As the use of ultrasound guided peripheral nerve block getting increasingly popular, peripheral nerve can be blocked visually, with good accuracy and ease. It has made specific nerve block in the axillary region possible. Can specific nerve block increase the blood flow of target artery (radial or ulnar artery) and benefit microvascular surgery, or will it decrease the blood flow in other arteries and have a negative impact on microvascular surgery? In this study, the investigators will measure the hemodynamic changes of radial and ulnar artery after performing specific nerve blocks of the musculocutaneous, radial, ulnar, or median nerves respectively in patients without vascular disease. The aim is to assess the effect of specific nerve block on hemodynamics of upper extremity, and provided evidence for further research in patients undergoing microvascular surgery.