UMEM Educational Pearls - By Zach Rogers

Category: Critical Care

Title: Arterial Line Square-wave Test

Keywords: arterial line, square wave test, overdamped, underdamped (PubMed Search)

Posted: 10/24/2022 by Zach Rogers, MD
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Takeaways

Arterial line waveform interpretation and troubleshooting are essential skills for any physician caring for critically ill patients. Overdamping and underdamping of the arterial line waveform leads to inaccurate systolic and diastolic blood pressure readings which can lead to unidentified hypertension or hypotension. In addition to scrutiny of the arterial waveform pattern, the square-wave test is a tool to identify overdamped or underdamped arterial lines. 

Overdamped arterial waveforms will underestimate systolic blood pressure and overestimate diastolic blood pressure. Underdamping will have the opposite effect and overestimate systolic blood pressure and underestimate diastolic blood pressure. In both cases, the mean arterial pressure (MAP) often remains the same.  

The square-wave test is a rapid flush that is applied to the arterial line for approximately 1 second. This rapid high-pressure surge results in vibration and oscillation of the arterial catheter. These oscillations are then read by the pressure transducer and the number and amplitude of these oscillations can be measured. 0 or 1 oscillations is suggestive of overdamping. 3 or more oscillations is suggestive of an underdamped system. 

Major causes of an overdamped arterial line waveform include low infusion bag pressure, loose connectors, air bubbles in the tubing, blood clot in the circuit, or kinking of vascular catheter. An underdamped arterial line, however, is caused by overly stiff circuit tubing or a defective transducer.   

 

Scrutiny of the arterial waveform and utilization of the square-wave test can be helpful to both identify erroneous arterial line blood pressure readings as well as suggest likely corrective measures.  

 

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Arterial_line_overdamped_and_underdamped_examples.jpg (112 Kb)


Category: Critical Care

Title: Ultrasound Central Line Confirmation

Keywords: ultrasound, central Line, confirmation, venous, cavoatrial junction, agitated saline, pneumothorax (PubMed Search)

Posted: 8/30/2022 by Zach Rogers, MD
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Takeaways

Traditionally, internal jugular and subclavian central line placement has required chest x-ray confirmation of correct placement (venous cavoatrial junction placement) as well as demonstrating lack of complication (no pneumothorax) prior to use of that central line. However, current evidence supports similar if not superior complication identification and placement confirmation with ultrasound,(1-7) allowing for a much quicker confirmation time than traditional chest x-ray, which can be vital in critically ill patients who need immediate medication administration.

Venous placement is confirmed with prompt visualization of microbubbles in the right atrium and ventricle with a rapid flush of 5-10 ml of agitated saline via the distal central line port. Additionally, if the opacification occurs <2 seconds after injection then the catheter tip is sufficiently distal in the central venous system to not require additional verification. Additional confirmation of lung sliding in both lung apices will rule out pneumothorax.

Some authors recommend checking the contralateral internal jugular vein to ensure that the central line catheter has not traveled up the internal jugular towards the head, however this may be redundant as long as the time from agitated saline injection to right atrial visualization of microbubbles is clearly less than 2 seconds.

Bottom Line: Utilization of ultrasound for central line placement confirmation is a relatively simple, rapid, safe, and accurate means of confirmation of venous catheter placement and catheter tip location, as well as ruling out pneumothorax complications.

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