Outline the components required to measure blood pressure from an intra-arterial catheter (75% of marks). What information (other than blood pressure) may be gained from an arterial line trace (25% of marks)?

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College Answer

Most of the marks were allocated to the components of the measuring system (as detailed in the question), hence a level of detail was required. An explanation of how the various components work was required; e.g. hydraulic coupling and transducers. Some candidates forgot to include heart rate as a piece of information derived from the trace.

Discussion

Components of the measurement system:

  • An intra-arterial catheter
    • Kink-resistant, biologically inert, incompressible
    • Accesses the arterial circulation and provides the interface between the arterial blood and the circuit fluid
  • Fluid-filled tubing
    • Produces the hydraulic coupling between the arterial circulation and the pressure transducer
    • Access points to allow sampling
    • Flush valve
  • Fluid in the tubing
    • Incompressible
    • Usually, normal saline or 
    • Under pressure from the pressure bag to prevent blood refluxing into the line
  • Counterpressure fluid bag
    • Pneumatically pressurised to ~ 300mmHg to sufficiently counteract systemic arterial pressure
  • Pressure transducer
    • Wheatstone bridge piezoresistive transducer which converts pressure into a change of electrical current 
  • Signal conditioning and monitoring software
    • Filters the raw signal from the transducer
    • Converts it into a human-readable waveform
    • Records the data in a storage medium for review

Information derived from from the measurements by the arterial line transducer:

  • Heart rate
  • Systolic pressure
  • Diastolic pressure (coronary filling)
  • Mean arterial pressure (systemic perfusion)
  • Pulse pressure (high in AR, low in cardiac tamponade or cardiogenic shock)
  • Changes in amplitude associated with respiration (pulse pressure variation)
  • Slope of anacrotic limb associated with aortic stenosis

Information derived from the waveform shape:

  • Slope of anacrotic limb represents aortic valve and LVOT flow
  • Slurred wave in AS
  • Collapsing wave in AS
  • Rapid systolic decline in LVOTO
  • Bisferiens wave in HOCM
  • Low dicrotic notch in states with poor peripheral resistance
  • Position and quality of dicrotic notch as a reflection of the damping coefficient

References

References

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 Monitoring. Volume 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.