Sensing and Pacing

Unipolar

Large sensing window between device and electrode in the heart

Bipolar

small, more specific sensing window between 2 electrodes at the tip

Can pacemakers function unipolar and bipolar?

Yes

Unipolar is _____ _______ when bipolar fails

backup option

Measure sensing in

mV

Measure pacing in

Volts

Minimum P wave value

greater than or equal to 2 mV

Minimum R wave value

greater than or equal to 5 mV

Sensing

When a device sees natural/intrinsic depolarization

Devices sense cardiac depolarizations by

measuring change in electrical potential of myocardial cells

Fence in sensing

tells us less or more sensitive and has an inverse relationship

Less sensitive fence means

device sees less, fence is higher

More sensitive fence means

device sees more, fence is lower

Oversensing

seeing too much (like r and t waves), fence needs to be raised

Undersensing

not seeing enough (won't sense r wave), fence needs to be lowered

Oversensing causes

Under pacing

- skipped/missed beats

Undersensing causes

overpacing

- faster/unnecessary paces and pro arrhythmic

Do we ever want to pace atrium in A flutter or A fib?

NO

Oversensing on RV lead can cause

pacing inhibition and device misinterpretation of VF

How to reproduce noise?

isometric exercises

Noise Oversensing

- can cause underpacing and misinterpretted VF

What happens if we undersense AF or VF?

could continue to pace during these fast rates

Near and farfield over sensing

sensing an extra signal that disrupts timing cycle

comes from same chamber or different chambers/outside the heart

Nearfield Oversensing

Oversensing in same chamber

- V channel senses R and T waves, double counting

Farfield Oversensing

- one channel senses signals from another chamber or from outside the heart

- A channel sensing V signals

Atrial lead shape

J shape in RAA

Farfield R wave

Oversensing

- A channel senses R waves from V channel

- can cause misinterpretation of atrial arrhythmias and false mode switch

Why would A channel see farfield R waves?

- A channel fence too low

- V output too high

T wave Oversensing

Nearfield

- counts T wave as R wave in V channel

- can cause false positive VT

Can device discern R v. T wave?

NO

Why might T waves be large?

Hyperkalemia

Atrial Lead Dislodgement

- usually happens in first 24h

- A lead falls into RV or dislodges from RAA

What do we need before discharging patient with new device?

X-ray for lead placement

What does atrial lead dislodgment look like on strip?

- atrial pacing impulse captures the V (lead has fallen into RV)

Polarization

Layers of negative and positive ions surrounding the electrode during pulse stimulus. It can slow the movement of a charge.

Capture

Successful depolarization of cardiac tissue from pacing stimulus

Stimulation threshold

minimum amount of energy required to cause depolarization

Conduction

Movement of charge under influence of electrical field

Voltage

V

- force that drives current (amplitude)

Pulse Width

ms

- duration of time in a pacing stimulus

Current

mA

- flow of electrons measured in milliamps, not programmable; is calculated

Output Capacitor

Stores and delivers pacing impulses

Impedence

Ohms

- resistance to the flow of current within an electrical circuit

T/F Thresholds can change

True

T/F Autocapture algorithms can be tricked

true

T/F A threshold can be checked on pacemaker dependent patient

True

T/F Afib always needs threshold check

false: can't outrun AF

Pacemaker battery made out of

lithium/iodine

Beginning of Life (BOL)

when the device is first put in

Elective Replacement Interval (ERI)

- device needs to be changed out

- 3-6 months left usually

End of Life (EOL)

pacemaker stops working and has to be changed out

Battery cathode

positive

Lead cathode

negative

Battery Anode

negative

Lead Anode

positive

Lead tip is always

cathode and negative

Depolarization with device

Electrons flow from pulse generator to negative cathode tip

- influence ions for big Na+ influx

In CHB what phase of AP are we in?

Phase 4

PM goal in AP

get to phase 0 to depolarize

Threshold purpose

to have enough energy to cause depolarization

Pacing is

programmable depolarization through amplitude and pulse width

Amplitude

how many volts are delivered per pacing impulse

Pulse width in pacing

duration of impulse, usually 0.4 or 0.5 ms

Goal of pacing

effective and safe output setting and maximizing battery longevity

Safety margin

2:1

- sensitivity: half

- amplitude: double

Threshold test

Control/outrun the heart rate

- may decrease AVD for ventricular threshold

Never run threshold when

patient is in VT

Rate limit for threshold test

120 bpm

Pacemaker dependent threshold test

Run in DDD, need ventricular support

How to know if patient is pacemaker dependent?

Run a sensing test

Can you run atrial threshold in AF?

No

In dual chamber device testing in DDD, how many chambers decremented at a time?

one

What to know before running threshold test

Presenting and Underlying rhythm

Overdrive Pacing requires

Power (adequate starting amplitude) and speed (faster than underlying rhythm)

Decrement power

decrease amplitude incrementally and watch for loss of capture

What is the threshold?

Not output where test stops

- It is previous output with successful depolarization

Factors affecting threshold

- Lead Integrity

- Lead location

- medications

- arrhythmias

- electrolyte imbalances

Causes of failure to capture

- lead dislodgment (physician fix)

- poor connection at connector block (least likely) (physician fix)

- lead maturation

- lead failure

- low output (change amplitude)

Fusion Beat

1/2 intrinsic beat, 1/2 paced

- timing cycle issue, AVD too short

Pseudofusion beat

- pace falls in the middle of intrinsic beat during absolute refractory and does not capture

- increase sensitivity (V) if d/t undersensing

Pseudo Pseudofusion beat

A pace in intrinsic R wave and V pace in T wave

- undersensing P and R waves

Reasons for Automatic Capture Algorithms

- more frequent testing

- prolong battery life