Other

Question 1

Electrical cardioversion/defib mechanism

Answer 1

• Shock delivered to critical mass of myocardium → atrial myocyte AP = coordinated alteration of membrane potential → refractory state
  o SA node regain control of rhythm

Question 2

Success in terminating arrhythmia with electric shock depend on

Answer 2

o Amount of E delivered
o Path of current vs position of heart
 Position paddles/patches at level of atrium
o Transthoracic impedance: ↑ impendance → ↓ probability of successful shock
 Determined by: chest conformation, water/fat content, pulmonary volume, size/position of paddles

Question 3

Synchronized cardioversion mechanism

Answer 3

Premature activation of all potentially receptive areas to terminate tachyarrhythmia and convert to sinus rhythm

Question 4

Indications

Answer 4

o Used with patients with pulse
o Unstable patients
o Unsuccessful chemical cardioversion

SVT, Afib, Aflutter, Vtach with pulse, unstable reentrant tachycardia

Question 5

Shock delivery with cardioversion

Answer 5

o Synchronous mode: time shock delivery to peak of R wave
 Absolute refractory period
 Prevent shock delivery at T wave peak

 After shock, default back to asynchronous mode

o Most arrhythmias stopped w 1 or 2 shocks

Question 6

Vulnerable period of ventricles

Answer 6

T wave peak
* Impulse reach ventricles during repol = electrical heterogeneity → ↑ risk of Vfib
* R on T phenomenon

Question 7

Cardioversion vs defib

Answer 7

requires < E vs defibrillation
 1st shock: 1-2J/kg
 Subsequent shocks: ↑ output until conversion occurs or max output fails to terminate arrhythmia
 Heart disease do not ↑ E required for cardioversion

Question 8

Indications for defib

Answer 8

• Treatment for immediate life threatening arrhythmia w/o pulse
• Technique to terminate ventricular fibrillation

Pulseless Vtach, Vfib, Cardiac arrest due to/resulting in Vfib

Question 9

Considerations defib

Answer 9

o Electrolyte imbalances: corrected before shock
o Ensure adequate O2
o Avoid opioids → effect on vagal tone
 Prone to maintenance/reoccurrence of Afib

Question 10

complications Defib

Answer 10

• Vfib induction: shock to synchronized to R wave
  o Should be rapidly treated w high E asynchronous shock
• General anesthesia
• Skin burns
• Thromboembolic events

Question 11

Defibrillator devices

Answer 11

multifct devices
o Monitoring + external pacing capabilities
 Synchronous/asynchronous mode
o ECG tracing, amount of energy
o Monophasic shocks: current of 1 polarity
o Biphasic shocks: direction is reversed near halfway point of electrical cycle
 Require less E + higher success rate

Question 12

Define overdrive suppression

Answer 12

Driving a PM cell faster than its intrinsic rate

Question 13

Mechanism of overdrive supp

Answer 13

o ↑Na+ enters the /unit of time
o ↑activation of Na+/K+ pump → Na+ efflux → hyperpolarization
 ↓ depolarizing If current
o If activity of driving PM  stops → pause to allow ↓[Na+]
 ↑rate or longer suppression → greater ↑ in pump activity → longer pause

Question 14

Degree of overdrive suppression depends on

Answer 14

membrane potential
 At ↑ membrane potential (less negative) → ↓ Na+ channels available → ↓ Na+ influx → ↓ activation of Na+/K+ pump
 At ↓ membrane potential (normal values) → ↑ Na+ channels available → ↑ Na+ influx → ↑ activation of Na+/K+ pump = ↑ overdrive suppression

Question 15

Overdrive suppression in normal heart

Answer 15

• Usually SA node > subsidiary PM
  o Intrinsic slope of phase 4 is faster → ↑ automatic rate

Question 16

Overdrive suppression w/ SVTs

Answer 16

o SA node can be suppressed by SVTs
 Less susceptible since depol is mostly dependent on ICaL
* ↓Na+ enter  during depol upstroke
* Accumulation of intra [Na+] and activation of Na+/K+ pump occur to a lesser degree
 Diseased SA node in SSS can be much more easily suppressed

Question 17

Abn automatic cells and overdrive suppression

Answer 17

lack of overdrive suppression

Question 18

Gap phenomenon

Answer 18

• Ability of premature impulse to propagate through AV node, while other “less premature” fail to reach ventricles
  o 2 levels: proximal = shorter refractory period and distal = longer refractory period
  o Premature impulses can travel proximal region, but are blocked at distal level
• Gap phenomenon = impulse with a shorter coupling interval
  o Proximal level during relative refractory period → delayed conduction
  o Distal level have time to recover → conduct impulse

Question 19

Supernormal conduction

Answer 19

• Conduction of an impulse at an unexpected time
  o Short period during repolarization where excitation is possible with a subthreshold stimulus
   At the end of phase 3 (end of T wave)
• Availability of fast Na+ channels
• Proximity of membrane potential to threshold potential
  o Smaller stimulus than normal required to depol
   Conduction is better earlier in the cycle than expected
   Normal QRS morphology or ↓aberrancy

Question 20

Duration of supernormal phase

Answer 20

constant even with RR changes
 Larger proportion of AP at faster HR

Question 21

Supernormal conduction associated with

Answer 21

o APC during sinus rhythm with BBB
o Narrow and wide QRS alternans during SVT
o Narrow QRS during Afib with BBB

Question 22

Physiologic mechanisms of supernormal conduction

Answer 22

a) Supernormal excitability in phase 3
b) Diastolic phase 4 depolarization: rapid conduction
c) Gap phenomenon (see previous)
d) Dual AV nodal pathways: early APC propagate through slow pathway
e) Peeling back refractoriness: shorter absolute AV node refractory period by VPC or JPC
f) Shorter refractoriness from changing preceding cycle length: proportional to length of cycling interval
g) Summation of subthreshold impulses
h) Wendensky facilitation: multiple impulses on blocked site → block is overcome → multiple stimuli summate and reach distal site → conduction
i) Bradycardia dependent blocks
j) Wenckebach phenomenon in BB

Question 23

Def: Effective inter-sinus interval

Answer 23

time between conducted sinus impulses

Question 24

Def: Escape capture bigeminy

Answer 24

bigeminal rhythm with an escape beat followed by captured beat

Question 25

When does escape capture bigeminy occurs

Answer 25

marked difference between escape interval and effective inter-sinus interval
o Sinus interval longer > escape interval
 SA node dz → low intrinsic rate
 Sinus rhythm associated with accelerated junctional rhythm
o Rare

Question 26

Forms of escape capture bigeminy

Answer 26

atrial or ventricular capture bigeminy
o Most commonly in Hi w SSS or 2AVB

Question 27

Requirementsof escape capture bigeminy

Answer 27

o Effective inter-sinus interval > sum of escape interval + refractory period
o Escape complex: do not alter SA node cycle (no retrograde conduction
o Intermittent block of sinus impulse at sinus or AV level

Question 28

Mechanisms of escape capture bigeminy

Answer 28

o SA block
 Sinus impulses occur just after recovery period of VPC (shaded)
* Normal conduction if relatively late
* Aberrant conduction if early after VPC
 If PP interval ↓ and become < escape rhythm interval → rhythm is abolished

o AV block
 Starts with a dissociated beat
 3:2 AVB→ block permit AV node to escape resulting in dissociated beat

o Reversed reciprocal rhythm
 Starts with a dissociated beat
 2nd impulse conducted from SA node
* 1st degree AVB present
* Conduct retrogradely by accessory pathway
o Negative P wave in ST segment
o Depol SA node → delay next impulse
o Postpone next sinus impulse
 3rd impulse: AV node escape from delay of sinus impulse

Question 29

Mechanism of atrial echo beat

Answer 29

VPC conducted retrogradely to atrium → provoke atrial impulse conducted via normal pathway (AV node, His Purkinje system)
o AV node: particularly if dual pathways
 Antegrade along slow pathway = long PR
 Retrograde along fast pathway
 Since long PR → atria recovered from previous depolarization
o Accessory pathway: WPW, concealed bypass tract
* Occurs when pacemaker and intact ventriculoatrial conduction
o Can result in bigeminal rhythm

Question 30

Factors necessary for re-entry

Answer 30

o Circuit: path followed by electrical impulse during re-entry
 Size and shape: determined by conduction velocity + myocyte refractory period
* Size must = or > length of re-entry
* Cycle length = refractory period x conduction velocity
 2 branches: α and β
* α: antegrade conduction
* β: retrograde conduction
* If same conduction velocity: impulse meet at the end and block in single wavefront
o Unidirectional block in a branch of the circuit
o Slow conduction velocity in a branch of the circuit
o Appropriate cycle length
o Trigger