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Flatline and defibrillator (Part II)

Last time, I started talking about how on TV shows, defibrillating the heart with the wired-up paddles always works beautifully, after the heart monitor has gone flat, emitting only a mournful beeeeeep.


When you're resuscitating people, heart problems are harder than breathing problems - both to fix, or even just to diagnose. It took until the 1st Century AD, for Pliny to describe his fellow Romans dying suddenly from heart problems.

The next big jump in knowledge happened in 1775, when Abildgaard (the Dutch Veterinarian and Physician) experimented with chickens. He showed that, by using electric shocks from a Leyden Jar, he could first stun the chickens, and then revive them. In 1850, Drs. Hoffa and Ludwig showed that they could excite the heart of a mammal into Ventricular Fibrillation (VF), and the animal would die. I'll explain VF shortly.

In 1911, Hoffman recorded the first ECG of a human as he suffered VF. In 1947, a Dr. Beck corrected Ventricular Fibrillation in a 14-year-old boy with Alternating Current (AC) directly to the heart muscle - which involved cutting open the chest.

In 1956, Dr. Zoll was able to correct VF with Direct Current (DC) electricity to the chest wall. Defibrillation for VF had finally arrived. The first advantage was that DC was much easier to deal with than AC. Second, it was much better, now that they did not have to cut open the chest wall.

But VF (Ventricular Fibrillation) is just one of several electrical heart problems that can happen.

Sometimes the electrical signals in the heart stop entirely. This is called "asystole". The ventricles stop contracting, and (of course) the heart stops pumping blood. The ECG shows a flat line. This is the close-up you usually see on TV.

Sometimes the electrical signals become uncoordinated. This is called ventricular fibrillation (VF). In VF, the individual tiny muscles in the ventricles keep beating, but they are not coordinated. If you cut open the chest and look at the heart, it's just like a bag full of wriggling worms. When one tiny muscle contracts, the one next to it might relax, so the blood just shifts around inside the ventricle, and doesn't leave the heart. I guess that it's like a car with a powerful engine, spinning the wheels on a slippery surface. There's lots of energy being used up, but it's all wasted internally. The ECG now shows a very chaotic rhythm, and again the heart stops pumping blood.

Another kind of electrical heart problem is that sometimes the heart beats very quickly. This is called ventricular tachycardia (VT). Again, the heart doesn't pump any significant blood into the arteries. This is because the blood doesn't have time to get into the ventricles - the filling time is too short. In VT, the ECG shows a very fast rhythm.

VF (Ventricular Fibrillation) and VT (Ventricular Tachycardia) are grouped together, because the treatments are very similar.

Now here's the important part. In both VF/VT and asystole, the heart stops pumping blood. But the treatments are very different.

In VF/VT, the remedy is the famous jumper cables and paddles - the defibrillator (though some ambos call it a "Packer Whacker", after Kerry Packer donated defibrillators to the NSW Ambulance Service). The paddles are really just electrodes. They are placed so that the heart is roughly between them. So when you hit the Go Button, the paddles deliver about 300 joules of electrical energy in a very short period of time - about 4-12 ms. The electricity passes in through one electrode, the skin, the heart and then out through the skin to the other electrode. The electrical shock delivered to the heart inside is similar to gently slapping a hysterical person on the face to bring them back to their senses. The electrical shock stops the rapid chaotic electrical activity - the VF/VT. Then, hopefully, a natural pacemaker, the Sino Atrial node in the heart restarts in its regular rhythm, and 80 ml of blood squirt out every second.

Defibrillation works only if there is already electrical activity going on in the heart.

In asystole, the ECG shows a flat line because there is no electrical activity going on, and the treatments are not very effective. They involve the full gamut of life support, including CPR and drugs. Blasting the heart with electricity is one treatment that definitely does not work.

One 1990 study in Sydney analysed some 1,300 cardiac arrests (heart attacks). In about 40% of the cases, the patients had VF or VT. In about 30% of cases, they had asystole. Only 97 left the hospital alive, and of these, all except 3 had VF - that is, some kind of electrical activity in the heart, when the doctors saw the patient.

So the treatment for asystole (flat line ECG) is not the defibrillator - and no matter what you see on TV, if it's asystole, it ain't good.

Tags: health, pseudoscience

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Published 08 August 2005

© 2017 Karl S. Kruszelnicki Pty Ltd