Cortical blood flow in controlled hypotension as measured by thermal diffusion

Where CBF P is flow in ml/100 g/min, AV is the voltage difference of the thermocouples, and AV O Is the voltage difference of the thermocouples with no flow, which was 342.8 ± 12.9 /iv. * describes the characteristics of the probe and was determined to be 52,431.2 ± 4796.3. The average deviation of the calculated curve from the experimental data points was ± 63.The calculated * differed markedly from the mean when Xe i u fast component flows were less than 35 ml/100 g/min. This is evidence that CBF as measured by Xe l u clearance analyzed by the bicompartmental technique loses accuracy at lower flows.The thermal diffusion flow probe is a good device for evaluation of flow in acute ischemia models since it can delineate abrupt flow variations. Theoretically the flow probe can accurately measure flow at iscbemic levels. Stroke, Vol 12, No 4, 1981THE STUDY of cerebral ischemia requires cortical blood flow assessment which is continuous and accurate at reduced flows. We have employed a thermal diffusion flow probe based upon a Peltier stack to assess flow in both the laboratory and clinical environments.1 "* This technique gives a dynamic flow recording and, theoretically, should be accurate at low flows since the thermal gradient is applied directly to the cortex. In order to evaluate these recordings better we have undertaken the following study to define further the calibration and physical characteristics of the device.The probe consists of a thermoelectric heat pump in the form of a Peltier stack with "L"-shaped gold plates soldered to the hot and cold surfaces. Copperconstantan thermocouples are fixed to the approximate center of the contact surface of each gold plate so that the output voltage from the thermocouples is proportional to the temperature difference between the plates. Thermocouple and power leads are encased in a pliable cable extending from the device which is encased in plastic. The probe weighs 1.5 gm, and is 13 mm in diameter and 4 mm thick. Activation of the stack produces a temperature gradient between the plates which encompasses the ambient cortical temperature. Changes in cortical blood flow cause the thermocouple voltage to vary and this voltage is correlated with the fast component of Xe 1M clearance (F,). Observation of the flow probe with the operating microscope has revealed that the weight of the probe causes obstruction of the pial circulation, thus leaving the probe in direct contact with the cortical capillary bed. Application of weights up to 5 g on the probe to increase probe pressure on the brain has no effect on the recording. It is therefore unlikely that the 1.5 g weight of the probe alters the flow in the cortical capillary bed. Since cortical flow is much greater than white matter flow and the probe is in direct contact with the cortex, it appears reasonable to correlate the probe with F g in the exposed brain. MethodEleven mongrel cats were anesthetized with intraperitoneal sodium pentothal (40 mg/kg), paralyzed with gallamine triethiodide (20 mg) and placed on a positi...

Section: Discussionmentioning

confidence: 89%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Cortical blood flow: thermal diffusion vs isotope clearance.

Carter¹,

Erspamer²,

Bro³

1981

“…The thermal probe used in these experiments is an updated version of the one decribed by Carter and Atkinson in 1973 2 and elaborated on elsewhere. [3][4][5] The probe consists of a Peltier stack with L-shaped gold plates arranged so that the voltage output is proportional to the temperature difference between the plates.…”

Section: Thermal Probementioning

confidence: 99%

Comparison of local cerebral blood flow determined by thermal and hydrogen clearance.

Gaines¹,

Carter²,

Crowell³

1983

SUMMARY Local cerebral blood flow (ICBF) was measured simultaneously in ten cats with (1) a large surface thermal diffusion probe resting on the cortex and (2) hydrogen clearance curves from implanted electrodes surrounding the thermal probe. A close correlation was found between ICBF values obtained by the two methods. Since hydrogen clearance is accepted as quantitative, the data suggest that the thermal diffusion technique is a reliably quantitative means of measuring local cerebral blood flow.

“…8 Carter and Atkinson, in 1973, revised the cortical surface probe using the Peltier thermoelectric effect as previously described by Brawley. 10 Calibration of this probe was carried out with '"Xenon clearance curves which demonstrated a linear relationship between the radioactive diffusion-washout rCBF and thermal diffusion local CBF measurements. These investigators believed that the Peltier flow probe was preferable to the heat clearance methods proposed by Betz and associates because it could quantitatively record continuous changes in CBF with reproducible re- 11 These investigators demonstrated that such a system could measure the thermal conductivity of tissue in the vicinity of a small heated thermistor maintained at a fixed temperature difference above a reference thermistor.…”

mentioning

confidence: 99%

Continuous quantitative local cerebral blood flow measurement. Calibration of thermal conductivity measurements by the hydrogen clearance method.

Cusick¹,

Myklebust²

1980

SUMMARY The capability of a miniaturized probe to measure local cerebral blood flow in a continuous and quantitative manner is described. The incorporation of thermal conductivity measurements using the isothermal principle with the hydrogen clearance method allows calibration of the thermal conductivity component in absolute terms. Evaluation of this system in 14 cats showed a linear relationship between both measurement methods. The major limitation of this combination probe system is the need for routine intermittent recalibration in order that changes of tissue thermal conductivity induced by physiologic alterations during the experimental procedure may be recognized and resolved.