Defibrillation - the working of an ICD
Patient Perspective Papers 7 and 8
These notes are a patient’s view on defibrillation and the working of an ICD. You might like to refer to the BHF and AA booklets on the subject and these are amongst the stock of information, which I bring to every meeting of the Group.
I must emphasise once again that notes such as these are very much the personal interpretation of a patient and I apologise
but he or I take no responsibility for any inaccuracy. If you are worried in any way concerning your own position, please consult
your own doctor or consultant.
Defibrillation …Pow!
Pow…somehow does not quite adequately describe the sensation of having eight hundred volts delivered directly to your heart. If one is asleep when this happens, or half asleep as I was, one is very much awake afterwards!
How did this happen, and why? Years after my heart attack I was thought to be at risk of suffering VT, ventricular tachycardia, a condition where the heart tries to beat so fast it cannot pump blood efficiently around the body. The left ventricle, the one of interest here, is the part of the heart that delivers blood to the rest of the body. VF, ventricular fibrillation has a similar effect and a similar therapy.
If one is awake and one’s heart goes into VT, or VF, there is an increasing fainting feeling because the brain is becoming starved of oxygen. An observer might see your face go pale as the blood literally drains out of the skin. Collapse happens quickly, brain damage occurs within a couple of minutes and brain death not much later.
Very quick treatment is necessary, either from medical equipment, a defibrillator that jolts the heart into its proper rhythm or from a person trained in cardiac massage that imparts an artificial pumping action to the heart by pressure on the chest.
If you are a “Casualty” fan you will recognise the scene where the heart patient is out cold, “He’s in VF. Defibrillator! Stand clear!” and with the paddles on his chest, POW!, the patient sort of jumps as a shock is delivered.
If you are in a shopping mall, cinema or a rail station they may have an AED, an Automatic External Defibrillator, and trained personal to hand. Elsewhere you will be very lucky if there is a person in sight who is trained in CPR, cardiopulmonary massage, who can render you aid.
Going into VT far from health professionals and their trusty defibrillator or a cardiac resuscitation trained person, is often fatal.
So my personal POW! was not delivered by any person. It was delivered automatically, at 3am one morning, whilst I was alone in bed. How come?
The working of the ICD
Back in May 2008 I was fitted with one of the cleverest and most beautiful pieces of medical technology in the world, an “Implantable Cardioverter Defibrillator” usually called an ICD for short.
Most of the time I forget it is there. There is this lump under the skin below my left collar bone. There is a small sensitive area where the leads from it to my heart come close to the surface. If I knock it though that’s as painful as a bang on the shin or funny bone and pressure on it, from a car seat-belt say, is uncomfortable. But one tries not to do anything that might move the ICD or its leads. That is a bad thing.
This marvellous little device, about the size of a small mobile phone, monitors my heart and tries to keep it beating regularly within set limits. In this respect it is just like a pacemaker, the difference is what happens when my heart goes outside of those set limits.
If there is a slow increase the ICD will give the heart little electric nudges to try to get it to behave. If it does not behave and goes into VT, trying to beat at maybe 260 beats per minute, the ICD gives it a slap – delivering eight hundred volts straight into the heart.
If one is fully asleep one will probably wake up with a memory that something happened. If awake it has been likened to a kick in the chest by a donkey but that is misleading. There is no residual pain (unless you bang yourself on something.). The shock does cause the body to jerk but it only lasts a small fraction of a second and is a memory almost as soon as you realise it has happened.
That is a good memory; it means that you are still alive to do the remembering!
If there is the slightest suspicion that there has been a shock event then a call to the clinic is necessary, leaving a message on the answer phone if it is out of hours. They get back first thing and usually ask you to come in to have the event checked out. If it happens again soon, or you have collapsed and hit your head, then it may be advisable to get someone to take you to A&E or call an ambulance if you live alone.
The ICD stores a memory of what has happened, recording the behaviour of the heart up to, during and after the VT and the shock. The monitor reads all this, along with other information that ensures the device is still working correctly. From those results the consultant can then twiddle with the programming and your medication to try to prevent the VT happening again.
I like my little electronic friend a lot, as the consultant said, “But for that device you would not be sitting here now.”
The only real down side is that is that one has to surrender one’s driving licence for six months. Suffering a fainting spell followed by an electric shock does not make for safe driving! If there is a subsequent event it becomes six months after that and so on. You just hope that the first shock was the last.
But, my little electronic friend has saved my life, giving up driving is a small, if inconvenient, thing in comparison. I probably need to do more walking anyway.
There was another side to the story. The technicians checked my ICD as soon as the clinic opened, before the people on the routine list. Then my consultant made room for me later that morning which made everyone else about half an hour late.
I will never complain about over-running clinics again, it may be due to another person, feeling all the concern and vulnerability that I felt, who needed that time. I can put up with reading “Country Life” or “Nature Watch” for an extra hour for that reason.
If you are offered an ICD, or even a simple pacemaker, do not hesitate. The discomfort and inconveniences are trivial compared to the alternative.
Joules* – what are they?
When the health care professionals talk about the shock that an ICD gives they often describe the strength it in terms of “Joules”; “Three hundred and sixty Joules!” shouts the doctor in Casualty as he applies the defibrillator paddles.
The Joule is the scientific unit of energy; any kind of energy, electrical, physical or even that in food.
In very round numbers one Joule is not a lot of energy, say about the amount needed to lift a medium sized apple from the floor onto a table. What we call “food calories”** is another way of talking about energy. A 10 Calorie sweet also contains about 40,000 Joules, equivalent to the energy in about 1000 shocks from an ICD. This does not seem a great deal per shock, but it’s a complicated story.
The electrical Joule is like a three-way seesaw, with the balance between voltage, current and time all determining what it does to the heart. Voltage: you know that if you put your fingers on the terminals of a 9V battery it is not going to hurt – but the muscles of your fingers will still jerk a very tiny amount, just too small to feel. Should you stick your fingers across the mains, 240 volts, you might find yourself on the floor – your muscles will have jerked a long way! And it will have hurt and might even kill you. The voltage can be thought of as “pushing” the electricity along; another term for voltage is “electro-motive force”. The higher the voltage the harder it pushes.
The current, that’s the “Amps” that the fuse in a mains plug is rated in, is how much electricity actually flows. A high current can heat things up – that’s how toasters and electric ovens work. But cook the food for too long and you risk burning it, so the time is also important.
As said above a higher voltage makes our muscles jerk further and when an ICD shocks, just as when using an external defibrillator, the patient certainly does jerk. So, as far as we are concerned it is the voltage and time, not the energy in Joules, which determines how we actually experience the shock. For a ICD that can be over 800 volts, but it lasts for less than one hundredth of a second.
It is not actually the muscular jerk that “resets” the heart to its proper rhythm, the shock “resets” the heart’s electrical system, but it certainly is that jerk that the patient experiences and is what may concern some prospective or new ICD recipients.
Others may tell you that the shock feels like anything from a punch to a kick in the chest. But if you have received such a blow you have residual pain, bruising etc. There is no residual pain from the ICD shock, it is a memory almost as soon as you realise it had happened. The only residual pain may be because, as the shock made you jerk, you banged into something.
You have to be alive to remember the shock - and that’s the whole idea! Your little electronic friend has done its job.
* James Prescott Joule, 1818 – 1889, was an English physicist and brewer who studied the relationship between heat and mechanical energy.
** “Food calories”, usually written with a capital “C”, are actually worth 1000 “real” calories (small “c”), the quantity that engineers and scientists work with. Check a packet of food and it will be marked something like “Energy: kJ - kcal (Calories): 132kJ - 31kcal”, so roughly 4 Joules per calorie or 4000 Joules per Calorie. (That’s a lot of apples to lift to keep the flab away!)
GCS/12.01.10
Dave Bailes