Sedation and analgesia

Sedation is a process of soothing. The concept of the ideal level of sedation is controversial and has changed over the last decade. A shift from deep sedation, often enhanced by muscle relaxants that completely detaches the patient from their environment, to light sedation rendering the patient sleepy but easily arousable has been widely accepted. This change in attitude has been brought about by sophisticated modes of ventilation allowing the ventilator to synchronize with the patient's own breathing pattern. In addition, the increasingly recognised adverse effects of over-sedation have contributed to the reduction in the depth of sedation.

Metabolism of remifentanil during liver transplantation

We have investigated the pharmacokinetics of remifentanil and its less potent metabolite, GR90291, in six adult patients undergoing orthotopic liver transplantation (OLT). A single bolus infusion of remifentanil 10 micrograms kg-1 min-1 was given at the beginning of the dissection and anhepatic phases of OLT. Remifentanil and GR90291 concentrations were measured in subsequent serial arterial and mixed venous blood samples. Mean arterial clearance of remifentanil was significantly greater (P = 0.02) in the dissection phase (79.54 ml min-1 kg-1) than in the anhepatic phase (39.57 ml min-1 kg-1). Steady state volumes of distribution were not significantly different. Clearance of remifentanil during the anhepatic phase was similar to that of healthy adult patients. Mean maximum concentration (Cpmax) of GR90291 was lower in the dissection phase than in the anhepatic phase (P = 0.026). There was no significant pulmonary metabolism of remifentanil.

Serum albumin and colloid osmotic pressure in survivors and nonsurvivors of prolonged critical illness

We retrospectively compared the changes in serum albumin concentration and colloid osmotic pressure between survivors and nonsurvivors of prolonged (> or = 7 days) critical illness over a 2-year period from 1 July 1995. All patients had serum albumin measured daily, and colloid osmotic pressure measured 5 days a week, throughout their ICU admission. They received crystalloid and colloid infusions as well as parenteral or enteral feeding. Infusions of albumin were not used to treat hypoalbuminaemia. One hundred and forty-five patients were included, 66 nonsurvivors and 79 survivors. Nonsurvivors were significantly older than survivors [mean (95% CI): 58 (3.8) and 49 (4.1) years, respectively] and had a greater risk of death [mean (95% CI): 0.44 (0.06) and 0.28 (0.05); p < 0.05]. There was no significant difference in gender, APACHE II score [mean (95% CI): 22 (2.7) (nonsurvivors); 18 (2.3) (survivors)] or length of stay [median (interquartile range): 14 (9-27) days (nonsurvivors); 15 (9-26) days (survivors)]. There was no difference between the two groups in the absolute minimum serum albumin concentrations reached, the time to reach that minimum or the minimum in the first 7 days. However, nonsurvivors had a significantly lower mean serum albumin concentration: [mean (95% CI): 15.7 (5.1) g.l-1 compared with 18.3 (4.6) g.l-1 in survivors; p < 0.05]. They also had a lower recovery mean (the weighted mean after the minimum value): [mean (95% CI): 13.3 (5.1) g.l-1 (nonsurvivors) and 18.6 (5.3) g.l-1 (survivors); p < 0.01]. Analysis of colloid osmotic pressure results showed no difference between the groups in mean, minimum or recovery mean. Regression analysis of mean colloid osmotic pressure and albumin revealed that albumin only contributed 17% of the colloid osmotic pressure in these patients. The similar decrease in albumin in nonsurvivors and survivors may reflect the acute inflammatory response and/or haemodilution. However, survivors showed an ability to increase serum albumin concentrations, possibly owing to resumption of synthesis. The colloid osmotic pressure varied little between or within either group of patients, possibly because of the use of artificial colloids. There was no relationship between death and colloid osmotic pressure.

Treatment of compulsive behaviour in eating disorders with intermittent ketamine infusions

We have previously shown that eating disorders are a compulsive behaviour disease, characterized by frequent recall of anorexic thoughts. Evidence suggests that memory is a neocortical neuronal network, excitation of which involves the hippocampus, with recall occurring by re-excitement of the same specific network. Excitement of the hippocampus by glutamate-NMDA receptors, leading to long-term potentiation (LTP), can be blocked by ketamine. Continuous block of LTP prevents new memory formation but does not affect previous memories. Opioid antagonists prevent loss of consciousness with ketamine but do not prevent the block of LTP. We used infusions of 20 mg per hour ketamine for 10 h with 20 mg twice daily nalmefene as opioid antagonist to treat 15 patients with a long history of eating disorder, all of whom were chronic and resistant to several other forms of treatment. Nine (responders) showed prolonged remission when treated with two to nine ketamine infusions at intervals of 5 days to 3 weeks. Clinical response was associated with a significant decrease in Compulsion score: before ketamine, mean +/- SE was 44.0 +/- 2.5; after ketamine, 27.0 +/- 3.5 (t test, p = 0.0016). In six patients (non-responders) the score was: before ketamine, 42.8 +/- 3.7; after ketamine, 44.8 +/- 3.1. There was no significant response to at least five ketamine treatments, perhaps because the compulsive drive was re-established too soon after the infusion, or because the dose of opioid antagonist, nalmefene, was too low.

The effect of propofol on midazolam metabolism in human liver microsome suspension

We have studied the inhibitory effects of propofol on the metabolism of midazolam using human liver microsomes. In addition, we also investigated whether the lipid in which propofol is solubilised inhibits the metabolism of midazolam. Only high concentrations of propofol (> 100 mmol), greater than those found in clinical practice, inhibited the metabolism of midazolam. The lipid had no effect on the metabolism of midazolam. This study differs from other laboratory studies looking at the inhibitory effects of propofol. These showed inhibition at concentrations similar to those seen in patients. The reasons for the differences may be explained by the use of different substrates or methodology. Propofol may be an enzyme inhibitor, but this remains to be shown to be important in patients.

Effects of inotropes on human leucocyte numbers, neutrophil function and lymphocyte subtypes

We have investigated the effects of inotropes with different adrenergic receptor specificity on differential white cell count, lymphocyte subtypes and neutrophil function in healthy volunteers. Six healthy, male volunteers were enrolled into this randomized, placebo-controlled pilot study. Each volunteer was studied on four separate occasions during a 2-h infusion of various agents, and for 2 h after stopping the infusion. The agents investigated were adrenaline 0.1 microgram kg-1 min-1, dobutamine 5 micrograms kg-1 min-1, dopexamine 2 micrograms kg-1 min-1 and 5% glucose 0.5 ml kg-1 h-1. Venous blood was sampled at 0, 30, 120 and 240 min. Haemodynamic monitoring was continued throughout the study. Full blood count, white cell differential count and enumeration of lymphocyte subtypes were performed. Neutrophil function tests included chemoluminescence, and assessment of neutrophil chemotaxis, phagocytosis and adhesion. The Wilcoxon signed rank test was used to compare differences between placebo and active drugs at each time compared with baseline. There was a significant increase in white cell count, lymphocyte count and neutrophil count with adrenaline, and a small but significant decrease in these variables with dobutamine and dopexamine. These changes were also apparent for absolute CD3+, CD4+ and CD8+ lymphocyte counts. Neutrophil respiratory burst in response to f-methionyl-leucyl-phenylalanine increased significantly only with adrenaline at 30 min (P = 0.046). There were no other significant changes in tests of neutrophil function. Infusion of inotropes was associated with changes in white cell numbers, lymphocyte subtypes and neutrophil respiratory burst. In healthy volunteers, adrenaline had effects different from those of dobutamine and dopexamine. The clinical relevance of such effects requires further investigation in critically ill patients.

The aliphatic oxidation of salmeterol to alpha-hydroxysalmeterol in human liver microsomes is catalyzed by CYP3A

Salmeterol xinafoate (Serevent) is a long-acting beta2-adrenoceptor agonist, used in the treatment of asthma, that has bronchodilator and anti-inflammatory action. Salmeterol is extensively metabolized by aliphatic oxidation in humans, with the major metabolite being alpha-hydroxysalmeterol. The aim of this investigation was to identify the specific cytochrome P450 (P450) isoform or isoforms involved in the formation of alpha-hydroxysalmeterol in human liver microsomes. [14C]Salmeterol was incubated with a pooled sample (N = 19) of human liver microsomes in the absence or presence of selective chemical inhibitors of the major human P450 isoforms. One microM ketoconazole, a selective inhibitor of CYP3A, substantially inhibited the metabolism of salmeterol to alpha-hydroxysalmeterol. Disulfiram caused a small but consistent decrease in the amount of alpha-hydroxysalmeterol formed, possibly reflecting less than total selectivity for CYP2E1 under the conditions used. Other selective inhibitors had no significant effect on the metabolism of salmeterol. The rates of formation of alpha-hydroxysalmeterol in 10 individual liver microsomal samples showed an approximately 10-fold variation and were found to be highly correlated (r2 = 0.94; p < 0.001) with rates of metabolism of midazolam to 1'-hydroxymidazolam, a marker of CYP3A activity, in the same microsomal samples. No significant correlation was evident for the metabolism of salmeterol with levels of total P450 or other markers of human P450 activities in the same microsomal samples, thus indicating that the formation of alpha-hydroxysalmeterol is catalyzed predominantly by CYP3A. Insect cell microsomes that coexpressed human CYP3A and NADPH-P450 reductase were able to metabolize [14C]salmeterol to alpha-hydroxysalmeterol, thus confirming the role of CYP3A in catalyzing this reaction. The therapeutic dose of salmeterol is very low, so it is unlikely that any clinically relevant interactions will be observed as a consequence of the coadministration of salmeterol and other pharmaceutical agents that are metabolized by CYP3A.

What changes drug metabolism in critically ill patients--III? Effect of pre-existing disease on the metabolism of midazolam

Liver samples were obtained at hepatectomy from patients with end stage alcoholic liver disease (n = 5), primary biliary cirrhosis (n = 5) and chronic rejection needing retransplantation (n = 5). Normal liver material was also obtained from five organ donors. From these samples microsomes were made containing cytochrome P450 3A. The amount of this enzyme was measured by Western immunoblotting and its function assessed by measuring the rate of production of two metabolites of midazolam, 1-hydroxy midazolam and 4-hydroxy midazolam. There was a wide range in all groups for both the expression and function of this enzyme. Liver tissue affected by cirrhotic disease showed greater preservation of enzyme function than that affected by hepatocellular disease. There was a good correlation between the expression of the enzyme and production of the 1-hydroxy metabolite, but a poor correlation between production of the 4-hydroxy metabolite and expression. This poor correlation may reflect the failure to measure the specific enzyme responsible for producing 4-hydroxy midazolam.

The plasma turnover of transfused antithrombin concentrate in patients with acquired antithrombin deficiency

Antithrombin concentrate, prepared from human plasma, has been used as replacement therapy in 35 patients with acquired antithrombin deficiency. The inhibitory activity of the concentrate, measured by chromogenic assay, correlates well with the manufacturer's quoted activity. The mean in vivo recovery of the product was 0.0124 iu mL-1 per iu of antithrombin (AT) concentrate administered by kilogram body weight. The recovery was similar in all diagnostic groups studied and did not vary during the course of treatment. Consumption of the antithrombin concentrate was monitored by measuring the production of thrombin-antithrombin complexes and the loss of plasma antithrombin activity. The mean concentration of thrombin-antithrombin complexes was elevated (23 ng mL-1) at the time of admission to the intensive care unit and fell progressively over the next 4 days. The mean time for the decay of half the antithrombin activity was 23 h during the first 24 h of therapy and rose to 42.1 h after day 1. The recovery and half-life measurements are necessary to plan an appropriate dosage regimen for the administration of this antithrombin concentrate in acquired deficiency states.

What changes drug metabolism in critically ill patients?--II Serum inhibits the metabolism of midazolam in human microsomes

Serum samples from five critically ill patients were incubated with microsomes prepared from three human livers. The activity of cytochrome P450 3A4 was assessed by measuring the disappearance of midazolam and the appearance of 1-hydroxy midazolam in the incubates. Significant inhibition of the ability of this enzyme to metabolise midazolam was seen. This occurred in incubates containing serum samples from critically ill patients and not in those containing serum from two normal volunteers. The mechanism of this inhibition is unknown, but several possibilities are discussed.

The role of flumazenil in the critically ill

Flumazenil is a specific benzodiazepine antagonist. It competitively inhibits the action of benzodiazepines at gamma aminobutyric acid (GABA) receptors in the central nervous system and thus reverses the sedative effects of benzodiazepines. Usually, it is given intravenously as a bolus dose or as an infusion. It has a short duration of action. Flumazenil is extensively metabolized to compounds that have minimal benzodiazepine antagonistic properties. Reversal of sedation can be useful in many conditions that are often encountered in the critically ill. The adverse effects of its use are usually predictable and, with sufficient clinical monitoring, are usually avoidable. These properties make it a useful and safe drug when used appropriately.

Anaesthetic drugs and bacterial contamination

We investigated the bacterial contamination of commonly used intravenous anaesthetic agents. Ten trainee anaesthetists drew up four syringes each of propofol, midazolam, thiopentone, 0.9% saline and a culture medium control using their normal practice. A set of syringes was cultured at the time of drawing up and at 2, 4 and 8 h afterwards. No anaesthetist washed his or her hands before drawing up the drugs. Six anaesthetists capped the syringes using sheathed needles. Eight syringes were contaminated with bacteria. Coagulase-negative staphylococci were isolated from six syringes (four sheathed and two unsheathed) and an Acinetobacter sp. from two syringes (one sheathed and one unsheathed). No bacteria were cultured from the midazolam syringes. However, two syringes from each of the other solutions were contaminated. This implies that one syringe was contaminated when it was drawn up, at 2 h a further two had detectable contamination, two more were contaminated at 4 h and three more at 8 h. We cannot recommend drawing up drugs at the start of a list for use later in the day. They should be used immediately after drawing up. Basic hand washing before drawing up drugs may reduce contamination.

Multiple forms of cytochrome P450 are involved in the metabolism of ondansetron in humans

Ondansetron is cleared primarily by metabolism in humans, with hydroxylation of the indole moiety in the 7- and 8-positions being the major identified phase I pathways. In vitro studies using lymphoblastoid cell lines expressing single human cytochrome P450 forms and hepatic microsomes were undertaken to investigate the forms involved in the metabolism of ondansetron in humans. The cell lines that expressed CYP1A1, CYP1A2, and CYP2D6 were shown to be capable of metabolizing [14C]ondansetron. Studies with human hepatic microsomes and the specific inhibitors furafylene, quinidine, and ketoconazole confirmed the role of CYP1A2 and CYP2D6 and also demonstrated the involvement of the CYP3A subfamily. The data in this study collectively indicate that multiple cytochrome P450 forms, including CYP1A1, CYP1A2, CYP2D6, and the CYP3A subfamily, are probably involved in the clearance of ondansetron in humans, with no single form of cytochrome P450 dominating the overall metabolism of ondansetron. The role played by CYP2D6 in the metabolism of [14C]ondansetron by human hepatic microsomes in vitro was shown to be minor. This finding is consistent with the lack of bimodality in the clinical pharmacokinetics of ondansetron. It is therefore concluded that ondansetron is metabolized by multiple forms of cytochrome P450, and this limits the likelihood of a clinically relevant interaction with ondansetron by a modulator of a single form of cytochrome P450.

Propofol versus propofol with midazolam for laryngeal mask insertion

We evaluated the addition of midazolam to propofol during induction of anaesthesia by assessing laryngeal mask tolerance, haemodynamic variables, recovery times and cost. Forty patients (ASA grades I-IV) undergoing elective surgery were allocated randomly to receive a standard dose of propofol or a smaller dose of propofol combined with midazolam. A laryngeal mask was inserted and any episodes of coughing or hiccuping during its insertion or removal were recorded. Anaesthesia was maintained with nitrous oxide and enflurane with fentanyl for analgesia. After surgery, recovery times, pain, shivering, nausea, vomiting and analgesic requirements were recorded. The cost of the drugs used was also calculated. No significant differences were detected in any variables, except that patients given propofol needed more morphine in the recovery ward. The average cost of propofol alone was 3.47 pounds per anaesthetic, while the midazolam plus propofol cost was 2.03 pounds. Adding midazolam to propofol allowed a reduced dose of propofol to be used without adverse effects, while reducing the anaesthetic costs.

Desflurane and the sensitivity of upper airway reflexes

Sixty consecutive patients who were given an anesthetic, not needing tracheal intubation, received desflurane as part of the anesthetic technique to assess the incidence of upper airway irritation. A variety of induction agents and artificial airways were used. The incidence of upper airway irritation was recorded. Eight patients coughed at induction. Five of these were related to insertion of a laryngeal mask and probably represent light anesthesia. One was unrelated to instrumentation, another caused by a partially obstructed airway and the final one occurred in a child with an upper respiratory tract infection. Six episodes of coughing and hiccoughs occurred after the end of anesthesia. Four were related to removal of an airway. Five episodes of coughing occurred in the ten smokers, confirming the importance of this as a risk factor. Six patients who coughed were given no premedication or were anesthetized before it could take effect. Coughing occurs during general anesthesia, but it is caused by many factors. In adults, using desflurane it does not appear to be a significant risk factor.