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06.04 Electrical Activity in the Heart

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Overview

  1. Structure and function of cardiac muscle cells
    1. Cardiac muscle cells
      1. One nucleus
      2. Cross-striations
      3. Branched cells
      4. Intercalated discs = gap junctions
        1. Rapid movement of AP’s between cells
    2. Functional syncytium
      1. ONE stimulus causes ENTIRE heart to contract
      2. “All or none” law
        1. Complete contraction or NO contraction
        2. ENTIRE heart…not just one cell

Nursing Points

General

    1. Electrical activity of heart and cardiac cycle
      1. Sinoatrial (SA) node = “pacemaker of the heart”
        1. Avg 70-80 beats per minute (bpm)
        2. Initiates APs in heart
        3. Located in Right Atrium
        4. AP sent to atrial conducting pathway then to both atria → Atrial systole
      2. Atrioventriculare (AV) node
        1. Located at base of Right Atrium
        2. Delays AP for a fraction of a second to make sure that atrial systole fully occurs
        3. Sends AP to ventricular conduction system
          1. Bundle of His
            1. Connects AV node to interventricular septum
          2. L & R bundle branches in IV septum send AP to respective ventricles
          3. Purkinje fibers
            1. Carry AP to the rest of ventricles
              1. Reaches all areas of ventricles quickly and simultaneously
        4. Ventricular systole
    2. EKG or ECG (Electrocardiogram)
      1. Basis
        1. Movements of APs create electrical current
        2. Parts of the EKG Waveform:
          1. P-wave
            1. Atrial systole
          2. PR Interval
            1. Time from SA to AV node
          3. QRS complex
            1. Ventricular systole
            2. Q-wave = downward wave right after P-wave
              1. Not always present
            3. R-wave = first upward wave
            4. S-wave = downward wave after R-wave
          4. ST segment
            1. Time between ventricular depolarization and repolarization
            2. Should be at isoelectric line
          5. QT interval
            1. Time from ventricular systole to completion of diastole
          6. T-wave
            1. Ventricular diastole
      2. Value of EKG
        1. Determine heart rate
        2. Reveals diseases or damage to any conducting pathways
      3. Arrhythmias (abnormal rhythms)
        1. Alteration of SA node rate
          1. Bradycardia — <60 bpm
            1. Normal – Well-trained athletes
            2. Normal – During sleep, activity over vagus nerve increases → HR and SA node are slowed
            3. Hypothyroidism (not enough thyroxine)
            4. Drugs → beta blockers –
              1. Excessive use → HR too slow
          2. Tachycardia— >100 bpm
            1. Normal – exercise
            2. Hypotension (low BP)
              1. HR increases to compensate
            3. Hemorrhagic anemia
              1. Internal bleeding
                1. Less oxygen delivered, therefore SA rate and HR up to compensate
            4. Drugs – excessive use:
              1. Anticholinergics inhibit vagus nerve effects on SA node
              2. Caffeine
              3. Nicotine
            5. High fever
        2. Ectopic focus – region of muscle becomes highly active → acts as an abnormal pseudo-pacemaker
          1. Causes:
            1. Ischemia to myocardium of heart (lack of blood supply)
            2. Severe emotional distress and anxiety
            3. Excessive caffeine or nicotine
          2. Effects:
            1. Single focus
              1. Premature Ventricular Contraction (PVC)
                1. Ventricle contracts early (before atria)
              2. Atrial flutter – atria contracting way faster than ventricles
                1. 300 bpm → not efficient pump
            2. Multiple foci
              1. Fibrillation
                1. Atria – no coordinated pumping action of atria → inefficient pump
                2. Ventricular – ventricles quiver, no coordinated pumping action
                  1. NOT COMPATIBLE WITH LIFE!!!
                  2. Common cause: myocardial infarction
                    1. Blood clot blocks coronary artery → ischemia → muscle tissue dies
        3. Interference of AP through ventricular conduction system
          1. Damage to:
            1. AV node
            2. Fibers of conducting system
            3. Purkinje fibers
          2. “Heart block”
            1. 2 : 1 heart block
              1. Atria beat twice as fast as ventricles because all APs are not being transmitted into ventricles
              2. More P waves than QRS complexes
              3. Inefficient pump
          3. Myocardial infarction
            1. Regions of dead muscle
            2. They don’t pass AP’s
            3. Common to ventricles (therefore slower)

References
Betts, J.G., et al. (2017). _Anatomy and physiology_. Houston, TX: OpenStax, Rice University. Retrieved from https://openstax.org/details/books/anatomy-and-physiology?Book%20details

Study Tools

Video Transcript

In this lesson we’re going to take a look at the electrical activity in the heart

So before we get started, let’s take a look at the cardiac cells.

Cardiac cells are muscle cell that is only located in the heart. They have one nucleus, they have cross striation, and they are branched cells. That means that they have specific interlocking groups. Cardiac cells also have these things called intercalated discs which help to provide synchronicity every time the heart beats. It wouldn’t make sense for the heart to beat in all sorts of crazy different ways by waiting for one action potential to go from one heart cell to another. What these intercalated discs allow the action potential to go across simultaneously.

Now there’s a concept of functional syncytium. The principle behind it is similar to the idea of one single movement across the single stimulus. It also involves the all-or-none principle, being that either all of the muscle cells contract from one action potential, or and none of them contract

So now let’s take a look at all of the electrical framing in the heart.

In the heart there are several different nodes that stimulate the contraction.

We’re going to start with the sinoatrial node or the SA node. The SA node is known as the pacemaker of the heart. It initiates the action potential. It’s located at the top of the right atrium. The action potential is sent to what’s called the atrial conducting pathway and then to both Atria. this creates atrial systole. This thing is on a timer, and it averages 70 to 80 beats per minute.

Then there’s something called the atrioventricular node or the AV node. It’s located at the base of the right atrium. Now this is different than the base of the heart. Remember the base of the heart is located at the second intercostal space, so more towards the head.

With the atrioventricular node there’s a delay in the action potential for a fraction of the second and the reason is is that it allows the heart to complete atrial systole. If not it would send that conduction down too Fast and the rest of the Heart would fire and not get adequate filling. Here it’ll send the action potential to the ventricular conduction system. There are several parts of this and the first is the bundle of His. This connects the AV node to the interventricular septum and it breaks off into the left and right bundle branches in the interventricular septum and to those respective ventricles. Then it sends out a final action potential to the rest of the purkinje fibers and that allows the ventricles to squeeze. As this action potential is sent down it creates the ventricular systole.

So how do we measure electrical conductivity in the heart? We do it with an EKG or an electrocardiogram. The most common type that you’re going to see is something called a lead to on an EKG and it gives us this diagram here. This is the P QRS complex. Now let’s break this down.

The P wave is this first hump. Then you have the big triangle that’s called the QRS complex. And then you had this thing called the T wave.

The P wave is this electrical conduction as atrial systole happens. Then there’s something called the PR interval. And it’s a measurement from the time the SA node send the action potential to the AV node. And you can see it here.

This is the QRS complex, and this is when ventricular systole happens. This is that action potential down the bundle of His, to the left and right bundle branches, and finally to those purkinje fibers and the ventricles squeeze.

The ST segment is the measurement between ventricular depolarization to repolarization. What’s this happens, the heart can get ready to fire again. And then you got the T wave which is actually ventricular diastole.

Now there a lots of benefits from the EKG. The EKG allows us to check out different types of problems at a person may have, such as diseases of conduction pathways, or even electrolyte imbalances.

Now a great function of the heart is that it can beat and Rhythm all the time. Now sometimes these rhythms get messed up and these are called arrhythmias. Now what happens is you can have alterations of the sinoatrial node.

Now there are a couple ways that this happens. The first one is bradycardia. Brady mean slow cardio refers to the heart. So specifically bradycardia is a heart Rate of Less than 60 beats per minute. It can occur normally in some people, cuz I’m one of them. My heart rate is normally anywhere between 40 and 50 beats per them. For some people that is normal especially in athletes. The reason this happens is because the heart is conditional well enough that it only needs to be 40 to 50 times per minute. However it can be caused by some abnormalities. Certain causes are hypothyroidism and certain drugs like beta blockers. Especially with excessive use that heart rate gets really slow and the heart can’t pump fast enough.

Tachycardia is the opposite. Tacky means fast, and tachycardia is a heart rate in excess of 100 beats per minute. Now this is normal in certain instances, like exercise. The other time that it happens is that it’s a compensatory mechanism, meaning that the heart rate increases to compensate for other issues. For instance if the blood pressure is too low, the heart rate will increase to help get out perfusion.

Another reason that this will happen is for something like anemia related to bleeding. If there is excessive bleeding, that means that the tissues are getting enough oxygen so the response from the body is to increase the heart rate hoping to get that blood out more quickly. They’re also a couple other reasons why there should happen. Excessive use of certain drugs like anticholinergics are one. Anticholinergics inhibit the vagus nerve stimulation. The biggest nerve is responsible for bringing that heart rate down, and if we’re inhibiting that process the heart rate stays High. Other things like stimulants such as caffeine and nicotine will also increase the heart rate. Also a high fever can bring that heart rate up.

There’s also another cause of an arrhythmia and it’s called an ectopic Focus. Basically it’s the another region of the heart becomes highly active and acts like a pacemaker. Certain reasons for this are decreased blood flow to the heart muscle, the stress or anxiety, and excessive stimulant intake like caffeine or nicotine.

Sometimes there’s one specific area of the heart that causes this pacemaker action. For instance one is called a premature ventricular contraction. What’s happening is that the ventricles are getting stimulated early before the Atria. Another time is something called atrial flutter, where the atria are contracting way faster than the ventricles are. Sometimes they can be in excess of 300 beats per minute and they are just not an efficient pump.

Sometimes you can have multiple areas that are being over-stimulated. This is called multiple coci. Fibrillation is a big one. You can have atrial fibrillation also known as afib, and it’s basically a not coordinated pumping action of the Atria. They are just not being together. Then you have another one called ventricular fibrillation, and this is a big problem. It’s not compatible with Life, there is no pump from the ventricles, and they just quiver. A common cause of this is like myocardial infarction. Basically a clot is blocking perfusion to the actual heart muscle itself, and the tissue starts to die and that creates this fibrillation.

Sometimes there can even be damage to the atrioventricular node, and this actually messes up the ventricular conduction system. You can have damage to the actual AV node, you can have damage to the fibers of the conducting system, or you can have damage to the purkinje fibers.

Sometimes there’s also an arrhythmia where the Hearts aren’t eating in unison. one example of this is a 2 to 1 heart block. The Atria beat twice as fast as ventricles because all the action potentials aren’t being transmitted into the ventricles. So you’ll get to stimulations to the SA node but it’s just not getting sit down. On an EKG you’ll see more p waves and QRS complexes and this is just an efficient pump.

The last thing that can occurs in myocardial infarction, which is a heart attack. To put it simply, a heart attack is where tissue dies as a result of lack of oxygen. The heart has a bunch of vessels that provide oxygen to it as a means of perfusion. With a heart attack, There’s an actual blockage in one of these arteries, and the tissue dies. This is where it becomes a problem with the arrhythmia. Because there is no dead muscle to continue conducting the action potential, it basically stops. And this prevents the Action Potential from passing down to a lower areas of The ventricle.
Okay so let’s recap.

We were talking about the SA node this is the sinoatrial node. This is the origination of that electrical impulse in the heart.

the AV node is atrioventricular node. This is responsible for waiting for the atrial contraction and then we’ll send it down to the interventricular fibers and down the rest of the way.

That conduction path, is the bundle of His, bundle branches, and the purkinje fibers.

The EKG is it really useful tool for determining abnormal heart rhythms and then gives more insight into what’s going on with potential underlying causes.

Arrhythmias are abnormal rhythms and they are caused by a disruption in that conduction pathway

That’s it for a lesson on electrical activity in the heart. Make sure you check out all the resources attached to this lesson. Now go out, and be your best selves today, and as always, happy nursing

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