What really happens to the heart during cardiac arrest and why is an AED useful?
I'm interested in studying medicine in the future and this question is really bothering me. So most sources would say that Cardiac Arrest occurs when the heart 'stops functioning normally' and hence cannot pump blood effectively to our organs, including the brain which causes us to lose consciousness and breathing. We also know that a defibrillator cannot be used to restart a heart that has no rhythm, only to reset a chaotic one. So why then, is an AED useful if cardiac arrest causes the heart to stop? (assuming that that's what happens during cardiac arrest...) Does the rapid CPR cause it to start having some sort of rhythm again? Or is it that cardiac arrest actually doesn't STOP the heart completely, but the slightest ventricular fibrillation could cause oxygen deficiency in the brain and cause us to lose consciousness?
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1) What happens to the heart in cardiac arrest?
During a cardiac arrest, the electrical and mechanical activity is compromised (either one or both) such that the heart does not pump blood sufficiently to sustain life.
This can be either due to an abnormal electrical rhythm, such as ventricular fibrillation, or a condition such as hypovolemic shock where the preload of the heart (the volume of blood in the chambers prior to the start of a contraction) is insufficient to have a useful amount of normal blood flow.
2) Why is an AED useful?
An automated external defibrillator is useful for two abnormal heart rhythms: pulseless ventricular tachycardia and ventricular fibrillation. The shock from an AED essentially resets the electrical system of the heart in hope that it will resume normal activity afterwards. In many cases, this may work.
The reason that an AED is not the cure-all for cardiac arrest is that there are many reasons for someone to go into an abnormal rhythm and only rarely are abnormal rhythms an isolated problem by themselves. Typically, someone goes into ventricular fibrillation because they are having a heart attack. Only by opening up the artery that has been blocked in a cardiac catheterization suite can the inciting event be treated. Shocking the patient may get them back to a normal rhythm for some time but until the blockage is fixed or the tissue beyond the blockage is completely dead and therefore electrically inert they will go back into the abnormal rhythm.
I remember one patient whom I had flown to the cath lab who was shocked 13 times, the last time being in the hallway prior to the doors of the cath lab. Once the blocked artery was opened he stopped going into abnormal rhythms.
3) Does CPR do something else?
In some cases, the heart can mechanically be defibrillated. The old "precordial thump", which was taught to those of us with grey hair back when advanced cardiovascular life support was a 3 full day class, can (every once in a while) shock the heart enough for return of spontaneous circulation. There is a reason it is no longer taught, along with "cough CPR" not being mentioned.
CPR, at best, produces maybe one third of normal cardiac output. It can support oxygenation of vital organs and prevent clinical death (pulselessness and apnea) from becoming biologic death (dead beyond any hope of resuscitation). A hypoxic heart is not likely to have enough oxygen for normal electrical function. It doesn't make sense to try to zap someone into oblivion without at least giving them a fighting chance with CPR. This is the reason for the old "shock first" with witnessed arrest and "shock fast" with unwitnessed arrest strategies that I believe were put into the ACLS protocols maybe around the year 2000.
4) Pacemakers
Defibrillation and cardioversion are distinctly different from internal or external pacemakers. Think of defibrillation and cardioversion as a sledgehammer and a temporary transvenous pacemaker as a jeweler's hammer. Ventricular fibrillation and pulseless ventricular tachycardia require a big whack. Really big. Sometimes two distinct defibrillator units at the same time big ("dual simultaneous defibrillation" - google scholar it, it's neat stuff). However, getting the heart to beat in an organized manner requires a much more gentle touch. We are talking 200 Joules of a big whack versus 0.2 mA of pacing current the last time I had floated a wire in someone's heart. Pacing is a firm but gentle zap. It gets the electrical system to beat but orders of magnitude less than what is required to reset the system from V-fib.
Finally, there is no "slight" ventricular fibrillation. That's an all or nothing rhythm. The reason that "cough CPR" is still useful to those of us board-certified in emergency medicine is that we have toys the majority of the population does not know how to use. Consciousness can be maintained many seconds beyond cardiac arrest. If I have you on a cardiac monitor and you go into asystole in front of me, I can likely keep you conscious for a minute or longer if you actually obey my instructions for cough CPR.
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