Biomedical Engineering Reference
In-Depth Information
RATE RESPONSIVENESS
The pacemakers described so far attempt to maintain the ventricular rate over a mini-
mum,
fixed programmed rate. However, most modern pacemakers have sensors that feed
algorithms that attempt to determine what the heart rate should be from moment to
moment. These sensors measure variables that are connected to the patient's activity
level and/or emotional state, making it possible to calculate whether a heart rate higher
than the programmed base rate should be maintained to supply the body with blood dur-
ing times of physical and/or emotional stress. These pacemakers are said to be rate
responsive and are identi
fi
fi
ed by the letter “R” following the operational mode (e.g.,
DDD-R).
Rate-responsive pacemakers make use of di
erent sensor technologies to determine the
optimal heart rate. The goal is to control the heart rate of a pacemaker patient in a manner
similar to the intrinsic heart rate of a healthy person with a normally functioning heart,
under various conditions of rest and exercise, in a physiologically appropriate manner. The
most common type of sensor used in pacemakers today is one that detects body movement.
The more the patient is moving, the faster the heart rate should be. The level of activity or
motion of the patient is picked up by an accelerometer or a piezoelectric crystal “micro-
phone.” Filtering and signal processing are applied to the signals detected by activity sen-
sors to reduce the e
ff
ff
ects of disturbances unrelated to exercise which would otherwise
a
ect the heart rate.
Another widely used method for rate responsiveness involves estimating a respiration
parameter called
minute ventilation
(MV). Minute ventilation is the air volume being
expired by the patient during 1 minute. Most minute ventilation sensors use the principle
of impedance plethysmography, where the electrical impedance of the lung tissues is mon-
itored and the changes in electrical impedance are interpreted as changes in the volume of
air in the lungs. Higher impedance typically results from more air and less blood in the
lungs following inhalation. Conversely, lower impedances result from less air and more
blood in the lungs due to expiration.
Despite advances in the technology, activity sensors are inherently limited to detecting
physical activity, leaving the pacemaker unresponsive to emotional stresses, which also
require added hemodynamic support. In addition, activity sensors also have to make strong
assumptions regarding what the patient's rate should be after exercise has stopped but
when the patient still needs high levels of blood
ff
flow to let the tissues recover. For this rea-
son, the very latest commercial pacemakers combine an activity sensor with an MV sen-
sor to get the rate response closer to that of a healthy person.
There is still place for further developments in sensor technology for pacemakers. This
is not only because better response to emotional stresses is desirable, but because the
heart's own state should be considered in the calculation of rate. As such, it would be desir-
able to tune the parameters of a pacemaker to improve it hemodynamic state: namely,
forces the heart has to develop to circulate blood through the cardiovascular system. The
hemodynamic state of the heart is represented by the relationship between blood pressure
and blood
fl
ow.
One promising hemodynamic sensor technology that we have worked on is the meas-
urement of intracardiac impedance. In essence, impedance signals derived from electrodes
attached to the heart contain information regarding the volume of blood held by the heart
as a function of time. Intracardiac impedance measurements have been used to estimate the
heart's stroke volume. The stroke volume of the heart is de
fl
fi
ned as the volume of blood
expelled by the ventricle in a single beat. It is equal to the di
ff
erence between the end dias-
tolic volume (volume of blood to which the heart is
filled when it relaxes) and the end sys-
tolic volume (volume of blood remaining in the heart when it reaches maximum
fi
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