Final answer:
The sign that numbing is effective during Vascular Access cannulation is when the skin at the site appears pale or blanched, indicating that the local anesthetic lidocaine has blocked voltage-gated sodium channels and stopped pain sensations from reaching the brain.
Step-by-step explanation:
The sign that numbing has taken effect and it's time to stop spraying pain ease into a Vascular Access (VA) cannulation site is when the skin at the site appears pale or blanched (D). The application of an anesthetic spray like pain ease typically contains lidocaine, a local anesthetic agent.
Lidocaine functions by blocking voltage-gated sodium channels on neuronal cell membranes. By doing so, it prevents sodium ions from entering the neuron and halts the propagation of the nerve impulse. This blockage stops the transmission of pain sensations to the brain, resulting in numbness and pain relief at the site of administration, making lidocaine a critical component in pain management during procedures such as VA cannulation.