Patent ID: 6868345

Claim:
An apparatus for extracting signals which are indicative of the level of consciousness of a patient comprising means for monitoring auditory evoked potentials (AEP) produced by the patient as a response to a repetitive acoustic click stimulus, means for extracting an AEP within a few repetitions, preferably more than 10 and less than 50 of the audio stimulus, means for using an autoregressive model with exogenous input (ARX), and means for calculating an index (AAI) indicative of anaesthetic, characterized in that the index is defined as: AAI - index = { k 1 ⁢ ∑ i = k 2 k 3 ⁢  x i - x i + 1  if ⁢ ⁢ y > 60 ⇒ k 4 ⁢ y + k 5 if ⁢ ⁢ y < 15 ⁢ ⁢ and ⁢ ⁢ y > 4 ⇒ k 6 + k 7 × ⅇ y p where k4, k 5 , k6, k 7 and p are constants, and where y is defined as: y = k 1 ⁢ ∑ i = k 2 k 3 ⁢  x i + x i + 1  where x 1 are the samples of a sweep, k 1 are a constant, which preferably is larger than 0.0100 and less than 0.02000, and which in particular may be chosen to be 0.0165, and where k 2 and k 3 are the start and the end samples, respectively, of the summation, chosen not to include the start and the end of the AEP window, or that the index is defined as: AAI - index = { 0.0165 ⁢ ∑ i = 17 69 ⁢  x i - x i + 1  if ⁢ ⁢ y > 60 ⇒ 0.2786 ⁢ y + 43.2857 if ⁢ ⁢ y < 15 ⁢ ⁢ and ⁢ ⁢ y > 4 ⇒ 9.3769 + 0.28 × ⅇ y 5 .