datex and a non invasive time of flight monitor vittamed the monitoring data from the invasive icp monitor and invasive abp monitor where processed in order to get slow icp and slow abp waves in the frequency range 0 008 hz to 0 033 hz these slow waves were used to calculate moving correlation coefficient r icp abp which has been taken as an index of cas status estimation the monitoring data from the non invasive time of flight monitor relative ultrasound speed was processed in order to get slow waves respiratory waves and pulse waves and to calculate phase shifts ps 1 and ps 2 these phase shifts were used to calculate non invasive index of cas status estimation icas non invasive cos u03c0 u2212 a 1 ps 1 u2212 a 2 ps 2 or icas f ps 1 ps 2 in order to compare the invasive icas and non invasive icas data the data obtained from the seven patients was plotted in fig3 the non invasive icas data was collected using an embodiment of the method 10 and apparatus 11 of the present invention the data from each patient is marked on the chart in order to fit a linear relationship between invasive icas and non invasive icas to find a correlation factor these data points were plotted against each other in fig4.