RESUMO
Objectives: The critical closing pressure (CrCP) defines arterial blood pressure below which cerebral arteries collapse. It represents a clinically relevant parameter for the estimation of cerebrovascular tone. Although there are few methods to assess CrCP, there is no consensus which of them estimates this parameter most accurately. The aim of present retrospective, experimental study was to compare three methods of CrCP estimation: conventional Aaslid's formula and methods based on the cerebrovascular impedance: the established continuous flow forward (CFF) and a new pulsatile flow forward (PFF) model.Methods: The effects of the following physiological manoeuvres on the CrCP were studied in New Zealand white rabbits: lumbar infusion of Hartmann's solution to induce mild intracranial hypertension, sympathetic blockade to induce arterial hypotension, and modulation of respiratory tidal volume to induce hypocapnia or hypercapnia.Results: During intracranial hypertension, all CrCP estimates were significantly higher than at baseline, decreased with decreasing ABP and increased with gradual hypocapnia. During hypercapnia, all CrCP estimates were significantly decreased but only in the case of CrCPA the negative, non-physiological values were observed (16% of the cases). The Bland-Altman analysis revealed that a good agreement between each impedance method and Aaslid's method deteriorated significantly in the low range of the average numerical value of the estimates.Discussion: Our results confirm the limited usage of Aaslid's formula for the calculation of CrCP. Although both impedance methods seem to be equivalent, the fact that PFF model better describes cerebrovascular hemodynamic allows the recommendation of this model for the calculation of CrCP.