Do infusions of midazolam and propofol pose an infection risk to critically ill patients?

In order to investigate bacterial contamination of i.v. anaesthetic agents, given by infusion to critically ill patients, we have cultured residual infusion fluid from infusion syringes, 50 containing midazolam and 50 propofol. The infusions had been prepared with routine aseptic precautions and had been running for between 0.75 and 21.25 h. Only scanty growths of Staphylococcus epidermidis were isolated from seven syringes (four midazolam and three propofol). Small volume samples were more likely to produce bacterial growth than large volume specimens. Midazolam infusions made up in 5% glucose were more likely to be contaminated than those made up in 0.9% saline. Antibacterial activity was detected in 18 midazolam and one propofol filtrate. Midazolam infusions inhibited the growth of all seven of the S. epidermidis isolates, whereas propofol supported similar rates of multiplication to that obtained with control broth medium. The results of this study imply that contamination of the infusions probably occurred after they were disconnected from the patient. Despite the ability of propofol to support microbial multiplication, we have no evidence to suggest that this is clinically significant when infusions are prepared with conventional aseptic precautions.

Characterization of the enzyme responsible for the metabolism of sumatriptan in human liver

Studies have been undertaken to investigate the enzymes responsible for the metabolism of [14C]sumatriptan in man. Oxidative deamination of sumatriptan to form the indole acetic acid derivative is the only phase 1 pathway evident in man and both cytochrome P450 (P450) and monoamine oxidase (MAO) are capable of catalysing this type of reaction. The metabolism of [14C]sumatriptan was therefore investigated in vitro in a preparation derived from human liver, which was shown, by the use of the probe substrates [14C]testosterone (P450), [3H]5HT (MAO-A) and [14C]benzylamine (MAO-B) to be a rich source of both enzyme systems. Incubation with clorgyline and deprenyl, probe inhibitors of MAO-A and MAO-B, respectively, showed that [14C]sumatriptan was metabolized by MAO-A; there was no evidence of P450 involvement in its metabolism. The data in this study therefore indicate that the enzyme MAO-A is the major enzyme responsible for the metabolism of sumatriptan in human liver.

7-Ethoxycoumarin O-deethylase kinetics in isolated rat, dog and human hepatocyte suspensions

1. A comparative study of the kinetics of the O-deethylation of 7-ethoxycoumarin has been carried out in freshly isolated rat, dog and human hepatocytes. 2. Biphasic kinetics were observed for all three species with apparent Km and Vmax values of 11.5, 2.2 and 3.9 microM and 0.30, 0.21, 0.007 nmol/min/10(6) cells from rat, dog and man, respectively, for the high affinity-low capacity component, and 560, 40, 470 microM and 1.52, 0.74, 0.057 nmol/min/10(6) cells, respectively, for the low affinity-high capacity component. 3. These observed kinetic parameters in hepatocytes from rat and man were similar to published values for microsomes for the same two species. 4. Values for intrinsic clearance of 7-ethoxycoumarin for the three species calculated from the Km and Vmax data were 152, 631 and 6 ml/min/kg for rat, dog and human hepatocytes, respectively. These intrinsic clearance values predict that 7-ethoxycoumarin would be subject to a high hepatic clearance in rat and dog, and low hepatic clearance in man. These values are supported by published data on rat which show that 7-ethoxycoumarin is subject to high clearance.

What changes drug metabolism in critically ill patients? Two preliminary studies in isolated human hepatocytes

The metabolism of many drugs is abnormal in the critically ill patient. The causes of this are unknown, and investigation in patients is difficult. We therefore used isolated, cultured human hepatocytes to study the effects of hypoxia and induction on cytochrome P450 3A, the cytochrome responsible for the metabolism of many drugs. When hepatocytes were exposed to 5% oxygen, the amount of 3A produced, after induction with rifampicin, was five to 10 times less than the amount produced in 21% oxygen. In another study, we exposed isolated hepatocytes for 4 days to serum from five critically ill patients or from volunteers. At the end of this time, the functional ability of the hepatocytes to glucuronidate 14C progesterone was measured. Four of the five patients had a substance in their plasma that reduced the ability of the hepatocytes to glucuronidate progesterone. The nature of this substance and the reason that the serum from the fifth patient did not affect metabolism are unknown. This model is able to simulate the abnormalities which occur in critically ill patients. Further studies are needed to explain our observations and to identify the substances in the serum from critically ill patients that alter drug metabolism.

Blood concentrations of dopexamine in patients during and after orthotopic liver transplantation

Blood concentrations of dopexamine were measured in five female patients during and after orthotopic liver transplantation. Each patient received a continuous infusion of the drug (2 micrograms kg-1 min-1), starting at induction of anaesthesia and finishing 48 h afterwards. Blood concentrations of dopexamine increased rapidly at the start of the infusion, to a mean (range) value of 64 (40-150) ng ml-1 after 20 min. Blood concentrations of dopexamine increased further during the anhepatic period of surgery, to 236 (180-410) ng ml-1. On reperfusion of the donor liver, concentrations of dopexamine decreased rapidly, reaching similar values to the maximum seen during the dissection period. Steady-state was not reached during either the dissection or anhepatic periods. From 1-2 h after revascularisation the mean (range) steady-state concentration was 85 (69-102) ng ml-1 corresponding to a mean (range) clearance of 24 (20-29) ml min-1 kg -1. These results suggest that the liver plays a significant role in the clearance of dopexamine.

Propofol and intralipid cause creaming of serum from critically ill patients

The aim of this in-vitro study was to investigate the incidence of propofol agglutination with serum from critically ill patients. Serum (400 microliters) from 58 critically ill patients and 30 healthy volunteers was incubated with 10 microliters of either propofol, Intralipid 10% or Intralipid 20%. Control incubations contained serum only. At 24 h, the serum was examined macroscopically and microscopically for agglutination. Agglutination was seen with Intralipid 20% in serum from all critically ill patients and 13.3% of volunteers. Serum from 91.4% of critically ill patients was agglutinated with Intralipid 10% and only 3.3% of the healthy volunteers. In comparison, propofol produced agglutination in 74.1% of critically ill patients and in none of the serum from healthy volunteers (p < 0.05 propofol versus Intralipid 10%, p < 0.0001 propofol versus Intralipid 20%). No correlation was seen between agglutination and age, sex, APACHE II score or plasma concentration of acute phase proteins. However, agglutination of propofol and Intralipid 10% was more frequent (p < 0.001) in serum from patients with pulmonary disease, than in patients with normal lungs. The clinical implications of these in-vitro findings are unclear and need further investigation.

Pharmacokinetics and pharmacodynamics in the critically ill patient

1. Until recently, when drugs were used in critically ill patients they were expected to behave in the same way as in less seriously ill patients. Now the unpredictability of even the most reliable drugs has been recognized. With this there is an awareness of the adverse effects drugs may have on organs other than the ones the drug was intended to act on. In patients with multiorgan dysfunction, poly-pharmacy is usually needed. The drugs may not only interfere with the action of each other at the receptor and enzyme level, but may also change protein binding and elimination. All these effects may be unimportant in less seriously ill patients, but may affect outcome in the critically ill. A high degree of awareness and suspicion of unknown drug-induced adverse reaction is needed by clinicians and pharmacologists alike.

Creatinine and 51Cr-EDTA clearance after liver transplantation

We have compared glomerular filtration rate measured by creatinine clearance with that measured by 51Cr-EDTA clearance after liver transplantation. Fourteen pairs of values were obtained from seven patients on the first and second days after operation. There were wide discrepancies between the values for glomerular filtration rate measured by the two methods, with a regression co-efficient of 0.43 (p = 0.12). Both methods assume a steady state, with no change of extracellular fluid volume or in the rates of exchange between physiological compartments, that does not apply in the immediate period after operation. The results show the difficulties of using clearance techniques to assess renal function after major surgery. Since drug therapy may be based on these measurements, we suggest that in this group of patients isolated clearance values should not be used.

Midazolam-induced amnesia: implications for the implicit/explicit memory distinction

In order to examine how the distinction between implicit and explicit memory might relate to theories regarding the organization of memory, two types of memory tests were administered in conjunction with the amnesia-inducing benzodiazepine midazolam. Performance on an implicit task (perceptual facilitation in identifying degraded pictures and words) was relatively unimpaired by midazolam, whereas performance on an explicit task (recognition memory) was severely impaired. Interpreting the results in both the implicit/explicit framework and an independent stage analysis suggests that the terms implicit and explicit may reflect something about the nature of the memory representation and need not be reserved solely to describe classes of memory tasks.

Morphine elimination and hepatic blood flow--a study in patients with portal hypertension

To investigate the influence of changes in portal venous blood flow on the clearance of morphine, the elimination of a single intravenous dose of morphine sulphate was studied in twelve patients with periportal fibrosis undergoing distal splenorenal shunt for portal hypertension. All patients had almost normal preoperative liver function. Six patients (Group A) received morphine after induction of anesthesia but before surgery; a further six patients (Group B) received morphine after completion of the splenorenal shunt and removal of all vascular clamps. Blood was sampled for 24 hours after morphine administration and analysed for plasma morphine concentration using high performance liquid chromatography. The maximum concentration of morphine was significantly greater in those patients in Group B who received morphine after the shunt (P < .002). Clearance of morphine during the operation was also significantly reduced in these patients compared with the initial clearance (before release of the clamps) in Group A (P < .004). As a consequence of these changes the AUC was greater in this group (P < 0.004). However, there were no differences with the elimination rates after surgery had finished. The results suggest that in individuals with normal liver function, the clearance of morphine is dependent on hepatic blood flow.

Comparison of the sedative and amnesic effects of midazolam and propofol

We have compared the sedative and amnesic effects of midazolam and propofol in 35 volunteers. Sedation was measured by simple reaction time immediately before and after a bolus injection and 1 h after the commencement of a subsequent continuous infusion. Memory was measured three times using two memory tests: perceptual facilitation provided an implicit memory measure and recognition provided an explicit memory measure. Propofol and midazolam had similar sedative effects both immediately after bolus doses and after 1-h continuous infusions of the drugs. In contrast, midazolam had a more profound amnesic effect than propofol on the recognition memory test. The drugs had little effect on performance with the implicit memory test. Performance on the memory tests was unrelated to sedation.

Surfactant and adult respiratory distress syndrome

In order to determine if the Adult Respiratory Distress Syndrome (ARDS) is associated with a shortage of pulmonary surfactant, we have examined a series of 155 undiluted tracheal aspirates obtained from 23 patients with ARDS and from 30 patients without ARDS, all 53 needing ventilatory support. The unfixed and unstained specimens were examined by polarized light microscopy for the presence of pulmonary surfactant. Free surfactant particles were present in the aspirates of 50 patients (95%). Within minutes of obtaining the specimen, in addition to showing surfactant, the examination of these undiluted, unfixed tracheal aspirates by light microscopy showed the presence of bacterial and fungal infection. As fibrinogen/fibrin inactivates surfactant, the undiluted aspirates of 21 patients with ARDS and of 30 patients without ARDS were stained for the presence of these proteins by a new slide method using 0.01 ml of tracheal aspirate. Fibrinogen/fibrin was found in the aspirates from 20 of 21 patients with ARDS and in only five of 30 patients without ARDS; these five had bronchopneumonia. The method has a sensitivity of 0.95 and a specificity of 0.83. The role of plasma proteins and of neutrophil leucocytes in causing ARDS is discussed.

Intravenous chlormethiazole

Intravenous chlormethiazole is widely used for its sedative and anticonvulsant action in patients with acute alcohol withdrawal, status epilepticus, pre-eclampsia and eclampsia. Concern remains over its safety if it is given carelessly and without careful monitoring.