Ventricular Tachycardia and Ventricular Fibrillation – Fatal Arrhythmias – Cardiac Arrest.
I am starting a series of posts on ICD’s (Implantable Cardioverter Defibrillators) to do so I need to firstly explain Ventricular Tachycardia and Ventricular Fibrillation.
Right, first please enlarge this graph below (click on it – but NOT on the red P)
Now although a little confusing to look at! I want to draw your attention to two key point, VF and Ventricular Tachycardia fall into the Potentially Fatal and Fatal classifications. They do this because during these rhythms the Cardiac Output (blue curve) reduces so much that the brain and organs are not receiving the blood that they require to function and survive. If the brain isn’t supplied with blood for even a relatively short length of time then a person will die. Both arrhythmias affect the Ventricles (the bottom of the heart and the main pumping chambers) by directly affecting this part of the heart they have a huge impact on cardiac output.
Ventricular Tachycardia (VT).
VT can very often be benign (not fatal but undesired) but it can also be a fatal arrhythmia as it may lead to asystole (the heart stopping altogether) or VF (I will come on to).
Conduction hits a dead end.
If we refer back to my earlier explanation about the hearts conduction system. It can be seen that the heart is made up of cells that if the cell next to them contracts then that follows suit. In a healthy heart and when this conduction is occurring as it should be. The order of contraction in the ventricles is shown by the green lines (image A). You can see the conduction hits a dead end and there for stops waiting for the next impulse to arrive. This means that the heart rate is being controlled by the top of the heart.
A Re-entrant Circuit.
In most Ventricular Tachycardia, a small ‘re-entrant’ circuit occurs (picture B) within the Ventricle, that continuously initiates a ventricular contraction, independent of the hearts normal conduction. (stick in there it will all become clear).
The top picture shows healthy tissue,
the lower image shows how a circuit can
form around damaged tissue.
OK so a re-entrant circuit is a loop, one that could continue indefinitely. How does this occur in the heart? Well the most common cause is damaged tissue as a result of a heart attack/Myocardial Infarction. If all pathways are healthy then a re-entrant circuit is unlikely, if there is some scar tissue then conduction can occur in a loop around it (Image C.).
A re-entrant circuit at work
in the Ventricles causing VT.
When this occurs in the ventricle, the circuit will provide continuous stimulation for the bottom of the heart to beat and becomes the ‘Focus’ of the heart rhythm (Image D). In VT this rate is over 100bpm. When this occurs cardiac output (the amount of blood being pumped around the body decreases). This is for two reasons, firstly because the faster rate does not allow proper filling time so less blood is pumped with each beat and secondly because this order of contraction is not the physiological one and less efficient at pumping blood. This arrhythmia ca stop spontaneously or requires medical intervention. In some people the arrhythmia is really rubbish at pumping blood, those people can pass out and the arrhythmia can be fatal, ultimately deteriorating into V.F.
Ventricular Fibrillation (VF) – If VT is bad news… this is disastrous.
Ventricular Fibrillation, Chaotic firing of
the cardiac cells.
VF is similar to Atrial Fibrillation but in the Ventricles. So the systematic and repetitive nature of the ventricles is replaced by chaos (Image E). This chaos means that cardiac output is pretty much zero. VF can occur as a result of deteriorating VT, where multiple re-entrant circuits begin to occur until there is no order or structure to the contraction of cells, cellular imbalances or R on T Phenomenon. R on T phenomena will require a post of its own but is essentially a contraction of the ventricles at a time where some of the cells are still resetting ready to contract again.
Some people can tolerate VT – No one can tolerate VF. Whilst there are some reports of spontaneously reverting VF but the general consensus is that, when you are in this arrhythmia, you need CPR until Defibrillation (An electrical shock delivered to the heart) which treats the arrhythmia, or you will die.
Now this all sounds a little terrifying and morbid, but help is at hand from medications, ablations and ICD’s. My Next post will explain what ICD’s are.
Time to do the washing up, I must invest in a dish washer!
Thanks for Reading
Image courtesy of fotographic1980 / FreeDigitalPhotos.net