of the 10 leads, which two leads might produce a net 0 deflections if the MAE travels top right to bottom left?
Limb lead I and aVL (moves beside these leads, neither toward or away
VOCAB: Mean Electrical Axis (MEA)
MEA is the net direction of electrical conduction during ventricular depolarization (normally top left to bottom right)
What does MEA tell us about the heart?
MEA tells us heart orientation, size of ventricular chambers, and presence of conduction blocks
Describe the location of the following lead in a standard 12 lead EKG: Limb lead III
Negative pole: Left arm Positive pole: Left foot
Describe the location of the following lead in a standard 12 lead EKG: Limb lead I
Negative pole: Right arm Positive pole: Left arm
Describe the location of the following lead in a standard 12 lead EKG: Limb lead II
Negative pole: Right arm Positive pole: Left foot
What does a EKG produced by second degree Mobitz Type II AV block look like?
No measurable lengthening of PR intervals, just a sudden lack of QRS following a P-wave Usually have a regularly irregular heartbeat (continuing ratio of beat, beat, beat, drop, repeat for example)
Describe normal MAE
Normal MAE points 0* to +90* (bottom right quadrant on axis [IV])
Where on the heart are the changes the ECG reflects?
On the surface of the heart
Describe what causes a AV block 2 type 1 (second degree, Mobitz 1, or Wenchebach).
Partial AV block, not all impulses conducted to ventricles
Describe what causes a AV block 2 type 2 (second degree, Mobitz 2).
Partial Bundle of HIS block, some atrial impulses lost
Prior to AP, is the outside of the heart cell mostly positive or negative?
Positive
Describe the location of the following lead in a standard 12 lead EKG: aVL
Positive node: Left arm Fictional midway point between right arm and left foot
Describe the location of the following lead in a standard 12 lead EKG: aVF
Positive node: Left foot Fictional midway point between right arm and left arm (chest)
Describe the location of the following lead in a standard 12 lead EKG: aVR
Positive node: Right arm Fictional midway point between left arm and left foot
Given the standard layout of the 10 leads, which four leads will produce positive deflections if the MAE travels top right to bottom left?
Precardial leads 3,4,5,6
Which type of leads in a standard 12 lead EKG have both only positive electrodes (unipolar) that record in a transverse plane?
Precordial leads V1, V2, V3, V4, V5, V6
What does a EKG produced by second degree Mobitz Type I AV block look like?
Progressively lengthening PR intervals until P-wave impulse fails to be conducted and no QRS is generated (= no ventricular contraction) PR then returns to normal and cycle repeats Usually have a regularly irregular heartbeat (continuing ratio of beat, beat, beat, drop, repeat for example)
Which type of MAE change is commonly seen with right ventricular hypertrophy
RAD
Describe an MAE with Right-axis deviation
RAD MAEs point +90 to +150* (bottom left quadrant on axis [III]
What does a regular ECG rhythm look like?
RR intervals are the same for each beat and rhythm is maintained
VOCAB: ECG segment
Region on an ECG of isoelectric neutrality, so lacking in waves
VOCAB: ECG interval
Region on an ECG with at least one wave
VOCAB: ECG PR segment
Segment on ECG between end of P-wave and start of Q Length represents the time required for depolarization to move through AV node, Bundle of His, and Purkinje system
VOCAB: ECG ST segment
Segment on ECG between end of S and start of T Length reflects time of complete ventricular depolarization
VOCAB: ECG TP segment
Segment on ECG between end of T and start of P, reflecting period of complete atrial and ventricular diastole
Which type of leads in a standard 12 lead EKG have both positive and negative electrodes (bipolar)?
Standard limb leads I, II, II
Why the are tachycardia and bradycardia 'sinus'?
Term sinus is used because the disruption causing these are from sinus node
Describe how the radial axis diagram for determining MAE is formed.
The lines of the three limb leads on Eithoven's triangle are crossed at their midpoints to form a asterisk-like start with 60* for each wedge The augmented limb lead lines are then also added so that their midpoints are at center, creating a 12-point star with 30* for each wedge
What does it mean to describe the heart as dipolar?
The wave of AP sweeps from the top to the bottom of the heart, making the inside top + and the bottom - first The heart repolarizes after the AP, making inside top - and bottom + after
What does a EKG produced by AV block 3 look like?
There are more P-waves than QRS (2 to 4 more P than QRS), as the ventricle has aquirred its own rhthym
What does an atrial fibrillation arrhythmia ECG look like?
There are no clear P-waves but still QRS, a lack of coordination between the atria and ventricles Usually not fatal
What does a regularly irregular ECG rhythm look like?
There is a predictable pattern of beats to missed beats (ratio of 3:1, 4:1, etc, in a second degree type 1 heart block)
What does a irregularly irregular ECG rhythm look like?
There is no predictable pattern at all (as in atrial tribulation)
Which is the most serious type of heart block?
Third degree heart block
On ECG trace paper, how many seconds does one tiny box and one large box represent on the X-axis?
Tiny boxes are 0.04 sec Large boxes are 0.2 sec
On ECG trace paper, how many mVs does one tiny box and one large box represent on the Y-axis?
Tiny boxes are 0.1 mV Large boxes are 0.5 mV
What does a ventricular fibrillation arrhythmia ECG look like?
Total absence of a pattern Usually fatal
T/F QUIZ: In a normal, healthy heart, the mean electrical axis is between 0 degrees and 90 degrees.
True, a normal MAE is 0 to +90
T/F QUIZ: The bipolar limb leads are each composed of a negative and positive electrode.
True, bipolar limb leads have + and - electrodes
T/F QUIZ: The six precordial leads are unipolar leads in the transverse plane.
True, precordial leads are unipolar in the transverse plane
T/F QUIZ: The QRS complex of the ECG corresponds to ventricular depolarization.
True, the QRS complex of the ECG corresponds to ventricular depolarization.
Describe the general placement of the six precordial leads.
V1 is placed right of the sternum V2-V6 wrap from left the sternum under the left arm
The QRS complex on an ECG triggers which mechanical event?
Ventricular depolarization and thus contraction
The T-wave on an ECG reflects which mechanical event?
Ventricular repolarization and thus relaxation
CARDIOVASCULAR QUIZ 1: If the ECG tracing shows that the QRS is isoelectric in Lead I and the net deflection of the QRS in aVF is positive, e.g. +2, then the mean electrical axis of the heart must be approximately: a) 90 degrees b) 60 degrees c) 0 degrees d) 110 degrees
a) 90 degrees
Given the standard layout of the 10 leads, which lead will produce a negative deflection if the MAE travels top right to bottom left?
aVR
CARDIOVASCULAR QUIZ 1: An increase in the R-R interval on the ECG indicates: a) an increase in heart rate b) a decrease in heart rate c) an increase in atrial contraction d) a decrease in atrial contraction
b) a decrease in heart rate
CARDIOVASCULAR QUIZ 1: Which of the following components of the ECG would be most likely to be altered by an area of cardiac ischemia that is interfering with ventricular repolarization? a) the P-wave b) the PR interval c) the RR interval d) the T wave
d) the T wave
