Cardiovascular effects of sedative infusions of propofol and midazolam after spinal anaesthesia

Neurosurgical Anesthesia: Optimizing Outcomes with Agent Selection

Anesthesia in neurosurgery embodies a vital element in the development of neurosurgical intervention. This undisputed interest has offered surgeons and anesthesiologists an array of anesthetic selections to utilize, though with this allowance comes the equally essential requirement of implementing a maximally appropriate agent. To date, there remains a lack of consensus and official guidance on optimizing anesthetic choice based on operating priorities including hemodynamic parameters (e.g., CPP, ICP, MAP) in addition to the route of procedure and pathology. In this review, the authors detail the development of neuroanesthesia, summarize the advantages and drawbacks of various anesthetic classes and agents, while lastly cohesively organizing the current literature of randomized trials on neuroanesthesia across various procedures.

[Sedation in surgical procedures using regional anesthesia in adult patients: results of a survey of Spanish anesthesiologists]

OBJECTIVE

To analyze the frequency and methods of sedation used in the context of regional anesthesia in adults by means of a national survey.

MATERIAL AND METHODS

We carried out a survey of participants at the courses of an anesthesiology training organization (Fundación Europea de Enseñanza en Anestesiología), held in Spain in 2006. The survey questionnaires asked about indications for sedation used during surgery under regional anesthesia as well as the form of administration, follow-up, and complications.

RESULTS

A total of 375 questionnaires were sent out and 185 responses were received (49.3%). Sedation is always used to accompany regional anesthesia by 69.2% of the respondents; 13.5% of them discuss the technique to be used with the patient and come to an agreement. The same type of sedation, regardless of the regional block performed, is used by 49.2% of respondents, and 64.3% use a scale to evaluate the level of sedation. The most favored sedation technique is continuous infusion, followed by target controlled infusion and boluses on demand. The most commonly used technique is sedation with bolus injections. Sixty percent use a single agent and 38.9% use combinations. The most commonly reported adverse effects are variability of patient response (53.5%) and respiratory complications (27%). In cases of ineffective regional blockade, 49.2% of those surveyed switch to general anesthesia.

CONCLUSIONS

Sedation is very often used to complement regional anesthesia in adult patients. Even though continuous infusion is considered to be the most appropriate form of administration, the most commonly used form is injection of boluses. Sedation with a single drug is used more frequently than drug combinations. Variability of individual response is the complication most commonly reported by the respondents.

Comparison of the effects of propofol and midazolam on the cardiovascular autonomic nervous system during combined spinal and epidural anesthesia

STUDY OBJECTIVE

To investigate the effect of propofol and midazolam on cardiac autonomic nervous system (CANS) activity during combined spinal-epidural anesthesia.

DESIGN

Prospective, clinical study.

SETTING

Operating room of a university hospital.

PATIENTS

Forty ASA physical status I and II patients scheduled for knee surgery.

INTERVENTION

Patients were randomized to receive sedation with either propofol or midazolam.

MEASUREMENTS

Heart rate (HR), HR variability (HRV), systolic arterial pressure (SAP), and SAP variability (SAPV) were used for the analysis. These values were measured at the preanesthetic period, after intrathecal injection for spinal anesthesia, after sedation with propofol or midazolam, and just before the end of surgery with sedation. Cross-spectral analyses of the HR and SAP data were assessed to quantify the frequency-related coherence spectra and phase spectra.

MAIN RESULTS

Spinal anesthesia itself had no effect on power spectral changes in both groups. After sedation, as for HRV, high-frequency (HF) power (HF, 0.15-0.40 Hz) did not change, whereas low-frequency (LF) power (LF, 0.04-0.15 Hz) and LF/HF, an indicator of CANS balance, significantly decreased with propofol. Further, coherence in cross-spectra presented depression in the LF band area after sedation with propofol. Before the end of surgery with sedation, LF and LF/HF in both HRV and SAPV were correlated with age in those with propofol; however, scarce relation was observed in those who received midazolam.

CONCLUSIONS

Propofol was more potent than midazolam in causing CANS activity to be sympatholytic during combined spinal and epidural anesthesia and which was correlated with age only with propofol.

The timing of intravenous crystalloid administration and incidence of cardiovascular side effects during spinal anesthesia: the results from a randomized controlled trial

We conducted a randomized clinical trial to evaluate the efficacy of crystalloids in preventing spinal-induced hypotension (SIH) and cardiovascular side effects (CVSE) in a group of surgical patients. Participants were assigned to receive lactated Ringer's solution at 1-2 mL/min (Placebo group, n = 142); lactated Ringer's at 20 mL/kg starting 20 min before spinal block (n = 130); or lactated Ringer's at 20 mL/kg starting at the time of spinal block (n = 132). SIH was defined as a decrease of > or = 30% in baseline systolic blood pressure, and CVSE as SIH plus nausea, vomiting, or faintness requiring treatment. The incidence of SIH was similar in all treatment groups. Compared to placebo, crystalloid administration at the time of spinal block resulted in a significant reduction in the proportion of patients developing CVSE from 9.9% to 2.3%. The corresponding relative proportion was 0.23 (95% confidence interval, 0.07-0.78; P = 0.019), and one additional case of CVSE was avoided for each 13 patients receiving crystalloids at the time of spinal block instead of placebo. Administration of crystalloids at the time of spinal block seems to be effective because it provides additional intravascular fluids during the period of highest risk of CVSE after spinal anesthesia.

IMPLICATIONS

Crystalloids are frequently administered to nonobstetric patients minutes before spinal anesthesia to prevent cardiovascular side effects (CVSE). This randomized controlled trial shows that although crystalloids administered before spinal block result in no clinical benefit, they significantly reduce the risk of CVSE when administered at the time of spinal block.

Effects of midazolam on heart rate variability during surgery under spinal anaesthesia

Effects of midazolam on the cardiac autonomic nervous system were studied by power spectral analysis of electrocardiographic R-R intervals in patients undergoing elective lower abdominal surgery under spinal anaesthesia. Patients were randomly assigned into two groups: 10 patients in group A received spinal anaesthesia only and 10 in group S received spinal anaesthesia and midazolam of 0.05 mg/kg when surgery started. In the frequency domain power spectra, low (Lo; 0.04-0.15 Hz) and high (Hi; 0.15-0.40 Hz) frequency components were integrated to ascertain sympathetic and parasympathetic activity, respectively. There was no intergroup difference in starting time of surgery, ventilatory frequency, arterial pressure, heart rate, mean and variance of R-R interval, and cephalad level of analgesia. In spectral component, Lo decreased (P < 0.05) and Hi/Lo ratio increased (P < 0.05) relative to their baselines in group S. These were also different from group A (P < 0.05). We concluded that intravenous midazolam depressed sympathetic activity to produce a vagotonic cardiac autonomic nervous system under spinal anaesthesia.

The role of vasopressors in the management of hypotension induced by spinal and epidural anaesthesia

Although spinal and epidural blocks provide excellent anaesthesia for many operations, they are frequently accompanied by hypotension. This is largely the result of sympathetic nerve blockade. Excessive hypotension may potentially produce myocardial and cerebral ischaemia, and is associated with neonatal acidaemia in obstetric practice. How to prevent and treat this hypotension has been the subject of much investigation and controversy. One of the mainstays of management is the use of vasopressor agents and those currently available are not perfect. In this review, the role of vasopressor agents is discussed and possible future management strategies are commented upon. Ephedrine was the first agent used for this purpose and it has withstood the test of time: it is the agent against which all others are compared. It remains the first-line agent in obstetric anaesthesia as it does not affect the fetus adversely, but it cannot be relied upon to be 100% successful and other agents must be considered when it is inadequate. It is best given by infusion. In non-obstetric practice, ephedrine has a good track record but again its success rate is less than 100%. As there is no fetus to consider, it may be more appropriate to consider using a pure vasoconstrictor agent such as methoxamine or phenylephrine as a first-line therapy in such cases. This judgment can only be made on an individual patient basis as ephedrine produces a tachycardia while phenylephrine and methoxamine both produce bradycardia.

Changes in blood flow and vascular resistance in the arm and leg during spinal anaesthesia and vasopressor therapy with methoxamine

The effects of subarachnoid block with hyperbaric bupivacaine 0.5% on forearm and calf blood flow and vascular resistance were examined using venous occlusion plethysmography in 10 fit patients. Following blockade there was an increase in mean (SD) calf blood flow from 2.0 (0.4) to 2.6 (1.2) ml.100 ml-1.min-1 (p = 0.057), a decrease in mean (SD) calf vascular resistance from 55 (15) to 38 (14) R units (p < 0.01), a decrease in mean (SD) forearm blood flow from 3.1 (1.2) to 1.8 (0.9) ml.100 ml-1.min-1 (p < 0.01) and an increase in mean forearm vascular resistance from 38 (16) to 62 (24) R units (p < 0.01). Methoxamine 2 mg was administered intravenously when the mean arterial blood pressure decreased by 15%. There was a resultant marked increase in mean (SD) forearm blood flow from 1.8 (0.9) to 3.0 (0.8) ml.100 ml-1.min-1 (p < 0.001) but a decrease in mean (SD) calf blood flow from 2.6 (1.2) to 2.1 (1.4) ml.100 ml-1.min-1 (p = 0.11). Correspondingly, the mean (SD) forearm vascular resistance decreased from 62 (24) to 46 (17) R units (p < 0.01) with an increase in mean (SD) calf vascular resistance from 38 (14) to 56 (22) R units (p < 0.01). These findings indicate that whereas methoxamine produces the anticipated constrictor response in the calf vessels it causes a simultaneous reduction in vascular resistance in the forearm.

The effects of propofol and thiopentone on forearm circulation

Some effects of propofol and thiopentone induction on the peripheral circulation of healthy patients are examined using mercury strain gauge venous occlusion plethysmography of the forearm. Results indicate that both drugs produce a statistically significant decrease in mean arterial blood pressure and forearm blood flow. Forearm vascular resistance remains unchanged after either drug. These data suggest that bolus dose induction of anaesthesia with propofol does not cause arterial vasodilatation of the limb and that a cause for the reduction in mean arterial pressure must be sought elsewhere.

Is crystalloid preloading useful in spinal anaesthesia in the elderly?

Sixty ASA grade 1 or 2 patients, aged 60 years or over, scheduled for surgery to the lower abdomen or lower limbs under spinal anaesthesia were allocated randomly to one of three treatment groups. Group A received 16 ml/kg of Ringer's acetate solution immediately before spinal anaesthesia, group B received 8 ml/kg and group C received no volume preload. Heart rate, arterial pressure and anaesthetic level were recorded by an independent observer. The overall incidence of systemic arterial hypotension (defined as a decrease of 25% or more in systolic arterial pressure) was 27%; there were no significant differences among groups. The overall incidence of hypotension was 60%, when temperature sensation was blocked to T7 and above (n = 25). The number of patients with hypotension which required treatment increased as block height increased above T7; at a level of T4 or higher, all patients required ephedrine. Crystalloid preloading had no effect on the incidence of hypotension after spinal anaesthesia in fit, elderly patients.

Pharmacology of drugs for conscious sedation

In endoscopic monitoring and treatment of gastrointestinal disease, it is important that patients will accept repeated examination. They are less likely to do so if the procedure is remembered as distressing or uncomfortable, as is likely when it is performed under topical anaesthesia alone. The aim of conscious sedation is a lightly sedated patient, who is awake, cooperative on demand, amnesic, and free from anxiety and fear. Various drugs in low doses can be used to meet these criteria. Among these are phenothiazines, butyrophenones, barbiturate and non-barbiturate hypnotics, benzodiazepines, and the hypno-analgesic, ketamine. As benzodiazepines offer both sedative and profound amnesic and anxiolytic effects, these drugs are used for conscious sedation worldwide. Diazepam has been the 'gold standard' of sedation, but the more modern benzodiazepines, particularly midazolam, are now more commonly used. In general, benzodiazepines demonstrate a broad therapeutic range. In accordance with dose, however, sedative drugs may induce side-effects, such as drowsiness, lowering of blood pressure, and respiratory depression. In addition, some may induce more wide-ranging side-effects, such as histamine liberation and anaphylactic reactions, thrombophlebitis, and pain on injection. They may have severe drug interactions when used in combination with local anaesthetics, hypnotics and opioids. In older patients, lower doses are necessary for sedation. Sedative drugs should be administered slowly, to avoid haemodynamic and respiratory side-effects.

The pharmacology of propofol

A review of the pharmacology of propofol, a new IV anesthetic agent, is presented. Solubilized in a soybean emulsion, propofol is one of a series of sterically hindered phenols that exhibit anesthetic activity. Induction of anesthesia with propofol may be associated with pain on injection, apnea, and a reduction in arterial blood pressure (BP) and cardiac output. Caution should be ascribed to its use in patients with coronary artery disease, where these effects may have the potential for producing myocardial ischemia. The hemodynamic responses to laryngoscopy and intubation are attenuated. The pharmacokinetic profile suggests suitability as an infusion for either maintenance of anesthesia or sedation. Use of propofol as an infusion during surgery may result in a further reduction in cardiac output, particularly with the concomitant administration of adjuvant increments of fentanyl. The ventilatory response to CO2 is depressed during such an infusion. The high clearance of propofol suggests that even after a prolonged infusion, recovery should be rapid. This finding has been confirmed in a series of studies establishing propofol as an ideal agent for use in a total IV anesthetic technique. Both the quality and speed of recovery, together with the absence of emetic sequelae, support the use of propofol in an outpatient setting. Propofol appears to have no long-term effect on adrenocortical function and appears safe for use in patients with acute intermittent porphyria and susceptibility to malignant hyperpyrexia.