Which arrhythmia has an irregular rate but is typically rapid?

The arrhythmia characterized by an irregular rate that is typically rapid is known as Supraventricular Tachycardia (SVT), also referred to as Atrial Tachycardia. This condition involves the heart beating at a rapid pace, often exceeding 100 beats per minute, and it can present with a variety of rhythms, leading to the irregularity observed in the rate.

SVT arises from reentry circuits originating above the ventricles, and while it typically features a rapid heart rate, the rhythm can fluctuate, leading to the irregularity. This can often cause symptoms such as palpitations, dizziness, or even syncope due to reduced cardiac output during episodes.

In contrast, other arrhythmias listed in the question exhibit different characteristics. Atrial flutter is generally more regular in terms of rhythm, albeit with a consistent rapid rate. Bradycardia involves a slower heart rate, which does not align with the criteria of being rapid. Third-degree AV block presents complete dissociation between atrial and ventricular activity, often leading to a slower heart rate as well.

Thus, the defining features of SVT as a rapid, irregular rhythm make it the correct choice in this case.