Intravenous administration of azumolene to reverse malignant hyperthermia in swine.

BACKGROUND: The efficacy of intravenous (IV) administration of azumolene (Az), an analogue 30-fold more soluble than dantrolene, on pigs susceptible to malignant hyperthermia (MH) is incompletely understood. OBJECTIVE: To evaluate efficacy of Az on MH crisis in pigs. ANIMALS: Eight normal (MHN) and 7 susceptible to MH (MHS) pigs (Landrace × Large White × Pietran). METHODS: Prospective, laboratory trial. Hypermetabolic crisis was observed in MHS pigs, but not in MHN pigs, after a combined administration of inhaled halothane (1.5%) and IV injection of succinylcholine (SCh; 2.5 mg/kg). Susceptibility was confirmed using a caffeine and halothane contracture test. Az was administered 15 minutes after administration of SCh. RESULTS: Respiratory acidosis (pH 7.16 ± 0.02; Pco(2) , 46.2 ± 9.1 mmHg, HCO(3) , 22.5 ± 2.3 mmol/L), fever (38.2 ± 1.1°C), cardiac arrhythmias, and muscle contracture were observed in MHS pigs. MHS pigs (n = 5) treated with Az (2 mg/kg IV) survived the crisis with attenuation of signs (pH 7.30 ± 0.10; Pco(2) , 36.3 ± 4.5 mmHg; HCO(3) , 22.9 ± 2.3 mmol/L) and recovery of normal muscle tone and cardiac rhythm. CONCLUSIONS AND CLINICAL IMPORTANCE: Az represents a possible substitute for dantrolene to reverse MH crisis in susceptible pigs.

Use of the caffeine-halothane contracture test for the diagnosis of malignant hyperthermia in Brazil

Malignant hyperthermia (MH) is a pharmacogenetic disease triggered by volatile anesthetics and succinylcholine. Deaths due to MH have been reported in Brazil. The first Malignant Hyperthermia Diagnostic and Research Center in Latin America was inaugurated in 1993 at the Federal University of Rio de Janeiro, Brazil. The center followed the diagnostic protocols of the North America MH Group, in which the contractures of biopsies from the vastus lateralis muscle are analyzed after exposure to caffeine and halothane (CHCT). CHCT was performed in individuals who survived, their relatives and those with signs/symptoms somewhat related to MH susceptibility (MHS). Here, we report data from 194 patients collected over 16 years. The Southeast (N = 110) and South (N = 71) represented the majority of patients. Median age was 25 (4-70) years, with similar numbers of males (104) and females (90). MHS was found in 90 patients and 104 patients were normal. Abnormal responses to both caffeine and halothane were observed in 59 patients and to caffeine or halothane in 20 and 11 patients, respectively. The contracture of biopsies from MHS exposed to caffeine and halothane was 1.027 ± 0.075 g (N = 285) and 4.021 ± 0.255 g (N = 226), respectively. MHS was found in patients with either low or high blood creatine kinase and also, with a low score on the clinical grading scale. Thus, these parameters cannot be used with certainty to predict MHS. We conclude that the CHCT protocol described by the North America MH Group contributed to identification of MHS in suspected individuals at an MH center in Brazil with 100 percent sensitivity and 65.7 percent specificity.

Use of the caffeine-halothane contracture test for the diagnosis of malignant hyperthermia in Brazil.

Malignant hyperthermia (MH) is a pharmacogenetic disease triggered by volatile anesthetics and succinylcholine. Deaths due to MH have been reported in Brazil. The first Malignant Hyperthermia Diagnostic and Research Center in Latin America was inaugurated in 1993 at the Federal University of Rio de Janeiro, Brazil. The center followed the diagnostic protocols of the North America MH Group, in which the contractures of biopsies from the vastus lateralis muscle are analyzed after exposure to caffeine and halothane (CHCT). CHCT was performed in individuals who survived, their relatives and those with signs/symptoms somewhat related to MH susceptibility (MHS). Here, we report data from 194 patients collected over 16 years. The Southeast (N = 110) and South (N = 71) represented the majority of patients. Median age was 25 (4-70) years, with similar numbers of males (104) and females (90). MHS was found in 90 patients and 104 patients were normal. Abnormal responses to both caffeine and halothane were observed in 59 patients and to caffeine or halothane in 20 and 11 patients, respectively. The contracture of biopsies from MHS exposed to caffeine and halothane was 1.027 +/- 0.075 g (N = 285) and 4.021 +/- 0.255 g (N = 226), respectively. MHS was found in patients with either low or high blood creatine kinase and also, with a low score on the clinical grading scale. Thus, these parameters cannot be used with certainty to predict MHS. We conclude that the CHCT protocol described by the North America MH Group contributed to identification of MHS in suspected individuals at an MH center in Brazil with 100% sensitivity and 65.7% specificity.

Motor nerve blockade potency and toxicity of non-racemic bupivacaine in rats.

BACKGROUND: Racemic [RS(+/-)] bupivacaine can be associated with severe cardiotoxicity. The S(-) isomer is known to be less neuro- and cardiotoxic, but demonstrates a lower potency to block motor activity than RS(+/-) bupivacaine. Thus, the potency and toxicity of a non-racemic bupivacaine mixture were studied. METHODS: Gastrocnemic muscle twitches induced by electrical stimulation of sciatic nerves in rats were used to compare the impact by bupivacaine solutions on motor activity. Field stimulation at 1 Hz eliciting ventricular muscle twitches was used to investigate the effects on cardiac contractility. The lethal dose of each local anesthetic agent was determined following drug infusions during general anesthesia in mechanically ventilated rats. RESULTS: Non-racemic (75S:25R) bupivacaine was more potent (P<0.05) than S(-) or R(+) enantiomers to block motor nerve activity. The concentrations of RS(+/-), 75S:25R, R(+) and S(-) bupivacaine to inhibit nerve conduction by 50% were 0.84 (0.37- 2.20), 0.84 (0.47-2.48), 2.68 (0.98-3.42) and 2.11 mM (1.5-4.03), respectively. Pronounced reductions in ventricular muscle twitches were observed with RS(+/-) and R(+) bupivacaine at low concentrations (0.5-4 microM). Lethal doses for 75S:25R (39.9 mg kg(-1)), and S(-) (34.7 mg kg(-1)) were higher (P<0.05) than for R(+) (16.2 mg kg(-1)) and RS(+/-) bupivacaine (18.4 mg kg(-1)), respectively. DISCUSSION: The potency of S(-) bupivacaine to block the motor activity in the sciatic nerve was enhanced when 25% of the S(-) isomer was replaced by the antipode R(+) bupivacaine. This effect was not associated with increased toxicity.

Is comparative cardiotoxicity of S(-) and R(+) bupivacaine related to enantiomer-selective inhibition of L-type Ca(2+) channels?

Cardiac toxicity can occur after accidental intravascular injection of bupivacaine. Racemic bupivacaine can inhibit both cardiac Na(+) and Ca(2+) channels, but the contribution of these actions to cardiac depression is not totally understood. We tested whether the effect of R(+) bupivacaine on cardiac electrical activity in isolated hearts and on L-type Ca(2+) channels (I(Ca-L)) in isolated cardiac myocytes could be responsible for its increased cardiotoxicity compared with S(-) bupivacaine. Cardiac electrical activity of spontaneously beating isolated hearts was recorded before and after exposure to increasing concentrations of R(+) and S(-) bupivacaine. An increase of the PR interval (80%) and the QRS duration (370%) by 10microM R(+) bupivacaine (80% and 370%) was significantly higher than for S(-) bupivacaine (25% and 200%, respectively). R(+) but not S(-) bupivacaine produced severe arrhythmias at concentrations larger than 2.5microM. The intensity of I(Ca-L) inhibition did not differ between bupivacaine isomers. At 0 mV, I(Ca-L) was irreversibly reduced by 40.2% +/- 8.8% and 51.4% +/- 3.8% in the presence of 10microM R(+) and S(-) bupivacaine, respectively. The arrhythmogenic effect of R(+) bupivacaine could not be explained by stereoselectivity on the I(Ca-L) inhibition. Thus, other mechanisms could contribute to the cardiac electrical and contractile dysfunction induced by R(+) bupivacaine.

Alterations in the functional properties of skinned fibers from denervated rabbit skeletal muscle.

Isometric tension was recorded in vitro from chemically skinned fibers obtained from normal and 14-day-denervated extensor digitorum longus muscles of the rabbit. Denervation potentiated the tensions elicited by pCa 6.0 but did not modify the pCa value (5.6) required for maximum tension. Ca2+ transport across the membranes of the sarcoplasmic reticulum (SR) was markedly affected by denervation. Thus the rate of ATP-dependent net Ca2+ uptake increased significantly, and the spontaneous release ("leakage") of the Ca2+ stored in the SR was significantly reduced in denervated fibers. These effects lead to increased accumulation of Ca2+ in the lumen of the SR. The dose-response curve for the halothane-induced contractures of Ca2(+)-loaded skinned fibers was displaced to the left after denervation. Thus 0.7 mM halothane, a concentration that elicited no tension in 10 control fibers, induced contractures in the 10 denervated fibers tested. The potentiation of the halothane-induced tensions is attributed mainly to the larger stores of Ca2+ in the SR of denervated fibers. The possibility that denervation may also affect the interaction of halothane with the SR membranes is discussed.

Potentiation of the halothane-cooling contractures of mammalian muscles by denervation.

Denervation potentiated the cooling-induced contractures and the halothane-cooling contractures of isolated extensor digitorum longus and soleus muscles of the mouse. These effects were more striking in extensor digitorum longus than in soleus muscles. Significant increases in the peak amplitudes of the halothane-cooling contractures of both muscles and of the cooling contractures of soleus muscle were observed within 2 and 7 days of denervation. The potentiation of the contractures persisted for 90 days, the period of this study. Denervation (greater than 2 days) endowed extensor digitorum longus with the ability to generate cooling contractures in the absence of halothane. The rate of tension development of cooling-induced contractures in the absence or presence of halothane was significantly greater in denervated (2-90 days) than in innervated muscles. Denervation also reduced the effectiveness of procaine in inhibiting the halothane-cooling contractures. It is proposed that the potentiation of cooling-induced contractures in denervated muscles results primarily from an increase in the rate of efflux and in the quantity of Ca2+ released from the sarcoplasmic reticulum, upon cooling and (or) when challenged with halothane.