Role of cranial bone mobility in cranial compliance.

Increases in intracranial pressure are normally buffered by the displacement of blood and cerebrospinal fluid from the cranium when there is an increase in intracranial volume (ICV). How much pressure increases with an increase in ICV is expressed in the calculation of cranial compliance (delta ICV/delta P, where delta P is change in pressure) and elastance (delta P/delta ICV). Data reported here indicate that the movement of the cranial bones at their sutures is an additional factor defining total cranial compliance. Using controlled bolus injections of artificial cerebrospinal fluid into a lateral cerebral ventricle in anesthetized cats and a newly developed instrument to quantify cranial bone movement at the midline sagittal suture where the bilateral parietal bones meet, we show that these cranial bones move in association with increases in ICV along with corresponding peak intracranial pressures and changes in intracranial pressure. External restraints to the head restrict these movements and reduce the compliance characteristics of the cranium. We propose that total cranial compliance depends on the mobility of intracranial fluid volumes of blood and cerebrospinal fluid when there is an increase in ICV, but it also varies as a function of cranial compliance attributable to the movement of the cranial bones at their sutures. Our data indicate that although the cranial bones move apart even with small (nominally 0.2 ml) increases in ICV, total cranial compliance depends more on fluid migration from the cranium when ICV increases are less than approximately 3% of total cranial volume. Cranial bone mobility plays a progressively larger role in total cranial compliance with larger ICV increases.

Proposed methods for the measurement of regional renal blood flow using heat transfer analysis.

The kidney, with its heterogeneous regional perfusion in the two anatomically and functionally distinct vascular beds of the renal cortex and medulla, and with its non-uniform blood vessel geometries, presents a unique challenge for measuring intrarenal blood flow distribution. Determining whole organ perfusion, on the other hand, is comparatively simple for the kidney, but it provides relatively little information about the suspected dependency of renal excretory function on local perfusion rate. Among the variety of methods proposed for gauging regional renal blood flow, some depend on measuring one or more of the tissue's thermal properties. The most straightforward, but least reliable, involve measurements either of focal tissue temperature alone, or of regional tissue thermal gradients. Simply using heat as a diffusible indicator, however, is unreliable as a measure of blood flow, for many of the same reasons that using an inert gas in a dilution technique is unreliable. Recently developed thermal analytical methods, though, hold promise for measuring local tissue blood flow with accuracy and precision. Two of them are reviewed here. One depends on measurement of the effective thermal conductivity of a small mass of tissue by evaluating the steady state ratio between regional unidirectional heat flux across it and the associated temperature gradient in one vector along a segment of it through an imposed spheroidal heat field. The other depends on analyses of tissue temperature decay subsequent to a controlled pulse of heat delivered through a small inserted thermistor bead. Both techniques use bioheat transfer equations to deduce regional blood flow by differentiating between heat dissipation due to local thermal conductivity and that attributable to the effects of regional convection. Although both methods are unavoidably invasive, neither produces debilitating damage in the tissue volume in which perfusion is measured, nor increases local temperature or metabolism enough to affect blood flow itself. Both techniques quantify local blood flow in small volumes of tissue by detailed evaluation of the many properties of tissue and blood which affect heat transfer, and both allow for a virtually unlimited number of nearly continuous sequential measurements at short (nom. 1 min) time intervals.

Cerebrospinal fluid transients induced by hypercapnia.

Ventriculocisternal perfusion studies using tracers have shown that hypercapnia causes a transient increase in cerebrospinal fluid (CSF) outflow rate (displaced CSF volume, Vd) and a decrease in CSF effluent tracer concentration (tracer-free CSF, CSFtf). This dilution could be due to an increase in CSF formation rate (Vf) and/or to displacement of unequilibrated CSFtf sequestered in poorly mixed compartments. To facilitate convection in the subarachnoid spaces, we used a "stop-flow" procedure (by clamping the cisternal outflow tube while infusion was constant) in anesthetized cats during ventriculocisternal perfusion with mock CSF containing [14C]dextran. Each animal spontaneously breathed air, then 5% CO2 both before and after stopflow. Although Vd and the times over which Vd and CSFtf were defined were unaffected, CSFtf was decreased by 50% after stop-flow. We conclude that during ventriculocisternal perfusion, mixing is incomplete in CSF spaces, and that unequilibrated CSF contributes significantly to the reduced tracer concentration in Vd during acute hypercapnia. To determine whether Vf transiently increases in response to CO2 breathing, or to any perturbation causing craniospinal fluid redistribution, homogeneity in CSF spaces must be verified.

Effect of hypercapnia and cerebral perfusion pressure on cerebrospinal fluid production in cat.

Brain ventricles of anesthetized cats were perfused with an artificial cerebrospinal fluid (CSF) containing inulin (or [14C]dextran) and 3H-labeled sucrose while each animal respired in turn either room air or an 8-11% CO2-in-air gas mixture. Perfusion inflow (Vi) and outflow (Vo) rates and concentrations of the test molecules were measured to calculate steady-state CSF production (Vf), CSF absorption (Va), and ependymal sucrose permeability (Ksuc). During respiratory acidosis Vf varied inversely as a function of normocapnic Vf, Ksuc increased, and Va was the same as during normocapnia. Vf increased with cerebral perfusion pressure (CPP) during normocapnia but was inversely related to it during hypercapnia. When a normocapnic animal's CPP is high in the range 70-105 Torr, its Vf will also be high, but it will increase its Vf little or not at all during hypercapnia. In the same range, if its CPP is low, its Vf will also be low, but its Vf will increase predictably fourfold or more when it breathes CO2. CPP is an influential determinant of Vf at any level of acid-base balance, possibly due to variations in blood flow at CSF production sites.

An improved method for water vapor detection.

We describe improvements in and details for the construction, calibration and use of a device using a thermal conductivity cell for the measurement of low-level rates of water evaporation (E) from a small surface area. E is measured from 0.0 to 1.0 mg . min-1 with a correlation coefficient of 0.999 between measured and independently verified rates and amounts of water evaporation. Data are available as a recordable analog d.c. voltage as well as in digital display for E and for the amount of water evaporated during an operator defined time period. The device we describe is noninvasive and it is designed to be constructed of conventional components. It is useful not only for measuring transcutaneous water diffusion in normal and diseased skin, but also it is adequately sensitive and rapidly responding to follow thermoregulatory and psychogenic sweating in small (nom. 1.0 cm2) skin areas. It can also be used to measure accurately and precisely the rates at which water is adsorbed by and removed from inanimate materials, as well as to determine how much water they store.

Thermodynamic technique for the quantification of regional blood flow.

A method is described to quantify regional blood flow by thermal analysis. A weak temperature field is established in a tissue and for a thermal steady state, unidirectional heat flux and the associated temperature gradient are measured simultaneously across a small fixed segment of the tissue. This information is evaluated with probe calibrations for homogeneous isotropic fluids, with data from ancillary measurements in the nonperfused tissue and with values of specific heat and density of blood to express local blood flow in heat transfer [effective thermal conductivity (W. degrees C-1 . cm-1 x 10(-3) and/or in perfusion (ml . min-1 . cm-3)] terms. The technique measures local perfusion in small tissue volumes and is usable in acute or chronic experiments. Its accuracy is not a function of the absolute steady-state temperature of the tissue or of its metabolic heat production.

Pressure-dependent PAH and creatinine transport from the brain ventricles.

The brain ventricular system of the adult dog was perfused with an artificial cerebrospinal fluid (CSF) containing inulin, creatinine and radioactively labeled p-aminohippuric acid (PAH) and mannitol. Inflow and outflow rates and concentrations of test molecules were measured at different intraventricular pressures, allowing calculation of their steady-state rates of removal from the ventricles. Clearance of inulin, a measure of CSF bulk absorption varied nearly with intraventricular pressure (- 15 to +12 cm H2O relative to the external auditory meatus). The efflux coefficient (Ko; representing clearance of a molecule by means other than bulk absorption) for mannitol was independent of intraventricular pressure. Ko's for PAH and creatinine were pressure dependent. PAH and creatinine efflux may be related to the amount of fourth ventricular choroid plexus surface exposed to the perfusion fluid. Ko's for creatinine and PAH (46 plus or minus 4 mul/min; 34 plus or minus 4mul/min, respectively) were significantly greater than mannitol (16 plus or minus 8 mul/min) at comparable intraventricular pressures, suggesting that both creatinine and PAH leave the CSF by an active process in addition to passive diffusion.

Creatinine, potassium, and calcium flux from chicken cerebrospinal fluid.

Brains of methoxyflurane-anesthetized chickens were perfused from a lateral cerebral ventricle to cisterna magna with an artificial cerebrospinal fluid (CSF) containing trace quantities of radioiodinated human serum albumin (RIHSA) or inulin (1.0 mg/ml) to measure CSF bulk absorption. In addition, it contained either trace quantities of 22Na, 42K, 45Ca or [14C]creatinine; the concentrations of the latter three were varied to determine permeability coefficients (K-D's) as a function of concentration. A mass balance for the tracer molecules was calculated to determine their movement into brain or blood. K-D's for 45Ca, 42K, 22Na, and creatinine (Cr) were unaffected by perfusion time and the latter two were larger than previously reported (3). The lack of effect of time on K-D and the large values for K-D22Na and K-D-Cr are attributed to anesthetic effects on brain blood flow. K-D-Cr and K-D42K were larger than K-D22Na or K-D45Ca and K-D's for 45Ca, Cr, and 42K were independent of their inflow concentrations. An active transport process is suggested for potassium and creatinine, but one that is located at sites other than the ependymal wall. Bulk flow clearance accounted for RIHSA movement from CSF, whereas nonbulk clearance accounted for 50% of 22Na and 45Ca movement and 90% of 42K clearance. Fifty percent of 42K and 25% of 22Na and 45Ca were found in brain. The large recovery of 42K in brain supports the hypothesis that intracellular potassium serves as an exchangeable pool for the tracer.