An arterial line, often referred to as an A-line, is a catheter placed directly into one of the body’s arteries. This device is distinct from an intravenous line, which enters a vein, because it allows for direct access to the high-pressure side of the circulatory system. Its primary function is to provide continuous, real-time blood pressure monitoring, which is significantly more accurate than a standard arm cuff. It also provides a convenient port for drawing frequent arterial blood samples necessary for testing blood gases and other laboratory values. This procedure is routinely performed in intensive care units and operating rooms when close physiological observation is required.
Clinical Necessity for Arterial Line Placement
The need for an arterial line arises when a patient’s hemodynamic status is unstable or requires precise management that cannot be achieved with non-invasive methods. Standard blood pressure cuffs can become unreliable in situations of severe low blood pressure (shock), where blood flow to the extremities is reduced. The cuff method can also produce inaccurate readings in patients with irregular heart rhythms or in individuals with morbid obesity.
The continuous data stream from an A-line is valuable when patients are receiving vasoactive medications—powerful drugs that rapidly constrict or dilate blood vessels to control blood pressure. Titrating the dosage of these medications requires immediate feedback to maintain blood pressure within a narrow, safe range. Complex surgical procedures, such as cardiac or neurological operations, also necessitate this level of monitoring to ensure adequate blood flow to delicate organs throughout the case.
Beyond continuous monitoring, the A-line offers a less traumatic way to obtain samples for arterial blood gas analysis, which measures oxygen and carbon dioxide levels in the blood. Patients experiencing respiratory failure or who are on mechanical ventilation often require these frequent samples to adjust ventilator settings or other respiratory therapies. The indwelling catheter avoids the need for repeated, painful needle sticks.
Common Insertion Sites and Selection Criteria
Arterial lines can be placed in several locations, but the choice of site is based on accessibility, the artery’s size, and the presence of collateral circulation. The radial artery in the wrist is the most frequently chosen site for placement due to its superficial location and the presence of collateral circulation.
Before cannulating the radial artery, medical staff often perform a circulation assessment, such as the modified Allen test, to confirm that the ulnar artery can adequately perfuse the hand. Other sites include the femoral artery in the groin, which is larger and often used in emergency situations or when peripheral access is difficult. The brachial artery in the arm is occasionally used, but it is less preferred because collateral circulation in that area is limited, raising the risk of complications.
The size of the artery also influences the length and gauge of the catheter used; smaller arteries in the wrist accommodate shorter, finer catheters compared to larger vessels like the femoral artery. Patient comfort and the risk of infection are also considered, as the radial artery site is easier to keep clean than the femoral site.
Overview of the Placement Procedure
Placement uses strict sterile technique, often involving full barrier precautions, to minimize infection risk. The clinician first identifies the target artery, most commonly by palpating the pulse, though ultrasound guidance is increasingly used to visualize the vessel and surrounding anatomy. After the site is cleaned with an antiseptic solution, a local anesthetic, such as lidocaine, is injected to numb the skin and surrounding tissue.
The actual insertion often employs the modified Seldinger technique, which utilizes a guidewire to safely thread the catheter into the artery. First, a fine needle is inserted through the skin at a shallow angle, typically between 30 and 45 degrees, until a flash of pulsatile blood confirms entry into the artery. A soft-tipped guidewire is then gently advanced through the needle and into the artery.
Once the guidewire is securely positioned inside the vessel, the insertion needle is carefully removed. The final arterial catheter is then threaded over the guidewire and into the artery. This technique is favored because it allows for a larger, more stable catheter to be placed through a small initial puncture, reducing trauma to the vessel. In some cases, a simpler catheter-over-needle technique, similar to a peripheral IV placement, is used for superficial arteries.
After the catheter is confirmed to be in place by the return of pulsatile blood, the guidewire is removed, leaving the catheter secured to the patient’s skin, sometimes with a suture. The line is immediately connected to a pressure transducer, an electronic sensor that converts the physical pressure waves from the artery into a visible tracing on the bedside monitor. This allows the medical team to view the real-time arterial waveform, confirming successful placement and function.
Post-Insertion Monitoring and Maintenance
After successful placement, continuous monitoring of the waveform is necessary to ensure the line remains patent and readings are accurate. The pressure transducer must be regularly “zeroed” to the atmospheric pressure and leveled to the height of the patient’s right atrium, which serves as the body’s reference point for central blood pressure. This calibration process ensures that the displayed numerical values are an accurate representation of the patient’s true pressure.
To keep the blood from clotting within the narrow catheter, a continuous flush system is used, delivering a slow infusion of normal saline, typically at a rate of 2 to 3 milliliters per hour. This fluid is contained within a pressure bag inflated to approximately 300 millimeters of mercury, ensuring the infusion pressure is higher than the patient’s systolic blood pressure to prevent blood from flowing backward into the line. The insertion site must also be checked regularly for signs of bleeding, swelling, or localized infection, and the dressing is kept clean and intact.
Recognizing Potential Complications
While arterial line placement is generally safe, adverse events can occur, requiring monitoring for potential complications.
Bleeding and Hematoma
Bleeding and hematoma formation at the insertion site are the most common minor complications, which often resolve with direct pressure and close observation.
Infection
Infection is a risk with any indwelling catheter, and while the risk is lower than with central venous lines, it increases the longer the line is in place. Clinicians look for signs like redness, tenderness, or pus at the site, which could indicate a localized infection or, less commonly, a bloodstream infection.
Thrombosis and Ischemia
A more serious concern is thrombosis, or the formation of a blood clot inside the artery, which can partially or fully block blood flow. If a clot forms, it can compromise circulation to the tissues beyond the insertion site, leading to distal ischemia, particularly in the fingers or hand. Medical staff check the extremity distal to the line for signs of impaired perfusion, such as coolness, pallor, or sluggish capillary refill.
Nerve Injury
Nerve injury is a rare complication, but it can occur if the needle or catheter inadvertently damages a nearby nerve, potentially causing numbness or tingling in the extremity.