Conduction Blocks

Table 4. In Vitro Conduction Blocking and Physiochemical Properties of Local Anesthetic Agents ...

A conduction block is a type of regional anesthesia produced by injection of a local anesthetic drug into or near a nerve trunk. Examples of a conduction block include an epidural block (injection of a local anesthetic into the space surrounding the dura of the spinal cord) a trails sacral (caudal) block (injection of a local anesthetic into the epidural space at the level of the sacrococcygeal notch) and brachial plexus block (injection of a local anesdietic into the brachial plexus). Epidural, especially, and trailssacral blocks are often used in obstetrics. A brachial plexus block may be used for surgery of the arm or hand. 

Labetalol 3-6 hour 5-10 minute 10-120 mg/hour Conduction block, heart failure, bradycardia, bronchospasm, exacerbate underlying pulmonary disease Rapid onset of action... 

Esmolol 10-20 minute <5 minute 2-21 meg /minute Conduction block, hypotension, bronchospasm Rapid onset, short half-life... 

Llinas, R. Walton, K. and Bohr, V. Synaptic transmission in squid giant synapse after potassium conductance block with external 3- and 4 - ami nopyri di nes. Bi ophvs J. 16 83-86, 1976. Mendelsohn, L.G. Kerchner, G.A. Kalra, V. Zimmerman, D.M. and Leander, J.D. Phencyclidine receptors in rat brain cortex. Biochem-Pharmacol 33-3529-3535, 1984. 

Sympathetic (sympatholytic) Heart Sinus node Atrioventricular (AV node) Slowing Increased refractory period Bradycardia Dysrhythmias, conduction block... 

Chang AP, England JD, Garcia CA, et al. 1998. Focal conduction block in -hexane polyneuropathy. 

Lidocaine (Xylocaine) was introduced as a local anesthetic and is still used extensively for that purpose (see Chapter 27). Lidocaine is an effective sodium channel blocker, binding to channels in the inactivated state. Lidocaine, like other IB agents, acts preferentially in diseased (ischemic) tissue, causing conduction block and interrupting reentrant tachycardias. 

The effects on the A-V node may result in a conduction block and the termination of tachycardias that use the A-V node as a Umb of a reentrant circuit. Adenosine does not affect the action potential of ventricular myocytes because the adenosine-stimulated potassium channel is absent in ventricular myocardium. 

Cardiac toxicity is generally the result of drug-induced depression of cardiac conduction (e.g., atrioventricular block, intraventricular conduction block) and systemic vasodilation. These effects may progress to severe hypotension and cardiac arrest. 

Contraindications for antipsychotic therapy are few they may include Parkinson s disease, hepatic failure, hypotension, bone marrow depression, or use of CNS depressants. Overdoses of antipsychotics are rarely fatal, except for thioridazine, which is associated with major ventricular arrhythmias, cardiac conduction block, and sudden death. For other agents gastric lavage should be attempted even if several hours have elapsed since the drug was taken, because gastrointestinal motility is decreased and the tablets may still be in the stomach. Moreover, activated charcoal effectively binds most of these drugs and can be followed by a saline cathartic. The hypotension often responds to fluid replacement or pressor agents such as norepinephrine. 

Procaine and the other local anaesthetic drugs prevent the generation and the conduction of the nerve impulses. Their main site of action is the cell membrane, since conduction block can be demonstrated in giant axons from which the axoplasm has been removed [25]. 

Raising the concentration of Ca in the medium pathing, a nerve may relieve conduction block produced by local anesthetics. Relief occurs because Ca alters the surface potential on the membrane, and hence the transmembrane electrical field. This, in turn, reduces the degree of inactivation of the Na channels and the affinity of the latter for the local anaesthetic molecule [25, 27]. 

It is also used in digitalis induced ventricular arrhythmia as it reverse the conduction block while accentuating the depression of automaticity (The detailed pharmacology is discussed in chapter Antiepdeptic agents ). 

Figure 15.7 Thermal conductivity block with filaments located out of the flow path.

Thioridazine in doses exceeding 300 mg daily is almost always associated with minor abnormalities of T waves that are easily reversible. Overdoses of thioridazine are associated with major ventricular arrhythmias, eg, torsade de pointes, cardiac conduction block, and sudden death it is not certain whether thioridazine can cause these same disorders when used in therapeutic doses. In view of possible additive antimuscarinic and quinidine-like... 

Bradycardia and hypotension are the most common manifestations of toxicity. Agents with partial agonist activity (eg, pindolol) can cause tachycardia and hypertension. Seizures and cardiac conduction block (wide QRS complex) may be seen with propranolol overdose. 

Clinical use Because of its long duration of action, bupivacaine is indicated for long surgical anesthesia where a considerable amount of postoperative pain is expected such as dental and oral surgeries. Infiltration using a 0.25 % solution of bupivacaine produces sensory anesthesia with an onset of 2 to 5 min and a duration of 2 to 4 h or greater (Tetzlaff, 2000). A nerve conduction block with a duration of between 4 to 8 h and occasionally up to 24 h is achieved with injection of 0.5 to 0.75 %... 

Clinical use The indications for levobupivacaine include wound infiltration (0.25 % solution), nerve conduction block (0.25 - 0.5 %), spinal analgesia (0.5 %) and epidural anesthesia (0.5 to 0.75 %). For labour analgesia, lower concentrations of levobupivacaine are recommended when administered as epidural injection (0.125 to 0.25 % up to 25 mg) or infusion (0.25 %). The maximum dose for ilioinguinal or iliohypogastric block in children is 1.25 mg/kg/side (0.25 to 0.5 % solutions). For postoperative pain management, levobupivacaine can be applied epidurally in combination with the opioids fentanyl or morphine or with the a2-agonist clonidine. 

Aldissi 586) has reviewed the stability of block copolymers of acetylene and other non-conducting blocks. The copolymers are somewhat more stable than polyacetylene but the effect is small. 

Cardiomyopathy Conduction blocks Wolff-Parkinson-White syndrome Inner ear... 

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