Experimental cerebral hemodynamics

Vasomotor tone, critical closing pressure, and vascular bed resistance

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✓ Application of Burton's concept of the critical closing pressure to experimental data on brain-blood flow in the monkey suggests that perfusion pressure, not vascular bed resistance, is the primary variable affecting cerebral blood flow. Perfusion pressure for the cerebral circulation is the mean arterial pressure minus the critical closing pressure (MAP — CCP). Vasomotor tone and intracranial pressure are the major determinants of the critical closing pressure. Changes in either of these variables, therefore, affect perfusion pressure and flow. Data on brain-blood flow at fixed vasomotor tone obtained over wide pressure ranges show little change in vascular bed resistance despite significant changes in flow. The diameter of resistance vessels probably does not change significantly throughout the normal physiological range of cerebral blood flow. The limits of the critical closing pressure in the anesthetized monkey are from 10 to 95 mm Hg. Using these limits, and beginning with the average values for MAP and CCP in 11 awake monkeys breathing room air, the authors present theoretical flow curves in response to changes in intracranial pressure and mean arterial pressure that closely approximate the data reported in man.

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Address reprint requests to: William E. Hunt, M.D., Division of Neurological Surgery, The Ohio State University Hospital, 410 West Tenth Avenue, Columbus, Ohio 43210.

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Figures

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    Dynamic pressure-flow relationships (DPFR's) for the cerebral vascular bed. Data were obtained by oscillating aortic pressures between 230 and 105 mm Hg. All data points are obtained within a 4-second period. Closed squares represent averaged values for each heartbeat during the entire pump cycle. Open triangles represent data obtained during a prolonged diastole occurring during the run. The overall mean arterial pressure (MAP) is shown by the large closed circle. The critical closing pressure in this case is 70 mm Hg. The perfusion pressure at any point along this line is the observed pressure minus the critical closing pressure. At the operating point, the perfusion pressure is MAP — CCP.

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    Components of tension in a theoretical arteriole. The active component is vasomotor tone and is fixed for each DPFR. The passive component is elastic, and is determined by the vessel unstretched diameter R0 and the modulus of elasticity. The more rigid the vessel, the more vertical the passive component.

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    Equilibrium diagram for a theoretical arteriole. The heavy line is the summation of active and passive tensions from Fig. 2. Several pressures Pb, Pa, P0 are superimposed according to the Laplace equation, T = P × R. At R0 the equilibrium is unstable. Slight decreases in pressure or in radius at this point (interrupted line) causes collapse of the vessel since active tension remains fixed. The P0 is the critical closing pressure (CCP) at the given level of active tension. Changes in active tension will produce similar changes in the critical closing pressure (CCP).

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    Effect of changes in the mean arterial pressure on DPFR's. The central operating point is shown by the heavy circle. Each DPFR represents data generated during a 4-second cycle. Blood gases show no significant change during runs. The MAP is shown by heavy closed circle.

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    Effect of changes in PaO2 on the DPFR. The central operating points are shown. Each DPFR represents data generated during 4-second cycle. The pH, PaCO2, and Hct. show no significant change during runs. The pO2 varies from 135 to 25 mm Hg.

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    Effect of changes in PaCO2 on DPFR's. The central operating points are shown. Each DPFR represents data generated during 4-second cycle. The PaO2, Hct. show no change during runs. The PaCO2 varies from 31 to 66 mm Hg. The pH varies from 7.48 to 7.10 with changes in PaCO2.

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    Four examples wherein the critical closing pressure was measured directly. Reversal of flow occurs when the diastolic part of the above DPFR crosses the vertical zero-flow lines. The CCP2 values between 60 and 90 mm Hg are seen. Case 1 is from Early, et al.,2 and is in an awake, sitting, hyperventilated animal. The following values apply to animals 2, 3, and 4.

    Animal 2Animal 3Animal 4
    PaO2 (mm Hg)460 440 450 
    PaCO2 (mm Hg)12 15 24 
    pH7.65 7.57 7.46 
    Hct.55 54 57 

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    DPFR's in four anesthetized animals to show range of extrapolated critical closing pressures. Central operating points are shown. Each DPFR represents data obtained between a 2.5- to 8-second period. The limits of arterial blood gas determinations are as follows.

    Animal 1Animal 2Animal 3Animal 4
    PaO2 (mm Hg) 25–135 47–122 65–460 
    PaCO2 (mm Hg)18–105 20–66 30–62 12–108 
    pH27–54 7.10–7.48 7.03–7.21 7.04–7.65 
    Hct.7.14–7.49 34–45 16–21 25–57 

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    DPFR for cerebral vascular bed. This is the same curve as in Fig. 1 showing that the value of vascular bed resistance (VBR) is related to the slope of the DPFR (ΔP/ΔF).

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    DPFR's for cerebral vascular bed. Above data are combined from 11 experiments in unanesthetized animals. The physiological conditions and central operating points for each DPFR are given. There is insignificant variation in vascular bed resistance (ΔP/ΔF) for each of these DPFR.

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    The relationship between critical closing pressure (CCP) and vascular bed resistance (VBR). Curves show the relationship between vasomotor tone as determined by the critical closing pressure (CCP) and vascular bed resistance (VBR) as determined by the slope of the DPFR. The upper curve is from Early's combined data in unanesthetized animals (Fig 10). Curves 1, 2, 3, and 4 are computed from Fig. 8.

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    Flow model of the cerebral vascular bed. See text for description.

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    Relationship between the intracranial pressure and cerebral blood flow. Solid arrows to the left indicate the starting values for MAP (90 mm Hg) and CCP (43 mm Hg) found in the awake animal in the sitting position. Intracranial pressure is raised continuously, as shown by the solid bars. Vasomotor tone compensates for this by decreasing to zero, as shown by the cross-hatched bars.

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    Relationship between the mean arterial pressure and cerebral blood flow. See text for description.

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