View Arterial Line Placement
As opposed to non-invasive blood pressure monitoring via a blood pressure cuff, an intra-arterial line (otherwise known as arterial lines, art lines, a-lines) is an invasive blood pressure monitoring method. Unlike NIBP, it offers real time information regarding pressure waves in the blood generated by ventricular contraction. Prior to interpreting numbers, waveforms have to be referenced to a zero point to ensure accurate data.
A-lines are most commonly placed in the radial artery, though you can use any peripheral artery (including the carotid, which I’ve done) for placement. Other common locations include the dorsalis pedis, ulnar, and femoral arteries.
In addition to providing real-time blood pressure measurements, arterial lines also facilitate the sampling of arterial blood, typically for blood gases. This is essential in intracranial neurosurgical operative procedures where strict control of the carbon dioxide content (pCO2) of blood is needed. It is also useful in patients undergoing brain death examination, where serial blood gas measurements are required.
General arterial line kits are available in various needle sizes and typically consist of the arteriotomy needle, a wire, and the intra-arterial catheter. It is preferable to use the largest you can. A 20G catheter is a good starting point. If you are unsuccessful in cannulating the artery with a 20G catheter, a 22G needle and catheter can be used. Larger bore catheters are usually used when cannulating the femoral artery.
Prior to arteriotomy, the wire should be inspected and tested to make sure it slides smoothly in and out of the needle.
A classic teaching point regarding arterial puncture in the wrist is the Allen test, which tests the arterial blood supply to the hand. The hand is fed by the radial and ulnar arteries. Because a-line placement carries with it the risk of arterial thrombosis, ensuring that collateral circulation exists prior to arterial puncture is recommended. The Allen test is performed by compressing both the radial and ulnar arteries and having the patient repeatedly open and close their fist. This drains the hand of any residual blood. One artery is then released resulting in rapid “pinking up” of the hand, indicating good blood flow. The test is then repeated for the other artery. If both arteries are patent, it is considered safe to place an arterial line.
There is no right answer. Generally, the hand is placed in the anatomical position (palmar surface up) and the wrist is gently extended and secured at an angle of 30-45 degrees. Abducting the arm may aid in extending the wrist. Some people like to sit but the general rule of thumb is to make sure that whatever position you are in, you are comfortable.
The skin is prepped in standard sterile fashion. Subcutaneous infusion of local anesthetic is optional, and if you choose to do numb the skin, be aware that excessive amounts of fluid can distort the anatomy and diminish the strength of the pulse.
The point of maximal impulse is then identified. It is helpful to find several points to get an idea of the course of the artery. A good point of entry for your arteriotomy is just proximal to the flexor retinaculum, though anywhere you can feel a pulse is fair game.
Some people like to make a superficial nick in the skin to facilitate passage of the needle and catheter, though this is not necessary. In patients with loose or tough thick skin, this is helpful.
With the bevel facing up, the needle should be passed at a 45 degree angle. Once the artery is entered, you should see arterial blood reflux and continue to fill the tubing on the back of the needle. At this time, it is helpful to lessen the angle between the needle and the skin to facilitate passage of the wire into the artery. It should slide easily. If not, it is probable that the wire is not in the lumen of the artery. Once the wire has been passed, the catheter is passed into the artery over the wire via the Seldinger technique. It is helpful to hold the catheter close to the skin and slowly twist it back and forth to allow it to slide easily through the soft tissue. Once the catheter is in, the needle and wire are removed. Pulsatile blood should be seen shooting out of the catheter prior to hooking the catheter up to monitoring equipment.
Some physicians like to penetrate the back wall of the artery with the needle and then slowly pull the needle back until the tip is in the lumen of the artery.
Keep in mind that the artery is typically not very deep (2-3 mm).
Use the fingers of your other hand to keep the artery from rolling while performing the stick.
When feeling the pulse, most beginners intuitively push down harder when the pulse is weak or difficult to palpate. This actually occludes the artery and you won’t feel the pulse at all. Instead, lessen the amount of pressure on the artery and the pulse is usually easier to feel.