Approximately how long is the impulse-delaying period in the AV node?

• A. 0.12 seconds
• B. 0.05 seconds
• C. 0.20 seconds
• D. 0.30 seconds

Answer: (A)

The main idea is that the AV node purposefully slows the electrical signal to give the atria time to contract and fill the ventricles before the signal reaches the ventricles. In a normal heart, this nodal delay is about a tenth of a second; the value around 0.12 seconds is what you’d typically memorize for this impulse-delaying period. That brief pause is what coordinates atrial and ventricular activity, tying atrial systole to ventricular filling as the impulse moves onward through the His-Purkinje system. If the delay were only 0.05 seconds, there wouldn’t be enough time for atrial contraction to contribute fully to ventricular filling; if it were 0.20 or 0.30 seconds, the delay would be too long, slowing the heart cycle more than normal.