Heart rate response to an i.v. test dose of adrenaline and lignocaine with and without atropine pretreatment

In order to evaluate the sensitivity of an adrenaline test dose for detecting intravascular injection and the effect of atropine pretreatment, 90 ASA physical status I and II patients were allocated randomly to two groups, to receive i.v. saline 1 ml (n = 46) or i.v. atropine 0.5 mg (n = 44). Five minutes later, all patients received an i.v. test dose of 2% lignocaine 3 ml with adrenaline 15 micrograms at a rate of 1 ml s-1. The groups were similar with respect to basal heart rate (HR). HR remained unchanged after saline injection, but increased slightly 5 min after atropine injection (mean 78 (SD 15) beat min-1 vs 87 (20) beat min-1 (P less than 0.05). After the test dose of lignocaine with adrenaline, HR increased significantly in both groups at 30 s, 1 and 2 min, and remained increased at 3 min in the atropine group. The maximum increase in HR was greater in the atropine group than in the saline group (31 (4) beat min-1 vs 26 (11) beat min-1 (P less than 0.05). However, when individual maximum HR changes are considered, five patients in the saline group and four in the atropine group had an increase less than or equal to 10 beat min-1, and three patients in the saline group had no change or a decrease in HR. Defining a positive result to a test dose as an increase in HR of greater than 10 beat min-1, the sensitivity of the adrenaline test dose was 83 (5.5)% in the saline group and 91 (3.5)% in the atropine group (ns).(ABSTRACT TRUNCATED AT 250 WORDS)

Extrahepatic metabolism of morphine occurs in humans

The pharmacokinetics of morphine was studied in six patients in whom a radiologic localization of an insulinoma was to be performed under general anesthesia. Sampling was done in the peripheral artery, the mesenteric vein in five of the six patients, the hepatic vein, and the peripheral vein, as well as in urine. Hepatic blood flow was estimated by an indocyanine green infusion technique at the end of the radiologic procedure. Morphine terminal half-life was 92 +/- 9 minutes, total body clearance was 1260 ml.min-1, and the hepatic extraction ratio was 0.65 +/- 0.11. No concentration gradient was observed between the artery and the superior mesenteric vein, showing that no gut wall metabolism of morphine occurred. The total body clearance exceeded the hepatic clearance by 38%. It was concluded that the extrahepatic extraintestinal clearance of morphine probably occurred through the kidney.

Myocardial uptake of lignocaine: pharmacokinetics and pharmacodynamics in the isolated perfused heart of the rabbit

1. The uptake kinetics and pharmacodynamics of lignocaine were studied in isolated perfused heart of the rabbit. 2. Six hearts were perfused with increasing concentrations of lignocaine in a modified Krebs-Henseleit buffer. The effluent concentration together with the increase in QRS duration were measured during lignocaine infusion and during 20 min after cessation of lignocaine infusion. 3. Lignocaine disposition and elimination were best described by a two-compartment open model. Terminal half-life was 11.0 +/- 2.9 min. The unidirectional transfer was slower from central to peripheral compartment than from peripheral to central compartment (T1/2.12 = 42.6 +/- 10.5 min whereas T1/2.21 = 10.7 +/- 2.8 min). The myocardium/perfusate concentration-ratio was 4.7 +/- 0.4. 4. The pharmacodynamic effect was best described in the central compartment by using the Hill equation. Calculated maximum QRS duration (Emax) was 77 +/- 8 ms. Emax was also directly measured in four additional rabbits by infusing ten times the dose of lignocaine used in the main experiment: the value of Emax measured in these conditions was 92.5 +/- 9.6 ms, i.e. a QRS widening of 150%. The steady-state perfusate concentration producing half the effect (C50) was 15.7 +/- 7.6 micrograms ml-1. 6. In conclusion, the specific lignocaine binding leading to increase in QRS duration appeared to be more closely related to the vascular stream than non specific binding leading to a deeper accumulation process.

Pharmacokinetics of bupivacaine after short and prolonged infusions in conscious dogs

A conscious dog model was used to study pharmacokinetics of bupivacaine after a short infusion (SI) (15 min) and a prolonged infusion (PI) (24 hours). Bupivacaine was infused in six mongrel dogs at least 10 days after implantation of femoral arterial and venous catheters. Each dog received both the SI and the PI in a random crossover design at a one week interval. Bupivacaine concentration was measured in serum sampled during the SI, during the last hour of the PI, and at frequent intervals during eight hours after cessation of infusion. Indocyanine green (ICG) clearance also was measured during the last hour of the PI and 90 min after cessation of both the SI and the PI. The terminal half-life (T1/2Z) of bupivacaine increased after the PI compared with the SI, 167 +/- 86 vs 53 +/- 13 min, respectively (mean +/- SD; P less than 0.05), and total body clearance (Cl) decreased, 3.4 +/- 1.2 vs 9.5 +/- 4.5 ml.min-1.kg-1, (P less than 0.05), although the volumes of distribution (VZ and VSS) did not change. A decrease in hepatic blood flow did not cause the decrease in Cl because ICG clearance did not change during the three sets of measurements. Thus, the observed increase in T1/2Z and decrease in Cl after the PI as compared with the SI are due to a decrease in hepatic intrinsic clearance of bupivacaine. Differences in the kinetic profile of the two enantiomers of bupivacaine cannot be excluded as a cause. We conclude that the extrapolation of kinetic data of bupivacaine obtained after a short infusion (or a bolus injection) to prolonged dosage must be done with care.

Pharmacokinetics of bupivacaine following caudal anesthesia in infants

Pharmacokinetics and protein binding of bupivacaine were studied after caudal injection of 2.5 mg/kg in 13 ASA PS 1 infants (1-6 months of age) scheduled for elective hernia repair. Blood was sampled at frequent intervals from 5 min to 600 min in all but one patients. Additional samples were taken at 720 and 840 min in five patients. Bupivacaine concentration was measured using gas chromatography. Protein binding was measured using ultrafiltration. Peak serum concentrations ranged between 0.55 and 1.93 micrograms/ml. The time to reach the peak ranged from 10 to 60 min. Terminal half-life (T1/2 beta) was 7.7 +/- 2.4h (mean +/- SD), the volume of distribution (Vss) was 3.9 +/- 2.01.kg, and the total body clearance (CL) was 7.1 +/- 3.2 ml.min.kg-1. The free fraction was markedly increased (0.16 +/- 0.07) when compared with published adult values, and showed a highly significant negative correlation with age. Alpha 1 acid glycoprotein measured in the same infants correlated significantly with age. In conclusion, pharmacokinetics of caudal bupivacaine in infants are characterized by Cmax of total drug similar to those observed in adults after epidural injection. The free fraction is increased at least until 6 months of life. This suggests caution in the use of bupivacaine in infants until we understand the clinical significance of this increased free fraction.

[Are combinations of local anesthetics dangerous?]

Although local anaesthetic mixtures are often used, there is often no scientific basis for this. Interactions between the local anaesthetics in the mixture can even increase their toxicity. Such interactions may be due to the physical chemical properties of each constituent (this is not a real problem for amide local anaesthetics), to a mutual displacement from the binding sites on the plasma proteins or to an additional effect on membrane receptors. This displacement would seem to benefit the least toxic drug: the toxicity of local anaesthetic mixtures may therefore be reduced; for example, lidocaine and mepivacaine are displaced by bupivacaine. Moreover, the fraction of drug which leaves the blood vessel is greater than the serum free fraction. During chronic administration, neurological toxicity and free fraction are narrowly linked. When using local anaesthetic mixtures, it must be remembered that potency and toxicity of a local anaesthetic are closely correlated and that the toxic effects are, at least, additive.

Pharmacokinetics of unchanged morphine in normal and cirrhotic subjects

Published reports of morphine pharmacokinetics differ, perhaps because of analytical discrepancies and different study protocols. A highly specific radioimmunoassay (N-carboxymethylnormorphine RIA) was used to measure unchanged morphine in plasma in six normal subjects and in eight cirrhotic patients with hypoalbuminemia, hyperbilirubinemia, and prolonged prothrombin time. All subjects received a single intravenous bolus of morphine hydrochloride (0.1 mg/kg). Peripheral blood samples were collected at frequent intervals from 2 min to 36 hr after injection. In cirrhotic patients as compared with normal subjects, terminal half-life of unchanged morphine was longer (201 +/- 39 vs 111 +/- 32 min (mean +/- SD), P less than 0.01) and total body clearance was slower (21 +/- 7.5 vs 33.5 +/- 8 ml X min-1 X kg-1, P less than 0.05). The apparent volume of distribution and the steady-state volume of distribution did not differ in the two groups. In conclusion, cirrhotic patients have a prolonged terminal half-life of unchanged morphine that is attributable to a decrease in total body clearance.