117 ECG 1

Question 1

Why are R and T waves usually positive?

Answer 1

The T wave is generally upright because the first cell to depolarize is usually the last to repolarize (ECG=AP 1-2). If the sequence were reversed (first to depolarize, first to repolarize), T waves would be down (inverted), as in AP 1-3.

Question 2

What do parts of ECG represent

Answer 2

• P-wave: atrial depolarization
• Note: sinus node cannot be recorded on the surface ECG because it is too small to generate an extracellular signal
• QRS complex: initial activation of ventricles
• T wave: ventricular repolarization
• PR interval: from initiation of atrial to initiation of ventricular activation (most of the delay in the AV node)
• QRS interval: total time of ventricular activation
• ST segment: plateau
• QT interval: sum total of AP duration in ventricle

Question 3

How to label QRS

Answer 3

Q wave: Always the initial deflection; always down.

R wave: Any upward deflection

S wave: Any downward deflection that is not the initial deflection

Question 4

Kirchoff’s law

Answer 4

•Kirchoff’s law: sum of all potential differences in a closed circuit=0:

LL+LA+RA=0

Question 5

Eintheoven’s triangle, positioning of ecg leads

Answer 5

So think about how the ECG waveform resulting from a propagated wave (thick arrow) that is horizontal and points to the right would look:

• •In lead I: upright because the waveform is going towards the + end of lead I
• •Leads II: Mainly upright, wavefront is going towards the + end of lead II
• •Lead III: Downward: wavefront is going away from the + end of lead III
• •aVR: negative: wavefront is going away from its + end.
• •aVL: Positive: wavefront is going towards the + end of aVL.
• •aVF: Isoelectric: average direction is neither up nor down because the wavefront is perpendicular to aVF.
• •For this arrow, we would say that the AXIS is +0º

Question 6

Positioning of precordial leads

Answer 6

• V1: 4th intercostal space, right sternal border
• V2: 4th intercostal space, left sternal border
• V3: midway between V2 and V4 (therefore, apply it AFTER applying V4)
• V4: 5th intercostal space, mid-clavicular line
• V5: horizontal with V4, anterior axillary line
• V6: horizontal with V4, mid-axillary line

Question 7

Positioning and “equals” of 12 leads

Answer 7

•3 bipolar leads

• I=LA-RA
• II=LL-RA
• III=LL-LA

•3 augmented unipolar limb leads

• aVR=RA-(LL+LA)/2
• aVL=LA-(LL+RA)/2
• aVF=LL-(RA+LA)/2

•6 unipolar precordial leads, V1-V6

• Vx= Vx-(LL+LA+RA)= Vx-0

Question 8

How does the sequence of depolarization in the ventricle explain the normal QRS waveform?

Answer 8

Question 9

ECG graph paper

Answer 9

Question 10

ECG paper lead alignments

Answer 10

Question 11

Quick easy estimate of HR based on ECG boxes

Answer 11

•Quick and easy estimate: Count big boxes between beats: Memorize: 300, 150, 100, 75, 60, 50, which=HR for 1,2,3,4 and 5 boxes between beats

Question 12

Standard ECG interval times

Answer 12

Question 13

QTc

Answer 13

Question 14

How to measure axis to the closest 90º quadrant

Answer 14

Question 15

Left axis deviation

Answer 15

Question 16

Right axis deviation

Answer 16

Question 17

Parallel leads in axis measuring

Answer 17

Question 18

Normal ECG things:

Answer 18

• ·intervals, QRS and T wave axes and rate are normal
• ·P waves are upright in lead II
• ·every P wave is followed by a QRS complex.
• ·normal sinus rhythm is present
• ·the TP, PR, and ST segments are all on about the same level
• ·the P, T waves, and QRS complex all point in approximately the same direction (put another way, their axes should be about the same)
• ·from V1 to V6, there is a smooth and progressive transition from a predominantly downward to a predominantly upward QRS waveform

Question 19

Causes of ST depression/elevation

Answer 19

because current flows from healthy cells to dead cells (dead cells have lower volutages) we can see changes in usual waveform

•ST depression when the ischemia or injury is subendocardial (panel A)

•ST elevation when it is transmural (panel B)

• ST depression (≥ 1mm) if it is subendocardial (typical in angina) and non-ST elevation MI (NSTEMI)
• ST elevation ≥1mm (≥ 0.5mm in V2-3) if it is transmural (as in a transmural MI or ST elevation MI, STEMI). The ST segment is often (not always) convex when viewed from above.

Question 20

T wave inversion

Answer 20

Question 21

Q wave in MI

Answer 21

Criteria for an Abnormal Q-wave:

Duration: ≥0.03sec, just about 1 small box; ≥0.02sec in V2 or V3

AND

• has an amplitude ≥ 0.1 mV (1 small box)
• Present in at least 2 contiguous leads (i.e. 2 inferior leads, 2 anterior leads, 2 lateral leads)

Not present in all MIs:

• For a Q wave to appear there must be a substantial area of necrosis.
• In smaller MI’s, Q waves do not appear on the ECG.
• The diagnosis is made from the other ECG changes (ST segment shifts, T wave inversion), the clinical situation and cardiac enzyme
• Older terminology for non q-wave MI is non-transmural or subendocardial MI. These terms have been abandoned by many but you hear them occasionally
• Non-Q wave MI has a better short term prognosis than Q wave MI (because it’s usually smaller) but may imply a high risk for subsequent MI

Question 22

Which ECG leads show which areas of heart best

Answer 22

Electrical activity in the anterior wall of the LV is best seen in leads V1-V4

Electrical activity in the lateral wall of the LV is best seen in leads I, aVL, V4-V6

Electrical activity in the inferior wall of the LV is best seen in leads II, III, aVF

Question 23

Markings of the age of MI

Answer 23

• •Earliest stage: Hyperacute phase, characterized by presence of tall, peaked, hyperacute T-waves, with or without ST elevation: minutes to a few hours…and usually gone by the time the patient arrives at the emergency department
• •Acute, ST Elevation MI (STEMI): ST segments elevated ≥ 1 small box; hyperacute T-waves resolve; q-waves may start to appear: minutes up to a few days.
• Eventually (hours to a few days), T waves invert. ST segments may remain elevated during this time, but gradually return to baseline

Old:

• •Eventually, ST segments return to baseline
• –usually!!
• •in some pts in whom an aneurysm develops, ST elevation persists indefinitely
• •Q waves persist indefinitely in most patients
• •T wave inversion may or may not persist
• •The only definitive proof that an MI is old is that it was present on an old ECG!!

Question 24

Other Causes of ST Elevation

Answer 24

• •Normal variant: “early repolarization” or “J-point elevation”
  
  • –Seen especially in young people
  • –ST segment is usually elevated in multiple leads (instead of in leads that localize the changes to a specific region)
  • –ST segment is usually concave (viewed from above)
• •Ventricular aneurysm
  
  • –Persistent ST elevation after an MI suggests the presence of an aneurysm (an area of scar that bulges out during systole…also called a dyskinetic area)
• •Pericarditis
  
  • –ST segment is usually elevated in multiple leads
  • –Associated with clinical picture of pericarditis instead of that of MI
  • –Sometimes associated with PR segment depression
  • –Sequential ECG changes differs from those of MI

Question 25

Atrial enlargement on ECG

Answer 25

Question 26

Left or right Atrial Abnormality (Enlargement): Criteria

Answer 26

Question 27

Ventricular enlargement ecg

RVH

LVH

Answer 27

RVH: Criteria:

• V1: R wave is larger than S wave and/or
• V6: S wave is larger than R wave
• Limb leads: Right axis deviation (axis between 90º and 180º)
• Usually, right atrial p-wave abnormality is present

LVH Criteria:

• S-wave in V1 + R-wave in V5 or V6 ≥ 35 mm
• R in I + S in III ≥ 25
• R in aVL ≥ 12 mm
• Know that other criteria exist (you’ll learn about them later) and that LVH is often accompanied by the strain pattern.