Biomedical Engineering Reference
In-Depth Information
Fig. 9.
Postganglionic vagal stimulation in the rabbit sinus node. The chrontropic effect of ten
stimuli locked to the spontaneous cycle is shown in the
top panel
left
. The
bottom panel left
shows the same response in the presence of 3 µM atropine. The
right panel
shows the
electrotonic potential during diastole. There is a 15-20% decrease of this potential during vagal
stimulation, which can only be explained when membrane resistance has decreased. This
implies that vagal stimulation increases the overal membrane conductance. This is more
compatible with an increase of
I
K-Ach
than with a decrease of
I
f
in response to vagal stimulation.
Compiled from [18].
I
K-Ach
, not closing of
I
f
in response to physiologically relevant vagal stimulation. This
supports my view that it is fortunate that the biological pacemaker of Rosen et al. [53]
(this issue) is solely based on
I
f
current.
A relatively high intrinsic heart rate based on a biological pacemaker, without the
possibility of deceleration (e.g. during rest or sleep) may also be useful from the point
of view that cardiac muscle with suboptimal contractile performance has impaired
capacitance to compensate low heart rate by a high stroke volume. For this reason the
management of recipients of a transplanted heart aims at maintaining a relatively high
resting heart rate, if possible above 100 beats/min [9].
7 Conclusion
Biological pacemakers have a long way to go before they will ever be superior to
electronic pacemakers. There is, however, important progress. It seems sufficient
