SM 130/131 ECG II and III Flashcards
In Left Axis Deviation, what are the net directions of Leads I and aVF?
I = Upright aVF = Downward
What is the order of Atrial depolarization?
Right Atrium depolarizes first, then Left Atrium
Right Atrium is first part of P wave, Left Atrium is second part of P wave
What does the RCA supply?
The inferior wall of the LV and posterior IV septum
What does the LAD supply?
The LAD supplies the anterior and lateral LV as well as the anterior 2/3 of the IV septum
What is the ST segment?
Describes the end of the S wave to the beginning of the T wave
What process can result in cellular depolarization of an ischemic area?
Loss of intracellular K to leaky membranes leading to loss of transmembrane gradient
What height corresponds to 1 mV on an ECG?
2 large boxes or 10 small 1mm boxes represent 1 mV
How is Heart Rate calculated from an ECG?
Find a QRS complex that lines up with a big box, and count however many big boxes until the next QRS
300 for 1, 150 for 2, 100 for 3, 75 for 4, 60 for 5, 50 for 6
Or count boxes between 6 seconds (30 big boxes) and multiple by 10 for irregular rhythms
Why is there a delay between the P wave and the QRS complex?
AV node conducts slowly, leading to delay that allows ventricles to finish filling
What leads visualize electrical activity in the lateral wall of the LV?
Leads I, aVL, and V4 - V6
What are the signs of an old MI?
ST segments return to baseline unless an aneurysm develops
Q waves persist indefinitely
T wave inversion may or may not persist
In a normal ECG, what lead has upright P waves consistently?
Lead II
What does Left Ventricular Hypertrophy look like on ECG?
On V1: larger, more negative S wave
On V6: larger, more positive R wave
What is the normal axis of the heart?
0 to +90 (bottom right quadrant)
What is Left Axis Deviation?
Axis is in the upper left quadrant from 0 to -90
What is Ventricular Aneurysm?
Persistent ST elevation after an MI suggests the presence of a thinned portion of the ventricle that bulges out
How do Left Atrium abnormalites alter the P wave?
L. Atrium abnormalities exaggerate the second part of the P wave
Causes two peaks on Lead II
Causes a deeper negative second peak on Lead V1
What type of ischemic injury leads to ST depression?
Subendocardial injury leads to ST depression
What is the PR segment?
Describes the end of the P wave to the beginning of the QRS complex
What does the LCA supply?
The Lateral LV
What does the PR segment represent
End of P wave to beginning of QRS complex
The time period between Atrial contraction and Ventricular contraction
When do the isoelectronic phases occur during a normal set of Action Potentials?
Isoelectronic phases occur during Phase 4 and Phase 2
Phase 4 = neither cell has depolarized, lasts until first cell depolarizes
Phase 2 = both cells have depolarized and entered plateau, lasts until second cell hyperpolarizes
How do QRS complexes change from V1 to V6?
From V1 to V6, QRS complexes transition from predominantly downward (V1) to predominantly upward (V6)
How does the IV septum depolarize?
Midseptum initially, from left to right
What is the difference between a segment and an interval on an ECG?
A segment connects the end of one wave with the beginning of another and describes the space between two waves
An interval connects the beginning of one wave to the beginning of another, thus containing one or more waves
What is the ST interval?
Describes the end of the S wave to the end of the T wave
When do T wave invert on an MI?
Within hours to days of onset
What is the QRS complex?
Describes the beginning of either the Q or R wave to the end of the S wave
What do the bottom 3 strips on an ECG tracing show?
The bottom 3 strips show the continuous activity of 3 leads over 10 seconds
How do stable angina and NSTEMI differ on ECG?
In stable angina, the T wave and ST segment return to normal after an oxygen demanding stimulus is removed
In NSTEMI, the T wave stays inverted and the ST segment stays depressed
What are the criteria for LV enlargement?
V1 S wave and V5/V6 R wave >= 35mm
R wave in I and S wave in III >= 25mm
R wave in aVL >= 12mm
Other criteria also exist
What is Right Axis Deviation?
Axis is in the lower right quadrant from 180 to +90
What does the QT interval represent?
Time period from beginning of QRS complex to end of T wave
Represents the total duration of Action Potentials in the ventricle
How long does the acute phase of an MI last?
Mutes to days
What lead is perpendicular to Lead II?
aVL
How does the net direction of Lead I set the axis of the heart?
If the net direction in Lead I is up, the axis in the right half of the circle (+90 to - 90)
If the net direction in Lead I is down, the axis is in the left half of the circle (+90 to -90)
What effect does increasing Heart Rate have on Action Potential Duration and why?
Increased Heart Rate leads to shorter AP duration due to several ion channel currents, such as Ik-s decreasing
When do voltage gradients between ischemic and healthy tissues emerge on an ECG?
Voltage gradients between healthy and ischemic tissue during Phase 4 and Phase 2, which are normally isoelectric, emerge and are visible on ECG
How does the net direction of Lead aVF set the axis of the heart?
If the net direction in Lead aVF is up, the axis is in the bottom half of the circle (180 to 0)
If the net direction in Lead aVF is down, the axis in the top half of the circle (180 to 0)
What is Ventricular Enlargment?
Hypertrophy of the left or right ventricle
Why does MI lead to elevated Q waves?
Using the lead facing the LV, normally the subendocardial to epicardial depolarization of the large LV outweighs the subendocardial to epicardial depolarization of the RV and IV Septum, resulting in net current flow towards the detector
In MI, tissue in the LV dies and the net current flow is away from the detector, causing an elevated negative Q wave
What leads visualize electrical activity in the anterior wall of the LV?
Leads V1 - V4 visualize the anterior wall of the LV
What is the Axis of the heart?
Average of the vectors from depolarization
What are the Right Atrial criteria for enlargement?
Lead II amplitude >= 2.5mm or Lead V1 amplitude >= 2.5mm
What characterizes Ischemic Heart Disease?
Oxygen Demand > Oxygen Supply
Shows up on ECG and clinical examination
What leads can diagnose Atrial Abnormalities?
Leads II and V1 (either/or, don’t need both)
How can the heart axis be measured from two leads?
Find the Isoelectronic lead, and the axis is perpendicular to it on the perpendicular lead
The axis will point in the same direction on the perpendicular lead
I - aVF
II - aVL
III - aVR
What are the criteria for RV enlargement?
V1: R wave > S wave
V6: S wave < R wave
Limb leads: Right axis deviation
R. atrial P wave abnormality normally present
What does the Left main artery branch into?
The LAD and the LCA
What conditions lead to transmural ischemic injury?
STEMI
In Right Axis Deviation, what are the net directions of Leads I and aVF?
I = Downward aVF = Upright
What is the PR interval?
Describes the beginning of the P wave to the beginning of the QRS complex
What leads are used to detect Ventricular Enlargement?
Leads V1 and V6
V1 = Right Ventricle
V6 = Left Ventricle
How does MI appear in the hyperacute phase?
Tall, peaked “hyperacute” T waves with or without ST elevation
What is Pericarditis?
ST segment elevated in multiple leads, but may also have PR segment depression and a different series of ECG changes
Also has different clinical presentation
What is a nonspecific repolarization abnormality?
Explains how the cause of an ST segment or T wave changes can vary, and needs to be taken in a clinical context
What does a normal QRS complex look like on V1?
Since V1 is closest to the Right Ventricle, the QRS complex has just a small positive R wave and a large negative S wave
Is the ST segment normally isoelectronic?
Yes
How does MI appear in the acute phase?
“Hyperacute” T waves have disappeared, ST segment elevated and Q waves may appear
What type of ischemic injury leads to ST elevation?
Transmural injury leads to ST elevation
What are the Left Atrial criteria for enlargement?
Lead II width >= 3mm or Lead V1 Area of negative component >= 1mm^2
What is a normal resting heart rate?
Between 60 and 100bpm, varies for kids
What are the phases of an MI?
Hyperacute and Acute phases
What does a normal QRS complex look like on V6?
Since V6 is closest to the Left Ventricle, the QRS complex has a small negative Q wave and a large positive R wave
What is Early/J-Point Elevation?
Seen in young people, ST segment elevated in multiple leads; not pathological
Why does Subendocardial injury show up as ST depression on Lead V5?
Because V5 measures from the exterior facing the LV, and subendocardial damage in the LV leads to depolarized cells in the layer farthest from the detector on V5
Electrons flow towards the higher voltage, more depolarized ischemic cells in the subendocardial layer, and because the current flows away from the detector, an ST depression is detected
How are QT intervals corrected and why?
QT intervals represent the time period from the beginning of QRS complexes to the end of T waves
Corrected because AP shortens with increased HR
QTc = QT/(RR)^1/2 - must be measured in seconds
What leads visualize electrical activity in the inferior wall of the LV?
Leads II, III, aVF
Why does ischemia cause T wave inversion?
Normally, a T wave is positive because the first cell stays depolarized longer than the second
Ischemia causes T wave inversion when slow response AP’s in the first, ischemic cell lead to a shortened AP duration, which causes it to hyperpolarize faster than the second cell and invert the T wave (making it negative)
What lead is perpendicular to Lead I?
aVF
How do Right Atrium abnormalities alter the P wave?
R. Atrium abnormalities exaggerate the first part of the P wave on both Lead II and V1
What does the P wave look like on Lead V1?
A wave with a positive peak and a negative peak
What does the P wave look like on Lead II?
A normal wave with 1 peak
What lead is perpendicular to Lead III?
aVR
What is the only definitive proof that an MI is old?
Presence of an MI on an older ECG
Where are Q waves normally found?
Q waves are normally found in Lead III, V5, and V6
What is the QT interval?
Describes the beginning of the QRS complex to the end of the T wave
What does the PR interval represent?
Beginning of P wave to beginning of QRS complex
The time period from the beginning of Atrial contraction the beginning of Ventricular contraction
What is the requirement for Q waves to appear?
Substantial area of necrosis
Which leads are used to determine the axis of the heart?
Leads I and aVF
What does Right Ventricular Hypertrophy look like on ECG?
On V1: larger, more positive R wave (about same size as S wave)
On V6: larger, more negative S wave (about same size as R wave)
Why does Transmural injury show up as ST elevation on Lead V5?
Because V5 measures from the exterior facing the LV, and transmural damage in the LV leads to depolarized cells in all layers of a given region
Electrons from neighboring regions flow towards the higher voltage, and the current is detected as flow towards the V5 detector causing an ST elevation
How long is each large box on an ECG?
Each large box represents 0.2 seconds or 200 msec
Each large box has 5 small boxes worth 0.04 seconds
How long does the hyperacute phase of MI last?
Minutes to hours, usually gone by the time the patient presents to the ED
How do the ventricles depolarize?
Subendocardium to epicardium
How does ischemia affect cardiac myocytes?
Creates partially depolarized cells with a slow response Action Potential that alters an ECG trace
What conditions lead to subendocardial ischemic injury?
Stable angina and NSTEMI
