Intracranial hypertension is an important obstruction to intracranial blood flow. At moderate intracranial pressure (ICP) elevation the pulsatility of the middle cerebral artery flow velocity increases. This is expressed in the pulsatility index or PI, but also in the difference between Sys1 and Sys2.
The PI has an important drawback: it varies with heart rate. Schaafsma (2012) has, therefore, advocated to use a diastolic flow velocity with a fixed time with respect to stroke onset. Somewhat arbitrarily the D560 was chosen: the average flow velocity calculated over an interval of 80 ms around 560 ms after stroke onset. This allowed D560 to be calculated down to a pulse interval of 600 ms, which equals a heart rate of 100 BPM. At higher heart rates the interval of 80 ms was shifted forward so that it was safely based upon diastolic flow.
Since Sys1 is based upon arterial acceleration (Schaafsma 2014) it is relatively resistant to an increase in ICP, whereas the ejected stroke volume (Sys2) and diastolic flow are negatively affected, the blood being forced into other parts of the circulation where output resistance is less. This explains why the difference of Sys1 minus Sys2 becomes larger, especially when compared to the systemic blood pressure by calculating the ratio pulsatile apparent resistance (or PaR).
When ICP increases further to levels associated with uncal or tegmental herniation the middle cerebral artery flow becomes pendular: systolic inflow but diastolic back flow and, ultimately, so called systolic spikes may be observed (Schaafsma 2025). These systolic spikes can be demonstrated in Neuromons CardioVascularSimulationApp for macOS, in particular, in the recent extension demonstrating the interaction of arterial acceleration and EDRF.