The Arterial Line Pressure Transducer Setup

Created on Mon, 06/29/2015 - 16:56
Last updated on Tue, 09/01/2015 - 19:05

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Arterial Line Setup

the arterial line setup diagram

The arterial pressure wave travels at 6-10 metres/sec.
The cannula in the artery is connected to the transducer via some non-compliant fluid-filled tubing.

The transducer is usually a soft silicone diaphragm attached to a Wheatstone Bridge. It converts the pressure change into a change in electrical resistance of the circuit. This can be viewed as waveform.

Priming the non-compliant pressure tubing

The idea is that the fluid in the tubing transmits the pressure wave to the transducer - The whole principle rests on a continuous cylinder of saline connecting the artery to the pressure transducer.

Why no more than 1.2 metres?

Long tubing underdamps the system. This may seem weird (surely, the longer the line the more plastic there is to absorb the pulse wave) but the 2013 ESICM haemodynamic learning package seems to suggest (p.36) that it is so. It is best explained in this AANA article. Essentially, the fluid-filled system has a certain "natural frequency" of resonance. The major determinant of this  natural frequency is the length of the tubing: the longer the tubing, the lower the natural frequency.  The patient's pulse oscillation is usually a fairly low  frequency phenomenon, and as the tubing length increases, the natural frequency approaches the patient's pulse wave frequency. The system then resonates, amplifying the signal.  Thus, the longer the tubing, the more resonance in the system, and consequently the system will be underdamped. For the same reason the tube lumen should always be no smaller than 1.5mm.

"Damping": anything that has a "shock absorber" effect on the art line

Air bubbles, long tubing, or compliant tubing – all of these absorb some of the force of the pulse wave decreasing the amplitude of the oscillations. This is one of the reasons normal IV tubing is not used to set up an arterial line transducer kit: the IV tubing is too soft and compliant; the elasticity of the plastic would absorb much of the pulse wave. Damping results in a slurred waveform with overestimation of the diastolic and underestimation of systolic; however the MAP value is usually preserved. In contrast, a kinked or clogged art line will see MAP systolic and diastolic all trending towards zero.

Zeroing and leveling the art line

Zeroing and leveling are occasionally used interchangeably, but they are not the same thing. They tend to occur together in the clinical setting, but the terms describe different processes. Zeroing exposes the transducer to atmospheric pressure via an open air-fluid interface, and leveling assigns this zero reference point to a specific position on the symbolic fluid-filled column that is the patient‘s body.

"Zeroing"can be defined as "the use of atmospheric pressure as a reference standard against which all other pressures are measured". The canonical college definition is "a process which confirms that atmospheric pressure results in a zero reading by the measurement system". The device is zeroed when the air-fluid interface is opened to atmospheric pressure (otherwise it would read diastolic blood pressures of ~ 760mmHg).  Atmospheric pressure varies little between the intensivists' eye level and the supine patients' aortic root level, and so strictly speaking the zeroing of an arterial line can take place with the transducer lying anywhere. Re-zeroing must occasionally take place as both the transducer and the atmospheric pressure will gradually drift away from the calibration point.

"Leveling" can be defined as "the selection of a position of interest at which the reference standard (zero ) is set". The canonical college definition is "a process which determines the position on the patient you wish to be considered to be your zero."  For reasons of convenience this tends to happen at the same time as zeroing the system to atmospheric pressure (which also sets the reference "0 mmHg" standard), but theoretically one could zero the transducer to atmosphere and then swing it wildly all around the room before levelling it against a reference point on the frightened patient.

The system is conventionally "leveled" at the phlebostatic axis, which is a reference level we have used since probably 1945. The phlebostatic axis corresponds roughly with the position of the right atrium, and his level has generally been accepted as the ideal reference level for measure the pressure of the blood returning to the heart. It was  therefore adopted as the reference level for CVP measurement. For arterial pressure measurements, at least since 2001 or so we have been also leveling the arterial lines at the phlebostatic axis. Prior to that, some units leveled their arterial lines at the level of the catheter insertion site. The specific reference point for the arterial transducer is actually the aortic root, but because it is very close to the right atrium the two reference levels are essentially the same.

For every 10cm below the phlebostatic axis, the art line will add 7.4mmHg of pressure.

One may sometimes be interested in leveling the arterial line at another point. Essentially, the level at which you zero the arterial line will measure the arterial pressure at that level. Which means that if your patient is in some sort of unconventional position (eg. sitting bolt upright) you may wish to measure at the level of the tragus instead. An art line leveled at the level of the external auditory meatus will measure the arterial pressure in the Circle of Willis, which is a representation of cerebral perfusion pressure. Various eminent society guidelines recommend that for the use of cerebral perfusion pressure as a therapeutic target, the reference level should be somewhere around the middle cranial fossa.  Whether this matters or not is a subject of some debate.

Flushing the art line

Apparently, using heparinised saline improves accuracy somehow, but does not prolong patency. I cant remember what my reference was for this tidbit. It seems trivial. The normal rate of flow is 3ml/hr, just to keep it from clotting.
The flush rate of the fast flush is 30-60ml/min, so in absence of a good cannula one can infuse the patient with a litre of flush fluid every 15-30 minutes.

 

References

I humbly thank Vincent Chen (President of AACN, I think) for helping me bring some sort of order and accuracy to this set of notes. The quality of this resource is enhanced by his contribution.

From Bersten and Soni's" Oh's Intensive Care Manual", 6th Edition; plus McGhee and Bridges Monitoring Arterial Blood Pressure: What You May Not Know (Crit Care Nurse April 1, 2002 vol. 22 no. 2 60-79 )

Scheer,Perel and Pfeiffer.Complications and risk factors of peripheral arterial catheters used for haemodynamic monitoring in anaesthesia and intensive care medicine. Crit Care. 2002; 6(3): 199–204.

For those who like hardcore physics, this excellent resource will be an enormous source of amusement. It appears to be a free online textbook of anaesthesia.

LITFL also link to this comprehensive FRCA self-assessment document:
Abby Jones, Oliver Pratt; PHYSICAL PRINCIPLES OF INTRA-ARTERIAL  BLOOD PRESSURE MEASUREMENT - ANAESTHESIA TUTORIAL OF THE WEEK 137 8TH JUNE 2009

McCanny, Peter, et al. "Haemodynamic monitoring and management." (2013). PACT, ESICM

This FRCA study document on arterial pressure monitoring is a goldmine of detailed information.

Lodato RF, Schlichting R: "Arterial pressure monitoring. Arterial catheterization: complications." In Principles and Practice of Intensive Care MonitoringVolume Part III. 2nd edition. Edited by Tobin MJ. New York: McGraw Hill; 1998::733-756.

Winsor, Travis, and George E. Burch. "Phlebostatic Axis and Phlebostatic Level, Reference Levels for Venous Pressure Measurements in Man." Experimental Biology and Medicine 58.2 (1945): 165-169.

McCann, Ulysse G., et al. "Invasive arterial bp monitoring in trauma and critical care: Effect of variable transducer level, catheter access, and patient position." CHEST Journal 120.4 (2001): 1322-1326.

Thomas, E., M. Czosnyka, and P. Hutchinson. "Calculation of cerebral perfusion pressure in the management of traumatic brain injury: joint position statement by the councils of the Neuroanaesthesia and Critical Care Society of Great Britain and Ireland (NACCS) and the Society of British Neurological Surgeons (SBNS)." British journal of anaesthesia (2015): aev233.

Gondringer, N., and J. D. Cuddeford. "Monitoring in anesthesia: clinical application of monitoring central venous and pulmonary artery pressure (continuing education credit)." AANA journal 54.1 (1986): 43-56.