Yagel - Electrophysiology for the Perinatologist

Fetal Electrophysiology Yagel31

...normal EP fundamentals

Normal fetal HR:

-atrial & ventric rates bn 100-180bpm

-declines from 140 +/-20 at 20 wks GA to 130 +/-20bpm at term, because of incr parasymp tone

-Average AV conduction time prolongs from 90msec at 15 wk GA to 110msec at term

Abnormal Impulse Generation

-automaticity

-automatic arrhythmias of the sinus node = sinus tachy that is abNl or sinus brady that is abNl though it is still the dominant pacemaker

-usually ppt by fetal stress, maternal Ab mediated fetal thyroxicosis, hypoxia, acidosis

-ectopic automatic arrhythmias- non SA pacemaker is dominant - AET, JET, VT, or heart block with jctl/ventricular escape

-triggered activity

Abnormal Impulse Conduction

-Reentry = propagation of an impulse through myocardium that has already been activated by that same impulse

-atrial flutter - fast clockwise/countercloswise rotating atrial macroeentry circuit

-reentrant SVT

-#1 fetal SVT is orthodromic AV reentrant tachycardia, usually initaited by a pAC

-dual AV nd physio also possible

-AV block

Intrauterine Investigation of Fetal Rhythm & AV Conduction

-must use M Mode & Doppler bc no ECG avail

-record atrial and ventricular systolic events simultaneously...

-SVC & Asc Ao

-SVC a-wave = beginning of retrgrade Q in SVC = atrial systole onset

-Aortic forward Q onset = ventricular systole

-cannon a wave = tall a waves seen when the atria and ventricles contract at the same time - eg orthodromic reentry tachycardia - AV vlvs didnt open normally so atria contracts against a closed valve

-Alternates: PA & PVn, or LV inflow and outflow

-time interval bn onset of mitral a-wave and ascending Ao Q = geginning of atrial and ventricular systole respectively

Echo Assessment of Fetal atrioventricular conduction system

-should have 1:1 inflow/outflow...

-There are GA matched reference values for the AV time interval (~to PR interval)

-should monitor AV time interval if fetal exposure to anti-Ro/SSA & anti-La/SSB auto-Ab

-found in 1-2% of all PG women!

-risk fetal AV block at about 20-24 weeks GA

-some say that if you catch it during the short period where it is still intermittent HB, it can be stopped from progression to complete HB

-1st degree HB - maint 1:1 AV relationship, but with prolonged AV interval

-but ? accuracy of US derived AV measurement for 1st degree AV block

-up to 25% of fetuses exposed to SSA/SSB mhave 1st degree AV block, so ? efficacy of the measure...

-one xx with using AV interval to approximate PR interval is that electromechanical delay may be affected by confounders- loading conditions, intrinsic myocardial properties, heart rate/size, US site/technique.

-must use a fast sweep speed (100mm/s) to be accurate...

Echocardiographic Assessment of Fetal Arrhythmias

-Arrhythmias seen in 2% of all pregnancies during routine OB exam

-must check:

-rate & regularity of the AA evens and VV events

-relationship of atrial mechanical events to ventric events (A:V conduction ratio, & VA chronology)

-Irregular Rhythm

-90% of of PG women have arrhythmias- brief, isolated, benign - eg isolated PACs, only some are assoc w other arrhythmias

-If PAC is conducted, must DDx the PAC from PVC, which is rare in fetuses

-If PAC is blocked, must DDx from 2nd degree HB- uncommon

-2nd degree HB denotes regular atrial rates with failure to conduct some, but not all, of the impulses. So, the atrial rate is regular, but ventric rate isn't.

Abnormal Rates

-Fetal brady = HR<100bpm, tachcyardia = >180bpm

--> xx of low CO state, hydrops, neuro xx, death

Bradycardia

-brief sinus brady, <1-2minutes are common & benign, espec in early gestation

-prolonged sinus brady, atrial bigeminy with blocked PACs, and high-degree AV block --> prolonged decr in HR

-sinus brady:

-may be bc of fetal long-QT syndrome, fetal distress

-atrial rate is regular and slow with normal 1:1 AV relationship

-blocked PACs:

-atrial rate is faster than ventricular rate

-nonconducted atrial bigemony: every 2nd atrial impulse occus early enough that it doesn't conduct thru refractory AV jct.

-high grade AV block- no signif variation in AA interval, but the propgation to the ventricles is intermittent (2nd degree AV block), or 2:1 AV block, or completely blocked (3rd degree HB)

-in 2nd degree AV block, ventric rate is regular only if there isa 2:1 block or a higher AV ratio... (regular rate of blocked impulses...)

-must DDx blocked PACs from true HB, bc blocked PACs causing bradycardia usually resolves w/o Tx, but higher grade block is assoc with bad xx...

Tachyarrhythmias

-usually due to atrial flutter, SVT, or sinus tachy

-Atrial Flutter- atrial rate >300bpm --> only every 2nd or 3rd atrial beat is conducted --> ventric response rate of 150-250bpm

-SVT-

-AV reentrant tachy w fast retrgrade AP conduction

-PJRT = permanent jctl reciprocating tachy - w slow retrograde conduction

-AET - enhanced focal automaticity

-DDx by their arrhythmia pattern and VA time relationship

-AV reentry- short VA tachyarrhythmia, retrograde activation of atria through the fast pathway (so short RP = short VA interval)...

-Long VA SVT- atrial contraction happens just before ventric contraction - seen w AET, PJRT, Sinus tachy

-sinus tachy usually slower than AET/PJRT - rate 180-200bom, normal 1:1 conduction, variability

-fetal distress, anemia, infection, maternal beta-stim, fetal thyrotoxicosis

-VT & JET are arre causes of fetal tachy

-if no retrgrade conduction across the AV nd or AP, the ventric rate wil be > atrial rate during the tachy. If there is retrograde 1:1 VA conduction, then it is hard to DDx fr SVT