Well, I'll be Damped – How Square Wave Testing Can Assure Accuracy in Your Hemodynamic Monitoring Interpretations


Since the inception of hemodynamic monitoring, square wave testing, also known as dynamic response testing, continues to intermittently confuse both new and experienced critical care providers.  Square wave testing can have a direct impact on the validity and accuracy of the hemodynamic values which are obtained from the invasive monitoring device.  It is imperative that critical care clinicians be competent in understanding not only the purpose of square wave testing, but how to interpret its results.

The primary utilization of the square wave test is with arterial lines – those invasive monitoring cannulations which reside within the lumen of a systemic artery – and can be transduced to reveal a beat by beat graphic of luminal arterial tension.  In order to properly and accurately interpret the values being transduced, it is recommended to perform a square wave test.  This test is nothing more than fast flushing (with high pressure) the non-compliant tubing with saline (or heparinized saline) – in terms of tangible work, that’s about it.  What comes next is the ability to interpret what is displayed on the monitor!

There are two factors to consider when evaluating the square wave test (or the dynamic response test).  The first is called the natural resonant frequency, and the second is referred to as the damping coefficient.  When the invasive monitoring system is flushed, it exposes the transducer to a pressure signal (a very high-pressure signal), which in turn causes the transducer to vibrate.  How quickly the system vibrates is, essentially, the natural resonant frequency.  This phenomenon is rapidly followed by the damping coefficient, which is the speed at which those vibrations stop and return to normal transduction.  The vibrations caused by the fast flush should be quickly quelled and the system should return to pressure transduction.

In broad terms, there are three responses that are typically seen with a square wave test; adequately-damped, over-damped, and under-damped.  An adequately-damped waveform is when there are only two oscillations that follow the fast-flush wave.  The two oscillations should be no more than one-third the height of the previous oscillation.  The subsequent transducing should demonstrate a clear arterial waveform with a discernable dicrotic notch.  This result requires no further intervention or evaluation, and the hemodynamic values displayed on the monitoring device, can be interpreted with legitimacy and accuracy.

Image from Deltex Medical Limited, 2015

An over-damped waveform is when there is only one oscillation (or little to no vibrations / ringing) following the square wave test.  A waveform that is damped will appear small in amplitude and flattened.  The dicrotic notch will be hard to visualize and appreciate.  Additionally, the systolic pressure will be poorly reflected, causing it to be reported lower than it actually is.  Conversely, the diastolic blood pressure will be over-estimated, and will be reported higher than it actually is.  There are a number of causes of an over-damped waveform.  Tiny air bubbles in the tubing, a clot at the tip of the catheter, tubing that is “too” stiff or kinked and / or a catheter that is positioned against the wall of the blood vessel.  Remember that air is easily compressible, and will almost always cause an over-damped waveform.  An over-damped waveform is a relatively common occurrence and can be fairly easy to correct.

Image from Deltex Medical Limited, 2015

Last, an under-damped waveform is where there is “ringing” or multiple oscillations / vibrations that follow the square wave test.  A waveform that is under-damped will appear saltatory in nature causing variations in the systolic and diastolic blood pressure values.  Typically, the systolic blood pressure will be reported higher than it actually is, and the diastolic blood pressure will be reported lower than it truly is.  The dicrotic notch will be visible and likely exaggerated in size in an under-damped waveform.  The causes of this type of waveform are limited and therefore, it is not as common to see in clinical practice.  Things like excessive tubing length, the use of multiple stopcocks, and patient conditions, such as tachycardia, or a high cardiac output, can all cause under-damping.  In the event that the patient condition is causing a under-damped waveform, it is acceptable to treat the underlying condition to ensure a more adequate and accurate waveform interpretation.

Image from Deltex Medical Limited, 2015

In order to ensure that invasive monitoring be used as a reliable resource, critical care providers should be challenged to master the skill of performing a square wave test and competently interpreting the ensuing waveform – whether it is adequately-damped, under-damped or over-damped.  The best practitioners know that information such as this is only as good as the technical quality will allow – therefore making it an additional priority when caring for a patient undergoing invasive hemodynamic monitoring.  Like a former mentor of mine once said, “damped if you do, and damped if you don’t”.

Be well and stay safe out there!!

 ** (all pictures taken from Deltex Medical Limited, 2015)

Peer Review: 

I think this can be a very difficult concept to grasp especially for those who may not have much experience with hemodynamic monitoring.  However, I also feel it is one of those concepts that is imperative to understand in order to fully provide competent care.  It is important to understand that these dynamic response artifacts (overdamping and underdamping) are encountered commonly in patient’s undergoing hemodynamic monitoring and being able to troubleshoot and fix the presenting problem (artifact) is imperative.  Bruce noted the differences in the SBP and DBP with each dynamic response.  I think it is important to note that despite the over- and underestimation of each, the patient’s MAP normally remains unchanged and is less sensitive.  From a clinical standpoint, this is relevant as medication changes and titration should not be based upon hemodynamic monitoring interpretation alone. The MAP is going to be less subject to errors of measurement in comparison to the artifacts discussed above.  The SBP and DBP should not be used alone to titrate therapy based upon these findings.  Therefore, if all steps have been taken to try and correct the damping issue but it still remains, then consider following your patient’s MAP or another alternative method of monitoring.

Ashley Bauer, MSN, MBA, APRN, FNP-C, CFRN 

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