IGH intracranial pressure (ICP) and hemorrhagic hypotension often coexist in patients with multi- ple injuries. 16,19 Several clinical and experimental studies describe the effect of hemorrhagic hypotension on the cardiovascular system, 10,15,21,25 and the effect of high ICP on the circulation has also been extensively docu- mented. 3,4,14,18,22,23 Both of these common pathophysiolog- ical states increase sympathetic nervous activity 15,18 and cause a redistribution of blood flow to vital organs. 14,21 Systemic hypotension greatly increases the mortality of head-injured patients. 16,19 It is therefore possible that the combination of hypotension and increased ICP has dele- terious effects on cardiovascular functions important to survival. There are, however, no specific studies of the systemic circulatory consequences of hemorrhagic hypo- tension during high ICP. The aim of this study was to mea- sure systemic circulatory variables and blood flow distri- bution in hemorrhagic hypotension during normal and increased ICP in an experimental animal model. Materials and Methods Animals and Operative Technique This study was performed with the consent of the Norwegian Council of Animal Research according to the Code for the Care and Use of Animals for Experimental Purposes. Eight juvenile Norwegian-bred landrace pigs of either sex weigh- ing 18 to 28 kg were used. The animals were anesthetized by in- traperitoneal injection of 25 mg/kg pentobarbital, and anesthesia was continued with 15 mg/kg/hour as a continuous infusion through an ear vein, supplemented with intermittent intravenous injections of 1 mg/kg pentobarbital as an analgesic. After tracheotomy, ven- tilation was established with 70% nitrous oxide and 30% oxygen by a servoventilator at 20 breaths/minute (5–7 L/min) adjusted to maintain normocapnia according to blood gas measurements. Mus- cular paralysis was achieved with pancuronium (0.1 mg/kg given intravenously in repeated doses as needed) after the surgical proce- dure had been completed. With the animal in the supine position, an indwelling bladder catheter was inserted via a cystostomy. Fluid- filled catheters were inserted into the aorta through the right femo- ral and brachial arteries to monitor the arterial pressure and for J. Neurosurg. / Volume 83 / December, 1995 J Neurosurg 83:1067–1071, 1995 Cardiovascular response to blood loss during high intracranial pressure OLE J. KIRKEBY , M.D., PH.D., INGUNN R. RISE, M.D., LARS NORDSLETTEN, M.D., PH.D., SIGMUND SKJELDAL, M.D., PH.D., AND CECILIE RISØE, M.D., PH.D. Institute for Surgical Research and Department of Neurosurgery, The National Hospital, University of Oslo, Oslo, Norway  The authors hypothesized that the combination of hemorrhage and increased intracranial pressure (ICP) has dele- terious effects on cardiovascular function. The effect of blood loss during normal and increased ICP was studied in eight pigs. The mean arterial pressure (MAP), pulmonary arterial pressure, pulmonary capillary wedge pressure, car- diac output, and cerebrospinal fluid (CSF) pressure were measured. The regional tissue blood flow was determined with radioactive microspheres labeled with four different nuclides. High ICP (80% of MAP) was induced by infusion of artificial CSF into the cisterna magna. The response to rapid arterial bleeding of 25% of blood volume was mea- sured. The decrease in blood flow to the intestine, skeletal muscle, and the kidneys after blood loss was significantly greater during high ICP. The decrease in blood flow to the spleen and pancreas tended to be greater during high ICP, whereas the changes in blood flow to the liver, adrenal glands, and heart muscle showed no such tendency. The fall in cardiac output and heart stroke volume after blood loss were more pronounced when the ICP was high, and the increase in systemic vascular resistance was considerably greater. These observations suggest that during high ICP the physio- logical protective mechanisms against blood loss are impaired in the systemic circulation, and a loss of 25% of the blood volume, normally well compensated for, may induce a state of shock. KEY WORDS • cardiovascular function • hemorrhagic hypotension • high intracranial pressure • organ blood flow • shock • pig H 1067