UMEM Educational Pearls

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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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.  




  1. Saugel B, Kouz K, Meidert AS, Schulte-Uentrop L, Romagnoli S. How to measure blood pressure using an arterial catheter: a systematic 5-step approach. Crit Care. 2020 Apr 24;24(1):172. doi: 10.1186/s13054-020-02859-w. Erratum in: Crit Care. 2020 Jun 23;24(1):374. PMID: 32331527; PMCID: PMC7183114.
  2. Ortega R, Connor C, Kotova F, Deng W, Lacerra C. Use of Pressure Transducers. N Engl J Med. 2017 Apr 6;376(14):e26. doi: 10.1056/NEJMvcm1513613. PMID: 28379806.



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