The Measurement of the Partition Coefficient of Krypton between Brain Cortex and Blood by a Double Isotope Method

A generic biokinetic model for noble gases with application to radon

To facilitate the estimation of radiation doses from intake of radionuclides, the International Commission on Radiological Protection (ICRP) publishes dose coefficients (dose per unit intake) based on reference biokinetic and dosimetric models. The ICRP generally has not provided biokinetic models or dose coefficients for intake of noble gases, but plans to provide such information for (222)Rn and other important radioisotopes of noble gases in a forthcoming series of reports on occupational intake of radionuclides (OIR). This paper proposes a generic biokinetic model framework for noble gases and develops parameter values for radon. The framework is tailored to applications in radiation protection and is consistent with a physiologically based biokinetic modelling scheme adopted for the OIR series. Parameter values for a noble gas are based largely on a blood flow model and physical laws governing transfer of a non-reactive and soluble gas between materials. Model predictions for radon are shown to be consistent with results of controlled studies of its biokinetics in human subjects.

Regional blood flow in human tumours measured with argon, krypton and xenon

Blood perfusion in 19 superficial tumour metastases was measured by injecting three tracers, 41Ar, 85Krm and 135Xe or 133Xe, simultaneously into the tissue. The blood flow rate was calculated from the constants of the biexponential functions fitted to the measured counts. The mean blood flow in the 19 tumours studied was 18 ml/(100 g min) (SD +/- 10). Alternatively, the relative partition coefficients of argon, krypton and xenon were calculated from the equations obtained using compartmental analysis. The mean values for the ratios of the partition coefficients of these nuclides were found to be between 0.82 and 1.07.

The effects of calcium antagonism on the epicerebral circulation in early vasospasm

We have studied the effects of the calcium antagonist verapamil on the epicerebral arteriovenous transit time and regional epicerebral circulation of dogs by direct measurement of arterial diameters, fluorescein angiography, and krypton-85 regional epicerebral blood flow analysis. A large craniectomy was performed and vasoconstriction was induced by the subarachnoid injection of human platelet-rich plasma (PRP) pretreated with 25 mu M of ADP to cause maximum aggregation. Once vasoconstriction was established, verapamil (0.1 mg/kg) was topically applied to the perforated arachnoid. The PRP-ADP produced a mean decrease in the arterial diameters of 38.2 +/- 1.6% (p less than 0.01) at 10 minutes after its injection and verapamil produced a mean dilatation of 19.5 +/- 2.5% (p less than 0.01), compared to control values. Regional epicerebral blood flow was 54.9 +/- 3.4 ml/100 g/min in the control state, 34.8 +/- 3.2 ml/100 g/min (p less than 0.01) during vasospasm, and 78.2 +/- 4.5 ml/100 g/min (p less than 0.01) after verapamil. Fluorescein angiography, after verapamil, demonstrated a mean acceleration of the arteriovenous circulation time of 4.5 +/- 0.8 seconds (p less than 0.01) compared to the spasm value. We concluded that the topical application of verapamil can dilate previously constricted cortical arteries and that this dilatation is associated with acceleration of the epicerebral transit time and increased cerebral blood flow.

CO2 responses of the cerebral circulation during drug-induced hypotension in the cat

Concern has often been expressed that hypocapnia produced by controlled hyperventilation might further reduce cerebral perfusion during drug-induced hypotension. In the present studies, hypotension was induced in cats with either practolol/trimetaphan (five experiments) or practolol/nitroprusside (five experiments) together with controlled haemorrhage. Arterial PCO2 was altered between 17 and 51 mm Hg by varying inspired CO2 during constant-volume ventilation, first during control conditions of light halothane/nitrous oxide anaesthesia and then during hypotension to mean blood pressure of 36-37 mm Hg. Cerebral cortical perfusion was measured by the krypton clearance technique and pial artery diameter by the image-splitting method. Cerebral cortical blood flow did not alter with PaCO2 changes during trimetaphan hypotension, but some responsiveness to CO2 persisted during nitroprusside hypotension, though at less than half control levels. No changes in pial artery diameter were seen with CO2 during hypotension under either technique. It is postulated that CO2 responsiveness persisted with nitroprusside because cerebral blood flow (CBF) values were higher when hypotension was produced with this drug, as compared with trimetaphan. It would appear that hypocapnia does not further reduce CBF during trimetaphan hypotension but does do so with nitroprusside. However, the combination of hypocapnia and nitroprusside hypotension did not in any instance lower CBF below the values found during trimetaphan hypotension.

Effects of hyperbaric oxygen on intracranial pressure and cerebral blood flow in experimental cerebral oedema

Increased intracranial pressure was induced in anaesthetized dogs by application of liquid nitrogen to the dura mater. Intracranial pressure and cerebral blood flow were measured, together with arterial blood pressure and arterial and cerebral venous blood gases.Carbon dioxide was administered intermittently to test the responsiveness of the cerebral circulation, and hyperbaric oxygen was delivered at intervals in a walk-in hyperbaric chamber, pressurized to two atmospheres absolute.Hyperbaric oxygen caused a 30% reduction of intracranial pressure and a 19% reduction of cerebral blood flow in the absence of changes in arterial PCO(2) or blood pressure, but only as long as administration of carbon dioxide caused an increase in both intracranial pressure and cerebral blood flow. When carbon dioxide failed to influence intracranial pressure or cerebral blood flow then hyperbaric oxygen had no effect. This unresponsive state was reached at high levels of intracranial pressure.