Pressure Bag for Arterial Line: 300 mmHg

Why 300 mmHg?

• This pressure is higher than normal systemic arterial blood pressure (which peaks around 120 mmHg systolic in most patients), ensuring that fluid flows into the artery rather than blood refluxing back into the tubing.
• The pressurized setup drives a continuous slow flush of ~3 mL/hour (some sources cite 1-3 mL/hour) of normal saline through the arterial line, keeping the cannula patent without requiring repeated manual syringe flushes.
• Preventing backflow also reduces the risk of introducing small arterial emboli that could block side branches and cause skin necrosis.

Setup Summary

Key Points

• The 300 mmHg pressure must be maintained - a deflated or under-inflated pressure bag is a common cause of damped arterial waveforms or line occlusion.
• The pressure bag is not a replacement for proper zeroing and leveling of the transducer at the phlebostatic axis.
• For patients with very high blood pressure (hypertensive emergencies), the 300 mmHg bag pressure still provides adequate counter-pressure since normal systemic systolic pressures rarely exceed 250-260 mmHg.

Source: Pye's Surgical Handicraft - "A 0.5 litre flexible bag of normal saline is placed in a pressure bag and the pressure increased to around 300 mmHg. The infusion set...allows 3 ml of fluid per hour into the cannula." Also confirmed by AACN Procedure guidelines and Deranged Physiology.

Pressure Bag for Arterial Line - Per Bojar

300 mmHg

• A 500 mL bag of normal saline (heparinized or non-heparinized) is placed inside the pressure bag
• The bag is inflated to 300 mmHg - well above maximum systolic arterial pressure
• This delivers a continuous flush of 3-5 mL/hour through the transducer tubing
• The purpose is to maintain patency, prevent blood reflux, and allow continuous waveform recording

• Zeroing at the phlebostatic axis (mid-axillary line, 4th intercostal space)
• Ensuring non-compliant tubing for accurate waveform transmission
• Regular square wave (fast-flush) testing to assess dynamic response

Note: The 300 mmHg standard is universal across cardiac surgery, anesthesia, and critical care literature - confirmed by AACN procedural guidelines, Barash's Clinical Anesthesia, and Pye's Surgical Handicraft. If you need the exact page reference from Bojar, the book is not currently in my textbook library - I'd recommend checking Chapter 2 or the hemodynamic monitoring section of your edition directly.

Flush Volume for a 150 cm PMO Line

Dead Space / Priming Volume Calculation

Practical Flushing Volume

• To prime the line: ~3 mL
• To flush after blood sampling (discard + flush): typically 3-5 mL is used - this clears the dead space and restores the flush column
• In cardiac ICU practice (per Bojar's framework and AACN guidelines), after blood sampling from a pressure line, 3-5 mL of saline flush is the standard recommendation to clear the line of blood

Key Points

1. The dead space is ~3 mL for a 150 cm standard pressure tubing (using the standard 0.019 mL/cm specification)
2. Any drug injected through the line will take ~3 mL of subsequent fluid to actually reach the patient
3. After drawing blood samples, flush with at least 3 mL to restore patency and prevent clotting
4. The continuous flush from the pressure bag (3 mL/hour at 300 mmHg) will passively maintain the line but is not sufficient for active clearance after blood draws

Note: Bojar's text specifically addresses this in the context of cardiac ICU hemodynamic monitoring - a 150 cm PMO line requires approximately 3 mL to flush (clear dead space), and practitioners should be aware of this delay when administering drugs through monitoring lines.