Clint is a 19 year old male that’s brought to the emergency department for acute chest pain. Upon examination, you notice that his heart rate is really fast, about 170 beats per minute. He has a history of depression, and is currently being treated with TCAs. He is otherwise healthy. A few minutes after arriving, Clint loses consciousness. His ECG shows this.
All right, so based on his presentation and ECG, Clint has some form of arrhythmia. The best way to approach arrhythmias is to one: know what a normal ECG looks like, and two: have a good classification system to narrow down the diagnosis. To help identify an irregular rhythm, you can look at the morphology of the waveform and make sure that there is a P wave before every QRS complex, and a QRS complex after every P wave.
Now let’s take a look at the heart rate. The resting heart beats at a rate between 60 to 100 times per minute, and each of those beats starts off with depolarization of the sinoatrial node, and so we call it a normal sinus rhythm. It's also important to know that there is typically a delay in the conduction at the AV node and the Bundle of His, which gives some time for ventricular filling before the ventricle contracts. On the ECG, this is represented by the PR interval, which should be less than 5 small boxes, or 200 milliseconds.
Now, any disturbance in the rate, rhythm, site of origin, or conduction of the cardiac electrical activity is called an arrhythmia. Arrhythmias could be completely asymptomatic, and be picked up incidentally on an ECG. Arrhythmias can also present with palpitations, which is a sensation of your heart beating too hard or fast, fluttering, or skipping a beat. Additionally, they may alter cardiac output, causing individuals to present with signs of hypotension and decreased brain perfusion, like dizziness, altered mental status, or syncope.
If an arrhythmia is really fast, the heart muscles now demand more oxygen, and if oxygen supply is not met, then the myocardium suffers from ischemia, which presents as angina. In people with underlying heart disease, the sudden onset of an arrhythmia can precipitate acute heart failure. Finally, some arrhythmias may also cause sudden cardiac death.