Modification of nitrous oxide-induced intracranial hypertension by prior induction of anesthesia

Monitoring and intraoperative management of elevated intracranial pressure and decompressive craniectomy

There are numerous clinical scenarios wherein a critically ill patient may present with neurologic dysfunction. In a general sense these scenarios often involve ischemia, trauma, or neuroexcitation. Each of these may include a period of decreased cerebral perfusion pressure, usually due to elevated intracranial pressure (ICP), eventually compromising cerebral blood flow sufficiently to produce permanent neuronal loss, infarction, and possibly brain death. Elevated ICP is thus a common pathway for neural demise and it may arise from a variety of causes, many of which may result in a neurosurgical procedure intended to ameliorate the impact or etiology of elevated ICP.

Inhibition of cerebral metabolic and circulatory responses to nitrous oxide by 6-hydroxydopamine in dogs

PURPOSE

To determine whether cerebral metabolic and circulatory consequences of N2O result from activation of the sympathoadrenal system. The effects of pretreatment with intracisternal injection of 6-OHDA, which produces chemical sympathectomy, were studied in dogs.

METHOD

Seven days before measurement dogs were pretreated with intracisternal injection of either saline vehicle (sham-group) or 100 micrograms.kg-1 6-hydroxydopamine (6-OHDA, group). Cerebral blood flow (CBF) was measured using an electromagnetic flow-meter probe and cerebral metabolic rate for oxygen (CMRO2) was calculated as the product of CBF and arterial-sagittal sinus blood oxygen content difference [C(a-v)O2].

RESULTS

In the sham group, N2O (60%) increased CMRO2 from 6.11 +/- 0.21 ml.100 g-1.min-1 to 7.10 +/- 0.39 ml.100 g-1.min-1 and CBF from 63 +/- 5 ml.100 g-1.min-1 to 173 +/- 26 ml.100 g-1.min-1. In the 6-OHDA group, CMRO2, did not change during N2O exposure, whereas CBF increased from 61 +/- 3 ml.100 g-1.min-1 to 135 +/- 19 ml.100 g-1.min-1 but less then in the sham group. The 6-OHDA group displayed a reduction in cortical noradrenaline (NA) concentration from 263.2 +/- 35.6 ng.g-1 to 102.7 +/- 16.5 ng.g-1. Cortical dopamine (DA) concentration was not affected by 6-OHDA administration.

CONCLUSION

These results suggest that most of the increase in CMRO2 and at least a part of the increase in CBF during N2O exposure in the sham-group are related to sympathoadrenal-stimulating effects of N2O.

Effect of nitrous oxide on cerebral blood velocity while reclining and standing

Cerebral blood velocity (CBV) (measured with transcranial Doppler, TCD) and other physiological and rating scale indices were measured before, during, and after inhaling a mixture of 40% nitrous oxide/oxygen and 40% nitrogen/oxygen, given during two separate visits in 7 normal male volunteers. During nitrous oxide/oxygen but not nitrogen/oxygen inhalation, CBV and euphoria increased significantly with minimal changes in other physiological indices except an increase in pulse rate after nitrous oxide/oxygen. In another 7 male volunteers, similar measurements were obtained while lying down and standing up during nitrous oxide/oxygen mixture and nitrogen/oxygen mixture inhalations, given during two separate visits. Nitrous oxide/oxygen inhalation produced significant CBV increase, which showed nonsignificant decrease of no clinical significance during standing up. There were no significant postural changes in blood pressure. Standing up during nitrous oxide/oxygen inhalation was associated with significant though mild dizziness.

Effects of nitrous oxide on cerebral haemodynamics and metabolism during isoflurane anaesthesia in man

Seven normoventilated and five hyperventilated healthy adults undergoing cholecystectomy and anaesthetized with methohexitone, fentanyl and pancuronium were studied with measurement of cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), and quantified electroencephalography (EEG) under two sets of conditions: 1) 1.7% end-tidal concentration of isoflurane in air/oxygen; 2) 0.85% end-tidal concentration of isoflurane in nitrous oxide (N2O)/oxygen. The object was to study the effects of N2O during isoflurane anaesthesia on cerebral circulation, metabolism and neuroelectric activity. N2O in the anaesthetic gas mixture caused a 43% (P less than 0.05) increase in CBF during normocarbic conditions but no significant change during hypocapnia. CMRO2 was not significantly altered by N2O. EEG demonstrated an activated pattern with decreased low frequency activity and increased high frequency activity. The results confirm that N2O is a potent cerebral vasodilator in man, although the mechanisms underlying the effects on CBF are still unclear.

Anesthesia for craniotomy: total intravenous anesthesia with propofol and alfentanil compared to anesthesia with thiopental sodium, isoflurane, fentanyl, and nitrous oxide

STUDY OBJECTIVE

To compare a total intravenous (IV) anesthetic technique based on propofol and alfentanil with a commonly used anesthetic technique for craniotomy.

DESIGN

Open-label, randomized, clinical study.

SETTING

Neurosurgical clinic at a university hospital.

PATIENTS

Forty patients, aged 18 to 55 years, scheduled for brain tumor surgery.

INTERVENTIONS

In 20 patients, anesthesia was induced with fentanyl and thiopental sodium and maintained with fentanyl, dehydrobenzperidol, isoflurane, nitrous oxide (N2O), and a thiopental sodium infusion. Twenty patients were anesthetized with a propofol loading infusion followed by a maintenance infusion at a fixed rate. In addition, alfentanil was administered as a loading bolus, followed by a variable-rate infusion, with additional doses as necessary to maintain hemodynamic stability.

MEASUREMENTS AND MAIN RESULTS

A decrease in blood pressure (BP) after induction with thiopental sodium was followed by a significant increase in BP and heart rate (HR) during intubation. BP and HR did not change during the propofol loading infusion. However, the administration of alfentanil was followed by a similar decrease in BP with a return to baseline values during the intubation period. Return of normal orientation (7 +/- 5 minutes vs 27 +/- 23 minutes) and concentration (12 +/- 12 minutes vs 35 +/- 37 minutes) was shorter and more predictable for the propofol-alfentanil-treated patients than for the thiopental sodium patients. Maintenance propofol concentration (nine patients) was between 3 +/- 0.69 micrograms/ml and 3.36 +/- 1.17 micrograms/ml, while the concentration at awakening was 1.09 microgram/ml. Alfentanil concentration at extubation (nine patients) was 79 +/- 34 ng/ml.

CONCLUSION

A total IV anesthetic technique with propofol and alfentanil is a valuable alternative to a more commonly used technique based on thiopental sodium, N2O, fentanyl, and isoflurane.

Effects of nitrous oxide on global and regional cortical blood flow

We conducted regional cortical blood flow studies using the xenon-133 clearance technique on 12 volunteers during the administration of 25% and 50% N2O and during baseline conditions (breathing room air or 100% O2). Global cortical blood flow was very highly significantly increased above baseline measures in all subjects by 50% N2O (mean increase 37% above 100% O2 condition). A smaller but still significant increase was observed with 25% N2O. Analysis of regional cortical blood flow revealed heterogeneity in the pattern of changes; that is, the baseline pattern was altered by the inhalation of N2O, most often resulting in an accentuation of relative frontal blood flow. The anterior-posterior gradient in N2O-induced blood flow changes differs from that observed with simple vasodilatory agents, such as CO2, with which the increase is purely systemic and the baseline pattern is preserved. This indicates that N2O has differential effects on cerebral metabolism that may well reflect the typical alterations in experiential state reported by subjects.

Reversal of the cerebral effects of diazepam in the dog by the benzodiazepine antagonist Ro15-1788

In six dogs initially anesthetized with halothane-N2O-O2 for surgery and maintained during the experiment with high spinal anesthesia and local infiltration, diazepam 3.0 mg X kg-1 converted the EEG from an awake to a sleep pattern. This was accompanied by a significant 21% reduction in cerebral metabolic rate for oxygen (CMRO2) and a 15% reduction in cerebral blood flow (CBF). Substituting nitrous oxide 70% for nitrogen had no additional cerebral effects. The benzodiazepine antagonist Ro15-1788, 2 mg, completely reversed the effects of diazepam on the EEG, CMRO2 and CBF. Cerebral biopsies taken at the end of the study revealed modest but significant decreases in ATP and the energy charge along with increases in AMP, lactate and lactate/pyruvate (L/P) ratio. These changes are unexplained and suggest a possible disturbance in oxidative phosphorylation produced by Ro15-1788 preceded by diazepam.

The influence of intravenous anaesthetic agents on primarily increased intracranial pressure

In the choice of anaesthetics and techniques the danger of a possible progressive increase of intracranial pressure (ICP) should be considered. Therefore the influence of intravenous anaesthetic agents on mean arterial pressure, ICP, and cerebral perfusion pressure (CPP) in patients with primarily increased ICP was observed under standard conditions for 20-40 minutes. Etomidate, thiopentone, propanidid, and ketamine showed remarkable effects on ICP, even in patients with disturbed cerebro-vascular reactivity. Etomidate and thiopentone cause a fall of ICP by 26%. Because of its stabilizing effects on circulation etomidate does not induce a reduction of CPP, whereas thiopentone will do so because of its depressing effect on blood pressure. Propanidid appears to be a less suitable agent when there is raised ICP, because it induces fluctuations of ICP and blood pressure up to the third minute after injection. According to our results, monoanaesthesia with ketamine cannot be recommended when there is increased ICP because it causes a prolonged increase in ICP, and reduction of blood pressure and CPP.