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Arterial Line Calibration

This section highlights the importance of maintaining and calibrating an Arterial Line

Incorrect setup of pressure readings can lead to inappropriate treatment.

Prior to any transduced pressure readings and then subsequent use with an ODM+, it is essential that the transducer has been:

Levelled to the phlebostatic axis to eliminate the effects of hydrostatic pressure on the readings:

  • The phlebostatic axis is on the 4th intercostal space along the mid axilla line.
  • The phleblostatic axis is relevant for supine and up to 60 degrees of head-up tilt.
  • The transducer should not be levelled to the site of arterial catheter access.
  • If the transducer has not been levelled to the phlebostatic axis, pressure readings will be either falsely high or falsely low.
  • It is not suitable for an abnormal shaped thorax.
  • Levelling should be done at every handover, prior to pressure and ODM+ readings and at any time where there is doubt about the readings. The literature suggests that for consistent readings of pressure trends, the patient bed should be at the same angle each time.

Zeroed to eliminate the effects of atmospheric pressure on the readings. It is sometimes known as calibration:

  • The transducer has to read zero when there is no pressure against it.
  • It is described as being similar to zeroing a set of scales before weighing.
  • This should be done at every handover, prior to pressure and ODM+ readings, if the line is disconnected from the patient monitor and at any time where there is doubt about the readings.

Tested for damping:

  • Damping in the pressure line system acts as shock absorber (like a car suspension).
  • In order to test the system dynamics, the user should carry out the Square Test.
    • The Square Test assesses how fast the system vibrates in response to a pressure signal.
    • Allows the transducer to ‘feel’ some of the 300mmHg in the pressure bag.
    • The user should squeeze the flush valve on the transducer for a few seconds and then let go.
      • Waveform should rise sharply, plateau and drop off sharply when released (Figure 1).

Inaccurate damping can lead to inappropriate treatment:

Overdamping (This is defined when the oscillations following the downstroke are sluggish and can underestimate systolic pressure or overestimate diastolic pressure) Causes include:

  • Loose connections
  • Air bubbles
  • Kinks
  • Blood clots
  • Arterial spasm
  • Narrow tubing

Underdamping (This is defined when the oscillations are too pronounced and can lead to a false high systolic or a false low diastolic pressure):

  • Catheter whip or artefact
  • Stiff non-compliant tubing
  • Hypothermia
  • Tachycardia or dysrhythmia