Local anesthetics in labor

Local anesthetics


There have been experiments with injections of various drugs into the epidural space. Currently, however, only two groups of drugs are used in routinely in clinical practice, local anesthetics and opioids.

Local anesthetics are powerful drugs used for preventing and treating pain. Many of us had experience with local anesthetics, most likely in much feared dentist’s chair during tooth extraction or treatment of root canal. Others may have a minor cut stitched under local anesthesia. Yet others had other experiences, such as removal of a mole or even something more significant, like carpal tunnel release or minor plastic surgery.

The mechanism by which nerves conduct electrical impulses is complex, but in essence depends on the movement of sodium ions into the nerve cell through the sodium channels situated on the surface. In resting state nerve cell is negatively charged. Sodium channels are activated sequentially, one by one, causing the change of the electrical charge of the cell to positive and thus propelling the nerve impulse along the neuron. Local anesthetic placed in the vicinity of the nerve cell binds to sodium receptors and inactivates them. As the result the impulse is not conducted along the nerve, and painful stimuli cannot reach the brain.

As time passes, local anesthetic injected around the nerve gets absorbed by the blood and is eventually metabolized. The speed of absorption and metabolism determines the duration of action of local anesthetic. In neuraxial blocks local anesthetics are placed in proximity to cauda equina and block nerve conduction in that area.

Like other drugs local anesthetics can cause undesirable side-effects. Two of them are clinically important.

Motor block

Most nerves in the body are mixed and consist of sensory and motor fibers. Sensory fibers carry impulses from the peripheral receptors to the spinal cord and the brain, eliciting various sensory effects. Motor fibers carry signals from the central nervous system to the periphery and ensure adequate movement. Blockade of sodium channels interferes with both motor and sensory fibers, resulting in impaired movement. Returning to the example from the dentist’s office, the numbness cause by local anesthetic injection is often associated with lopsided smile, due to the blockade of motor fibers that supply facial muscles.

When epidural or spinal anesthetic is administered for relieving pain of childbirth local anesthetics block nerve impulses traveling along the nerves of cauda equina that supply lower part of the abdomen and lower limbs. The degree of muscle varies, and if muscle strength in the legs is significantly affected it is uncomfortable for the patient. This side-effect is one of the most common reasons for patient dissatisfaction with labor epidurals. On the brighter side, currently recommended modern anesthetic technique allows to reduce this side-effect to the minimum. As the rule, sensory fibers are smaller and more sensitive to the effects of local anesthetics, and if these drugs are used at low concentrations it is possible to achieve pain relief with minimal effect on motor function. In addition, recently developed local anesthetic ropivacaine (see below) tends to affect sensory nerve fibers to larger degree than motor and preserves the ability to move in the area affected by epidural.

Local anesthetic toxicity

Local anesthetic toxicity is dangerous and potentially lethal. Sodium channels are located not only in the peripheral nerve fibers but also other organs, including the brain and the heart. If local anesthetics are given at large doses both of these organs will be negatively affected.

The cells of the heart muscle – myocardial cells – are activated similarly to nerve cells: sodium ions enter the cell through sodium channels. Sodium ions entering the cell trigger the chain of electrochemical reactions which lead to coordinated contraction of the heart. Drugs that reduce the activity of cardiac sodium channels are used for the treatment of arrhythmias, or disturbances of cardiac rhythm. One such drug is local anesthetic lignocaine, the same as used in epidurals. However, if local anesthetics are given in excessive doses sodium channels of the heart muscle will become blocked and cardiac contraction will not be possible. The result – cardiac arrest.

The systemic effects of local anesthetics on the brain are less dangerous, however not less dramatic. In simplistic form, there are two major systems of the neural pathways in the brain: stimulatory and inhibitory. If local anesthetics are given in sufficient amount the inhibitory pathways get blocked and stimulatory ones get out of control: the patient develops seizures, just like during an epileptic fit. As blood level of the drug continues to increase, all neural pathways become blocked, and the patient becomes unconscious. Neural centers that control breathing and cardiovascular system become dysfunctional, and without treatment the patient can die.

Even when administered directly into the circulation local anesthetic becomes dangerous only if it exceeds maximum safe dose, which is different for each individual drug. These doses have been established in animal experiments as well as in studies on human volunteers. Toxic reactions in clinical practice most commonly happen when the local anesthetic injected into the bloodstream inadvertently.

Epidural space is filled with veins, and the tip of the epidural catheter may inadvertently be placed into a blood vessel. To avoid such error several measures are taken by anesthetists when placing the epidural. One is gentle aspiration, by pulling the plunger of the syringe before injecting the blood and ensuring that no blood appears in the catheter. Second, test dose – small amount of local anesthetic mixture – is administered and the patient is observed for early signs of toxicity. Lastly, slow injection of local anesthetic mixture is another important aspect of safety. The severity of local anesthetic toxicity depends on the amount of the drug injected. Slow injection ensures that toxicity is detected early, when symptoms are mild, and further administration of the drug is terminated. Early symptoms of local anesthetic toxicity include nausea, numbness around the lips, dizziness, headache, disorientation and palpitations. If the woman complains of any of the above the injection is stopped immediately.

About a decade ago another technique of detecting intravascular epidural placement was practiced. Small amount of adrenaline (epinephrine) was added to the test dose. When injected into the bloodstream adrenaline causes tachycardia, and sudden increase in heart rate immediately after the test dose was suggestive of intravascular injection. However, this technique has been abandoned, mostly because there are many reasons for sudden changes of heart rate during labor, and it has not been shown to increase the safety of epidural.

The doses of local anesthetics required for effective labor epidural are relatively small and are unlikely to cause serious toxicity, even if given intravenously. The most popular local anesthetic in obstetric practice is ropivacaine at concentration of 0.2% mixed with fentanyl. Ten milliliters of this mixture are usually sufficient for achieving good pain relief in labor, resulting in total dose of the drug of 20 mg. For comparison, toxic dose of ropivacaine is 3 mg/kg, or between 200 and 400 mg for the woman of average size. Epidural dose in this case is less than one tenth of toxic. The doses of local anesthetics used for spinal anesthesia for caesarean section are even smaller and therefore safer.

Ropivacaine, the most popular local anesthetic used for labor epidural also has the best safety profile. Its dose that causes neurological symptoms is significantly lower than the dose that may cause cardiac arrest. Because of that toxic reactions to this drug are detected early and serious cardiac adverse effects are less likely to happen than with bupivacaine.


Currently the following local anesthetics are used in obstetric anesthesia:

Lignocaine

Fast onset of this drug makes it a popular choice when the patient with labor epidural has to undergo cesarean section. When given epidurally good quality block is often achieved within 10 minutes. Duration of action relatively short, about two hours.

Spinal lignocaine has been associated with transient neurologic syndrome and cauda equine syndrome, both discussed in separate chapters on this site.

Toxic dose is relatively large, 4 mg/kg when used epidurally and is safe from cardiovascular point of view.

Bupivacaine

Onset of action 10 – 20 minutes after epidural administration, duration 3 – 6 hours.

The most toxic local anesthetic of currently used. Toxic dose – 2 mg/kg. Cardiac arrest as the result of its toxicity is difficult to treat. Recent development in the treatment of toxicity is the use of Intralipid, the suspension of fat used for intravenous nutrition of critically ill patients. Intralipid binds highly soluble in fat bupivacaine and reduces its amount at sodium receptors. Survival of patients suffering from bupivacaine toxicity has been considerably improved after the introduction of Intralipid in treatment protocols.

Bupivacaine at low concentration is still popular for the use in labor epidurals. Small doses are effective and safe in labor.

Ropivacaine

This local anesthetic has two distinct features: it is safe and sensory selective. Its toxic dose is 3 mg/kg. Moreover, the dose necessary to cause adverse cardiac effects is significantly lower than that leading to neurological symptoms. Cardiac arrest with this drug is rare.

It also affects sensory nerve fibers to larger extent than motor. This results in better preservation of movement necessary for the second stage of labor.

0.2% ropivacaine mixed with fentanyl at concentrations between 1 and 4 ug/ml is probably the most popular local anesthetic mixture currently used for labor epidurals.

Levobupivacaine

This drug, L-isomer of bupivacaine, has been claimed the safest local anesthetic. Its onset of action is similar to that of bupivacaine, with considerably longer duration of action, up to 8 hours. In spite of claims of its superior safety profile its popularity remains low.


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Dr. Eugene Smetannikov is a practicing anesthesiologist with the interest in obstetric anesthesia. He is the author of the most comprehensive book on the subject, The Truth About Labor Epidural