Lidocaine pharmacokinetics during hyperbaric hyperoxia in humans

METHODS

The disposition of drugs may be influenced by hyperbaric conditions, in particular by changes of liver perfusion. The effect of hyperbaric hyperoxia on the pharmacokinetics of lidocaine, a drug eliminated in the liver with a perfusion-limited clearance, was investigated in human volunteers in a crossover trial.

METHODS

A single dose lidocaine i.v. bolus (0.69 or 0.75 mg x kg(-1)) was administered to two volunteers under normobaric conditions (NB: 1 bar or 0.1 MPa, air) and under hyperbaric/hyperoxic conditions (HBO: 2.5 bar or 0.25 MPa, alternating 100% O2-breathing for 20 min and air breathing for 5 min). Blood samples were serially collected for 5 h (NB) or 75 min (HBO), and lidocaine concentration in serum was measured by immunoassay. Data were analyzed assuming linear kinetics and an open two-compartment model.

RESULTS

At 1 bar or 0.1 MPa, lidocaine injection caused only slight dizziness and buzzing in the ear. Heart rate and blood pressure were not influenced. Under HBO, lidocaine injection caused marked dizziness and buzzing in the ears, sweating, tremor and coordination-disturbances, even though maximal lidocaine concentrations (0.63 mg x L(-1) and 0.70 mg x L(-1)) were far below therapeutic serum concentrations (1.5-5.0 mg x L(-1)). Pharmacokinetic parameters of lidocaine were similar to those published earlier (T1/2beta: 110+/-16 min; CI: 12.6+/-2.9 ml x min(-1) x kg(-1); Vss: 1.73+/-0.18 L x kg(-1)). There was no indication for effects of HBO on the disposition of lidocaine (p > 0.05).

CONCLUSION

The pharmacokinetics of lidocaine do not seem to be influenced in a clinically relevant way in humans by a single HBO-exposure under usual therapeutic conditions. Side effects of lidocaine at 2.5 bar or 0.25 MPa may be caused by pharmacodynamic interactions between lidocaine and hyperbaric/hyperoxic conditions.

Effects of hyperbaric and hyperoxic conditions on the disposition of drugs: theoretical considerations and a review of the literature

1. Hyperbaric or hyperoxic or both conditions may affect the disposition of drugs by (1) changes in the catalytic activity of drug metabolizing enzymes, (2) hemodynamic changes and (3) changes in membrane permeability, affecting drug distribution. 2. In isolated microsome preparations from rat liver, the metabolism rate of aniline, but not of amidopirin, is reduced by hyperoxia. In vivo, the clearance of salicylic acid is enhanced in the dog at 2.8 ATA and 100% O2, but not at 6 ATA and air, for reasons that are still unknown. The disposition of theophylline, pentobarbital or pethidine is not affected in dogs by hyperbaric or hyperoxic conditions. 3. In human volunteers, hyperbaric or hyperoxic or both conditions do not affect the disposition of gentamycin (2.4 bar, 100% O2), caffeine or lidocaine (2.5 bar, 100% O2). 4. In conclusion, a single exposure to hyperbaric or hyperoxic conditions does not seem to affect single-dose pharmacokinetics of drugs eliminated by the kidney (gentamycin) or by the liver with a capacity-limited clearance (pentobarbital, theophylline, caffeine) or with a perfusion-limited clearance (pethidine, lidocaine). The enhancement of salicylic acid clearance in dogs under hyperoxic conditions remains unclear.

Design and validation of an intelligent patient monitoring and alarm system based on a fuzzy logic process model

The process of patient care performed by an anaesthesiologist during high invasive surgery requires fundamental knowledge of the physiologic processes and a long standing experience in patient management to cope with the inter-individual variability of the patients. Biomedical engineering research improves the patient monitoring task by providing technical devices to measure a large number of a patient's vital parameters. These measurements improve the safety of the patient during the surgical procedure, because pathological states can be recognised earlier, but may also lead to an increased cognitive load of the physician. In order to reduce cognitive strain and to support intra-operative monitoring for the anaesthesiologist an intelligent patient monitoring and alarm system has been proposed and implemented which evaluates a patient's haemodynamic state on the basis of a current vital parameter constellation with a knowledge-based approach. In this paper general design aspects and evaluation of the intelligent patient monitoring and alarm system in the operating theatre are described. The validation of the inference engine of the intelligent patient monitoring and alarm system was performed in two steps. Firstly, the knowledge base was validated with real patient data which was acquired online in the operating theatre. Secondly, a research prototype of the whole system was implemented in the operating theatre. In the first step, the anaesthetists were asked to enter a state variable evaluation before a drug application or any other intervention on the patient into a recording system. These state variable evaluations were compared to those generated by the intelligent alarm system on the same vital parameter constellations. Altogether 641 state variable evaluations were entered by six different physicians. In total, the sensitivity of alarm recognition is 99.3%, the specificity is 66% and the predictability is 45%. The second step was performed using a research prototype of the system in anaesthesiological routine. The evaluation of 684 events yielded a sensitivity, specificity and predictability of the alarm recognition of more than 99%.

[Low-flow anesthesia with desflurane]

OBJECTIVES

Due to its low solubility and negligible metabolism, desflurane is assumed to be especially suitable for application by low-flow anaesthetic techniques. The aim of this clinical investigation was the development of a standardised dosing scheme for low-flow and minimal-flow desflurane anaesthesia.

METHODS

One hundred six ASA status I-II patient were assigned to six groups according to the duration of the initial high-flow phase, fresh gas flow, and fresh-gas desflurane concentration. The median age, height, body weight, and constitution of the groups was comparable. After an initial high-flow phase using 4.4 l/min, the fresh gas flow was reduced to 0.5 l/min (minimal-flow anaesthesia) or 1.0 1/min (low-flow anaesthesia). Inspired nitrous oxide concentrations were maintained at 60% to 70%. Using different standardised schemes of vaporizer settings, inspired desflurane concentrations were applied in the range from 3.4% to 8.7%, i.e., between 1 and 1.5 MAC. Inspired and expired desflurane concentrations were measured continuously by the side-stream technique and recorded on-line. Venous blood samples were taken immediately prior to induction and 45 min after flow reduction for measurement of carboxyhaemoglobin (COHb) concentration).

RESULTS

In the 10- to 15-min initial phase during which a high fresh gas flow of 4.4 l/min was used, the inspired desflurane concentration reached values in the range of 90%-95% of the fresh gas concentration. In low-flow anaesthesia this concentration could be maintained without any alteration of the vaporizer setting, whereas in minimal-flow anaesthesia with flow reduction the fresh gas concentration had to be increased by 1% to 2%: The quotient calculated by division of the inspired desflurane concentration by its fresh gas concentration (Q = CI/CF) ranges between 0.65 and 0.75 in animal-flow and between 0.80 and 0.85 in low-flow anaesthesia. If use was made of the wide output range of the desflurane vaporizer, the inspired concentration could be increased rapidly by about 5% in 8 min, although the flow was kept constant at 0.5 l/min. Compared with its value prior to induction (2.13 +/- 1.05%), the COHb concentration decreased statistically significantly by about 0.7% during the 1st hour of minimal-flow anaesthesia (1.42 +/- 1.01%). In no case was a COHb concentration observed that exceeded threatening or even toxic values, although the soda lime was changed routinely only once a week.

CONCLUSIONS

The pharmacokinetic properties of desflurane, resulting in especially low individual uptake, and the wide output range of the vaporizer facilitate the use of low-flow anesthetic techniques in routine clinical practice. Even in minimal-flow anesthesia, the duration of the initial high-flow phase can be shortened to min. If the flow is reduced to 1 l/min, the inspired desflurane concentration achieved in the initial high-flow phase can be maintained without any alteration of the vaporizer setting. In minimal-flow anesthesia, however, with flow reduction to 0.5 l/min, the fresh gas concentration has to be increased to a value 1%-2% higher than the inspired nominal value. Due to the wide dialing range of the desflurane vaporizer, the amount of vapour delivered into the breathing system can be increased to about 110 ml/min even at a flow of 0.5 l/min. The large amount of agent that can be delivered into the system even under low-flow conditions, together with the very low individual uptake, results in a time-constant that is sufficient short for the clinically required rapid increase in inspired desflurane concentrations. The short time-constant of low-flow desflurane anaesthesia improves the control of the anaesthetic concentration. If all measures are taken to safely avoid inadvertent drying out of the soda lime, there is no evidence that low-flow anaesthesia with desflurane is liable to increase the risk of accidental carbon monoxide poisoning. (ABSTRACT TRUNCATED)

Caffeine pharmacokinetics during hyperbaric hyperoxia in humans

The effect of hyperbaric hyperoxia on the pharmacokinetics of caffeine was investigated in human volunteers. Some 600 ml of coffee were administered to 2 volunteers and blood samples were serially collected for 24 h. The volunteers entered a hyperbaric chamber 2.5 h following coffee ingestion for a total period of 110 min (0.25 MPa, alternating 100% O2-breathing for 20 min and air breathing for 5 min). The concentrations of caffeine in serum was determined by high pressure liquid chromatography. The caffeine amount ingested was determined by analyzing an aliquot of the coffee beverage. Data were analyzed assuming linear kinetics and an open one-compartment model. Effects of hyperbaric hyperoxia on caffeine disposition were investigated using a runs test. Moreover, a one-population t-test was applied to residuals, separately for data from the initial normobaric period, the hyperbaric period and the terminal normobaric period. Pharmacokinetic parameters were similar to established literature data on caffeine [Volunteer 1: maximal concentration (Cmax: 6.13 mg.L-1 at Tmax: 55 min, half-time of elimination (T1/2: 180 min, total clearance (Cl): 3.41 ml.min-1.kg-1; volume of distribution (Vd: 0.88 I.kg-1; Volunteer 2: Cmax: 6.23 mg.L-1, Tmax: 94 min, T1/2: 283 min, Cl: 1.90 ml.min-1.kg-1, Vd: 0.77 L.kg-1). The runs test as well as the analysis of residuals gave no evidence for alterations of caffeine disposition by hyperoxia (p > 0.05). The pharmacokinetics of caffeine do not seem to be influenced in a clinically relevant way in humans during a stay for 100 min at 0.25 MPa, alternating 100% O2 and air breathing.

Knowledge-based decision support for patient monitoring in cardioanesthesia

An approach to generating 'intelligent alarms' is presented that aggregates many information items, i.e. measured vital signs, recent medications, etc., into state variables that more directly reflect the patient's physiological state. Based on these state variables the described decision support system AES-2 also provides therapy recommendations. The assessment of the state variables and the generation of therapeutic advice follow a knowledge-based approach. Aspects of uncertainty, e.g. a gradual transition between 'normal' and 'below normal', are considered applying a fuzzy set approach. Special emphasis is laid on the ergonomic design of the user interface, which is based on color graphics and finger touch input on the screen. Certain simulation techniques considerably support the design process of AES-2 as is demonstrated with a typical example from cardioanesthesia.

Hemodynamic variables and plasma levels of PGI2, TXA2 and IL-6 in a porcine model of recurrent endotoxemia

The pulmonary and systemic hemodynamic effects of recurrent endotoxemia were studied in pigs over a 48-hr period. Six pigs of the test group were given 0.5 micrograms/kg of an E. coli endotoxin (WO111: B4) over 60 min at the beginning and in the middle (22 hr) of the experiment. Three pigs given the same amount of physiological saline solution served as controls. The hemodynamic response to the first LPS injection was characterized by severe pulmonary hypertension, a significant increase in systemic vascular resistance, and a marked decrease in cardiac output. Circulating TxB2 levels were higher than those of 6-keto-PGF1 alpha levels, so that the first response to LPS is influenced by the vasoconstrictive actions of TxA2. With the second LPS application, the pulmonary response was attenuated, although a significant increase of pulmonary artery pressure and pulmonary vascular resistance occurred. Once again systemic vascular resistance rose and cardiac output decreased, but this time plasma levels of 6-keto-PGF1 alpha were greater than those of TxB2. Toward the end of the experiment, we noted the progressive onset of a hyperdynamic and hypotensive state. Systemic vascular resistant index decreased to 50% of the baseline value. IL-6, a cytokine of systemic importance during the course of septic shock, markedly and significantly peaked after each LPS injection. Circulating plasma levels in response to recurrent endotoxemia are described.

An interactive computer simulator of the circulation for knowledge acquisition in cardio-anesthesia

Knowledge-based decision support systems for use in cardio-anesthesia can provide online support to the anesthesiologist by generating intelligent alarms. However, the acquisition and validation of a consistent knowledge base for this application bears problems related to the transfer of clinical experiences into a rule system. An interactive simulator of the human circulation is presented that supports the process of knowledge acquisition and testing. The simulator can be controlled in realtime by an anesthesiologist during the simulation run thus providing a basis for interdisciplinary discussion of routine as well as critical situations. The output data can be transferred to a knowledge-based system for test purposes. The simulator is currently being used for the development of the Anesthesia Expert Assist System AES-2. With regard to the special application a model of the heart-function was integrated which enables the simulation of heart insufficiency. Simulation runs under various conditions are presented and discussed. The simulator was implemented on an ATARI ST personal computer.

[Particulate contamination of infusion solutions and drug additives in the framework of long-term intensive therapy. 2. An animal model]

When IV solutions are used, particulate matter may be introduced into the patient. Using X-ray studies we could demonstrate particulate matter consisting mainly of glass from ampules, rubber from rubber stoppers of infusion bottles, and plastic material from IV sets. With a recently introduced method of back-scatter pictures in scanning electron microscopy combined with X-ray studies--Jeol 35 CF/EEDS II Ortec (REM EDX)--we analyzed this phenomenon using an animal model (rabbit). It appears that particulate matter is deposited in the interstitial lung tissue and especially the pulmonary microcirculation. Pathogenetic effects could also be demonstrated (formation of thrombi, etc.).

[Particulate contamination of infusion solutions and drug additives within the scope of long-term intensive therapy. 1. Energy dispersion electron images in the scanning electron microscope-REM/EDX]

During use of i.v. solutions, particulate matter may be introduced into the patient. X-ray analyses have shown that the particulate matter consists mainly of glass from ampules, rubber from stoppers of infusion bottles, and plastic from infusion sets. A new method is introduced: scanning electron microscopy-SEM- in combination with energy-dispersive X-ray analysis-EDX. using this technique all foreign particles larger than 0.8 microns can be detected and analyzed in any tissue. Especially in critically ill patients, particularly matter is deposited in the microcirculation of the lung. The particles cause a variety of pathologic changes (formation of thrombi, thromboembolization of the microcirculation, destruction of the vascular endothelium, formation of granulomas and foreign body giant cells). The adult respiratory distress syndrome could be produced or respiratory insufficiency might be aggravated following circulatory shock. It is therefore recommended that terminal in-line filters with small pore sizes be used for all i.v. solutions.

[The filter effectiveness of common 15-micron filters (DIN 58362). II: Scanning electron microscopy and roentgen analysis]

When i.v. solutions are used, millions of foreign particles are washed into the organism. Our town studies could recently show that these particles may interact with the organism and thus cause injury. A new method is introduced here (energy dispersive X-ray analysis in scanning electron microscopy, SEM, Jeol 35 CF/EDX, EEDS II Ortec), with which the element combination of foreign particles can be analyzed qualitatively and quantitatively. As a first step an analysis of the particulate matter in filtrates of infusion solutions was carried out in the same way, materials used in infusion therapy (e.g. infusion sets, cannulas, etc.) were analyzed. The results show that most of the particulate matter consists of glass from ampoules, plastic materials from infusion sets and rubber from the rubber stoppers of infusion bottles.

[Filtering effectiveness of conventional 15 m micron filters (DIN 58362) and newly developed 15 m micron and 5 m micron filters. I: Particle count]

When i.v. solutions are used, a considerable amount of particulate matter is washed into the patient. This particulate matter consists mainly of glass from ampoules, plastic materials from infusion sets, rubber from the rubber stoppers of infusion bottles and various materials, partly derived from the manufacturing process of the solutions. A new method of particle counting is introduced here (modified thrombocyte counter, Becton Dickinson), whereby particulate matter (less than 2 microns, greater 0.8 microns) could be counted for the first time. Using this counting method, the efficiency of the standard filtration (DIN 58362) was tested. Also tested were 2 new final filtration systems (with 15 microns and 5 microns pore-size). All 3 filtration methods were shown to be ineffective; especially small particles (less than 2 microns) cannot be eliminated by these filtration methods.

Untersuchung über die Filtereffektivität herkömmlicher 15-μm-Filter (DIN 58362)

Bei einer intravenösen Behandlung mit Infusions- und Medikamentenlösungen werden mehrere Millionen Fremdpartikel pro Liter Lösung in den Organismus geschwemmt. In einigen Untersuchungen konnte bereits gezeigt werden, daβ diese Fremdpartikelchen zahlreiche Wechselwirkungen mit dem Organismus bewirken und so zu Schädigungen führen können. Es wird eine Methode vorgestellt (ener-giedispersíve Röntgenanalyse mit Rasterelektronenmikroskop – REM Joel 35 CF/EDX-EEDS II Ortec), mit der in In-vitro-Versuchen die Elementzusammensetzung der Fremdpartikel qualitativ und quantita-tiv analysiert werden kann. Zunächst erfolgte eine Analyse der Partikel im Filtrat von Infusions- und Medíkamentenlösungen. In gleicher Weise konnten die in der Infusionstherapie gebräuchlichen Materialien (z.B. Infusionsbestecke, Kanülen) analysiert werden. Dabei fíndet man, daβ Glaspartikel aus Medikamentenampullen, Kunststoffe aus den Infusionsbestecken sowie Gummi aus den Gummistopfen von Infusionsflaschen die häufigsten Partikel im Filtrat sind.

[CO2-response curves as a measure of opiate-induced respiratory depression. Studies with fentanyl]

The ventilatory response to CO2 after fentanyl 4 micrograms/kg was studied in a group of 10 healthy volunteers of both sexes, aged 20 to 41 years. Following randomization intravenous premedication with either placebo, diazepam 5 mg, droperidol 5 mg or etomidate 10 mg, a total of four test series were performed with each individual in two-week intervals, in order to assess the additive effect of central depressants on opiate-induced respiratory depression. CO2-response curves were obtained during six-minute rebreathing periods up to 60 min after fentanyl. Respiratory frequency as well as slope and position of the curves were compared with pre-drug control values. After fentanyl, CO2-response curves usually showed combined decreases in slope and displacements to the right, which had not completely returned to control levels after one hour. Due to enormous inter- and intra-individual variations, no clearly significant differences could be observed between the premedication groups. Fentanyl plasma concentrations, determined by radioimmunoassay at 1, 3, 6, 10, 20, 30, 45, 60, 75 and 120 min after injection, showed only a poor correlation to the changes of the CO2-response curves. To assess the clinical relevance of results derived from CO2-stimulation studies, the literature is reviewed with respect to the respiratory control system and the interactions of various anaesthetics drugs (including opiate analgesics) with respiratory regulation. The activity of the reticular formation at lower brain stem level seems to be of decisive importance to guarantee adequate ventilation. Despite quite different action sites, most central depressant drugs reduce reticular activity, thus inhibiting compensatory mechanisms which usually help control respiration. CO2-response tests are therefore suggested to be unable to give sufficient clinical information about the respiratory regulation system.

[Possible mechanisms, differential diagnosis and prophylaxis of vocal cords palsy following endotracheal intubation]

This report is based on a total of 53 vocal cord palsies after endotracheal intubation on the basis of a bibliographical review and 5 of our own observations. Pressure and extension injuries of the recurrent laryngeal nerve, especially of its terminal branches, are probably the cause of such damage. It seems that infection of the respiratory tract encourage vocal cords palsies. Possible aetiological mechanisms and questions concerning the innervation of the larynx connected with this, the clinical differential diagnosis and prophylactic measures are discussed.

[The present state of hypotension (author's transl)]

The study was made of the recent literature on controlled hypotension and its place in modern anaesthetic practice. The effects of the different techniques and drugs are outlined in this paper. The results of investigation revealed that the circulation of heart, brain, liver, and kidneys is able to tolerate a wide range of deviation of blood pressure. Hypotension induced by high doses of halothane alone, which used to be considered as an adequate measure, has recently been shown to cause such serious side-effects that it can no longer be recommended. Halothane lowers blood pressure almost exclusively by means of a reduction of the contractility of the heart muscle. Ganglionic blocking agents alone should not be used either because of their depressing effect on the heart; moreover their hypotensive effects are hard to control. The sole exception is trimetaphan but it produces other severe side-effects such as liberation of histamine. On the other hand ganglionic blocking agents combined with halothane can be recommended. The combined administration of these drugs allow an easy control of hypotension and the side effects are minimal. Only sodium-nitroprusside meets all the requirements of a hypotensive drug. It operates selectively in a peripheral vasoplegic manner without effecting the cerebral centres. Regarding the heart, no side-effect is known apart from tachycardia; cardiac output is not altered. The metabolism of sodium-nitroprusside requires precaution in dosage, as degradation takes place via the toxic compounds of cyanide. Therefore liver disease and disturbances in vitamin B12 utilisation are regarded as contraindications for the use of nitroprusside. That is also the reason why a maximum allowable dose of 10-15 gamma/kg body weight and minute should not be exceeded. Prophylactic administration of vitamin B 12a (hydroxocobalamin) should be considered in every case when nitroprusside is used.