An arterial line, often referred to as an A-line, is a small, flexible tube inserted directly into an artery for continuous, real-time blood pressure monitoring. This invasive method is the gold standard for measuring blood pressure with high precision, especially when a patient’s condition is rapidly changing. The line also offers a direct port for drawing frequent blood samples (e.g., for arterial blood gas analysis) without repeated needle sticks. This feedback is invaluable for managing patients who are hemodynamically unstable, undergoing major surgery, or requiring aggressive titration of vasoactive medications.
Preparing for Arterial Access
Preparation of the equipment and patient is required before the procedure. Specialized equipment includes the catheter kit, sterile barrier supplies, a pressure transducer system, and an intravenous bag of normal saline housed within a pressure infuser. The saline bag must be inflated to 300 millimeters of mercury to overcome arterial pressure, preventing blood from backing up and clotting. The transducer cable is connected to the bedside monitor, and the entire circuit is primed to eliminate air bubbles, which could introduce inaccuracies or complications.
The radial artery in the wrist is the preferred site for most adult placements due to its superficial location and the dual blood supply to the hand. The patient is positioned supine with the chosen arm extended and the palm facing upward. The wrist is stabilized in 30 to 45 degrees of dorsiflexion, often supported by a rolled towel, which minimizes movement and improves accessibility.
The Modified Allen Test must be performed to confirm adequate collateral circulation from the ulnar artery. The patient clenches their fist while the clinician manually occludes both the radial and ulnar arteries at the wrist. The hand should appear blanched after the arteries are compressed. Pressure is then released only from the ulnar artery, and the hand is observed for a return of color.
A normal result means the ulnar artery can adequately supply the hand, with color typically returning within five to fifteen seconds. If the hand remains pale, the collateral circulation is insufficient, and an alternative insertion site must be selected to prevent ischemic injury. Final preparation involves cleansing the insertion area with an antiseptic solution and donning full sterile barrier precautions, including a cap, mask, sterile gown, and gloves.
Step-by-Step Insertion Technique
Administration of a local anesthetic, typically 1% lidocaine, minimizes patient discomfort. This anesthetic is injected just beneath the skin, away from the arterial pulse to avoid vasospasm or distorting the artery’s location. A small skin nick can then be made with a scalpel to facilitate catheter passage. The artery is located either by palpation or with the aid of ultrasound guidance, which increases the success rate of cannulation.
The technique chosen for puncture depends on the type of catheter kit being used, with the catheter-over-needle approach and the Seldinger approach being the two most common methods.
Catheter-Over-Needle Technique
In this technique, the needle and catheter assembly is advanced toward the artery at a shallow angle of 30 to 45 degrees. The clinician watches the hub for a flash of pulsatile blood, confirming entry into the artery. Once the flash is observed, the angle is immediately lowered to 10 to 20 degrees, and the assembly is advanced slightly further to ensure the catheter tip is fully within the vessel lumen. The needle is held steady while the flexible catheter is gently threaded forward into the artery, the needle is removed, and pulsatile blood flow is confirmed from the catheter hub.
Seldinger Technique
For the Seldinger technique, a slender introducer needle is first used to puncture the artery until pulsatile flow is seen. A flexible guidewire is then carefully advanced through the needle hub and threaded into the artery. If resistance is felt, the wire must not be forced, as this may indicate incorrect positioning or vessel wall injury. The introducer needle is removed over the wire, and the arterial catheter is advanced over the guidewire until the hub is flush with the skin. Once the guidewire is removed, the catheter is capped with a stopcock to control bleeding, connected to the pre-flushed pressure tubing, and secured firmly to the skin.
Post-Procedure Monitoring and Maintenance
Once the arterial line is successfully placed and secured, the monitoring system must be calibrated for accurate readings.
Calibration: Leveling and Zeroing
The first step involves leveling the pressure transducer to the patient’s phlebostatic axis, the anatomical reference point corresponding to the level of the right atrium. If the transducer is positioned too high or too low, the pressure readings will be artificially altered; for example, a 10 centimeter deviation causes an error of approximately 7.4 millimeters of mercury.
The second calibration step is zeroing, which eliminates the influence of atmospheric pressure on the reading. The transducer stopcock is turned off to the patient and opened to the atmosphere, and the monitor is pressed to establish a zero reference point. This process must be repeated with every shift change, after major patient repositioning, or whenever the displayed blood pressure value appears questionable.
Waveform Analysis
The arterial pressure tracing displays a distinct waveform composed of three main components. The sharp, rapid upstroke corresponds to the peak systolic pressure as the left ventricle ejects blood. Following the peak, the dicrotic notch represents the momentary pressure increase caused by the closure of the aortic valve. The final, more gradual downslope reflects the diastolic pressure as blood flows out into the peripheral circulation.
The quality of this waveform is assessed by performing a dynamic response test, also called the square wave test. This test involves briefly pulling the fast-flush device on the transducer to produce a sharp, box-like pressure wave on the monitor. An accurate system shows the square wave followed by one or two rapid oscillations before immediately returning to the patient’s normal arterial tracing.
Monitoring issues are identified by a distortion of the square wave test, indicating a problem with pressure wave transmission. An underdamped system (excessive oscillations) overestimates systolic pressure, often caused by stiff tubing or a hyper-dynamic state. Conversely, an overdamped system (blunted waveform, absent dicrotic notch) underestimates systolic pressure. Overdamping is usually caused by physical problems like air bubbles, a blood clot in the catheter tip, or a loose connection, which must be immediately corrected.
Routine site care prevents infection and thrombosis. The insertion site is visually assessed hourly for signs of bleeding, pain, swelling, or compromised distal circulation. The transparent dressing is typically changed every seven days, or immediately if it becomes damp, soiled, or loose, maintaining strict aseptic technique.
Removal and Management of Common Issues
When continuous arterial monitoring is no longer needed, the line must be safely removed to prevent complications. Before removal, the patient’s coagulation status must be reviewed, especially if they are receiving anti-clotting medications. The monitoring equipment is disconnected, and any sutures or securement devices holding the catheter in place are carefully removed.
The catheter is gently withdrawn in one smooth motion, and immediate, firm pressure is applied to the puncture site with sterile gauze. Since arterial punctures are under high pressure, direct pressure is held for a minimum of five minutes in non-anticoagulated patients to achieve hemostasis. For patients with impaired clotting, pressure may need to be held for up to fifteen minutes to prevent hematoma formation.
Once bleeding has stopped, an occlusive dressing is applied, and the limb is kept straight and immobile for at least one hour. The most common complication is localized bleeding or thrombosis at the insertion site. Thrombosis occurs frequently in radial artery placements but rarely causes severe clinical consequences due to the ulnar artery’s collateral circulation. Signs of potential distal ischemia (cold, pale, or mottled limb) require immediate catheter removal and assessment for restored perfusion. Other issues include local infection or accidental dislodgement, which requires immediate direct pressure to prevent significant blood loss.