Damping is the influence within a system that is a dissipation of energy during an oscillation. In other words, think of damping like a shock absorber. Imagine a patient presenting with altered mental status. All we know about the patient is they have a history of hypertension. The patient is intubated for airway protection and due to the difficulties in obtaining blood pressures with the non-invasive blood pressure cuff an arterial line is placed (See image below).
The above waveform and pressure are what was seen on the monitor. This is one of the highest blood pressures I have ever seen, but the question is, how accurate is it?
Normal Arterial Waveform
- Peak of the Upstroke: The rounded part at the top of the waveform; Systolic blood pressure
- Dicrotic Notch: Closure of the aortic valve and subsequent retrograde flow
- Bottom of Downstroke: Bottom of the wave form just prior to the upstroke; Diastolic blood pressure
Damping:
- There are two main types of artifacts that can be seen on an arterial line tracing
- Underdamping
- Systolic pressure overshoot with a narrow peak and non-physiological oscillations during the diastolic phase
- Potential causes
- Artifact from catheter (catheter whip)
- Tachydysrhythmias
- Overestimation of the systolic blood pressure
- Underestimation of diastolic blood pressure
- Wider pulse pressure
- MAP not impacted
- Overdamping (More loss of energy)
- Waveform loses its characteristic landmarks and appears unnaturally smooth with a diminished or absent dicrotic notch
- Potential causes
- Air/Air bubbles in tubing
- Kinks in tubing
- Clots in tubing
- Underestimation of the systolic blood pressure
- Overestimation of the diastolic blood pressure
- Narrowed pulse pressure
- MAP not impacted
- Underdamping
Fast-Flush Test
- Also known as the square waveform test or the dynamic response test
- This allows clinicians to determine the natural frequency and damping coefficient of an invasive blood pressure monitoring system
- The assumption here is that this test activates the whole system including the distal catheter
- Performed by flushing crystalloid fluid that fills the tubing/transducer system with 300mmHg pressure via the flush system
- Activate the flush mechanism: This is done by squeezing the flush valve or pulling the pigtail on the transducer for a few seconds
- Square wave will appear on the monitor
- Count oscillations after square wave and before returning to baseline
- After the square wave a high amplitude oscillating wave that will fade exponentially after the flushing maneuver which can show:
- >2 oscillations before returning to baseline = underdamped
- 1 to 2 oscillations before returning to baseline = just right
- 0 to 1 oscillation before returning to baseline = overdamped
Underdamped Arterial Waveform (Image from [4])
Normal Arterial Waveform (Image from [4])
Overdamped Arterial Waveform (Image from [4])
The Basics of the Arterial Line Setup:
- There are two basic components to invasive hemodynamic monitoring:
- The electronic system
- The fluid-filled tubing system
- The Anatomy of the Setup
- Catheter: Access to the arterial system
- Catheter is connected to fluid-filled tubing
- Fluid-Filled Tubing: Fluid column in the tubing system carries the mechanical signal created by the pressure wave to the diaphragm of the electrical pressure transducer
- Wide-bore, high-pressure tubing
- Length limited to 122cm (48in)
- Transducer: Connects the fluid-filled tubing system and the electronic system (i.e. Converts the mechanical signal into an electrical signal)
- Catheter: Access to the arterial system
There are three steps to prepare the fluid-filled tubing system:
- Priming the Pressure Tubing
- Flush the entire tubing system with saline from the pressurized saline bag
- Ensure there are no bubbles or air in the line
- Leveling and Zeroing
- Transducer should be set at the level of the heart (phlebostatic axis)
- Turn 3-way stopcock toward the patient (“Off to the patient”)
- This allows the entire system to zero to atmospheric pressure
- Then the zero button is pressed on the monitor
- Once done the 3-way stopcock is then turned back to off toward the environment
- Dynamic Response Testing
- Frequency: How fast the pressure monitoring system vibrates when hit with a pulse wave
- Damping Coefficient: Measure of how quickly oscillations from a pulse wave dampen and come to rest
- Can be tested with the fast-flush test (also known as the square waveform test) by pulling and releasing the pigtail or compressing and releasing the squeezable fast flush valve on the pressure transducer
Back to our Patient
So back to our original question how accurate is the arterial pressure on the monitor for our patient? This is an underdamped waveform most likely from catheter whip. This means the pressures we were seeing most likely overestimated the systolic blood pressure and underestimated the diastolic blood pressure (See image below).
After giving 20mg of IV labetalol, a manual blood pressure reading was able to be obtained with systolic blood pressures in the 280s (Which makes me think the patient most likely did have a SBP >300mmHg). The patient was started on a nicardipine drip and taken for imaging. Head CT showed a basal ganglia bleed and CT angiogram of the head showed no AVMs or aneurysms.
References:
- Patey SJ et al. Processing, Storage and Display of Physiological Measurements. Anesthesia and Intensive Care Medicine 2020. [Link is HERE]
- Avolio AP et al. Role of Pulse Pressure Amplification in Arterial Hypertension: Experts’ Opinion and review of the Data. Hypertension 2009. PMID: 19564542
- Mcghee BH et al. Monitoring Arterial Blood Pressure: What You May Not Know. Crit Care Nurse 2002. PMID: 11961944
- Saugel B et al. How to Measure Blood Pressure Using an Arterial Catheter: A Systematic 5-Step Approach. Critical Care 2020. PMID: 32331527
Post Peer Reviewed By: Anand Swaminathan, MD (Twitter: @EMSwami)
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