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P2/P1 ratio’s key hypothesis is that when the P2 relative amplitude is higher than P1 it suggests a loss of compensatory reserve mechanisms and deterioration of intracranial compliance where P2 tends to increase if the compliance is reduced. The simple observation of ICP peaks by the physician may lead to misjudgment, since P2 may be overlapped with the respiratory cycle, given giving a false impression of elevation over P1. Hence, the automated estimation of P2/P1 in accordance with the cardiac cycle described below may provide more accurate information.
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Invasive ICP’s percussion wave (A1) timing (T1) is almost the corresponding ABP’s waveform first peak
Invasive ICP’s notch (A3) timing (T3) is closely related to ABP’s dicrotic notch
Invasive ICP’s tidal wave (A2) timing (T2) is estimated to be midway between ABP’s T3 and T1
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Representation of observed correlation between arterial blood pressure ABP (red) waveform and intracranial ICP (green) waveform. ICP’s percussion wave lag (A1) timing (T1) in the ICP pulse is almost the same as ABP’s first peak; ICP’s notch (A3) timing (T3) after its percussion wave is closely related to ABP’s dicrotic notch; ICP’s estimated tidal wave (A2) timing (T2) is observed to be midway between ABP’s T3 and T1
ABP based algorithm (research algorithm)
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Normalized time to peak’s key hypothesis is that it is affected by compensatory reserve and intracranial compliance. The derived parameters required to identify time to peak are defined in figure below.Normalizedtime to peak is identified from point A, the max slope of the calculated average waveform (moment in time where pulse slope is steepest) to point B, normalized duration from the identified average pulse’s Max slope to highest pulse amplitude.
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Normalized time to peak parameters. A - max slope (moment in time where pulse slope is steepest) and B - normalized time to peak, duration from Max slope to highest pulse amplitude
This parameter helps estimate when the highest peak of a pulse happened, normalized to the pulse's total time. The hypothesis is that if the time to peak is at a later duration of the pulse’s normalized timeline, it suggests being the waveform’s tidal wave P2. If the time to peak is at an earlier moment in the pulse’s normalized timeline, it suggests being the waveform’s percussion wave P1. Such interpretation should always be done by a trained health professional.