INTRODUCTION
In this article, we will discuss the use of intravenous narcotics for the treatment of labor pain. This is part two of a two part article; in part one, we covered intravenous agents in general and discussed the four other classes of intravenous agents briefly. Narcotics have been left for a seperate feature because they are by far the most widely utilized and most effective of these drugs.

Be sure that you read part one for some general information about intravenous agents and their effect on the mother and the fetus. In particular, it bears repeating that intravenous agents are a widely utilized option that offers a good alternative to the regional anesthetic techniques of spinal and epidural. It also is important to continue to stress that there is no ideal, universal analgesic or anesthetic technique and each person's desires and responses are different.

Systemic analgesics do offer some advantages: they are easily administered and do not require that an anesthesiologist be present, they can be given to patients that are unable to have a regional technique because of a medical condition or do not want a regional technique out of personal preference, they can be given during early labor when an epidural may cause a more pronounced slowing of labor, and they can be given by various "patient controlled" delivery mechanisms which returns some control to the patient.

GENERAL COMMENTS
As we mentioned before, all systemic medications cross the placenta and therefore have potential to depress the fetus. The narcotics are no exception. The amount of effect (pain relief) and side effects (respiratory depression, drowsiness, etc.) that result after administration of intravenous medications is a function of the dosage, timing and presence of other patient factors.

We made the point last week that the narcotics are the most effective systemic medications and, through increasing dosage of medication, these agents are able to provide complete analgesia (pain relief). However, we also stressed that the pain relief comes at a price, namely that as dosage increases so does the incidence and severity of side effects. The side effect that is most limiting is perhaps respiratory depression and the resulting decrease in oxygenation. Some of the newer agents that are becoming more popular (the most known is Stadol which is discussed below) have the advantage of a limited amount of respitatory depression. However, sedation, obtunded reflexes and decreased blood pressure are also serious side effects and can also limit the amount of drug that can be given.

As a result of these limitations, these drugs are used to reduce pain, not eliminate it. The best way to describe this is that prior to a contraction, the mother may feel the contraction starting at a pressure of 10 and feel the entire contraction up to a peak pressure of 60. After the narcotic is given, the mother may not feel the contraction until the pressure reaches 30 and the amount of pain after that is reduced as well. The result is less pain intensity as well as less pain duration (since the mother feels pain only the time it takes for the contraction to develop from 30 to 60 rather than from 10 to 60). Remember that the primary concern in any situation is the health and well-being of the mother and fetus; dosages high enough to give complete absence of pain are dangerous for both.

Other common side effects that are less life threatening are nausea, vomiting, pruritus (itching), constipation, etc.

EFFECTS ON LABOR AND FETUS
It is generally accepted that narcotics given during early labor decrease uterine activity somewhat, can slow cervical dilatation and slightly retard the progress of labor. There is no direct evidence that this is harmful, but some believe that this is an undesirable effect. Once labor is established, the fact that narcotics relieve the pain of labor seems to result in better coordinated uterine contractions and a shorter active phase of labor. Again, there is no proof that this is a good or bad thing, but many find a shorter labor to be a desired effect.

All the narcotics cross the placenta and decreased fetal heart rate variability. On delivery, narcotics can result in a neonate with respiratory depression (usually treated with Narcan) and decreased neurobehavioral scores, at least initially. There seems to be no long term effects on the child assuming proper breathing and oxygenation are provided upon delivery. The depressive effects of these drugs on the newborn seem to be most pronounced when the delivery occurs within 2-3 hours of the mother receiving the drug. This may reflect the time it takes for the drugs to reach a high level in the fetus.

SPECIFIC NARCOTIC AGENTS:

Meperidine (tradename Demerol) is by far the most popular drug of the narcotics and by far the most extensively used systemic labor medication in the world. Usual dosages when given intravenously are 25-50 milligrams which takes approximately 5-10 minutes to show peak effect. Demerol can also be given intramuscularly in the absence of an intravenous line. Again, neonatal respiratory depression is maximal if delivery occurs 2 to 3 hours after administration. However, sustained neurobehavioral effects are unlikely in the dosages mentioned above. Nausea is common with this drug; in fact it is so common that another drug called phenergan is often given automatically with demerol (I personally feel that no drug should be given "automatically" but should only be given if it is actually needed, in this case if nausea actually develops). The result of the phenergan is that patients often feel very sedated, "groggy", "out of it", etc.

Morphine is no longer popular, probably because it takes about 20 minutes to work once it is given and about 1-2 hours until it reaches its peak effect. The fact that it has a long duration of action (4-6 hours) makes it less flexible of a drug as well. The long duration seems to contribute to an increased amount of newborn respiratory depression versus the other drugs in this class.

Fentanyl is probably the most rapid acting of the commonly used agents. When given intravenously at its usual dosage of 25-50 micrograms it takes effect in about 3-5 minutes and its duration of action is 30-60 minutes. Fentanyl can also be given intramuscularly. At the dosages mentioned, there seem to be no significant effects on neonatal Apgar or neurobehavioral scores. Fentanyl is well-suited to a patient controlled delivery mechanism.

Sufentanil is a derivative of fentanyl with increased potency. It has a slightly more rapid onset of analgesia when given intravenously, but its potency has limited its systemic use during labor. It is often used in regional techniques to supplement the effect of other drugs and to provide a quick onset of analgesia.

Alfentanil is also a fentanyl derivative that has a very rapid onset and a short duration of action. Depite these benefits that make this drug very attractive in theory for use during labor analgesia, it has been assoiated with a higher degree of newborn depression when compared with other options. It is not widely utilized for this reason.

Two of the newer agents that are increasingly popular are butorphanol (tradename Stadol) and nalbuphine (tradename Nubain). These are synthetic drugs that belong in what is known as the agonist-antagonist class of narcotics. The major advantage of these agents is that they exhibit what is known as a "ceiling effect" for respiratory depression, meaning that once dosages increase past a certain level, no increase in respiratory depression is seen. Therefore, these drugs are theoretically safer than drugs that do not exhibit this property.

The major drawback of these agents is that there is also a ceiling effect on the analgesic level, meaning that once dosages increase past a certain level, there is also no increase in the amount of pain relief one can obtain from them. They may also limit the effectiveness of the other, more traditional narcotics once they are given. In addition, the other side effects still exist so that complaints of drowsiness, dizziness, weakness, nausea, vomiting, etc. are still common as dosages increase. In addition, these drugs cause a psychomimetic reaction (dysphoria or a feeling of unease) in some patients. These drugs should be used with extreme caution in patients that have a recent history of narcotic usage for any reason because they may precipitate a withdrawal type reaction.

Butorphanol is usually given in dosages of 1-2 milligrams which takes about five minutes to have an effect and lasts approximately 3-4 hours. Respiratory depression does not increase above 4 milligrams. In addition, this drug does not seem to have any depressant effects on the neonate. In some studies, it has been shown to have be associated with less nausea and vomiting than demerol. For all of these benefits, and also because of subjective preferences of many women who feel they do better with butorphanol, this drug is widely gaining acceptance and popularity.

Nalbuphine is usually given in a dosage of 5-10 milligrams requiring about five minutes to take effect and lasting about 3-6 hours. Maximal respiratory depression occurs at 30 milligrams. There seem to be minimal fetal effects and perhaps less nausea as well. Both butorphanol and nalbuphine can also be given intramuscularly.

CONCLUSION
Again, I will reiterate my comments last week that intravenous systemic medications provide an effective alternative to the use of regional techniques in women who either cannot receive regional techniques or prefer not to undergo regional techniques. These medications do cross the placenta and they do have side effects, but with careful administration they are very useful.

You may ask why a woman would prefer this route of labor analgesia versus some of the others. First of all, many patients do not need complete absence of pain. Many would prefer to avoid an invasive technique such as an epidural. Many patients feel more control when receiving analgesia in this manner and their use does not rule out other techniques should they be required. Again, there is no perfect choice for everyone and there are many options available. Proper planning and education combined with a frank dialogue with your healthcare providers will help in making a decision that is right for the individual.