Authors: Fraser CD 3rd, Brady KM, Rhee CJ, Easley RB, Kibler K, Smielewski P, Czosnyka M, Kaczka DW, Andropoulos DB, Rusin CG.
The frequency--response of pressure autoregulation is not well delineated, therefore, the optimal frequency of ABP modulation for measuring autoregulation is unknown. We hypothesized that cerebrovascular autoregulation is band--limited and delineated by a cutoff frequency for which ABP variations induce cerebrovascular reactivity. Neonatal swine (n=8) were anesthetized using constant minute ventilation while PEEP was modulated between 6 and 0.75 cycles per minute (min--1). The animals were hemorrhaged until ABP was below the lower limit of autoregulation (LLA) and PEEP modulations were repeated. Vascular reactivity was quantified at each frequency according to the phase lag between ABP and intracranial pressure (ICP) above and below the LLA. Phase differences between ABP and ICP were small for frequencies > 2 min-1, with no ability to differentiate cerebrovascular reactivity between ABPs above or below the LLA. For frequencies < 2 min--1, ABP and ICP showed phase shift when measured above LLA, and no phase shift when measured below LLA (above vs. below LLA at 1 min-1: 156° [139°-174°] vs. 30° [22°--50°]; p<0.001 by two---way ANOVA for both frequency and state of autoregulation). Data taken above LLA fit a Butterworth high---pass filter model with a cutoff frequency at 1.8 min-1 (95% C.I. 1.5---2.2). Cerebrovascular reactivity occurs for sustained ABP changes lasting 30 seconds or longer. The ability to distinguish intact and impaired autoregulation was maximized by a 60--second wave (1 min--1), which was 100% sensitive and 100% specific in this model.
Full text and source: Journal of Applied Physiology
J Appl Physiol. 2013 May 16.