Conduction of electrical activity through a healthy heart is highly organised and predictable. Heart muscle is conductive, unlike other muscles it can carry and transmit it's own action potential (electrical signal). There are tracts of nerves in the heart which act to regulate how the heart depolarises (makes an electric signal).

The Sinoatrial (SA) node is the primary intrinsic pacemaker of the heart. This node is where all the signals originate in a healthy heart. It ticks away at a rate between 60 and 100 beats-per-minute (bpm). If the SA node fails, another part of the heart will take over however, as you move further down the heart the intrinsic rate becomes slower.

After the SA node fires, the action potential moves down the internodal tracts and conducts through the atrial tissue. The action potential can't cross from the atria to the ventricles because of a layer of connective tissue so it is funnelled to the AV node.

At this point the heart takes a rest. Functionally this is to allow passive filling of the ventricles with blood. Electrically this is to maintain a coordinated depolarisation. After 0.12 to 0.2 seconds the AV node fires and depolarises the bundle of His.

This bundle splits into two branches, the left bundle branch and the right bundle branch. These in turn send signals up the Purkinje fibres and across the tissue of the ventricles.

The Going Rates

So if the SA node drops out for some reason, another part of the heart takes over. The table below shows the intrinsic rate of each part of the cardiac conduction system.

Node/Nerve	Rate
SA Node	60-100bpm
Internodal Tracts	55-60bpm
AV Node	45-50bpm
Bundle of HIS	40-45bpm
Bundle Branches	40-45bpm
Purkinje Fibres	30-40bpm

The rates get slower down the heart so that lower parts of the heart don't fire off prematurely before the higher ones, but they can if they need to.

Next up:

A Guy Called Einthoven

Willem Einthoven (In • doe • ven) was born in the golden era of electricity and science. He designed a non-invasive way to measure the electrical activity of the heart using buckets of saline, huge electromagnets, a tiny conductive wire and photosensitive paper. Modern ECGs have replaced the buckets with electrodes and the rest with electrical amplifiers and a CPU, but the design remains practically the same.

Limb Leads and Such

Lead	+ Electrode	- Electrode
I	Left Arm	Right Arm
II	Left Leg	Right Arm
III	Left Leg	Left Arm
aVL	Left Arm	(Right Arm + Left Leg)/2
aVF	Left Leg	(Right Arm + Left Arm)/2
aVR	Right Arm	(Left Arm + Left Leg)/2

The following story was told to me by one of our backup crew-members on after a big case, some of the details may not be accurate.

We were coming to the close of a long shift, no breaks, only two coffees and 11 hours of rubbish cases. We were called out to back-up the firies at a hotel. Apparently a male in his 30s had fallen 4 stories onto a glass railing after a few drinks. When we got there the firies had ceased CPR, and we soon found out why. The patient had managed to slice his occiput (the back of his head) clean off on the glass railing and brain matter was freely flowing from the massive wound on his head. My partner and I breathed a sigh of relief. "We won't have to pound on the chest of a doomed man" I thought to myself. We had a brief chat about the case, what paperwork needed to be done and, as with all gruesome cases, we made some light hearted comments.

"Talk about being glassed"
"Yeah, he must've been out of his mind drunk"
"Probably blind drunk now" (The part of your brain at the back deals with vision)

These comments weren't witty or particularly funny but when you're faced with mortality you cope however your can.

We liaised with the senior police on scene who said the wife of the patient needed to be assessed, she was "freaking out". On the way upstairs we knew she would be upset, but we remained distanced from the situation, after all this was a drunk idiot who brought this on himself.

From the doorway of the hotel room the somber scene hit me like a punch in the face. The scene of the gently weeping widow sitting alone on the couch, framed by the doorway to the balcony her husband had just fallen from. We sat down and began to unravel the events leading up to the fall.

She was celebrating with her husband, no event in particular just a romantic getaway. They had drunk a bottle of wine between them. Her husband was leaning over the balcony to try and attach a string to the bottom outside when suddenly he was dangling over the edge holding on to the railing by one hand. The wife ran over to him and tried desperately to pull him back over, but not having the strength she decided to run back into the hotel room and call for help. By the time she turned back, he had fallen. She didn't hear a scream, she didn't see him fall and she didn't get to say her last good-byes.

I've been to a lot of traumatic events. I've calmed a lot of family members. But something about this particular story struck a chord with me. These days I like to indulge my curiosity and get a full story before I judge a situation. Sometimes the most effective way to resolve a case in my mind is to give dignity where dignity is due.