Vasodilators during cerebral aneurysm surgery

Training anesthesiology residents in providing anesthesia for awake craniotomy: learning curves and estimate of needed case load

STUDY OBJECTIVE

To measure the learning curves of residents in anesthesiology in providing anesthesia for awake craniotomy, and to estimate the case load needed to achieve a "good-excellent" level of competence.

DESIGN

Prospective study.

SETTING

Operating room of a university hospital.

SUBJECTS

7 volunteer residents in anesthesiology.

MEASUREMENTS

Residents underwent a dedicated training program of clinical characteristics of anesthesia for awake craniotomy. The program was divided into three tasks: local anesthesia, sedation-analgesia, and intraoperative hemodynamic management. The learning curve for each resident for each task was recorded over 10 procedures. Quantitative assessment of the individual's ability was based on the resident's self-assessment score and the attending anesthesiologist's judgment, and rated by modified 12 mm Likert scale, reported ability score visual analog scale (VAS). This ability VAS score ranged from 1 to 12 (ie, very poor, mild, moderate, sufficient, good, excellent). The number of requests for advice also was recorded (ie, resident requests for practical help and theoretical notions to accomplish the procedures).

MAIN RESULTS

Each task had a specific learning rate; the number of procedures necessary to achieve "good-excellent" ability with confidence, as determined by the recorded results, were 10 procedures for local anesthesia, 15 to 25 procedures for sedation-analgesia, and 20 to 30 procedures for intraoperative hemodynamic management.

CONCLUSIONS

Awake craniotomy is an approach used increasingly in neuroanesthesia. A dedicated training program based on learning specific tasks and building confidence with essential features provides "good-excellent" ability.

Lawrence K. Using near-infrared spectroscopy to measure cerebral metabolic rate of oxygen under multiple levels of arterial oxygenation in piglets

Improving neurological care of neonates has been impeded by the absence of suitable techniques for measuring cerebral hemodynamics and energy metabolism at the bedside. Currently, near-infrared spectroscopy (NIRS) appears to be the technology best suited to fill this gap, and techniques have been proposed to measure both cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2). We have developed a fast and reliable bolus-tracking method of determining CMRO2 that combines measurements of CBF and cerebral venous oxygenation [venous oxygen saturation (CSvO2)]. However, this method has never been validated at different levels of arterial oxygenation [arterial oxygen saturation (SaO2)], which can be highly variable in the clinical setting. In this study, NIRS measurements of CBF, CSvO2, and CMRO2 were obtained over a range of SaO2 in newborn piglets ( n = 12); CSvO2 values measured directly from sagittal sinus blood samples were collected for validation. Two alternative NIRS methods that measure CSvO2 by manipulating venous oxygenation (i.e., head tilt and partial venous occlusion methods) were also employed for comparison. Statistically significant correlations were found between each NIRS technique and sagittal sinus blood oxygenation ( P < 0.05). Correlation slopes were 1.03 ( r = 0.91), 0.73 ( r = 0.73), and 0.73 ( r = 0.81) for the bolus-tracking, head tilt, and partial venous occlusion methods, respectively. The bolus-tracking technique displayed the best correlation under hyperoxic (SaO2 = 99.9 ± 0.03%) and normoxic (SaO2 = 86.9 ± 6.6%) conditions and was comparable to the other techniques under hypoxic conditions (SaO2 = 40.7 ± 9.9%). The reduced precision of the bolus-tracking method under hypoxia was attributed to errors in CSvO2 measurement that were magnified at low SaO2 levels. In conclusion, the bolus-tracking technique of measuring CSvO2, and therefore CMRO2, is accurate and robust for an SaO2 > 50% but provides reduced accuracy under more severe hypoxic levels.

Controlled hypotension in children: a critical review of available agents

Due to the potential for the transmission of infectious diseases with the homologous transfusion of blood products, there has been an increased interest in measures to limit intraoperative blood loss and avoid the need for homologous transfusion during high-risk surgical procedures including spinal surgery. Controlled hypotension (also referred to as deliberate or induced hypotension), defined as a reduction of systolic blood pressure to 80 to 90 mm Hg, a reduction of mean arterial pressure (MAP) to 50 to 65 mm Hg or a 30% reduction of baseline MAP, is one technique that has been used to limit intraoperative blood loss. In the adult population, several agents have been used alone or in combination for controlled hypotension including the inhalational anesthetic agents, direct-acting vasodilators such as nitroglycerin (glyceryl trinitrate) and nitroprusside, beta-adrenoceptor antagonists, and calcium channel antagonists. Despite clinical studies that have clearly demonstrated a reduction in blood loss with controlled hypotension when compared with the normotensive state and despite potential theoretical issues with each agent, there are no definitive studies demonstrating the preferred pharmacologic agent. When considering the pediatric-aged patient, studies have reported the use of the inhalational agent sevoflurane, the alpha(2)-adrenoceptor agonist dexmedetomidine as well as various vasodilators including sodium nitroprusside, nitroglycerin, fenoldopam, and alprostadil for controlled hypotension. Sevoflurane offers the advantages of easy dosage titration, no need for an additional intravenous infusion as well as providing anesthesia in addition to controlled hypotension. Disadvantages include a slightly higher cost than some of the intravenous agents and the inability to monitor evoked potentials with high sevoflurane concentrations. Whereas sodium nitroprusside, nicardipine and fenoldopam all provide the desired level of hypotension in pediatric-aged patients, nitroglycerin was not effective in this age group of patients in one study. When comparing nicardipine and sodium nitroprusside, nicardipine offers the potential advantages of fewer episodes of excessive hypotension, less rebound tachycardia and, in one study, less blood loss. Although fenoldopam has been shown to be effective, cost issues may limit is widespread application for this technique. The pharmacologic profile of dexmedetomidine indicates that this drug has potential in controlled hypotension and clinical data are needed to define its role.

Dose-dependent inversion of the effect of prostaglandin E1 on intraosseous pressure: adequate dose for reducing blood loss during operation on bone

Tibial intraosseous pressure and blood flow, along with arterial pressure, were measured in anesthetized rabbits before and during intravenous infusion of prostaglandin E1 at doses of 0, 100, 300, 1000, and 2000 ng/kg per min. The mean arterial pressure decreased dose-dependently. The intra-osseous pressure increased during infusion at doses of 300 and 1000 ng/kg per min, but decreased during infusion at the dose of 2000 ng/kg per min. Bone blood flow increased during infusion only at a dose of 1000 ng/kg per min. Calculated vascular and arteriolar resistances in bone decreased dose-dependently during infusion at doses of up to 1000 ng/kg per min, but no change was seen when the dose was increased from 1000 to 2000 ng/kg per min. These findings indicated that: (1) there was a turning point in the effect of prostaglandin E1 at a certain dose between 1000 and 2000 ng/kg per min, at which the arterioles in bone were fully dilated; (2) at a dose of 1000 ng/kg per min or lower, responsive dilatation in the arterioles of bone under mild arterial hypotension increased the influx of blood into the bone, thereby increasing the intraosseous pressure; and (3) at doses higher than 1000 ng/kg per min, dose-dependent arterial hypotension without further dilatation in the arterioles of bone decreased the influx of blood.

The effect of the nitric oxide donor glyceryl trinitrate on global and regional cerebral blood flow in man

Despite their potential use as cerebral vasodilatory agents there are few studies of the effect of nitric oxide (NO) donors on the cerebral circulation in non-anaesthetised man. We determined the effect of the NO donor glyceryl trinitrate (GTN) at clinically relevant doses on global and regional cerebral blood flow (CBF) in healthy non-anaesthetised volunteers, using H(2)(15)O PET, ultrasonic colour velocity flow imaging of carotid artery flow, and transcranial Doppler (TCD) of middle cerebral artery velocities (MCAv). Three rates of GTN infusion (0.1, 0.4, 1.0 microg/kg/min) were used. There was no significant change in common or internal carotid artery flow following GTN administration although a dose dependent fall in MCAv post GTN was observed. There was no significant change in either global or regional CBF following GTN. Thus intravenous GTN at therapeutic doses in awake humans does not alter global or regional CBF. However it does produce basal cerebral artery vasodilatation as evidenced by a fall in MCAv in the absence of a change in internal carotid artery flow.

Generation of reactive oxygen species by the redox cycling of nitroprusside

The formation of oxygen species during the redox cycling of sodium nitroprusside by rat liver microsomes and by chemical reductants was evaluated. The reduction of sodium nitroprusside by ascorbate and glutathione results in formation of the nitroprusside nitroxide radical which, on freezing at 77 K, results exclusively in the tetracyano [Fe(CN4)NO]2- and pentacyano [Fe(CN5)NO]3- forms of nitroxide radicals, respectively. The role of reducing agents on the inter-conversion of these two forms of nitroxide radical is discussed. The NADH and NADPH dependent microsomal reduction of nitroprusside results in the production of nitroprusside nitroxide radical, which in the presence of oxygen undergoes redox cycling to generate superoxide radical, and eventually hydroxyl radical is formed by a Fenton-type of reaction. Studies on the effect of several biologically or toxicologically relevant iron chelators on NADPH-dependent microsomal reduction of nitroprusside and subsequent formation of hydroxyl radical indicate that certain iron chelators such as isocitrate act as hydroxyl radical scavengers (depending on its concentration), but other chelators such as EDTA and DPTA function as good catalysts for the generation of hydroxyl radicals. The NADH and nitroprusside dependent microsomal production of hydroxyl radical is better in the presence of ATP, or equal in the presence of acetate, or diminished in the presence of DTPA when compared to the NADPH- and nitroprusside-dependent microsomal production of hydroxyl radicals. The effect of these chelates on the redox cycling of iron and nitroprusside by microsomes is discussed. Rat liver sub-mitochondrial particles and human hepatoblastoma cells (HepG2 cell line) also generated superoxide and hydroxyl radicals during the redox cycling of nitroprusside. These results provide direct evidence for the production of reactive oxygen species during the redox cycling of nitroprusside, The use of nitroprusside as a nitric oxide donor in biological systems may be complicated by the necessity to consider the generation of reactive oxygen species due to redox cycling of this compound by cellular reductases and low-molecular weight reductants.

[Peroperative risks in cerebral aneurysm surgery]

The perioperative complications associated with cerebral aneurysm surgery require a specific anaesthetic management. Four major perioperative accidents are discussed in this review. The anaesthetic and surgical management in case of rebleeding subsequent to the re-rupture of the aneurysm is mainly prophylactic. It includes haemodynamic stability assurance, maintenance of mean arterial pressure (MAP) between 80-90 mmHg during stimulation of the patient such as endotracheal intubation, application of the skull-pin head-holder, incision, and craniotomy. The aneurysmal transmural pressure should be adequately maintained by avoiding an aggressive decrease of intracranial pressure. Once the skull is open, the brain must be kept slack in order to decrease pressure under the retractors and avoid the risks of stretching and tearing of the adjacent vessels. If, despite these precautions, the aneurysm ruptures again. MAP should be decreased to 60 mmHg and the brain rendered more slack, in order to allow direct clipping of the aneurysm, or temporary clipping of the adjacent vessels. The optimal agents in this situation are isoflurane (which decreases CMRO2), intravenous anaesthetic agents (inspite their negative inotropic effect, they may potentially protect the brain) and sodium nitroprusside. Vasospasm occurs usually between the 3rd and the 7th day after subarachnoid haemorrhage. It may be seen peroperatively. The optimal treatment, as well as prophylaxis, is moderate controlled hypertension (MAP > 100 mmHg), associated with hypervolaemia and haemodilution, the so-called triple H therapy, with strict control of the filling pressures. Other beneficial therapies are calcium antagonists (nimodipine and nicardipine), the removal of the blood accumulated around the brain and in the cisternae, and possibly local administration of papaverine. Abrupt MAP increases are controlled in order to maintain adequate aneurysmal transmural pressure. Beta-blockers, local anaesthetics administered locally or intravenously, a carefully titrated level of anaesthesia, a maintained volaemia play a protective role. Cerebral oedema is sometimes already present at the opening of the skull or may arise later, due to a high pressure under the retractors, to the surgical manipulations of the brain or to brain ischaemia subsequent to temporary clipping. Its treatment is aggressive, with intravenous agents, mannitol, deep hypocapnia and/or lumbar drainage. Prophylaxis, according to the "brain homeostasis concept", is the preferred method to avoid these four peroperative accidents. It includes normal blood volume, normoglycaemia, moderate hypocapnia, normotension, soft manipulation of the brain and optimal brain relaxation.

Pharmacologic drugs for controlled hypotension

Due to the risks of transfusion reactions and the transmission of infectious diseases, there has been increased interest in measures to limit intraoperative blood loss and avoid the need for homologous transfusion. Controlled hypotension is one technique that has been used to limit intraoperative blood loss. Several drugs have been used alone or in combination for controlled hypotension, including the inhalational anesthetics, direct acting vasodilators such as nitroglycerin and nitroprusside, beta adrenergic antagonists, and calcium channel blockers. Various drugs available to the clinician for controlled hypotension are reviewed.

Effect of nicardipine and diltiazem on internal carotid artery blood flow velocity and local cerebral blood flow during cerebral aneurysm surgery for subarachnoid hemorrhage

STUDY OBJECTIVE

To determine the hemodynamic effects of a bolus injection of nicardipine 1 mg or diltiazem 5 mg on local cerebral blood flow (LCBF) and internal carotid blood flow velocity (ICBFV) with isoflurane anesthesia.

DESIGN

Randomized study.

SETTING

Inpatient neurosurgery and anesthesia clinic at a city hospital.

PATIENTS

26 patients with subarachnoid hemorrhage who were scheduled for cerebral aneurysm clipping.

INTERVENTIONS

A bolus injection of either nicardipine or diltiazem was administered to patients whose systolic blood pressure increased to over 150 mmHg after opening of the dura.

MEASUREMENTS AND MAIN RESULTS

After the bolus injection, both drugs rapidly decreased arterial blood pressure. Nicardipine increased LCBF [before injection, 42.1 +/- 12.3 ml/100 g/min; after injection, 47 +/- 10.7 ml/100g/min; (p < 0.05 vs control); after recovery, 42.4 +/- 11.1 cm/sec], but diltiazem did not change LCBF. Nicardipine 1 mg moderately increased ICBFV [before injection, 34.2 +/- 9.3 cm/sec; after injection, 40.6 +/- 8.7 cm/sec (p < 0.01 vs. control); after recovery, 34.1 +/- 8.9 cm/sec], but diltiazem did not change ICBFV. In addition, venous partial oxygen pressure and saturation of the internal jugular did not change throughout the study. There was a close correlation between presurgical neurologic status and LCBF (rs = -0.743; p < 0.01) and ICBFV (rs = -0.721; p < 0.01).

CONCLUSIONS

Nicardipine increased LCBF and ICBFV, but diltiazem did not change either. These results suggest that both drugs are useful and safe for the treatment of intraoperative hypertension during cerebral aneurysm surgery.