Pacemakers are often used to correct erratic heart rhythm, most notably abnormally slow heartbeat or brachycardia. People with brachycardia have heartbeat anywhere below 60, while on normal people, it is between 60 to 100. SA (sinoatrial) node is a natural pacemaker, and when it doesn’t work, brachycardia can happen. When the signals from the node are slow, heart can’t respond properly to the higher physical demands induced by stress or exercise and it is often called as sick sinus syndrome. Although this condition is actually part of a normal aging process, heart with slow beat rate may not be able to circulate enough blood. Common symptoms of brachycardia are activity intolerance, fatigue and in some cases, unconsciousness.
Pacemakers can provide a solution by providing enough electrical stimulus whenever SA node doesn’t work properly. Pacemakers are also often used to treat "heart block", which happen when electrical signals between ventricles (lower chamber) and atria (upper chamber) stop or are significantly slowed. AV (atrioventricular) node is the area where the signal travels. Without external stimulus, ventricles can only produce between 20 to 40 beats per minute (bpm), which is not enough to sustain steady blood flow throughout the body. This condition can also cause lingering fatigue and unconsciousness. People with brachycardia and heart block may eventually suffer from atrial defibrillation; very inefficient and fast atria contraction. When pacemaker detects this abnormal rhythm, it will fire certain electrical impulses to bring things under control. Once the defibrillation ceases, the pacemaker reverts to its normal mode.
Two main components of pacemakers are pulse generator and leads. A pulse generator is consisted of a digital processor, some electronic circuits and battery, which are enclosed in a small titanium case, while the electrical impulses are emitted by leads. The generator is equipped with a sensor to determine whether the heart works properly and the impulses are generated when the heart rate is erratic. Leads are made from double insulated and flexible wires that are inserted within the chambers of the heart so electrical impulses can directly reach the target area. There are two types of leads:
• Unipolar, if the tip of the lead is negative and the pacemaker case is positive.
• Bipolar, if both the positive and negative poles are on the tip of the lead, however, pacemakers with bipolar nodes are more vulnerable to external interference.
A pacemaker may only have one node and it is called as single chamber pacemaker. The lead can be placed in either right ventricle or the right atrium and effective for rectifying erroneous signals from VA node or SA node. Unfortunately, on some people, single node pacemakers may cause faintness, chest fullness and neck throbbing each time the device fires. This condition is called pacemaker syndrome. Double-node pacemakers are usually more favored because they eliminate these symptoms and also can pump larger amount of blood.
Another type of pacemaker is "rate-responsive". It is a more improved device, which can sense physical activity and adjust heart rate accordingly. Rate-responsive pacemakers use a number of sensitive sensors, to monitor blood temperature, motion and; rate and depth of breathing. The pacemaker is programmed to react to changes to any of these indicators, by increasing the firing rate, as the result, the device can more closely mimic normal heart behaviors.
Nowadays, surgeons can perform minor surgical procedure to install a pacemaker and it takes less than an hour. The procedure should be performed by the electrophysiologist, however many surgeons and cardiologists can also do task competently and efficiently. Patients are only given local anesthesia and a tiny incision is made, usually under the collarbone. The surgeon will then insert the lead(s) through the subclavian vein. To get a visual of the process, the doctor uses fluoroscopy. The pacemaker is connected with the heart surface by a tiny corkscrew (active fixation) or tined tip (passive fixation). Eventually a layer of blood protein (fibrin) covers the connection within 6 weeks of procedure and the pacemaker is attached more firmly. To speed up the recovery process, a pacemaker is inserted on left side for right-handed patients and vice versa.
Cardiologists who are trained to diagnose and treat heart conduction system problems are called electrophysiologist. It is preferable to have your pacemaker installed by an electrophysiologist and if possible, he should oversee the device programming tasks. During a surgery, he is assisted by an anesthesiologist and nurses. Occasionally, pacemakers manufacturers send a representative during the implantation and programming phases.
The pulse generator can also be embedded just under the chest skin, this will allow easier examination using an external device known as PSA (pacemaker system analyzer). In the recovery room, the pacemaker is fine-tuned using a programmer. On latest models, the pacemaker and the programmers can communicate wirelessly. Technicians check for two most important indicators, capture and sensing.
Capture involves the device ability to fire electrical signals with accurate pulse width and voltage. The pacemaker should be able to deliver 2 or 3 times the minimum voltage needed. On the other hand, if the capture is too high, the battery can be drained too soon and the device must be replaced early.
Sensing refers to the ability to detect signals from the patient’s heart and we can safely consider pacemakers as an internal, implanted electrocardiography unit. Single chamber pacemakers are more likely to have problems with sensing. To get proper sensing, technician need to adjust the pacemaker so any intra-cardial signal can still be seen at the highest voltage possible.
There are a number of environmental conditions that should be considered, as they can affect the normal functioning of the device:
• Powerful electromagnetic fields, for example arc-welding tools
• Contact sports, such as wrestling or football
• Shooting high-powered rifle on shoulder
• Mobile phones
• Some medical instruments, such as MRI (magnetic resonance imaging)
These tools are usually safe for pacemakers:
• Microwave oven only affects unshielded, older pacemakers
• Airport security equipments, some metal detector alarms can be set off, as the result you need to bring a card or document stating that you have a pacemaker implanted in the chest.
Generally, if the patient’s metabolic condition, drug intake and heart are stable, the pacemaker should be examined every two months, a special device is waved over the patient’s chest and certain signals are received by the technician. If there are changes in physical conditions or medications, the pacemaker can be adjusted using a programmer, internal settings can be changed remotely using a wand. Changes in metabolic conditions and drugs can affect pacemakers’ indicators, including its capture and sensing thresholds. For example, glucocosteroids or ephedrine can lower thresholds while anti-arrhythmics drugs can cause higher thresholds. Hypocapnia (carbon dioxide deficiency) and hyperoxia (too much oxygen in the system) can lower thresholds, while acid accumulation (acidosis) and base accumulation (alkalosis) in the body, can increase the thresholds. Reprogramming involves changes on capture and sensing values. If an examination indicates that the battery is almost depleted, a replacement procedure is scheduled. Because the battery is sealed inside the case, the whole signal generator will be replaced. The leads are attached to the new generator.