arterial line

**Definition and purpose** An arterial line is an intra-arterial catheter attached to fluid-filled, pressure-tubing and a transducer. It gives continuous invasive blood pressure (systolic, diastolic, MAP) as a waveform, and it allows repeated arterial sampling (ABG, lactate) without repeated punctures.

**Indications** - Hemodynamic instability: shock (septic, cardiogenic, hypovolemic), hypertensive emergency, and titration of vasoactive drugs (norepinephrine, vasopressin, nitroprusside) - Respiratory failure needing frequent ABGs (ARDS, mechanical ventilation) - Major surgery (cardiac, neurosurgery, major vascular, anticipated large blood loss) and patients with severe cardiopulmonary disease - Situations where a noninvasive cuff is inaccurate or impractical (e.g., morbid obesity, arrhythmias such as atrial fibrillation with beat-to-beat variability, severe burns) - Pulse pressure/stroke volume variation for dynamic assessment of fluid responsiveness

**Sites** - Radial artery is the most common. It is superficial and has collateral flow via the ulnar artery and palmar arch. - Others: femoral (large, good in shock, but higher risk of infection and retroperitoneal bleeding), brachial (end-artery with poor collaterals, so higher risk of ischemia), dorsalis pedis, and axillary. - Radial artery cannulation historically involved an Allen test to confirm ulnar collateral flow, though its predictive value is poor and many clinicians use ultrasound guidance instead.

**Waveform physiology** - Upstroke = systolic ejection - Dicrotic notch = aortic valve closure - Peripheral waveforms are narrower, with a higher systolic peak and a lower diastolic pressure than central aortic waveforms (pulse pressure amplification). MAP stays nearly the same throughout the arterial tree. - MAP = DBP + 1/3(SBP − DBP) - **Underdamped** (resonance) trace overestimates SBP, with spiky peaks and narrow bands (e.g., from stiff tubing or a short catheter). **Overdamped** trace underestimates SBP and has a flattened waveform (e.g., from air bubbles, clots, kinks, or low flush bag pressure). - Correct setup: zero (level) the transducer at the phlebostatic axis (4th intercostal space, mid-axillary line, roughly at the level of the right atrium). A pressurized flush bag (300 mmHg) with a continuous slow flush (about 3 mL/hr) keeps the line patent. - Arterial pulsus paradoxus, or respiratory variation of the waveform, can be seen in cardiac tamponade, severe asthma/COPD, and tension pneumothorax. Pulse pressure variation >12–13% in a passively ventilated patient suggests fluid responsiveness.

**Complications** - Thrombosis/occlusion and distal ischemia (the most feared complication, especially with brachial or radial lines). Signs are pallor, pain, and cool or cyanotic digits. - Hematoma, bleeding, and disconnection with hemorrhage - Infection/catheter-related bloodstream infection (higher risk with femoral and prolonged dwell) - Pseudoaneurysm, arteriovenous fistula, nerve injury - Air embolism, and retrograde cerebral embolism with radial or brachial lines from vigorous flushing - Blood loss and iatrogenic anemia from frequent sampling - Management: remove the line if there are signs of ischemia or infection.

**Boards pearls** - Arterial line = the most accurate real-time BP measure and the gold standard for titrating pressors. It reads higher in systolic and lower in diastolic pressures than cuff measurements, so interpret it by MAP. - Central venous catheter vs arterial line: a CVP waveform has a, c, and v waves with x and y descents, whereas an arterial line shows the dicrotic notch. - Pulmonary artery (Swan-Ganz) catheter is a different device: it measures PCWP, cardiac output, and mixed venous O2 saturation. - Never inject medications or vasopressors into an arterial line because of the risk of severe limb ischemia (this includes accidental intra-arterial injection of thiopental, which causes vasospasm and necrosis). Check blood color and pulsatility to distinguish arterial from venous cannulation.