Editing Pacemaker (section)

== Basic function ==

[[File:Biotronik Philos SR Single-chamber cardiac pacemaker 29.01.21 JM.jpg|thumb|Single-chamber VVIR/AAIR pacemaker]]
[[File:Boston Scientific Altrua 50 Dual-chamber cardiac pacemaker 29.01.21 JM (2).jpg|thumb|Dual-chamber DDDR pacemaker]]

=== Pacing modes ===
Modern pacemakers usually have multiple functions. The most basic form monitors the heart's native electrical rhythm. When the pacemaker wire or "lead" does not detect heart electrical activity in the chamber – atrium or ventricle – within a normal beat-to-beat time period – most commonly one second – it will stimulate either the atrium or the ventricle with a short low voltage pulse. If it does sense electrical activity, it will hold off stimulating. This sensing and stimulating activity continues on a beat by beat basis and is called "demand pacing". In the case of a dual-chamber device, when the upper chambers have a spontaneous or stimulated activation, the device starts a countdown to ensure that in an acceptable – and programmable – interval, there is an activation of the ventricle, otherwise again an impulse will be delivered.

The more complex forms include the ability to sense and/or stimulate both the atrial and ventricular chambers.

{| class="wikitable" style="margin: 1em auto 1em auto"
|+ {{anchor|mode}}The revised NASPE/BPEG generic code for antibradycardia pacing<ref>{{cite journal |vauthors=Bernstein AD, Daubert JC, Fletcher RD, Hayes DL, Lüderitz B, Reynolds DW, Schoenfeld MH, Sutton R | s2cid = 12887364 | title = The revised NASPE/BPEG generic code for antibradycardia, adaptive-rate, and multisite pacing. North American Society of Pacing and Electrophysiology/British Pacing and Electrophysiology Group | journal = Pacing Clin Electrophysiol | volume = 25 | issue = 2 | pages = 260–64 | year = 2002 | pmid = 11916002 | doi = 10.1046/j.1460-9592.2002.00260.x }}</ref>
! I || II || III || IV || V
|-
| Chamber(s) paced || Chamber(s) sensed || Response to sensing || Rate modulation || Multisite pacing
|-
| O = None || O = None || O = None || O = None || O = None
|-
| A = Atrium || A = Atrium || T = Triggered || R = Rate modulation || A = Atrium
|-
| V = Ventricle || V = Ventricle || I = Inhibited || || V = Ventricle
|-
| D = Dual (A+V) || D = Dual (A+V) || D = Dual (T+I) || || D = Dual (A+V)
|}

From this the basic ventricular "on demand" pacing mode is VVI or with automatic rate adjustment for exercise VVIR – this mode is suitable when no synchronization with the atrial beat is required, as in atrial fibrillation. The equivalent atrial pacing mode is AAI or AAIR which is the mode of choice when atrioventricular conduction is intact but the [[sinoatrial node]] of the natural pacemaker is unreliable – sinus node disease (SND) or [[sick sinus syndrome]]. Where the problem is [[atrioventricular block]] (AVB) the pacemaker is required to detect (sense) the atrial beat and after a normal delay (0.1–0.2 seconds) trigger a ventricular beat, unless it has already happened – this is VDD mode and can be achieved with a single pacing lead with electrodes in the right atrium (to sense) and ventricle (to sense and pace). These modes AAIR and VDD are unusual in the US but widely used in Latin America and Europe.<ref>{{cite journal |vauthors=Böhm A, Pintér A, Székely A, Préda I | title = Clinical Observations with Long-term Atrial Pacing | journal = Pacing Clin Electrophysiol | volume = 21 | issue = 1 | pages = 246–49 | year = 1998 | pmid = 9474681 | doi = 10.1111/j.1540-8159.1998.tb01097.x | s2cid = 23277568 }}</ref><ref>{{cite journal | author = Crick JC | title = European Multicenter Prospective Follow-Up Study of 1,002 Implants of a Single Lead VDD Pacing System | journal = Pacing Clin Electrophysiol | volume = 14 | issue = 11 | pages = 1742–44 | year = 1991 | pmid = 1749727 | doi = 10.1111/j.1540-8159.1991.tb02757.x | s2cid = 698053 }}</ref> The DDDR mode is most commonly used as it covers all the options though the pacemakers require separate atrial and ventricular leads and are more complex, requiring careful programming of their functions for optimal results.

Automatic pacemakers are designed to be over-ridden by the heart's natural rate at any moment that it gets back to a non-pathologic [[Sinus rhythm|normal sinus rhythm]] and can reinitiate influencing the electric activity in the heart when the pathologic event happens again.<ref>{{Cite book|url=https://books.google.com/books?id=EWVo9DT2-A4C&q=%22Demand+pacemaker%22&pg=PA322|title=Rapid Interpretation of EKG's: An Interactive Course|last=Dubin|first=Dale|date=2000|publisher=Cover Publishing Company|isbn=978-0-912912-06-6|language=en}}</ref> A "[[ventricle (heart)|ventricular]]-demand pacemaker" produces a narrow vertical spike on the [[ECG]], just before a wide [[QRS]]. The spike of an "[[atrial]]-demand pacemaker" appears just before the [[P wave]].<ref>{{Cite web|url=https://www.uptodate.com/contents/ecg-tutorial-pacemakers|title=UpToDate|website=www.uptodate.com|access-date=2018-09-10}}</ref>

Comparably, a '''Triggered Pacemaker''' is activated immediately after an electrical activity is commenced in the heart tissue by itself. A "ventricular triggered pacemaker" produces the impulse just after a pulse is created in the ventricular tissue and it appears as a simultaneous spike with QRS. An "atrial triggered pacemaker" is the mode in which an impulse is produced immediately after an electrical event in the atrium. It appears as a [[Electric discharge|discharge]] following the p wave but prior to the QRS which is commonly widened.<ref>{{Cite book | url=https://books.google.com/books?id=GXOQbDsCyLoC&q=%22Triggered+pacemaker%22&pg=PA421 |title = Basic and Bedside Electrocardiography|isbn = 978-1-4511-4791-9|last1 = Baltazar|first1 = Romulo F.|year = 2012| publisher=Lippincott Williams & Wilkins }}</ref>

=== Threshold ===
The heart is composed of an excitable tissue and an electrical pulse of a certain voltage is needed to "capture" the myocardium to make the heart function. The minimum voltage to achieve this is called the threshold. As this is a probabilistic property, the actual voltage used is higher than the threshold (usually by a 50–100% margin). Additionally, the electrical pulse has a time component and proper description of a threshold is a voltage and pulse width.

All electrical circuits require a complete connection between the cathode and the anode. For some pacing leads, both connections to the heart are provided in a single lead ("bipolar") and some only provide a single connection ("unipolar"). In unipolar, the second connection is internally from the heart to the generator through the body (blood, tissue, etc).

Also important is the [[electrical impedance|impedance]]. The lower the impedance, the more current is needed to achieve the threshold voltage and lowers the battery life. The impedance is affected by the integrity of the pacing lead and the electrode-tissue interface of the lead with the heart.

=== Sensing and sensitivity ===
In important aspect of pacemaker function is sensing the intrinsic heart activity. This is called sensing. Larger sensed voltages are desired and it is expected that the ventricle produces larger voltages than the atria.

The minimum voltage to sense an event is called the sensitivity. The higher the sensitivity, the less that is sensed, and vice-versa. Too low of a sensitivity can cause troubles with sensing P waves, T waves, and noise; sensing these things is called "over sensing". A sensitivity too high may result in missed sensing of P waves in the atria and QRS in the ventricles, and is called "under sensing."