Is a 2-second threshold for the RASS (Rapid Assessment of Sonographic sign) sign and a subxiphoid view sufficient to confirm central venous catheter (CVC) placement?

Ultrasound Confirmation of Central Venous Catheter Placement: RASS Sign and Subxiphoid View

The current evidence supports a 1-second threshold for the Rapid Atrial Swirl Sign (RASS), not 2 seconds, and while a subxiphoid view is valuable, it should be part of a more comprehensive ultrasound assessment for optimal confirmation of CVC placement. 1

RASS Sign Threshold: 1 Second vs. 2 Seconds

Recent evidence has updated the recommended threshold for RASS:

• The most recent high-quality study (2024) demonstrated that a 1-second push-to-bubbles time (PTB) threshold for RASS showed 100% sensitivity, specificity, positive and negative predictive values for detecting CVC malpositions compared to chest X-ray 1
• This 1-second threshold is more reliable than the previously used 2-second threshold
• The Society of Hospital Medicine's position statement recommends using ultrasound with rapid infusion of agitated saline to visualize RASS for detecting catheter tip misplacement 2

Ultrasound Views for CVC Confirmation

A subxiphoid view alone is not considered sufficient:

• Guidelines recommend a structured approach to ultrasound confirmation of CVC placement that includes multiple views 3

• The six-step approach to ultrasound-guided CVC placement includes:

  1. Identifying anatomy of insertion site
  2. Confirming patency of the vein
  3. Using real-time US guidance for puncture
  4. Confirming needle position in vein
  5. Confirming wire position in vein
  6. Confirming catheter position in vein 3

• Both short axis (transverse) and long axis (longitudinal) views should be used 3

• Visualization of the catheter tip is critical to ensure proper placement in the lower superior vena cava or upper right atrium 3

Benefits of Ultrasound Confirmation

• Ultrasound confirmation allows for immediate use of the catheter without waiting for radiographic confirmation 4
• LUS (lung ultrasound) can reduce CVC insertion-to-use time and improve patient safety 3
• Median point-of-care US completion time is 16 minutes compared to 32 minutes for chest radiographs 3
• Ultrasound is faster than radiography at identifying pneumothorax after CVC insertion 3

Limitations and Caveats

• RASS may not detect all types of malpositions, particularly those in the left brachiocephalic vein 4
• Ultrasound assessment requires proper training and expertise 2
• The Society of Hospital Medicine considers RASS detection an advanced skill requiring specific training 2
• Contrast-enhanced US has shown limitations in detecting CVC location compared to standard chest radiography 3
• Chest X-ray remains the gold standard for confirming CVC placement in many clinical scenarios 3

Algorithm for CVC Confirmation

1. Perform real-time ultrasound guidance during CVC insertion
2. Confirm catheter position using both short and long axis views
3. Perform RASS test with agitated saline flush (positive if swirl appears in ≤1 second)
4. If RASS is positive and no complications are suspected, the catheter may be used immediately
5. If RASS is negative or uncertain, or complications are suspected, obtain a confirmatory chest X-ray
6. Always obtain a chest X-ray for final confirmation when using left-sided approaches or in patients with anatomical variants

This approach balances the benefits of immediate ultrasound confirmation with the definitive assessment provided by radiography when needed.