Cardiac pacing tidbits

Question 1

Ohm’s law

Answer 1

V (voltage) = I (current) x R (resistance)

Question 2

Stimulation threshold

Answer 2

minimum energy required to active the heart electrically outside the refractory period

Question 3

Stimulus energy

Answer 3

E (energy) = (V^2 x time)/R

Question 4

Rheobase

Answer 4

lowest voltage resulting in capture at infinitely long pulse duration (usually less than 1.5 ms)

Question 5

Chronaxie

Answer 5

pulse width required to stimulate the heart at 2x rheobase voltage – closely approximates point of minimum threshold stimulation energy. (voltage usually set at 2x threshold value at chronexie pulse width)

Question 6

Impedance

Answer 6

total resistance to current flow

Question 7

Hysteresis

Answer 7

Allows ventricular pacing to occur only after a longer pause then programmed lower rate => Pacer will not begin to fire until patient’s HR falls below certain rate but when pacer begins firing, it fires at a rate faster than the escape rate (an attempt to preserve intrinsic sinus rhythm and avoid PM syndrome)

Question 8

Wedensky’s effect

Answer 8

when a voltage threshold is determined by measuring from higher to lower voltage outputs (capture to loss of capture), the threshold is lower than if the threshold is measured from lower to higher outputs (loss of capture to capture) – this difference is known as Wednesky effect and occurs due to hyperpolarized state across myocardial cell membranes when starting with a higher voltage.

Question 9

Slew rate

Answer 9

slope or rate of change in voltage

Question 10

Blanking period

Answer 10

immediate period of disable sensing following pacing spike (usually 25-100 msec)

Question 11

Refractory period

Answer 11

(noise sampling period) programmed to adjust to patient’s QT interval. Events sensed during this period reset refractory period but DO NOT reset the pacing interval

Question 12

Exit block

Answer 12

when progressively higher stimulation thresholds are required but loss of capture (with pacing spike) – usually secondary to fibrosis at lead tip

Question 13

Oversensing

Answer 13

when inappropriate signal is sensed resulting in loss of capture without pacing spike

Question 14

Undersensing

Answer 14

when pulse generator delivers stimulus at an inappropriate time

Question 15

Pseudofusion

Answer 15

pacing spike is superimposed on spontaneous QRS complex

Question 16

Pacemaker syndrome

Answer 16

Decrease in cardiac output and systemic blood pressure d/t loss of AV synchrony and VA conduction – usually result of single chamber ventricular pacing