Biomedical Engineering Reference
In-Depth Information
max
min
MSR
MSR
Accelerometer
[min
-1
]
[min
-1
]
Blended profile
Minute ventilation
LRL
Time
LRL
Low
High
Fig. 8.10
Blended pacing pro fi le
Activity
Fig. 8.8
Response factor parameter
and the device can, an electrical field modulated by breath-
ing is created across the thorax. The device detects the volt-
age between the distal electrode and indifferent electrode
placed on the pacemaker header. The respiration curve is
then processed to measure the total volume.
For the activation of the MV sensor, the system needs to
measure the basic level of MV at rest. Increase of MV above
the basic level value, caused by increased metabolic needs,
will be detected by the pacemaker, and the pacemaker will
translate it into a pacing rate increase using an algorithm. The
relation between the detected increase in the MV and final
acceleration of pacing is determined by the response factor
parameter. Pacemakers allow the slope to be programmed
even beyond the anaerobic threshold, simulating the physio-
logical relationship between MV and heart rate. This parameter
- the response factor beyond the anaerobic threshold - is deter-
mined as a percentage of the normal response factor.
MSR
[min
-1
]
LRL
0
min
Time
max
Fig. 8.9
Recovery time parameter
The effect of individual adaptive-rate parameters and their
specific settings may be estimated rather relatively, based on
their change from the nominal values of the devices. These
were defined as optimal in clinical studies, and their good
applicability can be expected in most patients. Today's devices
also have algorithms for setting an automatic pacing profile.
8.4.3
QT Interval
As a consequence of accelerated heart rate and the impact of
catecholamine, in particular noradrenaline, the QT interval is
shortened proportionally. However, the QT interval of a
paced cardiac cycle is also shortened. Therefore it is neces-
sary to pace the ventricle to make it possible for the device to
measure the QT interval, and the occurring QRS-T interval
can serve for pacing rate management. The subsequent elec-
trical response is filtered. This metabolic sensor increases the
pacing proportionally according to the noradrenaline level. It
can thus respond even to mental activity and changes in the
body's position. Nevertheless, considerably delayed onset of
response, long recovery, merely ventricular use, and relating
permanent pacing [64] can be seen as disadvantages.
8.4.2
Minute Ventilation
The use of respiration properties is another possibility of car-
diac rhythm management. MV is a product of respiration
frequency (breaths per minute) and tidal volume. The heart
rate is linearly related to the MV up to the anaerobic thresh-
old. At exercise levels beyond the anaerobic threshold, the
relation is still approximately linear, but at a reduced slope.
The relationship between both slopes is different in individ-
ual patients and depends on various factors such as sex, age,
and exercise frequency and intensity. An MV sensor for car-
diac rhythm management measures transthoracic impedance.
During inspiration, the transthoracic impedance is high; dur-
ing expiration, it is low. To measure this impedance, a certain
manufacturer's system applies a measuring current pulse
every 50 ms between the pacemaker can and the lead proxi-
mal electrode. This pulse is a flat signal with a low amplitude
that does not distort the surface ECG, even though excitation
waveforms can be detected and depicted in certain ECG
devices. During the flow of current between the electrode
8.4.4
Sensor Combination
Some systems allow two sensors - the accelerometer and the
MV or the accelerometer and the QT interval - to be com-
bined. If two sensors are selected for adaptive-rate pacing,
the signals from both sensors are combined, and an average
pacing profile is produced (Fig.
8.10
).
