What is the expected impact of a third-degree AV block on cardiac output?

A third-degree AV block, also known as complete heart block, results in a complete dissociation between the atrial and ventricular electrical activity. This condition occurs when the electrical impulses generated by the atria do not reach the ventricles, leading to independent pacing of the atria and ventricles.

In this situation, the atria may contract at a normal rate, but the ventricles will either rely on an escape rhythm, which is significantly slower than the normal heart rate. Because the ventricular rate is not adequate to sustain a sufficient cardiac output, the overall blood flow to the body is dramatically reduced. This compromised output can lead to symptoms such as fatigue, dizziness, or even syncope due to inadequate perfusion of vital organs.

While some patients might retain a degree of output due to the atrial contractions, the overall efficiency and effectiveness of heart function are severely diminished. Thus, the impact of a third-degree AV block on cardiac output is indeed severely compromised, leading to the correct answer.