notes by dr Claudio Italiano
The pacemaker, is a device that means " to make pace", that is a device, as small as
the battery of a cell phone of today, weighing 20 g, which is placed near the
heart and that replaces a function of the myocardium, that is, to produce the
heartbeat, that is to electrically stimulate the contraction of the heart when
this is not normally ensured by the cardiac conduction tissue. The heart, as we
know, is a muscle, but unlike other muscles of our body, it is also able to
contract itself, that is, by itself, thanks to two specific centers located in
it that are made of specific tissue.
They are constituted by a group of cells, precisely, specialized, located in the
upper part of the right atrium called "sinus node". From this tissue the
generated signal, which is essentially a natural electrical signal, of cell
depolarization, continues along routes of conduction to move further down,
towards another center called "atrio-ventricular node", from which they depart,
to in turn, two ways of conduction called His bundle and Purkinje fibers, which
have the task of bringing the signal to the ventricles (see
arrhythmias
Atrial fibrillation, a stupid and dangerous arrhythmia )
Certainly the automatic tissue of the heart must guarantee that:
-the heart is contracted regularly
-with a sufficient number of contractions, since the total volume of the blood
pumped is a function of the number of contractions for the quantity of blood
expelled at each individual contraction, ie 5-6 liters of blood per minute (cardiac
output), up to 20 liters / minute
- the frequency of contractions (heart rate) does not go down and does not rise
too much, otherwise the flow will be reduced or nothing, since in fact a heart
beating at 170 b / m, can not fill with blood in diastole, ie at the time of
pause and it's the same thing as if it were still!
For all these reasons, an intelligent doctor, the Canadian John Hopps in 1950
invented the pacemaker, that is an electronic contraption that was able to act
as "director of the orchestra of the heart", that is that he knew how to adjust
the beats, in this case of the heart, making it contract properly and taking up
the reins of the gallop, when there was the need. Basically, if the heart stops
and is blocked for 1-2 beats, here is the need for a pacemaker to intervene. On
the other hand, if the heart contracts vertically like ventricular arrhythmias (arrhythmias),
the pacemaker must intervene to resynchronize the heartbeat.
The physical dimensions of the pacemaker are very limited: weight about 20 gr.,
7x6x1 cm. It works powered by special batteries, usually lithium-iodine, which
ensure an autonomy of about 5-10 years (the figure is purely indicative since
the longevity can also vary considerably, depending on how the device is
programmed, the type, the clinical conditions of the patient, etc.) The
pacemaker is a pulse generator, a device that electrically stimulates the heart
and normalizes its rhythm; It is constituted by a casing in which a battery and
a series of electric circuits are placed and is in turn connected to the heart
by means of electric wires called leads, with which the contraction signal is
led where it serves the chambers of the contraction. , the ventricles. The
electrodes can be bipolar (anode and cathode both inside the heart) or monopolar
(cathode in the heart and anode usually constituted by the metal case of the
same pacemaker).
The circuits, the battery and the other internal components of the stimulator
are enclosed in a hermetic case made of titanium. The device is implanted, under
surgery, under the skin until, when the internal battery is exhausted, it must
be explanted and replaced with a new one.
The pacemakers, therefore, according to the stimulation catheters, are divided
into:
- MONOCAMERALS as they are connected to an electrode fixed to a single cardiac
chamber, usually the ventricle or the right atrium;
- BICAMERALS if the electrodes are two, for example, in the right atrium and the
other in the right ventricle.
However, there are also Implantable defibrillators or ICDs (Implantable
Cardioverter Defibrillator) that are capable of delivering an electrical
defibrillation shock every time this arrhythmia occurs, allowing a normal heart
rhythm to be restored. Generally a pacemaker electrically stimulates the right
ventricle, however in some particular forms of heart failure where there is a
contraction delay between the right ventricle and the left ventricle it is
possible to stimulate both ventricles (biventricular stimulation) in order to
synchronize the mechanical activity of the heart.
The intervention to place a device and the checks to be done.
The intervention is simple, implemented under local anesthesia. To place a
pacemaker there are two orders of problems:
It is necessary to incise 3 or 4 centimeters of length in the subclavicular, to
isolate a small vein, left subclavian vein or brachiocephalic and insert the
lead that is pushed until it is positioned in the cardiac cavity, atrium or
right ventricle, depending on the type of implant ( monocameral or bicameral),
following the X-ray practice (fluoroscopic control);
Incise again to create a subcutaneous "pocket", that is superficial, to place
the pacemaker.
Check the pacemaker at pre-established intervals, usually every six months; to
obtain information on the functioning of the stimulator (evaluation of any
arrhythmic events occurred), with a probe that communicates through a computer
with the pacemaker, to coordinate the variation of the stimulation parameters
according to clinical needs, control of the battery charge (generally hard 5-10
years), whose discharge is foreseeable many months in advance.
Precautions in the use of the device: autoscontro, horseback riding,
electrical interference, use of electrical appliances as long as they are
equipped with a three-pole plug with earth connection, an eye in the use of
mobile phones, which must be kept away (front in the jacket heart!), airport
(metal detectors), no possibility to perform nuclear magnetic resonance. Avoid
the use of radio or television transmitters or repeaters, radar, high voltage
lines and equipment, industrial equipment such as electric arc welding machines,
electric ovens. Do not cross areas where the "prohibition for PM carriers"
symbol is affixed.
Consult your doctor before undergoing diathermy, electrosurgery, electrocautery,
electrical cardioversion, lithotripsy.
Five letters are used for the naming of cardiac stimulators, the meaning of
which follows this pattern:
Place of stimulation: At atrium, V ventricle, D = A + V
Place of detection of autologous signal: as above
Type of function: I inhibition, T trigger (pulse originated during the
refractory period of the R wave), D = I + T
Type of programming: M multiprogrammable, R adaptable on the basis of the data
collected by the activity sensor
Antitachiarhythmia function: 0 absent, P antiarrhythmic stimulation, S
defibrillation D = P + S
VVI Stimulator: The most frequently used, indicated for bradyarrhythmia in
atrial fibrillation.
AAI Stimulator: Used in isolated deficits of sinus function with adequate
atrioventricular conduction. The patient should not have intermittent atrial
fibrillation.
Stimulation occurs if the minimum expected frequency is not met.
DDD stimulator: In a patient with AV block, the sequential double-chamber
stimulator allows intervention in case of sinus and / or nodal defaillance.
Improvement of cardiac output.
Stimulators R: VVI-R is indicated in bradyarrhythmias with atrial fibrillation.
DDI-R in diseases of both the SA and AV nodes
These allow a real-time adaptation to everyday activities.
Antitachycardia stimulators: Indicated for recurrent ventricular
tachyarrhythmias with risk of sudden death, especially ventricular fibrillation.
They act by defibrillation, that is, an electrostimulation of frequency higher
than that of the ectopic pacemaker.
Pacemakers are getting smaller and smaller, weighing only two grams and, through a vein, they can be screwed directly to the heart, thus avoiding the use of the cable that carries the impulse to the heart from the device. For now the City of Health has been implanted 3, 1300 worldwide.