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ECG Tutorial 1: Anatomy & Electrical System

ECG Tutorial 1: Anatomy & Electrical System

Introduction

This ECG Tutorial blog is the first in a series, designed for people who wish to learn something new or simply to refresh material already learned. It may be particularly useful to first responders, emergency medical technicians or student paramedics. During the course of this series we will be focusing on the basics of ECG along with discussing rhythm strips and 3 and 4 lead ECG's.

It's not going to turn you into a mega super ECG guru! But it will provide you with a good understanding of what exactly you are looking at on a rhythm strip and how that corrosponds to different actions within the heart. It is hoped that in the future we will also incorporate a 12 lead ECG section into this blog series. But thats a bit down the road yet. 

OK, enough talk let's kick off.

Gross Anatomy

The heart sits in the middle of the chest at a slight angle pointing downwards, to the left, and slightly anterior. Here's a cross-sectional view of the heart.b2ap3_thumbnail_Cardiac-Anatomy.jpg

The Pumping Heart

The heart consists of four main chambers: two atria situated towards the top, and two ventricles situated underneath the atria. The atria empty their chambers into their corresponding ventricles. The left ventricle pumps blood to the body (peripheral circulatory system), and the right ventricle pumps blood to the lungs (pulmonary system). Veins bring blood to the heart, while arteries take blood away from the heart. b2ap3_thumbnail_cardiovascular-system.jpg

Essentially what we have in the heart are four pumps working in sequence. The two smaller pumps (atria) whose sole purpose is to push a small amount of blood into the two larger ventricles. The ventricles differ in size and in the amount of pressure that they can generate. Because of the valves in the venous system, and within the heart, blood can only flow forward. 

Electrical Conduction

The electrical conduction system of the heart is made up of specialised cells. Some of these are for pacemaking functions and some for the transmission of the impulses that travel through them. The main function of the system is to create an electrical impulse and transmit it in an organised manner to the rest of the heart muscle (myocardium). This is an electrochemical process that creates electrical energy that is picked up by the electrodes when we carry out an ECG. 

The pacemaker dictates the rate at which the heart will cycle through its pumping action to circulate the blood. The pacemaker creates an organised beating of all the cardiac cells, in a specialised sequence, to produce an effective pumping action. The main pacemaker in the body is the SA Node (Sinoatrial node), found in the muscle of the right atrium. It responds to the needs of the body controlling the beat depending on information it receives from the nervous, circulatory and endocrine system. It fires at a rate of 60-100 beats per minute (BPM) with an average of approx 70 BPM

One thing we know about the body is that everything has a backup! Every cell in the conduction system is capable of setting the pace. However, the default rate of each type of cell is slower than the cells before it. The faster pacer cells set the pace because it causes all the ones that come after it to reset after each beat. Because of this the slower pacers will never fire. If however, the faster pacer doesn't fire, the next fastest will be there as a backup to ensure function is as close to normal as possible.

b2ap3_thumbnail_electrical-pathways.png

Default rates of pacer cells

  •  SA Node: 60 - 100 BPM
  •  Atrial Cells: 55 - 60 BPM
  •  AV Node: 45 - 50 BPM
  •  His Bundle: 40 - 45 BPM
  •  Bundle Branch: 40 - 45 BPM
  •  Purkinje Cells: 35 - 40 BPM
  •  Myocardial Cells: 30 - 35 BPM

 Conclusion

It is critically important to understand the anatomical and electrical systems of the heart. They are the core on which the rest of you ECG interperation skills are based. Without a good fundamental knowledge of these system it will be very difficult to fully breakdown and understand an ECG. 

 

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