The specified stimulation apparatuses also include in particular implantable cardiac pacemakers. They are usually connected by way of an electrode line to an electrode arranged in a heart, and adapted to deliver electrical stimulation pulses to the heart by way of the electrode. Those stimulation pulses serve to excite the cardiac tissue or myocardium and, depending on the respective kind of pacemaker, are delivered in particular when the heart does not contract in a natural fashion. In that case contraction is caused by electrical stimulation of the cardiac tissue.
Such an electrical stimulation pulse must involve a stimulation intensity which is above a respective stimulation threshold. In that respect the stimulation threshold is a measurement in respect of the minimum stimulation intensity which is sufficient to cause dipolarisation of the myocardium and thus contraction of a respective chamber of the heart. The stimulation threshold depends on various factors and in addition it is also variable under some circumstances in the course of time.
Besides the requirement to deliver a stimulation pulse of adequate stimulation intensity, there is a need for the energy to be applied for a stimulation pulse to be kept as low as possible. That energy is usually taken from a battery of the pacemaker, which becomes exhausted in the course of time. When that battery is exhausted the pacemaker has to be replaced by a new one by means of an operation.
There is therefore on the one hand the requirement that the stimulation intensity of a stimulation pulse must be sufficient to trigger contraction of the cardiac tissue. In that respect the stimulation intensity depends on the one hand on the duration of a stimulation pulse and on the other hand on the strength of a stimulation pulse. The strength of a stimulation pulse in turn depends on the electrical voltage with which a stimulation pulse is delivered to the cardiac tissue. This means that a greater energy consumption is usually linked to a greater stimulation intensity.
On the other hand there is the need for the energy consumption per stimulation pulse to be kept as low as possible as that energy is taken from a battery of the pacemaker, which becomes exhausted in that way. When the battery of the pacemaker is exhausted an operation is required to replace the pacemaker or the battery. Therefore, a reduction in the level of energy consumption of the pacemaker entails a longer service life for the pacemaker.
There is therefore a need to satisfy the requirements for a level of stimulation intensity which is as low as possible and at the same time a stimulation effect which is regularly successful, by optimisation of the stimulation intensity. In that respect, it is known from the state of the art, for example from U.S. Pat. Nos. 5,350,410; 5,411,533; 5,431,693 and 5,674,254, after delivery of a stimulation pulse, for the stimulation outcome (capture) to be detected (capture recognition), in order to trigger a backup stimulation pulse at least in the event of defective stimulation outcome.
In comparison with the known cardiac pacemakers with capture recognition and adaptation of stimulation intensity, there is in particular the wish for a lack of stimulation success to be recognised as quickly as possible in order to be able to deliver a backup stimulation pulse as quickly as possible in relation to the previously delivered stimulation pulse.
That involves the problem of reliable and rapid stimulation outcome monitoring.