Medical brain notes 7

  Regional anesthesia We can imagine future clinicians to prescribe treatments that would exclusively affect a single cell type or a specific organ without spillover effects. That type of explicit therapy would be the opposite to general anesthesia, the name of which implies generalized effects of the anesthetic drugs. Indeed, anesthetics delivered via the lungs or by intravenous injection flood all organs in the body, causing numerous undesired effects. How much better to pinpoint the effect with regional anesthesia. Here, we deliver the drug directly to the nervous tissue where we hope to cause a specific effect. We are closer to the ideal but not quite in heaven because we still have to contend with side effects that arise when the anesthetic drug appears in the circulation. We also lack the specificity of drugs that would block only one type of fiber and spare all others. Nevertheless, regional anesthesia provides a tool that can be used to great advantage for many patients. Fo...

  Neuraxial anesthesia Neuraxial anesthesia involves the placement of local anesthetics and/or opioids into the intrathecal (subarachnoid) or epidural space (Fig.  4.2 ), either by a single injection or by a continuous infusion catheter technique. The medications act directly on the spinal cord and, for epidurals, also on the spinal roots. This results in decreased transmission of impulses through the various nerves (Table  4.1 ).   Some local anesthetics have differential effects on various nerve types. For most applications, we would prefer to block only the pain impulses, but no agent is quite that specific. Bupivacaine blocks sensory more than motor fibers and is the agent of choice for labor analgesia where we desire maintenance of maternal mobility (“Push! Push!”). The dermatomal level (Fig.  4.3 ) achieved depends on several factors (Table 4.2 ) .  Consider a Cesarean delivery, for which we require a T4 sensory level tominimize discomfort with uterin...

  Peripheral nerve blocks With neuraxial anesthesia, it is difficult to block only the area of interest. Almost by definition, surgical anesthesia at the desired level includes everything “south” as well. Peripheral nerve blocks provide an alternative, interrupting nerve impulses at specific points in their course, rather than the entire spinal cord. Table  4.6  lists some of the blocks we perform.   While local anesthetics can diffuse a small distance, depositing the drug in close proximity to the desired nerve increases the likelihood of a successful block. Therefore, knowledge of anatomy is paramount. Sometimes, anatomic landmarks suffice; for example, we can deposit local anesthetic in the axillary sheath by traversing its artery (Fig.  4.7 ). For most other blocks, in order to ensure the needle tip lies within millimeters of the intended nerve (and not  in  the nerve), we use one of two common techniques: ·       ...

  Intravenous regional anesthesia (IVRA) Also called a Bier 2  block, this is perhaps the simplest, safest, most foolproof regional anesthetic technique. We replace the blood in the venous system of an extremity with local anesthetic (large volume, low concentration, i.e., 0.5% lido-caine WITHOUT epinephrine) by first exsanguinating the extremity (usually arm), applying a tourniquet, then infusing the local anesthetic distal to the tourniquet. We obtain excellent anesthesia within minutes. It will last until the tourniquet is deflated. The local anesthetic will flow retrograde through the venous system into the  vasa nervorum  that bathes each nerve fiber. Unfortunately, not infrequently the patient will be troubled by tourniquet pain. Therefore, this technique is best suited for operations lasting less than an hour. The technique is safe as long as the tourniquet holds tight, preventing the local anesthetic from gaining access to the circulation and causing systemic...

  Local anesthetic toxicity Local anesthetics exhibit dose-related toxicity. Therefore, concerns about poten-tial toxicity grow with increasing doses of local anesthetic (see also Pharmacology). Typical volumes of local anesthetics used for various blocks follow (we use lidocaine 1.5% as an example): Of these, intercostal nerve blocks lead to the highest local anesthetic blood levels and therefore are most likely to cause toxicity, because multiple small depots of the local anesthetic offer a relatively large surface for absorption of the drug into blood vessels. In order to reduce the rate of absorption, we often add 1:200 000 epinephrine (5 mcg/mL) to the local anesthetic, which not only reduces the absorption of the drug and thus the chance of toxicity, but also prolongs the anesthetic effect. An added advantage of the epinephrine: should the injection be inadvertently intravascular (as into an epidural vein), the prompt development of epinephrine-induced tachycardia will give a...