arterial line damping

Performing a rapid flush test is an important step to ensure the integrity of the arterial line system. In this clinical scenario, the arterial waveform with the rapid flush test is normal. There is a rapid rise in the pressure where the transducer is briefly exposed to pressure straight from the counter pressure bag, which is typically inflated at 300 mm Hg. When the rapid flush abruptly ends, the transducer system should oscillate no more than two or three times before returning to its natural frequency, and the arterial waveform must show a clear dicrotic notch (choice C is incorrect).1

  • use square waveform test
    • underdamping: > 2 oscillations
    • overdamping: 0 to 1 oscillation
  • underdamping
    • overestimates systolic but underestimates diastolic
    • wide pulse pressure
    • MAP same
    • narrow systolic peak, oscillations during diastolic
    • causes: long tubing, tubing that’s too elastic, too many stopcocks, artifact, or tachyarrythmia
  • overdamping
    • underestimates systolic and overestimates diastolic
    • narrow pulse pressure
    • normal MAP
    • waveform loses landmarks, becomes smooth, no dicrotic notch
    • cause: clots, kinks, air bubble, stiff tube, catheter against walls 2 3

It is important to understand both the properties of pressure measurement systems and the distribution of pressures throughout the vascular tree so that pressure measurements can be interpreted properly. Transmission of pressures through fluid-filled catheters is prone to artifacts, including underdamping, in which pressure vibrations cause excessive oscillations in the pressure tracing, and overdamping, in which waveforms are flattened. The fluid-filled tracings in the figures are underdamped, as can be seen from the oscillations in both the aortic and left ventricular tracings (choice A is correct; choice D is incorrect).

Underdamping (also known as catheter “whip”) can be caused by tubing that is excessively long or too elastic or by interposition of too many stopcocks. Underdamping tends to cause overshoot and undershoot, complicating interpretation of the waveforms; the mean pressures are not changed.

Overdamping occurs when the pressure waveform is flattened. It can be caused by air bubbles in the tubing, a clot at the tip of the catheter, tubing kinked or too stiff, or a catheter that is positioned against the wall of the blood vessel. Inappropriate tubing length is another cause. The degree of damping can be assessed by flushing the catheter and evaluating the square wave that results (square wave test). Two oscillations after the end of the square wave are optimal; one or none represents overdamping, and more than two suggests underdamping. Figure 3 shows an example of a square wave test with underdamping.4

Footnotes

  1. SEEK Questionnaires ↩

  2. internet ↩

  3. CHEST Board Review Critical Care? ↩

  4. Gardner RM, Hollingsworth KW. Optimizing the electrocardiogram and pressure monitoring. Crit Care Med. 1986;14(7):651-658. PubMed ↩