as used herein , the term “ pulse ” refers to a single occurrence of an electrical signal that has a defined shaped and period . as used herein , the term “ waveform ” refers to a repeating electrical signal that may include one or more pulses . the pulses that make up the waveform may be the same or may differ in any one of shape , polarity , duration and amplitude . for example , a biphasic waveform may include an anodal pulse and a cathodal pulse . the anodal and cathodal components may differ only in polarity or may be differ in shape , polarity , duration and amplitude . pulses making up a waveform may differ in shape , polarity , duration , and amplitude but be equivalent in power . as used herein , the term “ sub - threshold waveform ” refers to a waveform that does not result in stimulating the heart to beat . a waveform may be sub - threshold because the amplitude of the waveform is below an amplitude threshold value necessary to stimulate a heartbeat . a waveform may be sub - threshold because the duration of the waveform is below a duration threshold value necessary to stimulate a heartbeat . a waveform may be sub - threshold because the power of the waveform is below a power threshold value necessary to stimulate a heartbeat . as used herein , the term “ pacing waveform ” refers to a waveform that stimulates a heartbeat , results in depolarization and is by definition equal to or greater than a threshold necessary to simulate a heartbeat . fig1 shows a representative tracing 10 of electrical activity in a typical heartbeat . a p wave 11 represents the wave of depolarization that spreads from the sa node throughout the atria . a period of time from the onset of the p wave to the beginning of a qrs complex is known as the p - r interval 12 . the p - r interval 12 represents the time between the onset of atrial depolarization and the onset of ventricular depolarization ( typically lasting 20 - 200 ms ). if the p - r interval is & gt ; 200 ms , there is an av conduction block , which is also known as a first - degree heart block if the impulse is still able to be conducted into the ventricles . a qrs complex 13 represents the period of ventricular depolarization , which normally occurs very rapidly ( e . g ., typically lasting 80 - 120 ms ). if the qrs complex is prolonged , conduction is impaired within the ventricles . the isoelectric period ( st segment 14 ) following the qrs complex 13 is the period of time ( typically lasting 80 - 120 ms ) at which the entire ventricle is depolarized and roughly corresponds to the plateau phase of the ventricular action potential . the st segment 14 is important in the diagnosis of ventricular ischemia or hypoxia because under those conditions , the st segment 14 can become either depressed or elevated . fig2 is a schematic representation illustrating a multi - phase cardiac stimulus generator 120 implanted in a patient according to an embodiment . in an embodiment , one or more sensors sense rhythm and contractions of the patient &# 39 ; s heart 105 using at least one of atrial sensing and ventricular sensing , such as at least one of atrial sensor 110 and ventricular sensor 112 . the atrial sensor 110 and / or ventricular sensor 112 provide sensor data to a rules engine 122 . in an embodiment , the rules engine includes a processor 126 and a memory 124 for storing rules and receiving sensor data . the rules engine 122 may poll the one or more of the atrial sensor 110 and the ventricular sensor 112 to obtain sensor data and to apply the rules to the sensor data in order to determine whether to deliver electrical waveforms to one or more electrodes , and , if electrical waveforms are to be delivered , which of the one or more electrodes is to receive the electrical waveforms . in an embodiment , the one or more electrodes may be an atrial electrode 114 and a ventricular electrode 116 , and may provide electrical waveforms to at least one of an atrial chamber and a ventricular chamber of the heart 105 . the multi - phase cardiac stimulus generator 120 may generate an anodal waveform , a cathodal waveform , and a biphasic waveform above or below threshold depending on the sensor data and the rules applied by the rules engine 122 . in an embodiment , the memory 124 of the rules engine 122 of the multi - phase cardiac stimulus generator 120 is configured to store one or more anodal waveforms , cathodal waveforms , and biphasic waveforms . a waveform or a combination of waveforms may be selected from the memory 124 by the processor 126 based on sensor data and based on rules also stored in memory 124 . in an embodiment , the memory 124 may also store information about the patient 100 . the processor 126 may further select a waveform or a combination of waveforms from the stored waveforms based on the sensor data and data about the user . in an embodiment , the stored waveforms comprise waveform data that are used by the multi - phase cardiac stimulus generator 120 to produce waveforms for applying to the heart . fig3 is a schematic representation illustrating a cardiac and vagus nerve stimulation device according to an embodiment . fig3 includes elements from fig2 and additionally includes a vagus electrode 310 for stimulating the vagus nerve 305 . in an embodiment , the vagus nerve 305 is accessed in the neck . the carotid sheath is dissected and the vagus electrode 310 is an encircling electrode that is wrapped around the vagus nerve 305 to receive electrical stimulation . in an embodiment of the present invention , the one or more of atrial sensor 110 and ventricular sensor 112 provide sensor data that indicates the onset of tachycardia . in response to the heart sensor data , the rules engine 122 may apply an electrical waveform to the vagus nerve 305 via the vagus electrode 310 . the rules engine 122 monitors the sensor data to determine when a sinus rhythm has been reestablished in the cardiac tissue . if a sinus rhythm has been reestablished in the cardiac tissue , the rules engine 122 halts the stimulus to the vagus electrode 310 . if a sinus rhythm has not been reestablished in the cardiac tissue , the rules engine 122 continues the stimulation of the vagus nerve 305 . in an embodiment , the electrical waveform may be low frequency or high frequency electrical signal or a waveform made up of trains of electrical pulses . in an embodiment , the electrical waveform is a biphasic waveform . in another embodiment , the stimulation of the vagus nerve may be combined with the application of a sub - threshold waveform to the heart as previously described . a system and method for reducing stroke work in an artificially paced heart have been disclosed . it will also be understood that the invention may be embodied in other specific forms without departing from the scope of the invention disclosed and that the examples and embodiments described herein are in all respects illustrative and not restrictive . those skilled in the art of the present invention will recognize that other embodiments using the concepts described herein are also possible . further , any reference to claim elements in the singular , for example , using the articles “ a ,” “ an ,” or “ the ” is not to be construed as limiting the element to the singular .
a rules engine acquires sensor data from sensors applied to the heart to determine the presence of a rapid heartbeat . the rules engine applies rules to the sensor data to determine whether to deliver an electrical waveform to a vagus nerve . the rules engine further determines whether an electrical waveform should be applied to the heart and , if so , the type of electrical waveform .