If the nurse witnesses the conversation between the md and the pt, the md does not need to be present for the patient to sign consent as long as the pt has no further questions. As the RN, if i did not witness or find documentation that the md and pt spoke and the pt was consented, i do not have pt sign consent (md can come do it again or it can be done am of surgery). It is not my job to explain risks, benefits and the procedure to the pt, only to reinforce what the md previously stated. The md should and usually signs consent during or around the time of the convo with the patient, although im sure it differs per situation. Most important thing is if pt has questions or level of conciousness/awareness changes even after consent is signed you call the md and have them speak to the patient or reassess the patient.
Hope this helps