ST segment

Question 1

ST segment: def, what does it represent

Answer 1

• End of QRS → beginning of T wave
  o Membrane potential of ventricular myocytes is = → no potential difference on ECG
   Phase 2: plateau
   Ventricular systole
   Early phase of ventricular repol
  o End of QRS: J point
  o Should be at same level of PR and TP

Question 2

ST deviation normals

Answer 2

displacement above/below isoeletric line
o >0.2mV in limb leads
o >0.25mV in precordial leads
o Measured 40ms after J point

Question 3

St segment deviation: what does it represents

Answer 3

Reflects repolarization heterogeneity
o Slurring, coving, arching, dome-shape

o Can vary over time and follow pattern in relation to RR intervals
 Respiratory sinus arrhythmia: long RR → ↓ deviation
o Depend on location of ischemic zone (subendo/epicardial, transmural) → elevation or depression

Question 4

Slurring of ST

Answer 4

• LVH, RBBB, tachycardias, ventricular rhythms
• Marked RVE/RBBB: slurring upward toward T wave

Question 5

Coving/dome shaped ST

Answer 5

digoxin tx/toxicity

Question 6

Causes of ST segment changes

Answer 6

o Myocardial ischemia = ↓O2 levels
 Acute myocardial infarction
 Takotsubo CM: emotional distress → T wave inversion, ST elevation

o Pericarditis, myocarditis
 ↑intracranial pressure: associated w deep T wave inversion (cerebral T waves)

o Electrical cardioversion

o Hyper or hypoK+
 Hyper: tented P waves
 Hypo: downslopping ST, prominent U waves, flat T

o Pulmonary embolism (usually lead III)

o HypoCa and hypoT: J wave

o Trauma: cell membrane remain permeable → repol cannot occur

o Infectious process: damage muscle membrane

o Brugada syndrome: channelopathy of Na+ channels
 ST elevation + partial RBBB pattern

o Benign early repolarization: normal variant in young, healthy patients

o SVT: rate related ST segment depression → resolve w tx

Question 7

Distinguish between Ta wave alteration

Answer 7

o Measure 3x P wave duration from end of P wave
o ST segment deviation will extend beyond this point

Question 8

Ischemia pathophys

Answer 8

o Release K+ into local venous blood
 Accumulate outside ischemic cell = not removed since ischemia
 ↓ potential gradient of K+ → membrane depol
 Injured area = partially/ completely depolarized at rest
o ↑CO2, lactate, Pi + ↓pH
o ↓ intracell [ATP] → open K+ channel normally inhibited by ATP

Question 9

Changes in AP w/ ischemia

Answer 9

o Current of injury: current flow btw normal (polarized) and ischemic (depolarized) areas
 Diastolic K+ induced current from ischemic to normal tissue
 Systolic K+ induced current from normal to ischemic tissue
o From K+ ion loss:
 ↑ membrane potential = more positive than surrounding normal tissue
 ↓ AP amplitude and duration from ↑K+ channel activity

Question 10

Epi vs endocardial ischemia ECG changes

Answer 10

o ST segment elevation: epicardial or transmural ischemia
o ST segment depression: subendocardial
 Direction of current flow: away from electrode

Question 11

T wave inversion reflect

Answer 11

o Acutely inverted T waves → reflect
 Variation in rate of repol or myocardium
 AP variation: shorter with ischemia
 Inadequate production of ATP at cell membrane