Refractory periods

Pacemaker Timing in Single and Dual
Chamber Devices
Haran Burri, MD
University Hospital of Geneva
CME refresher course on Pacing, ICD and CRT
19.11.2014
Luzern
Agenda
•
•
•
•
•
•
•
General aspects
Single chamber timing cycles
Dual Chamber timing cycles
Hysteresis
Refractory periods
Upper rate behavior
Miscelaneous
2
The NBG Code
Position
I
II
III
Category Chamber(s) Chamber(s) Response
Letters
paced
sensed
to sensing
O= None
A=Atrium
V=Ventricle
D=Dual
S=Single*
O= None
A=Atrium
V=Ventricle
D=Dual
S=Single*
O=None
T=Triggered
I=Inhibited
D=Dual
IV
V
Rate
Modulation
Multisite
Pacing
O=None
R=Rate
Modulation
O=None
A=Atrium
V=Ventricle
D=Dual
* Manufacturer’s designation only
3
3
Converting Rates to Intervals
and Vice Versa
• Rate (bpm) to interval (ms):
– 60.000/rate (in bpm) = interval (in milliseconds)
– Example: rate 100 bpm → interval 600 ms
• (60.000/100 bpm = 600 ms)
• Interval (ms) to rate (bpm):
– 60.000/interval (in milliseconds) = rate (in bpm)
– Example: interval 500 ms → rate 120 bpm
• (60.000/500 ms = 120 bpm)
4
Agenda
•
•
•
•
•
•
•
General aspects
Single chamber timing cycles
Dual Chamber timing cycles
Hysteresis
Refractory periods
Upper rate behavior
Miscelaneous
5
Single chamber pacing - VVI
Chamber paced
Chamber Sensed
Response to sensing
Rate modulation
V=Ventricle
V=Ventricle
I=Inhibit
O=None (not active)
escape interval
escape interval
< escape interval
escape interval
< escape interval
Inhibit &
reset
counters
Pace &
reset
counters
Pace &
reset
counters
Inhibit &
reset
counters
Inhibit &
reset
counters
6
Differences between AAI and VVI
p
R
{
V Refr Per
A Refr Per
 Sensitivity value: Atrial < Ventricle
Refractory period: Atrial > Ventricle
Agenda
•
•
•
•
•
•
•
General aspects
Single chamber timing cycles
Dual Chamber timing cycles
Hysteresis
Refractory periods
Upper rate behavior
Miscelaneous
8
Dual chamber timing cycles
Chamber paced
Chamber Sensed
Response to sensing
Rate modulation
D=Dual
D=Dual
D=Dual
(inhibit+trigger)
O=None
<AVI
AVI
<AVI
VA interval
VRP
URI
AVI
VA interval
VRP
URI
VA interval
VRP
URI
VA interval
VRP
URI
In DDD, every time there is atrial activity – PACED OR SENSED – the device will start
ARP PVARP
ARP PVARP
ARP
ARP
PVARP
PVARP
an AV interval
9
AV interval
Paced AV delay
The P-wave starts at
the beginning of the
spike
Sense Offset
PAVD
Sensed AV delay
The P-wave starts
before the sensing of
the signal
AVD
Similar actual AV delay with atrial pacing or atrial sensing!
10
DDI pacing mode
May be used during modeswitching or to avoid ventricular pacing in sinus rhythm
Maintains AV synchrony if atrial pacing
Loss of AV synchrony if atrial sensing and AV block
Only paced AV delay can be
programmed (not sensed AV delay)
=VA interval
11
Agenda
•
•
•
•
•
•
•
General aspects
Single chamber timing cycles
Dual Chamber timing cycles
Hysteresis
Refractory periods
Upper rate behavior
Miscelaneous
12
Hysteresis
• Allows the intrinsic rate to go below the
programmed lower rate interval (hysteresis
interval)
• Favours intrinsic activation
• More physiological as long as the slower rates are
acceptable and not symptomatic for the patient
(e.g periods of rest)
• May help increase device longevity
• Both for single as well as dual chamber devices
13
Hysteresis
Heart Rate
Hysteresis Off
Lower rate
Hysteresis rate
Time
Heart Rate
Hysteresis On
Lower rate
Hysteresis rate
Time
Spontaneous rhythm
Pacing
14
Agenda
•
•
•
•
•
•
•
General aspects
Single chamber timing cycles
Dual Chamber timing cycles
Hysteresis
Refractory periods
Upper rate behavior
Miscelaneous
15
T-wave oversensing
LRI
LRI
LRI
LRI
T-wave oversensing
Actual pacing interval >> LRI
16
Far Field R-wave (FFRW)
oversensing
inappropriate mode switch
indicates Far Field R-waves
17
Sensing unwanted signals…
Far-field oversensing
≠
Cross-talk
Surface ECG
A EGM
V EGM
As
Vs
As
As Vs
As
Ap Vp
Vs As
Ap Vp
Vs As
AV crosstalk is potentialy lethal in a patient with complete AV block !
18
Refractory periods
• Absolute Refractory Period
– Also known as BLANKING period
– Initial portion of the refractory
period, usually very short
– Signals will be not be detected
– During this period of time, the
sensing channel is not active
– Forces the pacemaker to blind (in
one or both chambers) after a
pacing impulse, avoiding cross-talk
and far field oversensing
• Relative Refractory Period
– Signals will be “seen” but will not
reset timers
– Forces the pacemaker not to react
to unwanted intrinsic signals, e.g.
T waves and retrograde P waves
Refractory Period
Absolute
Relative
19
PVARP
• PVARP = Post Ventricular Atrial Refractory Period
• Typical value of PVARP = 270-310 ms (275 ms nominal)
• PVARP starts in the atrial channel after a paced or sensed ventricular
event
• An atrial event in the PVARP will not start the AV delay timer
• PVARP helps to prevent retrograde P-waves of being sensed and
tracked, thereby preventing Pacemaker Mediated Tachycardias
Atrial Channel
AS
VP
20
PVAB
•
•
•
•
•
•
PVAB = Post Ventrcular Atrial Blanking
PVAB is the first part of the PVARP
PVAB starts in atrial channel after paced or sensed ventricular event
Typical value of PVAB is e.g. 150 ms
During PVAB atrial events are blanked
PVAB helps to prevent Far Field R-wave oversensing
Atrial Channel
AS
VP
21
Ventricular Refractory Period
• A period in the ventricular channel following a paced or
sensed ventricular event during which the sense amplifier will
not respond to incoming signals
• To prevent T-wave oversensing or double counting of wide
QRS complexes
• Typical nominal value of Ventricular Refractory Period is e.g.
250 ms
Atrial Channel
Ventricular Channel
AP
VP / VS
22
Ventricular Refractory Period
Vref: 320 ms
Vref: 350 ms
Avoidance of T-wave oversensing
23
Ventricular blanking
• Blanking of the ventricular channel in response to
atrial or ventricular pacing (not for atrial sensing)
• Primarily intended to prevent cross-talk: detection of
the atrial or ventricular output by the ventricular
sense amplifier
Atrial Channel
Ventricular Channel
AP
VP / VS
24
VVI timing cycles and refractory
periods
LRI
VRP
LRI
VRP
URI
B
LRI
VRP
VRP
LRI
< LRI
VRP
B
URI
URI
< LRI
URI
VRP
URI
URI
Lower Rate Interval – defines the minimum heart rate
Upper Rate Interval – defines the highest pacing rate
Ventricular Refractory Period – prevents from restarting a new LRI when sensing the T wave
B Blanking Period– forces the device to be blind to the pacing spike
25
DDD timing cycles and
refractory periods
<AVI
AVI
<AVI
VA interval
VA interval
VRP
AB – Atrial
blanking
VA interval
URI
URI
ARP PVARP
AVI
ARP
PVARP
URI
URI
ARP
VRP
PVAB – Post ventricular
atrial blanking
VA interval
PVARP
VRP
PAVB – Post atrial
ventricular blanking
ARP
PVARP
VRP
VB – Ventricular
blanking
AVI: atrioventricular interval
URI: upper rate interval
PVARP: post-ventricular atrial refractory period
VRP: ventricular refractory period
26
Summary of DDD timing cycles
AS
AP
VS
VP
Usual duration (ms)
A Blanking




50-200
A Refractory




120-150 (post AP); 250-400 (post VP)



20-50 (PAVB); 150-250 (post VP)


150-300
V Blanking
V Refractory
Blanking and refractory periods may vary according to :
- manufacturer
- model
- paced or sensed event
- programmed pacing polarity
Aims:
Blanking: avoid crosstalk (=condition when pacing in one channel is sensed as
intrinsic activity in another channel) and oversensing of the afterpotential
PVARP: avoid sensing of FFRW and of retrograde P-waves
VRP: avoid sensing of T-waves
27
Agenda
•
•
•
•
•
•
•
General aspects
Single chamber timing cycles
Dual Chamber timing cycles
Hysteresis
Refractory periods
Upper rate behavior
Miscelaneous
28
Upper Rate Behavior
• Upper rate behavior relates to
– Programmed Upper Tracking Rate (UTR)
– Total Atrial Refractory Period (TARP)
• TARP = AV delay + PVARP
• Determines the maximum UTR
29
2:1 block
– Occurs when PP intervals are shorter than TARP
ARP
PVARP
ARP
PVARP
TARP
A
S
TARP
A
R
SAV
A
S
TARP
A
R
SAV
V
P
ARP
A
S
SAV
V
P
V
P
30
30
2:1 Block
Example
Sensed AV Delay = 150 ms, PVARP = 275 ms, therefore TARP = 425 ms
2:1-blockpoint: 60000 / 425 = 141 bpm
Stimulated
ventricular rate
160
140
120
100
80
60
Sensed
Atrial Rate
80
100
120
140
160
31
Ventricular Rate
MTR
Upper Rate Behavior
UTR
LR
1:1 Response
LR
= Ventricular Pacing
Wenckebach
window
UTR
2:1 Response
MTR
Atrial
Rate
32
32
Wenckebach
Wenckebach behaviour
• Each AS (P-wave) is followed by an increasing SAV, and then the VP
• Eventually an atrial beat is not tracked, and a ventricular beat is dropped
Produces gradual change in tracking rate ratio
Occurs if UTR interval > TARP interval
33
33
Wenckebach Behavior Example
• Dual Chamber pacemaker
– UTR = 100 bpm (600 ms)
– Sensed AV delay = 150 ms, PVARP = 250 ms
– TARP = 150 + 250 = 400 ms (150 bpm)
• As a result
– atrial rates >100 bpm (600 ms) but < 150 bpm will show
Wenckebach behavior
– Max AV delay prolongation is 200 ms (600 – 400)
– Sensed AV delay ranges from 150 - 350 ms
• Wenckebach window is UTR interval minus TARP = 200 ms
34
Achieving a Higher UTR without
Block
•
•
•
•
•
Decrease SAV interval
Program rate-adaptive AV intervals
Decrease PVARP
Program PVARP to “auto” (decreases with increasing HR)
Increase upper tracking rate
AVI
PVARP
TARP
A
S
A
R
35
35
Agenda
•
•
•
•
•
•
•
General aspects
Single chamber timing cycles
Dual Chamber timing cycles
Hysteresis
Refractory periods
Upper rate behavior
Miscelaneous
36
Rate adaptative pacing (R)
Indicator
Sensor
Algorithm
Respiration
Activity
Intra-cardiac impedance
Impedance
Quartz
Variable capacitance
Mathematical formula that
converts sensor data into heart
rate (pacing rate)
37
Modeswitching
The device switches from a tracking mode (DDDR) to a
non-tracking mode (DDIR or VDIR) at the start of an atrial
arrhythmia
38
Ventricular Safety Pacing
If a VS event follows shortly (e.g. within 110ms) of an AP event, this may
potentially be crosstalk
 triggers a VP at the end of the venricular safety pacing window
39
39
Endless loop tachycardia
A
V
Ap Vp
Causes:
Ap Vp
Atrial
Vs As Vp As Vp As Vp As Vp As Vp As Vp As Vp As Vp As Vp As
- non-capture
- overdetection
- under-detection
VPB
Long AVI
40
Endless loop tachycardia
termination – PVARP increase
To prevent conduction of retrograde P waves, the device increases the PVARP. The
P wave, falling inside the PVARP won’t be conducted to the ventricle, terminating the
pacemaker-mediated tachycardia
PVARP
PVARP
PVARP
VA interval
As Vp
As Vp
As Vp
VA interval
Ar
41
Noise Reversion
• Continuous refractory sensing will cause asynchronous pacing at
the lower or sensor driven rate
Lower Rate Interval
Noise Sensed
VP
SR
SR
SR
SR
VP
SR=sense refractory
42

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