Patent ID: 7499744

Claim:
A method performed by an implantable medical device, comprising: measuring time intervals between successive heart beats, referred to as RR intervals, over a predetermined long-term period, where N is the total number of RR interval measurements during the predetermined long-term period and the measured RR intervals RR 1 through RR N are referred to as an RR time series; computing mean values M x1 through M xL of successive x-second segments of the RR time series, where x is a predetermined number and L is the total number of such x-second segments in the RR time series, and computing a variance of the mean values M x1 through M xL , referred to as [SD x ] 2 ; computing mean values M y1 through M yK of successive y-second segments of the RR time series, where y is a predetermined number greater than x and K is the total number of such y-second segments in the RR time series, and computing a variance of the mean values M y1 through M yK , referred to as [SD y ] 2 ; computing a mean value of squared successive differences between the RR intervals in the RR time series, referred to as [rMSSD] 2 ; and, estimating a ratio of the low frequency content in the RR time series between approximately 1/x Hz and 1/y Hz and higher frequency content in the P.R time series, referred to as LF/HF, as: LF/HF=K ([ SD x ] 2 −[SD y ] 2 )/[ rMSSD] 2 wherein K is a defined constant.