Download Arrhythmia Management - Practical Considerations

Basics of Pacemaker
Functioning
T K Govindarajan
Medtronic
Topics
• Components of a Pacemaker System
• Types of Pacemakers & their Operation
• Functions of a Pacemaker System
– Stimulation of cardiac tissue
– Sensing of intrinsic heartbeats
– Single chamber timing cycle
The Pacemaker System
Cardiac Pacing is the artificial electrical stimulation of the heart in the absence of
intrinsic heartbeats causing it to contract
A Unipolar Pacing System
• Flows through the tip
electrode (cathode)
• Stimulates the heart
Anode
• Returns through body
fluid and tissue to the
IPG (anode)
-
Cathode
A Bipolar Pacing System
• Flows through the tip
electrode located at
the end of the lead
wire
• Stimulates the heart
• Returns to the ring
Anode
Cathode
electrode above the
lead tip
Tip electrode coil
Indifferent electrode
coil
NBG Code Revised - 2002
I
Chamber
Paced
II
Chamber
Sensed
III
Response
to Sensing
V: Ventricle
V: Ventricle
T: Triggered
V: Ventricle
A: Atrium
A: Atrium
I: Inhibited
A: Atrium
D: Dual (A+V)
D: Dual (A+V) D: Dual (T+I)
O: None
O: None
S: Single
S: Single
(A or V)
(A or V)
O: None
IV
Programmable
Functions/Rate
Modulation
V
Multi Site
Pacing
D: Dual (A+V)
R: Rate modulating
O: None
O: None
Single Chamber Pacemakers
Ventricular Single Chamber Pacing or VVI pacing
Pacing Rate
Pacing Rate
Pace
Pace
Sense
Pace
Rate Responsive Pacemakers
Rate-Responsive Pacing
Fixed-Rate Pacing
Normal Heart Rate
Running
Heart Rate (bpm)
150
Walking
100
Wake-up
Sleeping
Resting
Sitting
50
0
Daily Activities
Activity Sensors
• Activity sensors employ a
piezoelectric crystal that
detects mechanical
signals produced by
movement
• The crystal translates the
mechanical signals into
electrical signals that in
turn increase the rate of
the pacemaker
Piezoelectric
crystal
Pacing system - a standard electrical circuit
The pacemaker provides the voltage.
Current (electrons) flow down the
conductor to the lead tip or cathode (-)
Where the lead tip touches the
myocardium, electrical resistance is
produced.
The current then flows through the
body tissues to the anode (+) and back
to the battery.
All of the above are required for
current to flow.
Voltage, Current, and Impedance
• Voltage: The force moving the current (V)
– In pacemakers it is a function of the battery chemistry
• Current: The actual continuing volume of flow of
electricity (I)
– This flow of electrons causes the myocardial cells to depolarize
• Impedance: The sum of all resistance to current flow (R)
– Impedance is a function of the characteristics of the conductor (wire),
the electrode (tip), and the myocardium (tissue).
Ohm’s Law
• Describes the relationship
between voltage, current,
and resistance
(impedance)
V
• V=IXR
V
=
I X R
• I=V/R
V
I
I
R
• R=V/I
= R
V
I =
R
Lead Impedance Values
Electrical Analogies
• Normal resistance – friction caused by the hose and nozzle
Normal 300 – 1200 Ohms
• Low resistance – leaks in the hose reduce the resistance
Similar to a pacemaker lead with an insulation breach which
results in low resistance and high current drain; may cause
premature battery depletion.
• High resistance – a knot results in low total current flow
Similar to a pacemaker lead with a lead conductor break - impedance will
be high with little or no current reaching the myocardium.
High Impedance Conditions
A Fractured Conductor
• A fractured wire can cause
Impedance values to rise
– Current flow from the battery
may be too low to be
effective
• Impedance values may
exceed 3,000 W
Lead wire fracture
Increased resistance
Other reason for high impedance: Lead not seated properly
in pacemaker header (usually an acute problem).
Low Impedance Conditions
An Insulation Break
• Insulation breaks can cause
impedance values to fall
– Current drain is high and can
lead to more rapid battery
depletion
– Current can drain through the
insulation break into the body or
other lead wire, not through
myocardium
• Impedance values may be
less than 300 W
Stimulation Threshold
• Pacing Voltage Threshold
– The minimum pacing voltage
– at any given pulse width
– required to consistently stimulate the heart
– outside the myocardial refractory period
causing it to contract
Capture – Loss of Capture
Capture
VVI / 60
Non-Capture
The Pacemaker Stimulus
2.5 Volts
0.5 ms
1 sec
Time
Pacing Stimulus Voltage or Amplitude – 2.5 Volts
Pulse Width – 0.0005 seconds or 0.5 milliseconds
Pacing Rate – One stimulus per second or 60 stimuli (beats)
per minute
•
•
•
Stimulus Voltage & Pulse Width
have an exponential relationship
At short pulse widths
(<0.25 ms) the curve rises sharply
At long pulse widths
(>1.0 ms) the curve is flat
Stimulation Threshold (Volts)
The Voltage-Strength Duration Curve
2.0
1.5
1.0
Capture
.50
.25
0.25
1.0
1.5
Duration
Pulse Width (ms)
The Pacing Pulse
V = Pulse Amplitude in Volts (V) (say 2.5 V)
t
Output Voltage
Pacing Pulse
t = Pulse Duration or Width in milliseconds (ms)
(say 0.5 ms)
R = Impedance of Pacing Circuit (ohms)
(say 500 ohms)
V
I = V/R = Current through pacing circuit (mA)
= 2.5 V/ 500 ohms = 0.005 A = 5 mA
t
Pulse Duration (Width)
E = Energy delivered by Pulse to the Pacing Circuit
and Cardiac Tissue
= V . I . t = I2Rt = V2t/R
= 2.5 V . 5 mA . 0.5 ms = 6.25 micro Joules
If V = 5 V Energy = 25 micro Joules
Why measure stimulation threshold?
• To enable programming stimulus voltage
amplitude and pulse width such that
– Consistent capture & Patient Safety is ensured
– Battery drain minimized, Pacemaker longevity
maximized
– Good thresholds
•
Ventricle - <1V @ 0.5ms
•
Atrium - <1.5V @ 0.5ms
Evolution of Pacing Threshold
6
Voltage
Threshold (V)
s
Safety Margin
4
3
2
Chronic Phase
1
Acute Phase
0
4
8
Observation Time (weeks)
12
16
Sensing of intrinsic heartbeats
• Sensing is the ability of the pacemaker to “see” when an
intrinsic depolarization is occurring
– Pacemakers record the Intracardiac Electrogram (EGM)
by constantly recording the potential difference
between the cathode and anode
Depolarization Wave
Processed by
Device
Intrinsic R wave Amplitude
• Intrinsic R wave amplitude 5 mV
• Intrinsic P wave amplitude 2 mV
Intrinsic R wave in EGM
The Intrinsic R wave amplitude is usually much greater than the T wave amplitude
Sensitivity Setting
2.5
1.25
Time
5.0
Amplitude (mV)
Amplitude (mV)
5.0
2.5
1.25
Time
Sensitivity settings less than 2.5 mv – High sensitivity – can lead
to oversensing
Sensitivity settings greater than 2.5 mV – Low sensitivity – can
lead to undersensing
Undersensing . . .
• Pacemaker does not “see” the intrinsic beat, and
therefore does not respond appropriately
Intrinsic beat
not sensed
Scheduled pace
delivered
VVI / 60
Oversensing
Marker channel
shows intrinsic
activity...
...though no
activity is
present
• An electrical signal other than the intended P or R
wave is detected
• Pacing is inhibited
Refractory & Blanking Periods
•
Refractory period
– Prevent lower rate timer reset due to oversensing
•
Blanking Period
– The first portion of every refractory period
– Pacemaker is “blind” to any activity and no events can be sensed
– Designed to prevent oversensing of pacing stimulus & after-potential
Lower Rate Interval
VP
Blanking Period
Refractory Period
VP
VVI / 60
Values to remember

Pacing Thresholds





Atrium - <1.5V; Ventricle <1V @ 0.5ms PW
Outputs

2 X threshold Voltage

Doubling Voltage output = 4 X Energy drain from battery
P / R wave amplitudes

P Wave > 2mV

R Wave > 5mV
Sensitivity

R / P wave > 2 X Sensitivity setting

Oversensing – Increase sensitivity ( Reduce value)

Undersensing – Reduce sensitivity ( Increase value)
Lead impedance

300 – 1200 ohms – Normal

> 3000 Ohms - lead fracture / Loose set screw / inadequate lead pin insertion

< 300 Ohms – Insulation break

If impedance is out of range – try unipolar configuration
Dual Chamber Pacemakers
Advanced type of pacemakers that
closely mimic the natural heart
Work on both – RA and RV
Usually 2 leads
Maintain AV Synchrony
AV Interval
Provide rate response
Dual Chamber Pacemakers
• DDD, DDDR – sense & pace both atrium and
ventricle
• VDD – Sense atrium, pace ventricle
The four faces of dual chamber pacing
AV SEQUENTIAL PACING Atrial Pacing Rate – 60, AV Interval – 200 ms
AV Synchronous Pacing :NATURAL ATRIAL CONTRACTION & VENTRICULAR PACING : VDD
AV Interval = 150 ms
Spontaneous Atrial Rate – 55
Spontaneous Atrial Rate – 110
The four faces of dual chamber pacing
Atrial Pacing Rate = 70, Natural AV conduction
NATURAL ATRIAL CONTRACTION WITH NORMAL AV CONDUCTION
Spontanoeus Atrial Rate = 65, Spontaneous PR interval = 160 ms
Thank You