Dynamic Pressure–Flow Velocity Relationships in the Human Cerebral Circulation

Abstract

Background and Purpose-The pressure-flow velocity relationship in the cerebral circulation is characterized by the critical closing pressure (CCP), which is the pressure at which flow ceases, and the linear slope of a plot between pressure and flow velocity. It has been suggested, but not validated, that CCP can be determined from arterial blood pressure (ABP) and transcranial Doppler (TCD) recordings during the cardiac cycle. We studied a group of patients in whom ventricular fibrillation (VF) was induced. The time interval before defibrillation enabled calculation of CCP from data in which flow approached zero. These estimates were compared with values calculated before and after fibrillation and during regular heartbeats. Methods-TCD velocities and ABP in the radial artery were recorded before, during, and after 28 episodes of VF in 13 patients. CCPs were calculated by 3 different methods: (1) linear extrapolation from data during VF (gold standard); (2) linear extrapolation from normal heartbeat data; and (3) first harmonic Fourier filtering of normal heartbeat data. Results-The CCP during VF calculated from long diastoles was 32.9Ϯ11 mm Hg (meanϮSD). The regular heartbeat estimate was 6.0Ϯ4.3 mm Hg lower (PϽ0.05). The CCP estimate with the use of a Fourier filter was 1.4Ϯ3.9 mm Hg (PϭNS) lower than during VF. During hyperemia after defibrillation, the CCP decreased by 13.3 mm Hg, while velocity increased by 63%. The decrease in CCP could explain half of the increase in flow velocity during hyperemia. Conclusions-CCP can be accurately estimated from regular heartbeat data and is an important factor in regulation of the cerebral circulation. Key Words: cerebrovascular circulation Ⅲ ultrasonography, Doppler, transcranial Ⅲ vasoreactivity T he classic concept defining cerebrovascular tone is cerebral vascular resistance. This concept was developed from cerebral blood flow (CBF) determinations with the use of indicator methods such as nitrous oxide or 133 Xe. Basically this concept assumes that perfusion pressure and flow are linearly and proportionally related. It follows that flow stops only when the perfusion pressure is zero. However, when dynamic measurement techniques are used, such as electromagnetic flowmetry or ultrasound Doppler, the limitations of this concept become manifest. 1,2 Dynamically, flow may stop at pressure levels significantly higher than zero. The arterial blood pressure (ABP) level at which flow stops is defined as the critical closing pressure (CCP) 1-3 or, in cardiac literature, the zero-flow pressure (P fϭ0 ). 4 Above the CCP, an approximately linear slope, sometimes referred to as the inverse flow resistance, defines the relation between pressure and flow, when these variables are plotted as an x-y function 1,2 ( Figure 1). It follows that flow is linearly (but not proportionally) related to pressure and that it can be regulated by changes in both CCP (the x intercept) and slope. The pressure-flow relation is mainly a function of the peripheral resistance cerebral vascular bed. It...

Critical Closing Pressure: Comparison of Three Methods

“…However, strength of correlation was weaker than the correlation between CCP LDF and CCP TCD . It was undoubtedly lower than documented by Aaslid et al (2003), where comparison of CCP waveform and CCP calculated from 'long diastole' decrease in mean ABP and FV resulted in correlation generally greater than R = 0.9. Values of CCP waveform are greater than values of CCP TCD and CCP LDF .…”

“…These values were compared with CCP assessed from the TCD and ABP waveforms just before cardiac arrest adopting the algorithm of Aaslid (1992) and Aaslid et al (2003).…”

Ontogeny of cerebrovascular critical closing pressure

“…This method renders a CrCP that is bounded to be nonnegative. The impedance method has excellent agreement with the traditional method, except for instances where the traditional method renders clearly nonsensical low and negative values (16,17). In these cases, the new method gave results that made sense (4,18).…”

Effects of inhaled nitrous oxide 50% on estimated cerebral perfusion pressure and zero flow pressure in healthy volunteers*

“…Recent studies have provided preliminary data and validation of this concept [14]. Using a bench model [15], we have shown that despite large variability in ZFP values obtained with different configurations of flow velocity and blood pressure pulsatility, this method can be reliably used to assess changes in CPP and ZFP.…”