How often should you take vitals on a critical patient?

[Brandon O]

The quantitative value does not take a minute but for perfusion accuracy the number you are looking for will. Usually the value will be a 3 second average much like the ECGs.

Sometimes it is easy to see the delay on the pleth in the ICU by palpating arrhythmias and watching the pleth. Some machines have a solid 1 - 2 second delay. But, if you go to the Masimo website for their literature on this when they established their specs you can read about it.

Masimo has a nice site with a good collection of relevant literature; I recommend checking it out, for anyone who has such inclinations. I am particularly impressed that they seem to include studies with unfavorable findings for their products.

In any case, the most salient study is probably this: http://iopscience.iop.org.ezp-prod1.hul.harvard.edu/0967-3334/26/4/017/

which suggests that in most products, the PTT (or Pulse Transit Time, the lag between electrical activity -- as measured by the R-wave on the ECG -- and detection of a pleth fluctuation at the fingertip) is typically around .25 seconds. This is fast, and since what we're looking at is actually a shorter trip, not from electrical activity but only from arterial occlusion just proximal to the probe, our method should actually be even more rapid than this.

As a caveat, there seems to be a general understanding that lower BP is associated with slower PTT (so much so that PTT can be used as an indirect measure of pressure). So in the more hypotensive patient, this method will probably become more and more difficult, requiring slower bleeding of the cuff to stay close to real-time results. There is also at least one case report (http://www.ncbi.nlm.nih.gov/pubmed/15875136) of an approximately 10-second delay between electrical asystole, confirmed by ECG, EtCO2, arterial waveform, and palpated pulses, and the loss of the pleth waveform from the non-invasive pulse oximeter. This seems to be a software artifact resulting from the fancy algorithm used by the Masimo, however.

(If anybody wants the full-text for either of these studies, PM me.)

This is hardly an exhaustive literature review, but I suppose the takeaway message is that there is typically a very quick transit between arterial pulsation and the visualized waveform; however, in sick patients and certain other cases it may not be as reliable. This closely mirrors the usual rules for interpreting pulse oximetry, so it probably shouldn't be too surprising.

With that said, in most cases I would probably not rely on this method to make real decisions. Even at best it's too finicky; I just think it's an interesting academic approach.

 

[Melclin]

Hmmm.. interesting, I'll have to give it shot at work tomorrow.