Tox: ECG principles

Question 1

DDx for bradysrhythmias/AV blocks

Answer 1

BB, CCB, cardiac glycosides (e.g. digoxin), clonidine, other antidysrhythmics (e.g. amio)

Question 2

DDx for tachydsyrhythmia agents (tach/SVT/VT)

Answer 2

Anticholinergics, stimulants, sympathomimetics

Question 3

DDx for QRS widening drugs

Answer 3

Na channel blocking drugs, quinidine, antihistamines, cocaine, TCAs

Question 4

DDx for QTc prolongation drugs

Answer 4

Antidepressants (SSRI/SNRI), antidysrhythmics, antipsychotics, antifungals, antibiotics

Question 5

Digoxin findings on ECG

Answer 5

Digitalis effect (seen at therapeutic levels):
- T wave changes (flattening or inversion)
- QT interval shortening
- Scooped ST segments with depression in lateral leads
- Increased U wave amplitude
Rare: biventricular VT
Afib with slow ventricular response.

Question 6

TCA findings on ECG (Na+ channel blockade)

Answer 6

1) Sinus tach (most common)
2) PR, QRS, QTc prolongation
3) deep slurred S in I and aVL
4) RBBB
5) R axis deviation (of terminal 40ms)
- Terminal R wave in aVR, S wave in I, aVL
6) R:S ratio >0.7 in aVR