syringe with heparin flush solution (10 U/mL); transparent sterile dressing Consider use of ultrasound to identify artery (static approach) or track cannulation (dynamic approach) (see Chapter 131 Ultrasound ) Procedure Assess the adequacy of both radial and ulnar arterial flow by palpation before the puncture The modified Allen test assesses ulna arterial flow more completely and is performed as follows Compress both the radial and ulnar arteries firmly with three fingers on each artery Have the patient clench and unclench the fist 10 times until the palm is blanched Have the patient relax the hand without fully extending the fingers Release the pressure on the ulnar artery If the thumb and thenar eminence regain normal color within seconds, then the ulnar collateral circulation is sufficient to support radial artery cannulation For arterial puncture, attach the butterfly to the syringe and flush the system with the heparin solution and empty it Have the assistant firmly restrain the infant or child by holding the arm just proximal to the wrist in supination and hyperextending the hand approximately 20 to 30 degrees Using gentle pressure with the gloved palpating fingers, locate the vessel Ultrasound can be useful to identify the vessel or dynamically follow the needle Cleanse the skin overlying the radial artery with the antiseptic solution Hold the needle as shown in Figure 130.3 and pierce the skin between index and middle fingers of the palpating hand, directing the needle at 30 to 60 degrees from the horizontal plane When the needle enters the radial artery, blood will flow into the butterfly tubing Slight suction should be applied to fill the syringe If the initial thrust is unsuccessful, attempt to enter the artery from a different angle, either medially or laterally as determined by careful palpation To reduce the likelihood of vessel injury, partially withdraw the needle before advancing during reattempts and minimize the number of punctures After obtaining the specimen, remove the needle and apply manual pressure to the puncture site for minutes Any air bubbles must be immediately removed from the sample syringe to achieve accurate blood gas results For arterial cannulation, consider a 24-gauge over-the-needle catheter for patients weighing less than 10 kg, 22 gauge for 10 to 40 kg, and 20 gauge for more than 40 kg Secure the child’s hand and forearm to an arm board with the wrist in moderate extension using a gauze roll under the wrist Cleanse the wrist with antiseptic solution and allow the area to dry Locate the radial artery by palpating for the pulse over the distal volar forearm as shown in Figure 130.3 Alternatively, ultrasound may be used Assess collateral circulation by palpation of the ulnar pulse and performance of the Allen test After achieving local