Clinical Correlations of RMPs, APs, and Conduction System

Question 1

Bradycardia (general)

Answer 1

SA Nodal Failure

• unmasks slower, latent pacemakers in the AV node or ventricular conduction system
• escape beats or rhythms

ie. Junctional rhythm - ectopic focus at AV junction becomes pacemaker

Question 2

Accessory Conduction Pathway

Answer 2

Wolff-Parkinson-White (WPW) Syndrome

Alternate path around AV node
- Bundle of Kent

AP conducted directly from atrium to ventricle
- conduction is faster than via normal AV nodal pathway

Ventricular depolarization occurs more slowly than normal
- can result in reentry and cause SVT = bad

Question 3

Widening of the QRS complex

Answer 3

Ventricular depolarization that spreads only cell-to-cell via gap jxns

• inc. PVCs, V-tach
• much slower than normal Purkinje-fiber mediated propagation

Question 4

Biomarkers of Myocardial Injury

Answer 4

Troponin
- cTnT, cTnl
- used as biomarker for cardiac damage
(proteins normally sequestered inside cells are now in your bloodstream, which generally indicates some sort of cell injury has occurred)

CK-MB
- creatine kinase isoform specific to cardiac mm

Question 5

CAD and MI-mediated hyperkalemia

Answer 5

Reduced blood flow and ischemia

• less ATP to power Na+/K+ ATPase
• ATP-dep K+ channels open when ATP is decreased
• initial memb hyperpolarization turns into hypopolarization as hyperkalemia develops

MI: infarcted cells release intracell K+ stores

Resulting memb depolarization and higher [K+] outside slows conduction velocity and disrupts rhythm

Question 6

What are some anatomic variables that can alter conduction velocity?

Answer 6

Congenital accessory pathways

Degeneration of conduction system

Question 7

What are some other variables that can affect conduction velocity?

Answer 7

Premature excitation
- memb not fully repolarized

Ischemia/hypoxia - CAD

Autonomic

• sympathetic activation - beta1 R
• parasym (vagal) activation - M2 R

Chemical

• circulating hormones - catecholamines
• autonomic drugs - beta-blockers
• antiarrhythmic drugs - Na+ or Ca++ channel blockers
• hyperkalemia

Question 8

What are ectopic foci?

Answer 8

Generation of AP from a source other than the SA node

• cause of most premature contractions
• gen don’t follow normal conduction pathways

Myocytes take longer to depolarize cell-to-cell via gap junctions
- ventricular ectopic foci: wide QRS (PVCs, v-tach)

Question 9

What are some causes of ectopic foci?

Answer 9

Local areas of ischemia

Mildly toxic conditions can irritate fibers of the AV node, purkinje system, or myocardium

Calcified plaques irritating adjacent cardiac fibers

Cardiac catheterizations - mechanical initiation of premature contractions