In this discussion, we will be looking at using synchronized cardioversion on pulseless ventricle tachycardia. Defibrillation and electrical cardioversion are interchangeable and mean the same thing, a transthoracic electrical current to the cardiac cells. I have been looking at the ACLS cardiac arrest and tachycardia protocols and was wondering why we defibrillate (unsynchronized) pulseless ventricle tachycardia (V-Tach), but synchronize cardiovert unstable V-Tach? Isn’t pulseless V-Tach virtually unstable V-Tach? It’s the same mechanism, but one is just too fast to push blood effectively through the heart.
There are two types of V-Tach: focal V-Tach and re-entrant V-Tach. Focal V-Tach happens when a group of cells in the ventricle are irritated (abnormal automaticity) and fires over the SA and AV node. It is widely accepted that the most lethal ventricular tachycardia is re-entrant ventricular tachycardia. In reentrant V-Tach, there is an area of dead heart cells that are usually caused by an old myocardial infarction (MI). This group of dead cells do not conduct electricity. The electrical signals travel around the dead cells on both sides. Depending on the length of the refractory period of the live cardiac cells around the dead cells, the electrical signal can be caught in a loop around the dead cells causing ventricular tachycardia.
The exact mechanism of how electrical cardioversion works to stop tachycardic arrhythmias and ventricular fibrillation (V-Fib) is unclear, but there are two theories: The first by Zipe et al suggests that after depolarization of a critical mass of cardiac muscles by electrical defibrillation, the cardiac tissues are unable to maintain the re-entrant tachycardia. The other theory is that the shock waves of electrical cardioversion or defibrillation cause a prolonged refractory period, terminating the arrhythmia. We use synchronized electrical cardioversion during V-Tach with a pulse for a very specific reason: When we synchronize cardiovert, the defibrillator times out the R wave of the QRS complex. When we press the shock button, the defibrillator waits for the next R wave and delivers the shock on the R wave before the T wave. During the T wave, there is a relative refractory period known as the “vulnerable period”. If a shock is given during this vulnerable period, it can produce what is known as the R-on-T phenomenon and lead to resistant V-Fib. Without synchronized cardioversion for V-Tach with a pulse, you risk making a patient with a pulse, pulseless. The success rate of converting V-Tach with a pulse using synchronized electrical cardioversion is around 95%. That number is huge. If the there is a high success of converting V-Tach with a pulse, can this carry over to a pulseless V-Tach? Do we want to risk shocking on the T wave of pulseless V-Tach and possibly put them into a resistant V-Fib? Is this what we are doing now?
I was only able to find one research article on the subject: Turner, Turner, and Grace, the authors of the study, suggest the current guidelines recommend unsynchronized cardioversion due to the delay in shocks by hooking up the ECG leads. Some monitors, however, such as the Phillips Heartstart MRX, can sense the R wave using only the defib pads. It was a small study, and only 42 V-Tach pulseless/severely hemodynamic compromised patients were included. Thirty patients were shocked within a 100ms window of the peak of the QRS. Of those 30, 28 were converted into a perfusing rhythm; the other two went into V-Fib. The other 12 were shocked outside of the 100ms window, 7 went into V-Fib m, and 5 converted into a perfusing rhythm. It is a small study, and more studies will need to be done, but it makes me think: Should we be trying synchronized cardioversion on pulseless V-Tach? What do you think? Please let me know, as I am interested in your thoughts.
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