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22. The patient has natural rhythm – analysing the ECG part 1

22. The patient has natural rhythm – analysing the ECG part 1    April 2015

Keywords: prehospital ambulance paramedic ECG

The pulse is a key part of the perfusion assessment. The cardiac output and blood pressure will be very dependent on the strength and speed of the pulse. But the pulse only reflects half of what is happening in the heart. The heart is a muscle and the pulse is the contraction of this muscle that can be felt as a wave of blood moving through the circulation. Like all muscles, an electrical impulse must occur first that then stimulates the particular muscle response. The electrical impulse can be detected on an electrocardiograph (ECG) and analysed.

Ideally the electrical impulse should have two key features. The first is that it should form a very predictable pattern to look at. The second is that this electrical pattern should be directly related to the subsequent pulse.

The first features of the predictable pattern is that when nothing is happening, a straight line simply flows out on the ECG. Movement up and down on the paper reflects actual electrical movement away from the zero. Movement along the paper reflects the progression of time.

Normal electrical impulse begins in the sinoatrial (SA) node located at the top of the right atrium. This impulse gets passed downward like a wave along specific conduction pathways that run through the atrium. These pathways, like all the normal conduction system, are the most efficient at passing on the electrical impulse to the next cells. As the impulse moves downward and is passed on to muscle cells it also spreads out to get to all of the other muscle cells of the atrium. Cardiac muscle cells are closely packed. This allows all of the atrial cells to contract much at the same time. This is important as it creates the most efficient way of ejecting blood from the atria.

As each cell becomes active and passes on the flow of electricity it is said to depolarise. We’ll come back to that. The overall flow of electricity can be seen as a deflection up or down from the zero baseline. This deflection will occur over a short period of time so will appear as a lump or wave rather than a narrow line or spike. This is the P wave.

The heart sits slightly leaning backward and slightly pointing to the left when viewed in the chest from the front. As such, this impulse from the SA node moving normally can be best seen if watching it from the right shoulder but looking down toward the left hip. The most common ECG lead to watch this unfold is lead II which just happens to be pointing in that direction.

At the bottom of the atria, impulses come back to one point, the atrioventricular (AV) node. The atria are separated from the ventricles below by a band of tissue called the coronary sulcus. This does not conduct any electrical impulse. As such all flow must be through the AV node. This allows the impulses to pass but slows them for a short time. There is a good reason for this. The delay allows time for the atria to contract. This blood will fill the ventricles. On the ECG this period of relative electrical inaction and delay will appear as more flat baseline and is known as the PR interval. It has a normal expected duration. There is a problem if it is too long or too short.

Once just the right delay occurs, the impulse is allowed to continue on. Just like the atria, the ventricles are meant to contract efficiently and together. Just like the atria there are specific pathways that allow the impulse to travel most efficiently. These are first a single bundle of His which then divides into the left and right bundle branches that travel firstly down the septum before many smaller branches fork off to circle the ventricles. These smaller branches are the Purkinje fibres.

The flow of electricity spreading throughout the ventricles is the QRS complex. There is typically a small movement in one direction on the ECG as the impulse moves down the bundles. This appears as the first deflection or Q wave. Then at the bottom (apex) of the heart, the impulse spreads out through the Purkinje fibres. This is the bulk of the QRS and moves back up the outer walls of the ventricles. This makes the QRS now appear to move in the other direction to the initial burst. This is the R part. At the end there is another small movement in the same direction as the top of the ventricle is finally reached by the Purkinjes causing the impulse to head back into the centre of the heart. The S part.

Each QRS should be preceded by a P wave and a normal PR interval each time. The QRS should be fairly narrow since all this movement through the bundle branches and Purkinje fibres happens quickly. If it is wider there is a problem with the impulse moving through the ventricles.

Finally, the electrical cells will have to recharge for the next contraction. This is called repolarisation (the initial contraction is depolarisation). Depolarisation involves an inrush of sodium into the cell that causes a change in the electrical charge of the cell membrane. There is normally a lot more sodium outside the cell than inside it. This is then maintained by a slower calcium inrush into the cells that helps prolong the impulse to cause muscle contraction. This calcium stimulates release of more calcium already inside the cell. Other nerve cells don’t have this latter factor. There is also an exodus of potassium from the cell where there is much more of it than outside. This helps to try to restore the imbalance created by the sodium and calcium. Eventually the objective is achieved.

Repolarisation then follows as with near neutrality attained, sodium and calcium is moved back out of the cell and potassium back in again. All of these are against the concentration gradient as they are moving from lower concentration into a higher one. Diffusion and osmosis cannot achieve this. This takes energy. There are gated pumps in the cell membrane that achieve this. The gate opens inside of the cell membrane and sodium moves in. Energy from ATP then causes the gate to close inside the cell and open on the outside allowing the sodium to leave. It then allows potassium to enter while it is open. This then in turn re-enters the cell as the gate is closed on the outside and reopened where it started back inside the cell. This is the sodium/potassium pump. Eventually the energy is reset back to be ready to depolarise again.

Repolarisation of the ventricles appears on the ECG as the T wave since it too is a flow of electrical current. There is repolarisation of the atria as well but this is usually lost with the bigger QRS appearing at the same time.

The SA node usually runs the whole show. Its ability to depolarise and repolarise is fastest so it is always ready to go first. It can initiate an impulse without any other instruction. To guide its activity, input from the autonomic nervous system and hormones such as adrenaline make it go faster or slower. Parasympathetic nerves slow it up and sympathetic make it go faster. Its normal rate of firing is between 60 and 100 per minute.

So to analyse the ECG, we first analyse the rhythm. Print off a nice long ECG strip over at least twenty seconds if not longer. Start with the flat nothing happening baseline. The QRS will usually be the tallest up or down deflection of it so identify it first. There may be more than one type so look for the one that is the narrowest and most ‘normal’ looking. Is it nice and narrow or wider than it should be? A different ECG lead may be needed to find the best view. Then look for the other similar QRS beats. Are they regular and neatly spaced or all irregular? If irregular, is there any pattern to it? You may need to get a piece of paper and hold it under the rhythm strip to tell. Make a few marks on the paper where the QRS complexes are. Then move it one complex to the left or right. Then again and again. If the QRS complexes keep lining up with the marks, it is regular. If not, it is not regular. Really fast and really slow rates can begin to look regular even when they are not so assess carefully.

How fast are they going? That is, what is the heart rate? Then compare this to the pulse that can be felt. Is the electrical rate matching the contracting pulse? Note any difference. Back to the rhythm. Are the QRS complexes related to a P wave out the front? All of them or only some or does it change? Is the P wave the right distance in front of the QRS or not? Finally identify the T wave. Make sure you can tell it apart from the QRS complexes, particularly if the QRS complexes are wider than normal or there are QRS complexes with different shapes. Finally, is the T wave the right distance from the QRS in front and not taking too long to occur?

Jeff Kenneally – www.prehemt.com

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