Arterial baroreflex attenuation during and after continuous propofol infusion

Analyzing the Loss and the Recovery of Consciousness: Functional Connectivity Patterns and Changes in Heart Rate Variability During Propofol-Induced Anesthesia

The analysis of the central and the autonomic nervous systems (CNS, ANS) activities during general anesthesia (GA) provides fundamental information for the study of neural processes that support alterations of the consciousness level. In the present pilot study, we analyzed EEG signals and the heart rate (HR) variability (HRV) in a sample of 11 patients undergoing spinal surgery to investigate their CNS and ANS activities during GA obtained with propofol administration. Data were analyzed during different stages of GA: baseline, the first period of anesthetic induction, the period before the loss of consciousness, the first period after propofol discontinuation, and the period before the recovery of consciousness (ROC). In EEG spectral analysis, we found a decrease in posterior alpha and beta power in all cortical areas observed, except the occipital ones, and an increase in delta power, mainly during the induction phase. In EEG connectivity analysis, we found a significant increase of local efficiency index in alpha and delta bands between baseline and loss of consciousness as well as between baseline and ROC in delta band only and a significant reduction of the characteristic path length in alpha band between the baseline and ROC. Moreover, connectivity results showed that in the alpha band there was mainly a progressive increase in the number and in the strength of incoming connections in the frontal region, while in the beta band the parietal region showed mainly a significant increase in the number and in the strength of outcoming connections values. The HRV analysis showed that the induction of anesthesia with propofol was associated with a progressive decrease in complexity and a consequent increase in the regularity indexes and that the anesthetic procedure determined bradycardia which was accompanied by an increase in cardiac sympathetic modulation and a decrease in cardiac parasympathetic modulation during the induction. Overall, the results of this pilot study showed as propofol-induced anesthesia caused modifications on EEG signal, leading to a "rebalance" between long and short-range cortical connections, and had a direct effect on the cardiac system. Our data suggest interesting perspectives for the interactions between the central and autonomic nervous systems for the modulation of the consciousness level.

Propofol for sedation in neuro-intensive care

Interventions in the intensive care unit often require that the patient be sedated. Propofol is a widely used, potent sedative agent that is popular in critical care and operating room settings. In addition to its sedative qualities, propofol has neurovascular, neuroprotective, and electroencephalographical effects that are salutory in the patient in neurocritical care. However, the 15-year experience with this agent has not been entirely unbesmirched by controversy: propofol also has important adverse effects that must be carefully considered. This article discusses and reviews the pharmacology of propofol, with specific emphasis on its use as a sedative in the neuro-intensive care unit. A detailed explanation of central nervous system and cardiovascular mechanisms is presented. Additionally, the article reviews the literature specifically pertaining to neurocritical care use of propofol.

Baroreflex control of heart rate during and after propofol infusion in humans

BACKGROUND

This study was designed to determine cardiovagal baroreflex gain during propofol infusion and to characterize its recovery profile using the pharmacological and spontaneous sequence methods in 13 healthy volunteers without cardiovascular or autonomic disorders.

METHODS

After an 8- to 10-h fast and no premedication, measurements of RR intervals obtained from the electrocardiogram and non-invasive beat-to-beat systolic blood pressure (SP) were made at conscious baseline, at 60 and 120 min after induction of general anaesthesia using propofol, and at 20, 60, 120 and 180 min after emergence from anaesthesia. During propofol anaesthesia, ventilation was mechanically controlled to maintain normocapnia and calculated propofol concentration was adjusted by a TCI system at 5 microg ml(-1). Baroreflex responses were triggered by bolus i.v. injections of phenylephrine and nitroprusside to alter SP by 15-30 mm Hg. The linear portions of the baroreflex curves relating RR intervals and SP by least-square regression analysis were determined to obtain pharmacological gains. In addition, spontaneous sequence baroreflex gains were calculated from spontaneously fluctuating SP and RR intervals.

RESULTS

Baseline pressor and depressor test gains before propofol anaesthesia were 29.1 (SD 14.9) and 12.5 (7.8) ms mm Hg(-1), respectively. They were significantly depressed by 65-73% during propofol infusions. Similarly, baseline up- and down-sequence baroreflex gains were 33.8 (28.9) and 27.3 (19.8) ms mm Hg(-1), respectively, and were significantly depressed by 71-87% during propofol anaesthesia. Pressor test and up-sequence baroreflex gains returned to the baseline values 20 min after emergence from propofol anaesthesia, but depressor test and down-sequence baroreflex gains did not recover until 60 min after emergence.

CONCLUSIONS

We conclude that heart rate responses to both lowering and elevating blood pressure were depressed by propofol anaesthesia, and 60 min was required for their full recovery after discontinuation of propofol infusion.

Autonomic Mechanisms in the Age-Related Hypotensive Effect of Propofol

We hypothesized that age-related differences in cardiovascular regulatory processes play a role in the augmented vasodepressor response to anesthetic induction with propofol in older subjects. To test this hypothesis, differences in baroreceptor responsiveness (BR) were first demonstrated in young adult (6-12 mo, n=12) and aged (>42 mo, n=12) New Zealand rabbits, and then the vasodepressor effect of propofol was compared in both the absence and presence of ganglionic blockade. For each age group, half of the animals were pretreated with 20 mg/kg IV hexamethonium (HEX) with the remaining half designated as controls. BR was first assessed by plotting cardiac cycle length as a function of the decline in mean arterial blood pressure (MAP) produced by multiple IV boluses of tri-nitroglycerine. Propofol was then given as an IV bolus of 4.5, 6.4, or 8.4 mg/kg over 3 s. Each animal was studied three times, receiving a single dose in variable order with at least 7 days between injections. In control animals, marked age-related differences in BR were evident and propofol produced larger peak decreases in MAP in older rabbits at all doses. HEX pretreatment abolished BR for both young and aged rabbits. However, after HEX administration the vasodepressor response to propofol in young animals was enhanced by 150% at 4.5, 125% at 6.4, and 61% at 8.4 mg/kg, respectively, whereas the impact in aged animals was only 25%, 30%, and -10%, respectively. These data support the hypothesis that age-related enhancement of propofol-induced hypotension is largely a reflection of diminished BR.

Comparison of the Effects of Propofol and Pentobarbital on Left Ventricular Adaptation to an Increased Afterload

The purpose of this study was to compare the hemodynamic effects of pentobarbital and propofol and their effects on cardiovascular adaptation to an abrupt increase in left ventricular afterload. Experiments were performed in 12 open-chest pigs instrumented for measurement of aortic pressure and flow, and left ventricular pressure and volume. In one group (n = 6), anesthesia was obtained with sodium pentobarbital (3 mg x kg(-1) x h(-1)), and, in the second group B (n = 6), with propofol (10 mg x kg(-1) x h(-1)). Both groups received sufentanil (0.5 microg x kg(-1) x h(-1)) and pancuronium bromide (0.1 mg x kg(-1)). Left ventricular function was assessed by the slope of end-systolic pressure-volume relationship and stroke work. After baseline recordings, left ventricular afterload was increased by aortic banding. The cardiovascular adaptations triggered by the aortic banding, such as tachycardia, vasoconstriction, and augmentation of myocardial contractility were prevented with propofol, suggesting interference with the baroreflex. Increase in left ventricular afterload decreased mechanical efficiency, regardless of anesthetic agent. These results showed that pentobarbital at 3 mg x kg(-1) x h(-1) has less deleterious hemodynamic effects than propofol at 10 mg x kg(-1) x h(-1).

Effects of ketamine and propofol on autonomic cardiovascular function in chronically instrumented rats

In this study C. we systematically examined the effects of ketamine and propofol at various doses (5-20 mg/kg) on blood pressure, heart rate and renal sympathetic nerve activity in chronically instrumented Wistar rats. We also assessed the effects of these anesthetics on the baroreflex control of heart rate and renal sympathetic nerve activity. Ketamine (10 mg/kg) increased blood pressure by 30.0+/-4.5%, heart rate by 17.7-3.3% and renal sympathetic nerve activity by 38.8+/-14.6%, while propofol (10 mg/kg) decreased blood pressure by 18.9+/-3.5%, heart rate by 5.5+/-2.5% and renal sympathetic nerve activity by 7.5+/-2.1%. These variables showed dose-dependent responses to both agents. Both ketamine and propofol decreased the range and maximum gain of the logistic function curve obtained by relating mean blood pressure to heart rate and blood pressure to renal sympathetic nerve activity. In conclusion, ketamine and propofol had different effects on autonomic cardiovascular function, but attenuated the baroreflex sensitivity of heart rate and renal sympathetic nerve activity in a dose-dependent manner. These results suggest the possibility that baroreflex sensitivity may reflect the depth of anethesia.

Autonomic circulatory and cerebrocortical responses during increasing depth of propofol sedation/hypnosis in humans

PURPOSE

To describe the relative effects of graded central nervous system (CNS) depression, using increasing propofol infusion rates, on neurovegetative brainstem-mediated circulatory control mechanisms and higher cortical activity in healthy humans.

METHODS

Propofol was administered using an infusion scheme designed to achieve three target blood concentrations in ten healthy volunteers. Blood propofol concentrations and sedation scores were determined at baseline, during the three propofol infusion levels, and 30 min into the recovery period. Electroencephalographic (EEG) power was measured in three frequency bands to quantify cortical activity, and autonomic heart rate control was quantified using spontaneous baroreflex assessment and power spectral analysis of pulse interval.

RESULTS

Sedation scores closely paralleled propofol blood concentrations (0, 0.53 +/- 0.34, 1.24 +/- 0.21, 3.11 +/- 0.80, and 0.96 +/- 0.42 microg x mL(-1) at baseline, three infusion levels and recovery respectively), and all subjects were unconscious at the deepest level. Indices of autonomic heart rate control were decreased only at the deepest levels of CNS depression, while EEG effects were apparent at all propofol infusion rates. These EEG effects were frequency specific, with power in the beta band being affected at light levels of sedation, and alpha and delta power altered at deeper levels.

CONCLUSIONS

The results of this study support a relative preservation of neurovegetative circulatory control mechanisms during the early stages of CNS depression using gradually increasing rates of infusion of propofol. Indices of circulatory control did not reliably reflect depth of sedation.

Dose-dependent effects of propofol on renal sympathetic nerve activity, blood pressure and heart rate in urethane-anesthetized rabbits

To evaluate the role of the autonomic nervous system in hemodynamic changes after propofol bolus injection, we used direct recordings of renal sympathetic nerve activity to examine the dose-dependent effects of propofol (2.5, 5, 10, and 20 mg/kg) on heart rate, mean blood pressure and renal sympathetic nerve activity in urethane-anesthetized rabbits. The animals were divided into four groups: animals with an intact neuraxis (intact group), cervical vagal nerve-sectioned animals (vagotomy group), carotid sinus and aortic-nerve sectioned animals (SAD group), and animals with SAD plus vagotomy (SADV group). Heart rate did not change significantly even after administration of 2.5 and 5 mg/kg but decreased markedly on 20 mg/kg injection in all groups. The intact and vagotomy groups had augmented renal sympathetic nerve activity with insignificant changes in mean blood pressure after 5 mg/kg injection of the agent. Insignificant changes of renal sympathetic nerve activity but a remarkable decrease of mean blood pressure appeared after 10 mg/kg propofol. Sustained hypotension in parallel with a profound depression of renal sympathetic nerve activity developed at the dose of 20 mg/kg. In SAD and SADV groups, however, dose-dependent depressions of renal sympathetic nerve activity were accompanied by decreases of mean blood pressure. These results suggest the following: (1) propofol-induced hypotensive effects are probably produced by the central-mediated sympathetic depression. (2) The baroreceptor reflex may be preserved at the lower dose of the agent. (3) Heart rate does not change significantly unless a large dose of propofol is used. The difference in effects on heart rate and on mean blood pressure may denote a greater inhibition of sympathetic vascular outflow than of the cardiac sympathetic outflow regulating cardiac rate and contractility. This hypothesis needs further clarification.

Propofol does not affect the canine cardiac conduction system under autonomic blockade

PURPOSE

To determine the effects of propofol on the cardiac conduction system in dogs with pharmacological autonomic blockade.

METHODS

In eight mongrel dogs receiving 6 mg.kg-1.hr-1 propofol and vecuronium under pharmacological autonomic blockade with atropine and propranolol the infusion rates of propofol were increased from 6, (baseline), to 12, 18 and 24 mg.kg-1.hr-1 at 60-min intervals. An electrophysiological study assessed sinus rate, sinus node recovery time, corrected sinus node recovery time, intraatrial conduction time, AV nodal effective refractory period, Wenckebach cycle length and AV conduction times. Electrocardiographical variables and arterial pressures were also measured. All measurements were repeated at 30 min after the beginning of each infusion of propofol.

RESULTS

Propofol did not produce direct effects on the electrophysiological or electrocardiographical variables at any infusion rates. Heart rates did not change at higher infusion rates in the presence of decreases in arterial pressures.

CONCLUSION

Propofol did not affect the cardiac conduction system in the presence of autonomic blockade. Thus, the direct cardiac effects of propofol do not play a causative role in the genesis of propofol-associated bradyarrhythmias.

Propofol in patients with cardiac disease

Propofol is an intravenous anaesthetic which is chemically unrelated to other iv anaesthetics. Most anaesthetists are now becoming familiar with propofol's pharmacokinetic and pharmacodynamic properties. It has proved to be a reliable drug that can be used safely for induction and maintenance of anaesthesia for most surgical procedures and unlike other anaesthetic agents, it can especially be extended into the postoperative setting or intensive care unit for sedation. Propofol's greatest attributes are its pharmacokinetic properties which result in a rapid, clear emergence and lack of cumulative effects even after prolonged administration. Compared with other iv anaesthetics, the induction dose of propofol has a relatively higher incidence of respiratory depression, short-lived apnoea and blood pressure reduction that may occasionally be marked. Possible mechanisms for the hypotension may relate to (1) its action on peripheral vasculature (vasodilatation), (2) decreased myocardial contractility, (3) resetting of the baroreflex activity and (4) inhibition of the sympathetic nervous system outflow. In vitro studies indicate that propofol depresses the immunological reaction to bacterial challenge as well as the chemotactic activity. Clinical studies, in cardiac surgery, have demonstrated that propofol, in association with an opioid, is a logical anaesthetic choice. Propofol is about to receive approval for continuous iv sedation. Comparative studies of propofol and midazolam have clearly demonstrated the superiority of propofol in terms of rapid recovery and precise control of the level of sedation.