9. Cardiac pacemakers
The humble cardiac pacemaker has been around so long that, like heart bypass surgery, it is prone to being taken for granted. Since the first pacemaker was devised in 1958 with the intention of managing severe bradycardia there have been a continual series of innovations and progressions in its capabilities. Today’s pacemaker has a range of options with some perhaps little known to first responders and paramedics.
First, some basic revision may help to start with. A pacemaker is a small battery powered device that is implanted into the left or right upper chest or the upper abdomen. The left upper chest is most common. If you know someone has one implanted you will be able to feel the small lump under the skin. A small opening under a muscle is made and the implanted device connected to the heart by guiding electrode wires through a vein and allowing them to float down and attach into the right atria and/or the right ventricle. The option chosen will depend on the problem being treated. If the wire attaches to the atrium, this part of the heart can continue to contract and contribute to filling the ventricle. Attaching to the ventricle as well then allows this chamber to contract as well. Typically life will be ten years but technology is sophisticated enough now for the device to be ‘interrogated’ in hospital periodically to check on it.
Once implanted, the device will simply live in the background of the recipient’s life and go about its normal business. But what is its business and, more importantly, what implications do ‘pacemakers have for unsuspecting emergency medical responders’?
The first basic role of the pacemaker is to add in missing heart beats. The original and still the most common indication for a pacemaker is when the underlying heart rhythm fails, either occasionally or completely, to produce sufficient heart beats. The pacemaker can then be called upon to fill in on either those occasions or even if needed to take over completely. To allow that the pacemaker has the ability to perform certain functions. These are sensing, pacing and capture.
Sensing is the ability for the pacemaker to know what the heart is doing by itself. If the pacemaker is to fill in the missing gaps, it must first be able to tell where these gaps are. This is called sensing. By being able to detect the spontaneous activity of the heart ensures that the pacemaker doesn’t discharge on top of the hearts own efforts. It also ensures that the pacemaker can add heartbeats at just the right time to help maintain a normal heart rate. Pacing is just what it sounds like – once the device has ‘sensed’ a missing heart beat it then sends its own stimulus instead. What occurs is a short electric impulse through the wires attached to the heart. On an ECG trace this appears as a straight vertical line and is known as the pacemaker spike. The final feature is capture. Once the pacemaker spike is sent, the critical part is that the spike then causes the heart muscle to respond by contracting. Whilst the heartbeat cannot be seen on the ECG, the electrical activity that travels the heart causing contraction can be seen. This is the equivalent of the normal QRS. When a heartbeat follows the spike it is said to have been ‘captured’. All heart beats are the result of connection between electric stimuli, whether natural or artificial, and the following muscle response.
Pacemakers were once fixed to provide a pre-determined heart rate. This has changed significantly with pacemakers now capable of great variation in how fast they can run the heart. Some have the ability to measure what is going on within the recipient and can pick up or slow down to suit. One example is the ability to monitor the levels of oxygen and carbon monoxide. Another is to measure the QT interval. This can shorten with catecholamine release indicating increasing stress such as exercise. This allows recipients to exercise or change activity and have their heart rate change to suit just like a normal heart would. It is not uncommon to feel the pulse of a patient with pacemaker in action and feel it well over 100 beats per minute.
Further to this, some pacemakers have the ability to pace very fast if they detect an over rapid underlying heartbeat. This will be a short run of ‘over-ride’ pacing seeking to retake over control and re-set the normal heart rhythm up. Feeling the pulse of such a patient might show occasional short runs of rapid pulse. Without an ECG it will be unable to tell if it is the pacemaker or another abnormal heart rhythm though.
A big concern for recipients of pacemakers is the impact sources of electromagnetic interference have on the device. For most people this will be none at all. There are loads of dangers in hospitals including MRI scans and diathermy and cauterising devices. Hospital staff will be prepared to deal with these. Outside though is much safer with the risk of cooking appliances, metal detectors and the like all being safe in normal life activity. About the only notable recommendation is to keep the mobile phone more than a few centimetres from the pacemaker device.
One big innovation in pacemakers is the implantable defibrillator. For those who manage sudden cardiac arrest the value of rapid defibrillation will be well known. The implantable defibrillator is the Rolls Royce of pacemakers and provides a mini defibrillator wherever the patient goes. They are undoubtedly successful and have saved many lives. There are safety concerns with defibrillation so are there any with these? Firstly, the electricity delivered is very small in comparison – the wires are attached directly to the heart and don’t have to push electricity through the chest wall as well. Though the patient may jump with discharge or yell out if awake, others will be safe if touching the patient. The routine wearing of gloves is more than enough protection though you are still likely to jump from reflex each time the patient does. The defibrillator may go off just once or may continue to periodically discharge. It depends on the hearts electrical activity. Though the report of devices going off is not rare for recipients people should still be treated as though they have just had a significant event and transported to a suitable hospital for follow up. If the patient is in pain from the device going off pain relief is a good idea. What is really needed is to take away the reason causing it to go off such as an antiarrhythmic.
What if the patient is in cardiac arrest and there is a pacemaker? This is an important question for first responders. Firstly, even with a pacemaker in place patients can still suffer cardiac arrest. Still start CPR as per usual guidelines and apply the external defibrillator. If it recommends shock then do so. Not everybody will have an internal defibrillator and even when there is one it will not do all the work. Pacemakers do not like having electricity applied through them so it is important to make sure the defibrillation pads are not placed directly over the device. If you see or feel the pacemaker lump the pad is placed a few centimetres above, below or to either side of it. If the pacemaker is also a defibrillator continue to manage the cardiac arrest with it sometimes going off. This is okay just continue to try to manage the arrest as if wasn’t there. If it does go off it is best to wait one further minute before you apply defibrillation through your own device.
There are occasional times when pacemakers can fool automatic defibrillators. Sometimes analysis mode can mistake pacing spikes for heart beats even if the patient is in VF. You cannot do anything about this without a monitor screen so don’t worry about what you cannot fix. Just follow the analysis prompts as usual and continue with effective CPR until a monitor with a screen is available. These occasions will be infrequent. It will be careful to assess monitor screens where available to assess for this.