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    I just heard on city hospital that a heart attack (I assume mycocardial infarction?) and cardiac arrest are not the same thing. Apparently you can have a heart attack but not a cardiac arrest and vice versa. So what are the differences between these?
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    In a heart attack, a coronary artery is blocked by a clot so blood and oxygen can not get to part of the cardiac muscle.
    Cardiac arrest is where the heart stops beating completely - ie loss of the mechanical function of the heart. This may be caused by a heart attack, or merely an abnormal heart rhythm.
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    Yes, myocardial infarction (heart attack) may not lead to cardiac arrest (stopping of heart beat) if it's minor.
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    oh equally, cardiac arrest does not have to be caused by a MI, can be other things like fibrillation
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    (Original post by darkenergy)
    oh equally, cardiac arrest does not have to be caused by a MI, can be other things like fibrillation
    So fibrillation can lead to a cardiac arrest (stopping of the heart?). How does that happen?

    I was under the impression that the heart couldn't be completely stopped, but instead would go into fibrilation....which is why a defibrilator would be used to intentionally stop the heart so that sympathetic innervation restarts the heart in synchrony again?
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    (Original post by nikk)
    So fibrillation can lead to a cardiac arrest (stopping of the heart?). How does that happen?

    I was under the impression that the heart couldn't be completely stopped, but instead would go into fibrilation....which is why a defibrilator would be used to intentionally stop the heart so that sympathetic innervation restarts the heart in synchrony again?
    Well it is completely stopped by fibrillation in the sense that the overall mechanical action of the heart is stopped. Yes, there are still electrical signals being transmitted in an irregular fashion but there is no overall beating of the heart, which is cardiac arrest.
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    (Original post by oxymoron)
    Well it is completely stopped by fibrillation in the sense that the overall mechanical action of the heart is stopped. Yes, there are still electrical signals being transmitted in an irregular fashion but there is no overall beating of the heart, which is cardiac arrest.
    Ah I see what you mean. So a minor heart attack caused by a partial block of a branch of the coronary artery might simply damage a small proportion of cardiac muscle, whereas a large heart attack could induce fibrillation i.e. cardiac arrest.

    And in the same manner, fibrillation may not be the result of a heart attack (so the artery is not blocked, but fibrillation is caused by another mechanism).

    Is that right? Thanks for explaining!
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    (Original post by nikk)
    Ah I see what you mean. So a minor heart attack caused by a partial block of a branch of the coronary artery might simply damage a small proportion of cardiac muscle, whereas a large heart attack could induce fibrillation i.e. cardiac arrest.

    And in the same manner, fibrillation may not be the result of a heart attack (so the artery is not blocked, but fibrillation is caused by another mechanism).

    Is that right? Thanks for explaining!
    Yes, as I understand it, that is correct. I do not claim to be a medic though so I am happy to be proven wrong by someone who knows better!
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    (Original post by nikk)
    So fibrillation can lead to a cardiac arrest (stopping of the heart?). How does that happen?

    I was under the impression that the heart couldn't be completely stopped, but instead would go into fibrilation....which is why a defibrilator would be used to intentionally stop the heart so that sympathetic innervation restarts the heart in synchrony again?
    I *think* fibrillation leads to cardiac arrest beacuse of K+ release (? from lysis of dead cardiac muscle cells), this leads to rapid contraction of heart muscles, as the rhythm of conduction pathway is messed up.
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    Cardiac arrest is defined as pulseless unconsciousness - the heart can still be beating, and is caused by either electrical or mechanical defects of the heart, it can be any one of 4 basic arrest rhythms:

    Shockable: Ventricular Fibrillation (VF) and Ventricular Tachycardia (VT)
    Non-shockable: Pulseless Electrical Activity (PEA, aka Electromechanical dissociation, EMD) and Asystole

    Shockable rhythms often respond to defibrillation wheras the non-VT/VF rhythms do not, and as a result have a poorer prognosis.

    VF is when the electrical activity of the heart totally loses synchronicity and sinus rhythm - it looks like scribble on an ECG/heart trace, rather than the normal PQRST complex (see http://www.biologydaily.com/biology/...KG_complex.png)

    Tachycardia is basically when the heart is beating at greater than 120bpm. VT is when this fast beating originates from the ventricles (as opposed to the atria in a normal rhythm) - and can be diagnosed on ECG as 'broad-complex tachycardia' (the QRS comples lasts longer than 0.12ms

    PEA/EMD is when the electrical system of the heart is working normally, so PQRST is seen on ECG. However the heart is not effectively pumping blood, so there is no pulse.

    Asystole is when there is no electrical activity at all in the heart (i.e. 'flatliner')

    Reversible causes of cardiac arrest are below (NB this is not an exclusive list of ALL causes, but does include the most common of all causes of cardiac arrest, ie not just reversible):

    Hypovolemia (ie low blood pressure due to low blood volume, often due to shock)
    Hypothermia
    Hyper/Hypokalemia (either too much or too little potassium - causing malfunction of depolarisation/repolarisation of neurones)
    Hypoxia (ie not enough oxygen delivery to the tissues of the heart, as occurs in MI)
    Thromboembolism (causing a pulmonary emoblism, which will cause congestive right-sided heart failure and hypoxia)
    Tension pneumothorax (collapsed lung which leaks air into the thorax, causing increased pressure on the heart, limiting it's ability to pump)
    Toxins/Drug overdose
    Cardiac Tamponade (where the heart leaks blood into the pericardial sack that surrounds it, causing pressure buildup, and arrest)
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    (Original post by darkenergy)
    oh equally, cardiac arrest does not have to be caused by a MI, can be other things like fibrillation
    Just to clarify, fibrillation does not cause cardiac arrest, it is a form of cardiac arrest.

    (Original post by nikk)
    Ah I see what you mean. So a minor heart attack caused by a partial block of a branch of the coronary artery might simply damage a small proportion of cardiac muscle, whereas a large heart attack could induce fibrillation i.e. cardiac arrest.

    And in the same manner, fibrillation may not be the result of a heart attack (so the artery is not blocked, but fibrillation is caused by another mechanism).

    Is that right? Thanks for explaining!
    The second part is, however the first part is a bit wrong as even damage to a small part of the myocardium could induce fibrillation or another arrest rhythm
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    (Original post by oxymoron)
    Well it is completely stopped by fibrillation in the sense that the overall mechanical action of the heart is stopped. Yes, there are still electrical signals being transmitted in an irregular fashion but there is no overall beating of the heart, which is cardiac arrest.
    Fibrillation is the ineffective beating of the heart, so it can still technically be pumping, just not in a manner that allows blood to ciculate.

    Electrical activity without any beating is PEA (see above post). Lack of beating and electrical activity is Asystole

    (Original post by darkenergy)
    I *think* fibrillation leads to cardiac arrest beacuse of K+ release (? from lysis of dead cardiac muscle cells), this leads to rapid contraction of heart muscles, as the rhythm of conduction pathway is messed up.
    Fibrillation can be caused by apoptosis of dead myocardium due to potassium release
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    Wow, thanks for all the information! I don't need to know half of that kind of detail, but it is interesting nonetheless.

    Just to clarify, if fibrillation is a form of cardiac arrest, how would you define cardiac arrest? (i.e. what are the exact properties of cardiac arrest and how does it arise?) Thanks.
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    Fibrillation leads to cardiac arrest not a form of it?
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    (Original post by nikk)
    Wow, thanks for all the information! I don't need to know half of that kind of detail, but it is interesting nonetheless.

    Just to clarify, if fibrillation is a form of cardiac arrest, how would you define cardiac arrest? (i.e. what are the exact properties of cardiac arrest and how does it arise?) Thanks.
    See the start of my first post, caridac arrest is the cessation of electical, mechanical (or both) activity of the heart, and is a clinical diagnosis of unconsciousness and pulselessness
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    (Original post by darkenergy)
    Fibrillation leads to cardiac arrest not a form of it?
    No, as I said, VF is a form of cardiac arrest. AFAIK atrial fibrillation can lead to an arrest, but ventricular fibrillation is technically already an arrest (though VF can progress to further arrest, commonly asystole, and failure of resuscitation)
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    (Original post by j00ni)



    PEA/EMD is when the electrical system of the heart is working normally, so PQRST is seen on ECG. However the heart is not effectively pumping blood, so there is no pulse.



    Sorry to be a picky ******* - but pqrst is not always seen on the ECG trace with a PEA arrest. The electrical activity may not be "normal", but sufficient to normally have a cardiac output - i.e - atrial fibrillation, various types of heart block and these variations are commonly seen in PEA.

    Hope that makes sense. Was nowhere near as good an explanation as your posts.
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    (Original post by Ataloss)
    Sorry to be a picky ******* - but pqrst is not always seen on the ECG trace with a PEA arrest. The electrical activity may not be "normal", but sufficient to normally have a cardiac output - i.e - atrial fibrillation, various types of heart block and these variations are commonly seen in PEA.

    Hope that makes sense. Was nowhere near as good an explanation as your posts.
    I know exactly what you mean, and you are absoloutely correct, i just purposely left it out so as not to cause confusion. Perhaps a better wording would be that the ECG in PEA will show a normal (for the patient) rhythm or that the electrical activity alone is not abnormal enough to cause arrest
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    (Original post by j00ni)
    I know exactly what you mean, and you are absoloutely correct, i just purposely left it out so as not to cause confusion. Perhaps a better wording would be that the ECG in PEA will show a normal (for the patient) rhythm or that the electrical activity alone is not abnormal enough to cause arrest
    Yup - you put it much better.
 
 
 
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