Pacemaker

Artificial Cardiac Pacemakers

General:
- device used 2 regulate the beating of the heart by mimicking the actions of the nodes and conducting system
- successfully used to treat sick sinus syndrome (inability of the sinus node to regulate a steady heart rate due damage to the sinus node resulting in atrial rhythm disturbances) and heard heart block (atriaoventricular system, the conducting system of both fail to communicate with the ventricles). Also for patients with prolonged PR intervals, left bundle branch block, severe mitral regurgitation.

Types:
Type of pacemaker used depends on patient’s needs

o For temporary use:
§ external pacemakers for initial stabilization of a patient
- 2 pads placed on chest, one on upper portion of sternum, other along left axilla
- electrical impulse travels between pads and stimulates muscles between them including cardiac muscles and muscles of the chest wall to contract
- muscles twitch at a pre-determined rate the pacemaker is set to
- should not be relied upon for an extended period of time. If patient is conscious, frequent stimulation of chest wall muscles may cause discomfort
- also, stimulation of chest wall muscles does not necessarily indicate cardiac muscles are stimulated as well.

§ internal pacing wire
- wire that is place under sterile condition; distal tip placed into either right atrium or right ventricle, proximal tip attached to pacemaker generator outside of the body

o For permanent use (usually required for most conditions):
- made up of:
o pulse generator/battery pack implanted with a computer chip under the skin just deep to the collarbone
o coiled metal conductor leads insulated with plastic and anchored to appropriate chamber(s). inserted using X-ray control via a vein found in this area
o a bare metal electrode fixed against the heart
- pulse generator sends electical signal down the lead to the electrode and targeted heart chamber is prompted to contract.
- ‘listens’ to the heart and supplements the heart’s natural heart rate. E.g. During physical exertion, sensors in pulse generator can detect changes in breathing and boost the heart rate to appropriate level
- two types:
o single chamber: only one chamber is regulated, usually ventricles
o dual chamber: 2 leads are used. Information from atria regulates the contraction of the ventricles

Person living with an artificial cardiac pacemaker can live a normal life and still engage in moderate strenuous activity such as sex and preferably non-contact sports. Medication, mobile phones, and household electrical appliances such as microwaves are safe to use. However, MRIs should not be performed

Patients require one or two checks a year. Adjustments can be made non-invasively using a radio frequency programmer

Shanthini

Sinoatrial Node (Natural Pacemaker)

General:
- also known as the sinoatrial node
- modified myocytes
- in the right atrium, under epicardium, near superior vena cava
- self-excitable, sets the rate and timing (sinus rhythm) of other cardiac cells to contract

Physiology
- do not have stable RMP
- MP starts at -60, drifts upwards – depolarization due to slow inflow of Na+ without outflow of K+
- At -40mV, fast calcium channels open – depolarization continues till slightly above 0mv
- K+ channels open. K+ leaves the cell – repolarization.

In Short:
When SA node fires, rhythmically sets off a wave of action potentials which triggers the conduction of heart chambers. Each depolarization of the SA node sets off one heartbeat.

Saladin, Anatomy and Physiology, 4th Ed