USMLE® Step 1 style questions USMLE
A 67-year-old male presents to the emergency department after a confirmed cardiac arrest in the field. Per EMS, the patient was found down by several bystanders without a pulse, and CPR was initiated immediately. The patient has no past medical history and is not on any medications. He is currently without a pulse, and CPR is ongoing. His 12 lead electrocardiogram is demonstrated below:
Image reproduced from Wikimedia Commons
The patient is shocked multiple times and dies despite resuscitation efforts. Post-mortem analysis is most likely to demonstrate which of the following as the etiology of this patient’s clinical presentation?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Ventricular refers to the bottom chambers of the heart, the right and left ventricles, rather than the top chambers, the right and left atria. Fibrillation means quivering from uncoordinated muscle fiber contraction. So, ventricular fibrillation, sometimes called v-fib or VF, means that the heart’s muscle fibers start quivering because they’re not contracting at the same time.
Normally, an electrical signal spreads fast enough that all the muscle fibers in the ventricles contract at almost the same time, which essentially acts like a single, coordinated contraction. If they don’t all contract at about the same time, not much gets accomplished. It’s kind of like a rowboat; it works best when everyone rows at the same time, right?
Well, with VF, all the rowers stop rowing together, and just row whenever they want; the rowboat just moves in circles, and eventually sinks.
Just like a rowboat, if the heart’s not squeezing anymore, and it’s just squirming around “like a bag of worms” — a common description of what it looks like — then you can probably guess that this situation is extremely dangerous. Because your body, and especially your brain, isn’t getting fresh oxygen, ventricular fibrillation can lead to death within minutes of onset, which is called sudden cardiac death.
Now, the exact mechanisms leading up to VF aren’t always super clear because it’s hard to know what’s happening in the heart immediately before VF. Most often, however, the heart cells become stressed or damaged in such a way that different areas of tissue are structurally and electrically changed, and thus have different properties; this known as tissue heterogeneity. When they’re homogenous, or the same, they all behave in the same ways and can depolarize and contract at nearly the same time.
When the tissues have different electrical properties, they aren’t as good as working together, and are more prone to abnormal behavior and depolarizing on their own. In general, tissue heterogeneity can happen after something stresses the ventricular muscle, such as: certain medications; illicit drugs, like methamphetamine or cocaine; electrolyte imbalances; and ischemia to the ventricular muscle.
When a group of ventricular pacemaker cells starts firing at the wrong time, if the surrounding cells have the same or nearly the same properties, that signal radiates outward simultaneously, causing the ventricles to contract; this is called a premature ventricular contraction, or PVC. Although it is not the norm, it is a relatively benign phenomenon and can even be seen sometimes in otherwise healthy hearts. However, having more than three PVCs in a row is called ventricular tachycardia. Ventricular tachycardia can sometimes progress to ventricular fibrillation.
In our example, let’s imagine that surrounding cells have very different properties; maybe some cells don’t conduct the signal as fast as other cells. Instead of radiating outward uniformly, the wave of conduction can start to curve, and ultimately ends up literally spiraling out of control; therefore, the wavefront circles back on itself, and initiates another depolarization. This situation is called functional reentry. That spiral can spread and break down into other mini spirals, leading to fibrillation and to the different areas of the heart depolarizing and contracting at different times.
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- "The dynamics of cardiac fibrillation" (2005)
- "Premature ventricular contraction" (2017)
- "Ventricular fibrillation in emergency medicine treatment & management" (2015)
- "Pathophysiology and etiology of sudden cardiac arrest" (2016)
- "Re-entry ventricular arrhythmia" ()
- "Ventricular fibrillation" ()