MD/PRD/IN16.ANIMOC9 V.02 - 12/03/2020
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Chapter 1: Technology overview
ANI (Analgesia Nociception Index) is a standardized continuous measurement of the relative
pΣ tone (parasympathetic tone). Each respiratory cycle (spontaneous and artificial) induces a
fast, temporary decrease of the pΣ tone, which accounts for Respiratory Sinus Arrhythmia,
and leads to a transient shortening of the R-R intervals (increased heart rate). ANI quantifies
these "respiratory patterns" in order to
measure the "relative quantity" of pΣ tone.
The series of normal, non-ectopic, R-R intervals is processed after normalization, resampling
and filtering. The amount of pΣ tone is measured in relation to the total window surface
through the area comprised between the lower and the upper envelope of the RR series. The
higher the pΣ, the higher the shaded surface is, and reciprocally.
The ANI is expressed between 0 and 100. Each ANI value is computed on one time window
of 64 sec. This number shows the relative pΣ activity as a part of ANS activity: it expresses
the relative amount of pΣ tone present as compared to sum of sympathetic and pΣ activities.
ANI MOC-9 displays two averaged ANI measurements: ANIi results from the average of ANI
measured over the previous 56 sec, and ANIm results from the average of ANI measured
over the previous 176 sec.
There are multiple ways of interpreting an ANI value: one is probabilistic, as this index has
been developed in order to predict hemodynamic reactivity during nociceptive stimulation.
When surgical stimulation was constant, all hemodynamic reactivity episodes (20% increase
of heart rate or systolic blood pressure compared to a reference) were associated with a
decreased ANI up to 10 min beforehand. The predictive thresholds need yet to be
established, but preliminary studies suggest:
•
that an ANIm measure between 50 and 70 during surgery makes a hemodynamic
reactivity episode unlikely in the following 10 minutes;
•
that an ANIm lower than 50 makes hemodynamic reactivity very likely in the following
10 minutes.