Detailed waveform analysis of arterial blood pressure and blood flow to the brain reveals that the relation between these two may vary within the scope of a single heart beat. Increased intracranial pressure may severely impair diastolic flow whereas it may leave the first systolic peak or S1 largely unaffected. This peak S1 is hypothetically due to arterial acceleration (a brief peristaltic contraction in the body's conducting arteries). The differences in relation between pressure and flow translate to variation in the so called apparent resistance.
The working of the S1-PaR is demonstrated in Neuromon's cardiovascular simulation: S1-PaR explanation.
Read more in our publication published in Neuro Critical Care: an international collaboration between investigators in Brasil and the Netherlands.