Outline the factors associated with the accuracy of central venous pressure measurement by a central venous catheter

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College Answer

Accuracy of central venous pressure measurements depend on a number of factors. These include placement of device (tip in RA, RV, femoral vein etc), levelling (usually to phlebostatic axis), zeroing (zero means atmospheric pressure), calibration (measurement above zero is accurate when compared with gold standard [was mercury sphygmomanometer]), damping (not over or under, assessed by square wave or balloon bursting, prefer coefficient approximately 0.7). Frequency response of the system (intrinsic plus additional tubing) may significantly impact on damping (prefer shorter and stiffer tubing). Running averages also significantly alter ability to interpret spontaneous readings or variability associated with intra-thoracic pressure (better with printed waveform). Water column measurement is rarely done.

Discussion

The topic of CVP measurement is discussed in greater detail elsewhere.

In brief, the following factors influence the accuracy of CVP measurement:

Device factors

  • Zeroing
  • Calibration of the transducer
  • Selection of zero level
  • Dynamic response of the circuit

Artifact

  • Transducer drift
  • Infusions running through the monitored lumen
  • Damping in the system (eg. the presence of air bubbles)

Patient factors

  • CVC position (femoral vs. IJ, SVC)
  • Tricuspid regurgitation

Note that the question asked about accuracy. We were not expected to produce a list of different factors which influence the CVP, for instance PEEP, intravascular volume, etc. This was purely a device-oriented measurement question.

References

Alzeer A et al. Central venous pressure from common iliac vein reflects right atrial pressure. Can J Anaesth 1998 Aug 45 798-801.

 

Most of this material can be found in From and Soni’s” Oh's Intensive Care Manual”, 6th Edition, as well as the CVC section from The ICU Book by Paul L Merino (3rd edition, 2007)

 

Additionally, I have made use of the amazing Essentials of Critical Care, 8th ed.(ch.3 - Monitoring in the ICU)

 

Marik, Paul E., and Rodrigo Cavallazzi. "Does the central venous pressure predict fluid responsiveness? An updated meta-analysis and a plea for some common sense*." Critical care medicine 41.7 (2013): 1774-1781.