We have constructed low-cost equipment for the measurement of the optical density of x-ray films. This equipment is based on a video system connected via an analog to digital converter to an Apple microcom puter. The optical density of an x-ray film can thus be
The dynamic relationships among mean flow velocity, its pulsatile amplitude (FVa), cortical cerebral blood flow (CBF), and cerebral perfusion pressure (CPP) were studied in normal rabbits and rabbits with subarachnoid hemorrhage using 8-MHz pulsed transcranial Doppler ultrasound and hydrogen clearance under conditions of systemic hypotension and intracranial hypertension. A two-slope relationship was observed between FVa and CPP with a break point that correlated closely with the lower limit of CBF autoregulation in each animal. Below this CPP break point, FVa varied directly with CPP, and above the break point FVa varied inversely with CPP. In this experimental model, an inverse correlation between FVa and CPP indicates intact CBF autoregulation, whereas loss of that correlation implies exhaustion of autoregulatory reserve. Simultaneous recording and computation of FVa, CPP, and the correlation coefficient between FVa and CPP may be a means of monitoring CBF autoregulation in clinical practice.
Regional pituitary blood flow has been studied in adult female Fischer 344 rats by [14C]iodoantipyrine autoradiography. A general mathematical solution has been derived to allow the calculation of blood flow in the second compartment of a portal system and the proportion of blood "shunted" through the first compartment without exposure to tissue uptake from a knowledge of (a) the volume ratios of the two compartments, (b) the tissue tracer uptakes of the two compartments, and (c) the arterial tracer concentration with respect to time of a freely diffusible tracer. Significant diffusion limitation and/or arteriovenous shunting has been demonstrated in the neurohypophysis, suggesting that the majority of incoming blood is "shunted" unchanged to the adenohypophysis. The mean value of the shunt is 89% (range of 84-93%) for the median eminence and lies between 72% (range of 52-82%) and 73% (range of 59-81%) for the posterior pituitary. Neurohypophysial flow rates of 1.20 (range of 0.99-1.55) ml g-1 min-1 for the median eminence and 1.68 (range of 0.83-3.53) ml g-1 min-1 for the posterior pituitary were measured. These values represent "tissue-available" (nonshunted) flow; estimated mean total (shunted plus nonshunted) neurohypophysial flow rates were 11.7 (range of 9.5-17.5) ml g-1 min-1 for the median eminence and 6.1 (range of 3.1-8.9) ml g-1 min-1 (minimum) for the posterior pituitary. Adenohypophysial blood flow is heterogeneous. In the long portal territory, the flow rate was 1.18 (range of 0.95-1.75) ml g-1 min-1 but short portal territory flow calculation is complicated by an unquantifiable nonportal venous drainage; using the natural limits of zero and 100% gives a minimum adenohypophysial flow rate of 1.42 (range of 0.76-2.07) ml g-1 min-1 and a maximum value of 1.97 (range of 1.03-2.82) ml g-1 min-1.
The dynamic relationships among mean flow velocity, its pulsatile amplitude (FVa), cortical cerebral blood flow (CBF), and cerebral perfusion pressure (CPP) were studied in normal rabbits and rabbits with subarachnoid hemorrhage using 8-MHz pulsed transcranial Doppler ultrasound and hydrogen clearance under conditions of systemic hypotension and intracranial hypertension. A two-slope relationship was observed between FVa and CPP with a break point that correlated closely with the lower limit of CBF autoregulation in each animal. Below this CPP break point, FVa varied directly with CPP, and above the break point FVa varied inversely with CPP. In this experimental model, an inverse correlation between FVa and CPP indicates intact CBF autoregulation, whereas loss of that correlation implies exhaustion of autoregulatory reserve. Simultaneous recording and computation of FVa, CPP, and the correlation coefficient between FVa and CPP may be a means of monitoring CBF autoregulation in clinical practice.
SUMMARY Serial measurements of global cerebral blood flow (CBF) were made in 15 patients undergoing elective neurosurgical procedures in the sitting position, using a modified intravenous 133Xenon technique.' The mean supine CBF rose from 43 (+/-3) ml/lOOg/min to 62 (+/-6) ml/1OOg/min in the sitting position and remained elevated at the end of surgery at 62 (+/-5) ml/1OOg/min. Both increases in CBF were statistically significant with respect to baseline supine values.The threat of cardiovascular instability and air embolism dissuades many neurosurgeons from using the sitting position.2 Even in the absence of these complications the cerebral perfusion pressure and cardiac output are known to fall when patients are sat up, and in these circumstances a reduction in CBF might be anticipated. A sustained peroperative fall in CBF could prejudice the outcome of an otherwise uncomplicated procedure. This study was undertaken to examine changes in CBF during anaesthesia and surgery in the sitting position and their relationship, if any, to outcome.
Materials and methodsThe patients studied were undergoing elective surgery under the care of a single consultant neurosurgeon (JB) Following premedication with atropine 0-6 mg IM, general anaesthesia was induced using intravenous sodium thiopentone 250-500 mg, fentanyl 100-150 rg, and alcuronium or pancuronium and maintained with 60% nitrous oxide/-oxygen, fentanyl, alcuronium or pancuronium, supplemented by occasional use of 0-25-0 5% halothane or 0 2-0 5% trichloroethylene. In every case the electrocardiogram, radial arterial blood pressure and end tidal pCO2 were continuously monitored (Hewlett Packard; ECG-HP0300A, Pressure transducer-HP1290A, Capnometer-47210A). In 12 cases an oesophageal stethoscope and precordial Doppler probe (Sonicaid Doppler) were employed. A right atrial catheter was inserted in five cases. After endotracheal intubation the patients were mechanically ventilated using a Cape-Wayne Mk 111 ventilator, maintaining the end tidal pCO2 between 30 and 35 mm Hg.Positive and expiratory pressure (PEEP) was added at the discretion of the anaesthetist. All patients had compression bandages applied to their lower limbs and 400-500 ml crystalloid fluid were given intravenously before changing position. A standard neurosurgical chair was used in which the knees were kept flexed to avoid sciatic nerve traction and the head supported in a horseshoe headrest. Rigid cranial fixation was not used.The global cerebral blood flow was measured using a modified intravenous 1 33Xenon technique, deriving the data for cerebral and arterial clearance from single mid-frontal and apical chest probes as previously described from this centre.
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