Lidocaine elimination: effects of metoprolol and of propranolol

Drug Interactions Affecting Antiarrhythmic Drug Use

Antiarrhythmic drugs (AAD) play an important role in the management of arrhythmias. Drug interactions involving AAD are common in clinical practice. As AADs have a narrow therapeutic window, both pharmacokinetic as well as pharmacodynamic interactions involving AAD can result in serious adverse drug reactions ranging from arrhythmia recurrence, failure of device-based therapy, and heart failure, to death. Pharmacokinetic drug interactions frequently involve the inhibition of key metabolic pathways, resulting in accumulation of a substrate drug. Additionally, over the past 2 decades, the P-gp (permeability glycoprotein) has been increasingly cited as a significant source of drug interactions. Pharmacodynamic drug interactions involving AADs commonly involve additive QT prolongation. Amiodarone, quinidine, and dofetilide are AADs with numerous and clinically significant drug interactions. Recent studies have also demonstrated increased morbidity and mortality with the use of digoxin and other AAD which interact with P-gp. QT prolongation is an important pharmacodynamic interaction involving mainly Vaughan-Williams class III AAD as many commonly used drug classes, such as macrolide antibiotics, fluoroquinolone antibiotics, antipsychotics, and antiemetics prolong the QT interval. Whenever possible, serious drug-drug interactions involving AAD should be avoided. If unavoidable, patients will require closer monitoring and the concomitant use of interacting agents should be minimized. Increasing awareness of drug interactions among clinicians will significantly improve patient safety for patients with arrhythmias.

Tumescent anaesthesia: its applications and well tolerated use in the out-of-operating room setting

PURPOSE OF REVIEW

Tumescent anaesthesia is a method of administering dilute local anaesthetic into the subcutaneous tissue. Many anaesthesiologists are unfamiliar with the technique, its applications and potential risks.

RECENT FINDINGS

The maximum safe dose of lidocaine with epinephrine in tumescent anaesthesia for liposuction is probably between 35 and 55 mg/kg. Without liposuction, the maximum dose of lidocaine with epinephrine should be no more than 28 mg/kg. After tumescent infiltration for liposuction, serum lidocaine concentrations peak between 12 and 16 h after injection. When tumescent lidocaine without epinephrine is used for endovenous laser therapy, peak serum lidocaine concentrations are observed much earlier, between 1 and 2 h after injection. Slow administration of more dilute concentrations of local anaesthetic decreases the risk of local anaesthetic systemic toxicity.

SUMMARY

Although appealing because of its ability to provide prolonged analgesia, high doses of local anaesthetic are frequently administered using the tumescent technique, and absorption of local anaesthetic from the subcutaneous tissue is variable. When caring for patients having procedures in which tumescent anaesthesia is used, the risk of local anaesthetic toxicity should be acknowledged and lipid emulsion should be available for prompt treatment if needed.

Do children have the same vulnerability to metabolic drug–drug interactions as adults? A critical analysis of the literature

Many drug–drug interactions (DDIs) in the pediatric population are managed based on data generated in adults. However, due to developmental changes in elimination pathways from birth to adolesence, and variable weight‐adjusted dose of interacting drugs, the assumption of DDIs being similar in adults and pediatrics might not be correct. This study compares the magnitude of reported DDIs in pediatric and adult populations. A systematic literature review was undertaken to identify reports of DDIs in pediatric subjects. A total of 145 reports of DDIs were identified over the age range of birth to 20 years. The magnitude of DDIs for 24 drug pairs from 31 different pediatric studies could be assessed and compared with those in adults where corresponding data existed. The magnitude of the DDI, as measured by a relevant parameter (e.g., AUC, CL) in the presence and absence of inhibitor,were higher (>1.25‐fold), similar (0.8‐ to 1.25‐fold) or lower (<0.8‐fold) than the corresponding ratio in adults in 10, 15, and 8 cases respectively. An age‐related trend in the magnitude of DDIs could not be established. However, the study highlighted the clear paucity of the data in children younger than 2 years. Care should be exercised when applying the knowledge of DDIs from adults to children younger than 2 years of age.

Adverse drug reactions in patients with cardiovascular disease

Adverse drug reactions (ADRs) occur frequently in modern medical practice, increasing morbidity and mortality and inflating the cost of care. Patients with cardiovascular disease are particularly vulnerable to ADRs due to their advanced age, polypharmacy, and the influence of heart disease on drug metabolism. The ADR potential for a particular cardiovascular drug varies with the individual, the disease being treated, and the extent of exposure to other drugs. Knowledge of this complex interplay between patient, drug, and disease is a critical component of safe and effective cardiovascular disease management. The majority of significant ADRs involving cardiovascular drugs are predictable and therefore preventable. Better patient education, avoidance of polypharmacy, and clear communication between physicians, pharmacists, and patients, particularly during the transition between the inpatient to outpatient settings, can substantially reduce ADR risk.

Porcine FAD-containing monooxygenase metabolizes lidocaine, bupivacaine and propranolol in vitro

Lidocaine, bupivacaine and propranolol are amines that can be expected to act as substrates for FAD-containing monooxygensae (FMO) (EC 1. 14. 13. 8). We found that FMO metabolizes lidocaine, bupivacaine and propranolol. The Km and Vmax values of lidocaine, bupivacaine and propranolol for FMO are 143, 408 and 210 microM, and 145, 119 and 135 nmol/min/mg FMO protein, respectively. The lipophilicity of the drugs decreased in the following order: lidocaine>propranolol>bupivacaine, under our experimental conditions. Furthermore, the metabolic products of FMO were separated by high-performance liquid chromatography and analyzed by gas chromatography-mass spectrometry, and were found to be the N-oxides and N-hydroxylamines of the respective drugs. These findings suggest that lidocaine, bupivacaine and propranolol are substrates for FMO, and the enzymatic toward lidocaine or bupivacaine may be inhibited exclusively and competitively by propranolol.

β-Adrenergic Blockade Affects Initial Drug Distribution Due to Decreased Cardiac Output and Altered Blood Flow Distribution

Beta-adrenergic receptor blockers decrease intravenous anesthetic dose requirements. The present study determined the effect of propranolol on indocyanine green and antipyrine disposition from the moment of rapid intravenous injection. Anti-pyrine is a physiological marker that distributes to a volume as large as total body water in a blood flow-dependent manner and is a pharmacokinetic surrogate for many lipophilic drugs, including intravenous anesthetics. Antipyrine and indocyanine green disposition were determined twice in five healthy adult males in this Institutional Review Board-approved study, once during propranolol infusion. After rapid indocyanine green and antipyrine injection, arterial blood samples were collected frequently for 2 min and less frequently thereafter. Plasma indocyanine green and antipyrine concentrations were measured by high-performance liquid chromatography. Indocyanine green and antipyrine disposition were characterized, using SAAM II, by a recirculatory pharmacokinetic model that describes drug disposition from the moment of injection. Parameters were compared using the paired t test. The disposition of indocyanine green demonstrated that propranolol decreased cardiac output at the expense of the fast peripheral (nonsplanchnic) intravascular circuit. The area under the antipyrine concentration versus time relationship was doubled for at least the first 3 min after injection due to both decreased cardiac output and maintenance of nondistributive blood flow at the expense of a two-thirds reduction of blood flow (intercompartmental clearance) to the rapidly equilibrating (fast, splanchnic) tissue volume. The increase in antipyrine area under the curve due to propranolol-induced alteration of initial antipyrine disposition could explain decreased intravenous anesthetic dose requirements in the presence of beta-adrenergic receptor blockade.

Use of lidocaine metabolism to test liver function during the long-term follow-up of liver transplant recipients

BACKGROUND/AIMS

The aim of this study was to assess the usefulness of the monoethylglycinexylidide (MEGX) test to monitoring the long-term function of liver allografts.

METHODS

MEGX production was measured prospectively in 60 consecutive liver transplant recipients undergoing their annual review.

RESULTS

Median MEGX values in liver recipients (54 ng/mL; range 10-146) were lower than those found in healthy controls (78 ng/mL; range 44-118). MEGX values correlated negatively with alanine aminotransferase (ALT) activity (p = 0.004) and with the overall histological score (p = 0.01), and positively with sulfobromophthalein (BSP) and indocyanine green (ICG) clearances (p = 0.0002 and p = 0.002, respectively). A stepwise decline was observed with worsening liver fibrosis, from 71 +/- 5 microg/L in patients with no fibrosis to 27 +/- 9 microg/L in patients with bridging fibrosis (p = 0.002). BSP and ICG clearances correlated more closely than the MEGX test with the overall histological score (p = 0.001 and p = 0.001, respectively) and portal fibrosis (p = 0.002 and p = 0.001).

CONCLUSIONS

The measurement of MEGX formation is a simple and non-invasive method to monitor liver graft function. It may constitute a valuable tool for assessing the degree of fibrosis.

Prediction of lidocaine tissue concentrations following different dose regimes during cardiac arrest using a physiologically based pharmacokinetic model

BACKGROUND

The purpose of our study was to develop a physiologically based pharmacokinetic (PBPK) model describing the behavior of lidocaine in humans by scaling up physiological variables from animal models of cardiac arrest. We attempted to identify the optimal dose regime for lidocaine during cardiac arrest using this model.

METHODS AND RESULTS

We designed a flow-dependent PBPK model representing nine body tissues for lidocaine. Physiological organ flow rates, tissue volumes, and plasma-tissue partition parameters for lidocaine in humans were taken from the literature. Data from published animal studies were used to estimate loss of organ blood flow during cardiac arrest and lidocaine tissue partition coefficients. The model assumed a 70 kg cardiac arrest patient. The following five lidocaine dose regimes were simulated: (1) 4 mg/kg i.v. push (IVP) (2) 1.5 mg/kg IVP then 1.5 mg/kg IVP in 4 min, (3) 3 mg/kg IVP, (4) 2 mg/kg IVP, and (5) 1.5 mg/kg IVP. A simulation of Regimen 2, which is the current American Heart Association (AHA) recommendation, suggests that the concentration of lidocaine is suboptimal at the decision point (3-5 min) to administer another dose. Regimen 4 offers a slightly more rapid progress towards optimal cardiac concentrations and more acceptable brain concentrations compared to regimes 1-3.

CONCLUSION

Simulations from our PBPK model suggest that the current AHA lidocaine dose regime for cardiac arrest may not result in optimal lidocaine concentrations in the heart and brain. Simulations suggest that 2 mg/kg IVP may be the most acceptable lidocaine dose regime during cardiac arrest.

The effects of epidural insertion site and surgical procedure on plasma lidocaine concentration

We compared the plasma lidocaine concentrations associated with continuous epidural infusion at different insertion sites in patients during surgery using epidural plus general anesthesia. In Study 1, there were 12 patients in each of four surgical groups in whom blood loss was expected to be <400 mL. The four groups were as follows: the lower extremity, the lower abdomen, the upper abdomen, and the lung. Liver surgery was excluded from Study 1. Study 2 comprised patients undergoing radical hysterectomy or radical prostatectomy (a radical operation group, n = 12) and hepatectomy (a hepatectomy group, n = 12) in whom the expected surgical blood loss was more than 1500 mL. All patients initially received 0.1 mL/kg followed by a continuous infusion of 0.1 mL. kg(-1). h(-1) of 1.5% lidocaine, and plasma concentrations of lidocaine were measured at 15, 30, 60, 90, and 120 min and every 60 min thereafter to 300 min. The plasma lidocaine concentration during surgery did not change regardless of the infusion site or the surgical site, other than the liver. The plasma concentrations of lidocaine in the hepatectomy group increased significantly at 180 min (2.9 +/- 0.6 microg/mL, P < 0.01), 240 min (3.5 +/- 0.7 microg/mL, P < 0.01), and 300 min (3.6 +/- 0.74 microg/mL, P < 0.01) compared with that at 15 min (2.0 +/- 0.3 microg/mL), and these values were significantly larger than those in all other groups.

Antiarrhythmic agents: drug interactions of clinical significance

The management of cardiac arrhythmias has grown more complex in recent years. Despite the recent focus on nonpharmacological therapy, most clinical arrhythmias are treated with existing antiarrhythmics. Because of the narrow therapeutic index of antiarrhythmic agents, potential drug interactions with other medications are of major clinical importance. As most antiarrhythmics are metabolised via the cytochrome P450 enzyme system, pharmacokinetic interactions constitute the majority of clinically significant interactions seen with these agents. Antiarrhythmics may be substrates, inducers or inhibitors of cytochrome P450 enzymes, and many of these metabolic interactions have been characterised. However, many potential interactions have not, and knowledge of how antiarrhythmic agents are metabolised by the cytochrome P450 enzyme system may allow clinicians to predict potential interactions. Drug interactions with Vaughn-Williams Class II (beta-blockers) and Class IV (calcium antagonists) agents have previously been reviewed and are not discussed here. Class I agents, which primarily block fast sodium channels and slow conduction velocity, include quinidine, procainamide, disopyramide, lidocaine (lignocaine), mexiletine, flecainide and propafenone. All of these agents except procainamide are metabolised via the cytochrome P450 system and are involved in a number of drug-drug interactions, including over 20 different interactions with quinidine. Quinidine has been observed to inhibit the metabolism of digoxin, tricyclic antidepressants and codeine. Furthermore, cimetidine, azole antifungals and calcium antagonists can significantly inhibit the metabolism of quinidine. Procainamide is excreted via active tubular secretion, which may be inhibited by cimetidine and trimethoprim. Other Class I agents may affect the disposition of warfarin, theophylline and tricyclic antidepressants. Many of these interactions can significantly affect efficacy and/or toxicity. Of the Class III antiarrhythmics, amiodarone is involved in a significant number of interactions since it is a potent inhibitor of several cytochrome P450 enzymes. It can significantly impair the metabolism of digoxin, theophylline and warfarin. Dosages of digoxin and warfarin should empirically be decreased by one-half when amiodarone therapy is added. In addition to pharmacokinetic interactions, many reports describe the use of antiarrhythmic drug combinations for the treatment of arrhythmias. By combining antiarrhythmic drugs and utilising additive electrophysiological/pharmacodynamic effects, antiarrhythmic efficacy may be improved and toxicity reduced. As medication regimens grow more complex with the aging population, knowledge of existing and potential drug-drug interactions becomes vital for clinicians to optimise drug therapy for every patient.

Small-dose dopamine increases epidural lidocaine requirements during peripheral vascular surgery in elderly patients

We studied 20 patients over the age of 65 yr undergoing prolonged peripheral vascular surgery under continuous lidocaine epidural anesthesia, anticipating that the increased hepatic metabolism caused by small-dose IV dopamine would lower plasma lidocaine concentrations. Subjects were assigned (random, double-blinded) to receive either a placebo IV infusion or dopamine, 2 microg. kg(-1). min(-1) during and for 5 h after surgery. Five minutes after the IV infusion was started, 20 mL of 2% lidocaine was injected through the epidural catheter. One-half hour later, a continuous epidural infusion of 2% lidocaine at 10 mL/h was begun. The epidural infusion was temporarily decreased to 5 mL/h or 5 mL boluses were added to maintain a T8 analgesic level. Arterial blood samples were analyzed for plasma lidocaine concentrations regularly during and for 5 h after surgery. Plasma lidocaine concentrations increased continuously during the epidural infusion and, despite wide individual variation, were similar for the two groups throughout the observation period. During the observation period, the mean maximal plasma lidocaine concentration was 5.8 +/- 2.3 microg/mL in the control group and 5.7 +/- 1.2 microg/mL in the dopamine group. However, the mean hourly lidocaine requirement during surgery was significantly different, 242 +/- 72 mg/h for control and 312 +/- 60 mg/h for dopamine patients (P < 0.03). At the end of Hour 4, the last period when all 20 patients were still receiving the epidural lidocaine infusion, the total lidocaine requirement was significantly different, 1088 +/- 191 mg for the control group and 1228 +/- 168 mg for the dopamine group (P < 0.05). Despite very large total doses of epidural lidocaine (1650 +/- 740 mg, control patients, and 1940 +/- 400, dopamine patients) mean maximal plasma concentrations remained below 6 microg/mL, and no patient exhibited signs or symptoms of toxicity. We conclude that small-dose IV dopamine increased epidural lidocaine requirements, presumably as a consequence of increased metabolism.

IMPLICATIONS

We tested dopamine, a drug that increases liver metabolism of the local anesthetic lidocaine to determine if it would prevent excessively large amounts of lidocaine in the blood during prolonged epidural anesthesia in elderly patients. Dopamine did not alter the blood levels of lidocaine, but it did increase the lidocaine dose requirement to maintain adequate epidural anesthesia.

Lidocaine elimination and monoethylglycinexylidide formation in the dehydrated camel

The elimination kinetics and the formation of the monoethylglycinexylidide (MEGX), a major metabolite of lidocaine, were studied in camels deprived of water for 14 days. The study was conducted on four camels in a crossover design. Lidocaine was administered intravenously at a dose of 1 mg/kg to adult female camels when water was given ad libitum (stage 1) and to the same camels after 14 days of dehydration. Blood samples were taken up to 6 h after dosing. Serum lidocaine and MEGX levels were analysed by polarization fluorescence immunoassay. The elimination profiles of lidocaine and the formation of the metabolite MEGX in the two phases of the study were essentially identical. No difference in any pharmacokinetic parameter was noticed between normally hydrated and water-deprived camels. It is thus concluded that dehydration does not affect the cytochrome P450 isozymes involved in degradation of lidocaine to MEGX nor does it affect the hepatic blood flow, which is a major determinant in the clearance of lidocaine. The very low clearance of lidocaine in the camel in comparison with other ruminant or monogastric mammals may be associated with the camel's ability to survive drought in the desert.

Effect of dehydration and hyperosmolal hydration on lignocaine and metabolites disposition in conscious rabbits

1. The present study aimed to investigate the effect of dehydration and hyperosmolal hydration on the disposition of lignocaine and two of its metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX). 2. Lignocaine was infused to three groups of conscious rabbits: controls, rabbits previously deprived of water for 48 h and rabbits receiving an infusion of 2.5% NaCl. 3. In dehydrated and hyperosmolal-hydrated rabbits, plasma osmolality was 321 +/- 1 and 313 +/- 1 mOsm kg-1, respectively (P < 0.01 compared to controls, 285 +/- 1 mOsm kg-1). In dehydrated animals, baseline values of plasma arginine vasopressin (AVP) concentrations and plasma renin activity (PRA) were higher than in controls, i.e. 12.4 +/- 1.4 pg ml-1 and 15.4 +/- 1.7 ng AI ml-1 h-1 vs. 3.4 +/- 0.2 pg ml-1 (P < 0.01), and 5.1 +/- 0.6 ng AI ml-1 h-1 (P < 0.01), respectively; atrial natriuretic peptide (ANP) decreased from 55 +/- 11 to 32 +/- 4 pg ml-1 (P < 0.05). Compared to controls, hyperosmolal hydration only increased AVP to 15.5 +/- 0.7 pg ml-1 (P < 0.01). 4. Under both experimental conditions, lignocaine plasma concentrations were almost double (P < 0.01) those in controls, due to a lower systemic clearance, e.g. 54 +/- 3 and 59 +/- 1 vs. 96 +/- 5 ml min-1 kg-1, respectively. Plasma levels of MEGX increased (P < 0.01) only in dehydrated animals, although GX plasma concentrations were augmented (P < 0.01) about three fold in both groups of animals. The changes in lignocaine plasma concentrations were correlated with AVP levels (R2 = 0.5168, P<0.001).5. To document the effect of AVP on hepatic plasma flow, another group of rabbits received on separate occasions two doses of AVP (17 and 84 ng kg-1) while receiving an infusion of in docyanine green. AVP reduced hepatic plasma flow from 38.9 +/-2.7 ml min-1 to 19.6 +/-2.5 ml min-1 (P<0.01).The predicted maximal AVP-induced decrease in hepatic plasma flow was 19.6 ml min-1 kg- 1(Emax), and AVP concentration eliciting 50% of Em.. (ED50) was 28.7 pg ml-1.6 It is concluded that both dehydration and hyperosmolal hydration alter the disposition of lignocaine and two of its metabolites.

Kinetic analysis of propranolol-induced impairment of its own metabolism in rats

The effect of repetitive oral administration of propranolol (100 mg kg-1 day-1, 5 days) on the kinetics of liver microsomal propranolol metabolism was investigated in the rat. Vmax values of the high-affinity phase for biphasic kinetics of propranolol 4- and 5-hydroxylase activities were decreased by propranolol pretreatment, while those of the low-affinity phase were unchanged. The Vmax value of monophasic 7-hydroxylase activity was also decreased. On the other hand, the Vmax value of N-desisopropylase activity in the propranolol-treated rats was increased more than 2-fold compared with non-treated (control) rats, resulting in a change from monophasic in control rats to biphasic kinetics in propranolol-treated rats. These findings indicate that repetitive administration of propranolol selectively impairs a CYP2D isozyme that is involved in the high-affinity phases for propranolol ring-hydroxylations.

Kinetic analysis of mutual metabolic inhibition of lidocaine and propranolol in rat liver microsomes

The metabolic interaction between lidocaine (LD) and propranolol (PL) was analysed kinetically in rat liver microsomes. Employing a very short incubation time of 30 sec, we demonstrated that PL competitively inhibited liver microsomal 3-hydroxylation of LD, but did not affect either the formation of monoethylglycinexylidide or methylhydroxylidocaine from LD in PL concentrations up to 1 microM. On the other hand, LD competitively inhibited PL 4-, 5- and 7-hydroxylations, but the inhibition type of LD for PL N-desisopropylation could not be clarified. Comparison of the kinetic data for liver microsomes from Wistar and Dark Agouti rats indicated that among the primary metabolic pathways of LD, the Vmax value for 3-hydroxylation was markedly less in female Dark Agouti rats. The results suggest that LD 3-hydroxylation and PL ring hydroxylations are mediated by the same isozyme(s) belonging to the CYP2D subfamily.

A possible mechanism of the impairment of hepatic microsomal monooxygenase activities after multiple administration of propranolol in rats

The mechanism of selective inhibition of propranolol hydroxylations after multiple administration of the drug was investigated by metabolic inhibition studies in rat liver microsomes. The time course of irreversible binding of a reactive metabolic intermediate(s) of propranolol to liver microsomal protein, which was proposed as the cause of the impairment of enzymatic activities, had a delayed phase followed by a rapid linear rise, while the unmetabolized propranolol remaining in the reaction mixture showed a rapid linear decrease immediately after the onset of incubation. Thus, it was conceivable that the reactive intermediate(s) was not always formed directly from the parent drug, propranolol. Among four primary metabolites of propranolol, 4-hydroxypropranolol was the most potent inhibitor of propranolol hydroxylase activities, and this inhibition was much enhanced by preincubation of 4-hydroxypropranolol with NADPH. The type of inhibition kinetics of propranolol 5- and 7-hydroxylase activities by 4-hydroxypropranolol was changed from a competitive type to a non-competitive type by the preincubation. These results suggest that a reactive metabolite(s) of propranolol which impaired propranolol hydroxylase activities is a further metabolite(s) of 4-hydroxypropranolol.

Pharmacokinetic interaction between propranolol and the HMG-CoA reductase inhibitors pravastatin and lovastatin

1. Single oral 20 mg doses of the HMG-CoA reductase inhibitors pravastatin and lovastatin, with and without concomitant propranolol (40 mg twice daily), were administered to 16 healthy male subjects participating in a randomized, four-way crossover study. 2. Serum concentrations of total and active inhibitors were measured by bioassay and concentrations of pravastatin, two pravastatin metabolites and lovastatin acid were measured by gas chromatography/mass spectrometry. 3. Coadministration of propranolol with pravastatin reduced the mean area under the serum concentration-time curve (AUC) of total inhibitors by 23%, of active inhibitors by 20% and of pravastatin by 16%. 4. Coadministration of propranolol with lovastatin also resulted in decreases in the mean serum AUC of total inhibitors by 18%, of active inhibitors by 12% and of lovastatin acid by 13%. 5. These decreases in systemic drug concentrations may reflect enhanced drug first-pass hepatic clearance in the presence of propranolol. 6. The clinical significance of these changes is likely to be small.

Tumescent technique for regional anesthesia permits lidocaine doses of 35 mg/kg for liposuction

The tumescent technique for local anesthesia permits regional local anesthesia of the skin and subcutaneous tissues by direct infiltration. The tumescent technique uses large volumes of a dilute anesthetic solution to produce swelling and firmness of targeted areas. This investigation examines the absorption pharmacokinetics of dilute solutions of lidocaine (0.1% or 0.05%) and epinephrine (1:1,000,000) in physiologic saline following infiltration into subcutaneous fat of liposuction surgery patients. Plasma lidocaine concentrations were measured repeatedly over more than 24 hours following the infiltration. Peak plasma lidocaine levels occurred 12-14 hours after beginning the infiltration. Clinical local anesthesia is apparent for up to 18 hours, obviating the need for postoperative analgesia. Dilution of lidocaine diminishes and delays the peak plasma lidocaine concentrations, thereby reducing potential toxicity. Liposuction reduces the total amount of lidocaine absorbed systemically, but does not dramatically reduce peak plasma lidocaine levels. A safe upper limit for lidocaine dosage using the tumescent technique is estimated to be 35 mg/kg. Infiltrating a large volume of dilute epinephrine assures diffusion throughout the entire targeted area while avoiding tachycardia and hypertension. The associated vasoconstriction is so complete that there is virtually no blood loss with liposuction. The tumescent technique can be used with general anesthesia or IV sedation. However, with appropriate instrumentation and surgical method, the tumescent technique permits liposuction of large volumes of fat totally by local anesthesia, without IV sedation or narcotic analgesia.

The effect of propranolol on paracetamol metabolism in man

Ten healthy volunteers were treated for 4 days with 160 mg propranolol HCl and placebo in random order. At the end of each treatment salivary antipyrine kinetics and the plasma kinetics and urinary excretion of paracetamol and its major metabolites were measured following a 1500 mg oral dose. Propranolol prolonged the half-life of antipyrine by 11 +/- 5% (mean +/- s.e. mean) and lowered its clearance by 14 +/- 3% (P less than 0.05). Propranolol increased the half-life of paracetamol by 25 +/- 12% (P less than 0.05) and lowered its clearance by 14 +/- 3% (P less than 0.05). Propranolol decreased the partial clearance of paracetamol to its cysteine and mercapturate derivatives by 16 +/- 3% (P less than 0.05) and 32 +/- 7% (P less than 0.05), respectively. The partial clearance to the glucuronide conjugate was decreased by 27 +/- 6% (P less than 0.05), whereas that to sulphate was not changed significantly. Propranolol inhibits paracetamol metabolism predominantly through inhibition of the oxidation and glucuronidation pathways.

Inhibition of lignocaine metabolism by beta-adrenoceptor antagonists in rat and human liver microsomes

1. The inhibition of lignocaine metabolism by beta-adrenoceptor antagonists (beta-blockers) was investigated in rat and human liver microsomes. 2. Thirteen beta-blockers (concn. 50 microM) incubated with substrate (4.27 microM) and rat liver microsomes, showed a strong linear correlation between percentage inhibition of lignocaine metabolism and the distribution coefficients of the beta-blockers (r2 = 0.842, P less than 0.001). Similar results for four beta-blockers were obtained using human liver microsomes. 3. In rat liver, which metabolizes lignocaine by aromatic hydroxylation and N-dealkylation, inhibition was selective for the former route. Human liver microsomes metabolize the drug mainly by N-dealkylation and inhibition of this pathway was observed. 4. Liver microsomes from rats treated orally with beta-blockers (0.34 nmol kg per day for 5 days) showed impaired metabolism of lignocaine and impaired formation of 3-hydroxy-lignocaine, despite the absence of significant residues of beta-blocker. 5. 14C-Propanolol was bound irreversibly to rat liver microsomal protein; binding accounted for 4.1 +/- 0.3% (n = 4) dose after 30 min incubation. Exclusion of co-factors and addition of glutathione (GSH, 1 mM) lowered binding by 96% and 70%, respectively. Propanolol inhibited lignocaine metabolism to the same extent in the presence or absence of GSH. The 14C-propanolol bound to liver microsomes from propranolol-treated rats decreased in parallel with inhibition of lignocaine metabolism at 18 to 48 h after pretreatment. 6. These studies indicate at least two mechanisms for the inhibition of lignocaine metabolism by beta-blockers, namely, a 'lipid solubility hypothesis', where the effects may be related to the unchanged drug and a 'metabolite hypothesis', with the possible involvement of an irreversibly bound species.

Treatment of lithium induced tremor with atenolol

This is the first report on the successful treatment of one patient with lithium induced tremor with hydrophilic atenolol, which is a relatively selective beta 1 adrenergic receptor blocker. Atenolol's advantages over lipophilic beta blockers in the treatment of lithium induced tremor are discussed.

Anesthesia for liposuction in dermatologic surgery

Liposuction is now a well-established procedure in dermatologic surgery. The relative advantages and risks of the various forms of primary anesthesia and supplemental analgesia used for liposuction surgery in the office by dermatologic surgeons is described. Effective anesthetic techniques include infiltration of local anesthesia (LA) with or without intramuscular (IM), intravenous (IV), or nitrous oxide sedation, cryoanesthesia, and IV or inhalation general anesthesia (GA). Local anesthesia, using large volumes of dilute anesthetic solution containing lidocaine (0.05%), epinephrine (1:1,000,000), and sodium bicarbonate (12.5 meq/L), is a safe and effective modality for liposuction by dermatologists. In a study of 12 liposuction patients treated with this technique, the average lidocaine dose was 1181 mg (9.4 mg/kg/hr). The highest peak lidocaine blood level among all patients was 0.484 microgram/ml. Dermatologists should not assume the dual responsibility of surgeon and of monitoring patients given IV sedation. Any form of anesthesia has the potential for serious complications. The surgeon and office staff must be well trained and equipped to perform emergency resuscitation.

Pharmacokinetic interaction between nifedipine and propranolol

In 8 healthy volunteers, single-dose nifedipine pharmacokinetics were compared with and without the coadministration of propranolol. An elevation of the mean Cmax was found, from 73.9 +/- 14.1 when nifedipine was taken alone, to 115.7 +/- 12.1 (SE) ng/ml (P less than 0.02) when the agent was combined with propranolol. The AUC0----infinity increased as well, from 287.1 +/- 33.5 to 363.0 +/- 54.3 (SE) (micrograms.hr)/l (P less than 0.01), indicating an increase in bioavailability. Propranolol treatment did not significantly affect the nifedipine half-life (alpha or beta phase) or the estimated volume of distribution, whereas systemic clearance tended to decrease in 6 of the subjects. The most likely explanation for increased bioavailability of nifedipine when coadministered with propranolol is by a reduction of the hepatic "first-pass" clearance, as a result of changes in hepatic blood flow.

[Acute toxicity of local anesthetics as a function of the patient's condition]

A patient's condition may alter the pharmacokinetic and pharmacodynamic characteristics of local anaesthetics and so increase the risk of toxicity. In the elderly patient, the elimination half-life is increased for both lidocaine and bupivacaine; the risk of overdose is therefore increased when the local anaesthetic agent is given in repeated doses and as a continuous infusion. Cardiotoxicity due to bupivacaine seems to be worsened by pregnancy. In the foetus and newborn, local anaesthetic toxicity gives the same clinical picture as in the adult and is increased in the presence of acidosis and anoxia. Bupivacaine depressive effects are increased by tachycardias, intraventricular blocks and all the conditions which are known to depolarize the cardiac cell membrane (e.g. hyperkaliemia, acidosis, severe hypoxia, myocardial ischaemia). Drug interactions may also potentiate the toxicity of lidocaine and bupivacaine, such as calcium blockers and diazepam. The effects of other conditions (cirrhosis, renal failure, epilepsy) and other drug interactions, specially those modifying free fraction and elimination of local anaesthetics, are also discussed.

The pharmacokinetics of lignocaine and beta-adrenoceptor antagonists in patients with acute myocardial infarction

Lignocaine (lidocaine) and beta-adrenoceptor antagonists are widely used after acute myocardial infarction. The therapeutic value of these agents depends on the achievement and maintenance of safe and effective plasma concentrations. Lignocaine pharmacokinetics after acute myocardial infarction (MI) are controlled by a number of variables. The single most important is left ventricular function, which affects both volume of distribution and plasma clearance. Other major factors include bodyweight, age, hepatic function, the presence of obesity, and concomitant drug therapy. Lignocaine is extensively bound to alpha 1-acid glycoprotein, a plasma protein which is also an acute phase reactant. Increases in alpha 1-acid glycoprotein concentration occur after an acute MI, decreasing the free fraction of lignocaine in the plasma and consequently decreasing total plasma lignocaine clearance without altering the clearance of non-protein-bound lignocaine. Complex changes in lignocaine disposition occur with long term infusions, and therefore early discontinuation of lignocaine infusions (within 24 hours) should be undertaken whenever possible. Because the risk of ventricular tachyarrhythmia declines rapidly after the onset of an acute MI, lignocaine therapy can be rationally discontinued within 24 hours in most patients. Lignocaine has a narrow toxic/therapeutic index, so that pharmacokinetic factors are critical in dose selection. In contrast, beta-adrenoceptor antagonists' adverse effects are more related to the presence of predisposing conditions (such as asthma, heart failure, bradyarrhythmias, etc.) than to plasma concentration. The pharmacokinetics of beta-adrenoceptor antagonists are important to help assure therapeutic efficacy, to provide information about the anticipated time course of drug action, and to predict the possible role of ancillary drug effects (such as direct membrane action) and loss of cardioselectivity. Lipid solubility is the main determinant of the pharmacokinetic properties of a beta-adrenoceptor antagonist. Lipid-soluble agents like propranolol and metoprolol are well absorbed orally, and undergo rapid hepatic metabolism, with important presystemic clearance and a short plasma half-life. Water-soluble drugs like sotalol, atenolol, and nadolol are less well absorbed, and are eliminated more slowly by renal excretion. Clinical assessment of beta-adrenoceptor antagonism is more valuable than plasma concentration determinations in evaluating the adequacy of the dose of a particular beta-adrenoceptor antagonist.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of verapamil on antipyrine pharmacokinetics and metabolism in man

The effect of verapamil pre-treatment on the pharmacokinetics and metabolism of antipyrine was studied in eight healthy male volunteers. The oral clearance of antipyrine was decreased from 2.18 to 1.95 l h-1 (P less than 0.01) by verapamil (80 mg three times daily for 2 days prior to antipyrine administration and 2 days following) while half-life was increased from 13.2 to 15.6 h (P less than 0.01). The urinary excretion of norantipyrine, 4-hydroxyantipyrine and 3-hydroxymethylantipyrine was decreased by 19.2%, 23.1% and 16.7% respectively (P less than 0.05) in the presence of verapamil. In addition, the rate constants for formation of each of these metabolites were significantly decreased by an average of approximately 30%. These results suggest that verapamil is capable of inhibiting oxidative metabolism, a finding which could be of clinical significance for drugs highly dependent upon pathways such as those inhibited in this study for elimination.

Metoprolol. An updated review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy, in hypertension, ischaemic heart disease and related cardiovascular disorders

During the intervening years since metoprolol was first reviewed in the Journal (1977), it has become widely used in the treatment of mild to moderate hypertension and angina pectoris. Although much data have accumulated, its precise mechanisms of action in these diseases remain largely uncertain. Optimum treatment of hypertension and angina pectoris with metoprolol is achieved through dose titration within the therapeutic range. It has been clearly demonstrated that metoprolol is at least as effective as other beta-blockers, diuretics and certain calcium antagonists in the majority of patients. Although a twice daily dosage regimen is normally used, satisfactory control can be maintained in many patients with single daily doses of conventional or, more frequently, slow release formulations. Addition of a diuretic may improve the overall response rate in hypertension. Several controlled trials have studied the effects of metoprolol administered during the acute phase and after myocardial infarction. In early intervention trials a reduction in total mortality was achieved in one moderately large trial of prolonged treatment, but in another, which excluded patients already being treated with beta-blockers or certain calcium antagonists and where treatment was only short term, mortality was significantly reduced only in 'high risk' patients. Overall results with metoprolol have not demonstrated that early intervention treatment in all patients produces clinically important improvement in short term mortality. Thus, the use of metoprolol during the early stages of myocardial infarction is controversial, largely because of the requirement to treat all patients to save a small number at 'high risk'. This blanket coverage approach to treatment may be more justified during the post-infarction follow-up phase since it has been shown that metoprolol slightly, but significantly, reduces the mortality rate for periods of up to 3 years. Metoprolol is generally well tolerated and its beta 1-selectivity may facilitate its administration to certain patients (e.g. asthmatics and diabetics) in whom non-selective beta-blockers are contraindicated. Temporary fatigue, dizziness and headache are among the most frequently reported side effects. After a decade of use, metoprolol is well established as a first choice drug in mild to moderate hypertension and stable angina, and is beneficial in post-infarction patients. Further study is needed in less well established areas of treatment such as cardiac arrhythmias, idiopathic dilated cardiomyopathy and hypertensive cardiomegaly.

Selectivity and dose-dependency of the inhibitory effect of propranolol on theophylline metabolism in man

The effects of separate 5 day pretreatments of propranolol 120 mg day-1 and 720 mg day-1 on theophylline clearance and metabolism at steady-state were determined in seven healthy males. Propranolol 120 mg day-1 decreased theophylline plasma clearance (CL) by 30%. Clearance of theophylline to each metabolite was reduced by this treatment, clearances to the two demethylated products by 42-43% and clearance to the 8-hydroxylation product by 27%. Propranolol 720 mg day-1 decreased theophylline CL by 52%. Again, clearance of theophylline to each metabolite was reduced by this treatment, clearances to the two demethylation products by 73-77% and clearance to the 8-hydroxylation product by 44%. These data are consistent with a dose-dependent and selective inhibitory effect of propranolol on the separate forms of cytochrome P-450 involved in theophylline demethylation and 8-hydroxylation.

Prophylactic lidocaine in the early phase of suspected myocardial infarction

Four hundred two patients with suspected myocardial infarction seen within 6 hours of the onset of symptoms entered a double-blind randomized trial of lidocaine vs placebo. During the 1 hour after administration of the drug the incidence of ventricular fibrillation or sustained ventricular tachycardia among the 204 patients with acute myocardial infarction was low, 1.5%. Lidocaine, given in a 300 mg dose intramuscularly followed by 100 mg intravenously, did not prevent sustained ventricular tachycardia, although there was a significant reduction in the number of patients with warning arrhythmias between 15 and 45 minutes after the administration of lidocaine (p less than 0.05). The average plasma lidocaine level 10 minutes after administration for patients without a myocardial infarction was significantly higher than that for patients with an acute infarction. The mean plasma lidocaine level of patients on beta-blocking agents was no different from that in patients not on beta blocking agents. During the 1-hour study period, the incidence of central nervous system side effects was significantly greater in the lidocaine group, hypotension occurred in 11 patients, nine of whom had received lidocaine, and four patients died from asystole, three of whom had had lidocaine. We cannot advocate the administration of lidocaine prophylactically in the early hours of suspected myocardial infarction.

Penbutolol and propranolol: a comparison of their effects on antipyrine clearance in man

The effects of two beta-adrenoceptor antagonists, penbutolol (administered on separate occasions as (+/-)- and (-)-forms) and propranolol, on the kinetics of antipyrine were studied in eight normal subjects. At the same degree of beta-adrenoceptor blockade, as assessed by the lowering of exercise tachycardia, propranolol decreased antipyrine clearance by 31 +/- 11 s.d.% (P less than 0.001) whereas neither of the two penbutolol formulations had a significant effect. The volume of distribution of antipyrine was unchanged following any of the beta-adrenoceptor antagonist treatments. The lack of effect of penbutolol on oxidative drug metabolism is not consistent with in vitro data suggesting a relationship between the lipid solubility of beta-adrenoceptor antagonists and inhibition of metabolism.

The impairment of lignocaine clearance by propranolol--major contribution from enzyme inhibition

To account for a 40% lowering of the systemic clearance of lignocaine by propranolol treatment it has been proposed that propranolol reduces liver blood flow by 25% and causes a 50% decrease in the intrinsic clearance of lignocaine by enzyme inhibition. In theory, the contribution of direct enzyme inhibition is best evaluated using oral administration of lignocaine when models of hepatic drug clearance predict that propranolol could increase the AUCpo of lignocaine by 100-140%. This hypothesis was tested in six healthy men who received 200 mg lignocaine HCl X H2O orally with and without propranolol pre-treatment (80 mg twice daily for 3 days). Propranolol treatment increased the mean plasma AUCpo of lignocaine by 113 +/- 58 s.d.% (P less than 0.005); it increased the peak plasma lignocaine concentration by 79 +/- 50 s.d.% (P less than 0.025) and it prolonged the elimination half-life of lignocaine by 20 +/- 13 s.d.% (P less than 0.05). Propranolol treatment lowered indocyanine green clearance by 11 +/- 15 s.d.%, but this change was not significant statistically. These experimental results are in accord with the theoretical predictions suggesting that propranolol lowers the systemic clearance of lignocaine mainly by direct inhibition of its metabolism rather than by a lowering of the hepatic blood flow.

Failure of 'therapeutic' doses of beta-adrenoceptor antagonists to alter the disposition of tolbutamide and lignocaine

The effects of separate 1 week pre-treatments with each of the beta-adrenoceptor antagonists, propranolol (80 mg every 12 h), metoprolol (100 mg every 12 h) and atenolol (50 mg once daily), on the disposition of a single i.v. dose of tolbutamide were studied in six healthy volunteers. In addition, the effects of a 1 week pre-treatment with metoprolol (100 mg every 12 h) and atenolol (50 mg once daily) on the disposition of orally and i.v. administered lignocaine were determined in seven healthy subjects. Tolbutamide clearance, half-life, volume of distribution and plasma protein binding were not altered by the beta-adrenoceptor blocker pre-treatments. Similarly, neither metoprolol nor atenolol had a significant effect on the systemic clearance, apparent oral clearance or other dispositional parameters of lignocaine. 'Therapeutic' plasma concentrations of the beta-adrenoceptor blockers were confirmed on each study day. It is concluded that the inhibition of oxidative drug metabolism previously reported for lipophilic beta-adrenoceptor blockers may be selective for different forms of cytochrome P450 and possible concentration-dependent.

Interaction between warfarin and propranolol

The interaction between propranolol and warfarin was investigated in six healthy volunteers using a low dose warfarin protocol and in New Zealand white rabbits. Warfarin concentration (Cmin) in the volunteers increased by 14.7% on addition of propranolol (P less than 0.02). There was no significant change in prothrombin time. In the rabbit study, elimination half-life was shorter in the propranolol treated animals (8.20 +/- 0.44 h) than in controls (10.89 +/- 0.62 h). This was due to a fall in its volume of distribution from 1.13 +/- 0.10 1 kg-1 in controls to 0.81 +/- 0.08 1 kg-1 in the propranolol treated group. Clearance was not significantly altered. The small change in warfarin concentration could be of clinical importance in some patients because warfarin has a low therapeutic index. This requires further investigation.

Effects of beta-adrenoceptor antagonists on the pharmacokinetics of lignocaine

In theory, beta-adrenoceptor antagonists could lower the clearance of free lignocaine in three ways (a) by decreasing hepatic blood flow, (b) by competing for plasma binding sites or (c) by inhibiting the enzymes responsible for metabolising lignocaine. The first mechanism has been demonstrated for propranolol and is probably common to all agents lacking intrinsic sympathomimetic activity. The second mechanism is discounted by data showing that propranolol, one of the more highly bound beta-adrenoceptor antagonists, does not alter the free fraction of lignocaine in plasma. In vitro studies support the third mechanism for the more lipid-soluble beta-adrenoceptor antagonists, as does the fact that observed decreases in the clearance of lignocaine in vivo are generally greater than the anticipated maximum lowering of hepatic blood flow.

Study of the influence of nifedipine on the pharmacokinetics and pharmacodynamics of propranolol, metoprolol and atenolol

The influence of chronic therapy with nifedipine on the pharmacokinetics of propranolol 80 mg twice daily, metoprolol 100 mg twice daily and atenolol 100 mg once daily was investigated in eight healthy volunteers. Nifedipine 10 mg three times daily did not affect the pharmacokinetics of metoprolol and atenolol whereas nifedipine shortened the time to peak plasma concentration for propranolol by about 1 h. Propranolol, metoprolol and atenolol provoked comparable decreases in heart rate measured at rest and during exercise. The beta-adrenoceptor blocking properties of propranolol, metoprolol and atenolol were not affected by concomitant therapy with nifedipine. The present study did not show significant pharmacokinetic and pharmacodynamic interactions between nifedipine and lipophilic beta-adrenoceptor blockers.

Pharmacokinetic and pharmacodynamic interaction study of diazepam and metoprolol

In 6 normotensive, healthy male volunteers the pharmacodynamic responses (blood pressure, heart rate; sedation index, tracking test, reaction time) to metoprolol (100 mg bid orally), diazepam (0.1 mg/kg intravenously) and to their combination were studied. The pharmacokinetics of diazepam were also compared in a cross-over experiment, with and without pretreatment by the beta-adrenoceptor antagonist to evaluate the possibility of a drug interaction. The pharmacodynamic and pharmacokinetic investigations indicated that metoprolol only slightly impaired the elimination of diazepam (18% decrease in total clearance, 25% increase in elimination half-life). The pharmacodynamics of metoprolol (17% decrease in heart rate, 17% decrease in diastolic RR) was not significantly altered by the bolus injection of diazepam. The extent of prolongation in choice reaction time (RT2) induced by diazepam was significantly (p = 0.001) more pronounced following the co-administration of metoprolol. However, the results of RT1, the tracking test and the sedation index did not indicate any increased effect due to the beta-blocking agent. It is concluded that concomitant treatment with metoprolol and diazepam causes only minor and clinically irrelevant changes in drug metabolism and drug response.

Prediction of metabolic drug interactions involving beta-adrenoceptor blocking drugs

There is evidence, from human and animal studies, that drug-metabolising enzymes exist in multiple forms, the individual enzymes having selective, but not specific, substrate requirements. Consequently drug interactions may arise when two drugs bind to the same enzyme. The degree of enzyme inhibition will be partly dependent on the relative affinities of the drugs for the enzyme and on their rates of turnover. The decrease in drug clearance produced by enzyme inhibition is dependent on the fraction of the drug normally metabolised by the inhibited pathway(s). Cimetidine, a P-450 enzyme inhibitor, increases the systemic bioavailability of propranolol and labetalol, which undergo extensive metabolism, but does not affect the clearance of atenolol, which is excreted largely unchanged. In this situation, both the extent and type of biotransformation are important. Thus, cimetidine has no effect on the clearance of penbutolol, even though the drug is eliminated almost entirely by biotransformation. The major metabolite is penbutolol glucuronide, and it has been shown recently that cimetidine does not inhibit glucuronylation. Beta-adrenoceptor blockers also act as enzyme inhibitors themselves. For example, antipyrine clearance is decreased by propranolol and to a lesser extent by metoprolol, whereas atenolol has no effect. It has been suggested, therefore, that there is a relationship between the lipid-solubility of beta-adrenoceptor blockers and their ability to inhibit drug metabolism. The clearance of lipophilic beta-adrenoceptor blockers is dependent on hepatic enzyme activity, and is therefore sensitive to enzyme induction. For drugs with high hepatic clearance and subsequent high presystemic elimination, a moderate increase in the extraction ratio will produce a marked decrease in systemic bioavailability. (ABSTRACT TRUNCATED AT 250 WORDS)