Hawkins and colleagues (1997) reported that four of 129 maternal anesthetic-related deaths were from parenteral sedation-one from aspiration, two from inadequate ventilation, and one from overdosage. Opioids used during labor may cause newborn respiratory depression. Naloxone is a narcotic antagonist capable of reversing this respiratory depression. It acts by displacing the narcotic from specific receptors in the central nervous system. Withdrawal symptoms may be precipitated in recipients who are physically dependent on narcotics. For this reason, naloxone is contraindicated in a newborn of a narcotic-addicted mother.
Inhaled nitrous oxide has a rapid onset and ofset that provides analgesia during episodic contractions. It can be self-administered as a mixture of 50-percent nitrous oxide and 50-percent oxygen premixed in a single cylinder (Entonox) or using a blender that mixes the two gases from separate tanks (Nitronox). he gases are connected to a breathing circuit through a one-way valve that opens only during inspiration. he use of intermittent nitrous oxide for labor pain is generally regarded s safe for the mother and newborn, but pain control is less efective than epidural analgesia (Barbieri, 2014; Likis, 2014). In many cases, nitrous oxide simply serves to delay more deinitive neuraxial analgesia. For maximal eicacy, nitrous oxide is inhaled 30 seconds prior to the start of a contraction, although this prevents adequate rest for the mother. Nitrous oxide is also associated with nausea and vomiting. The environmental and health risk of its use without proper scavenging remains to be carefully evaluated (King, 2014).
Various nerve blocks have been developed over the years to provide pain relief during labor and/or delivery. These include pudendal, paracervical, and neuraxial blocks such as spinal, epidural, and combined spinal-epidural techniques.
Some of the more commonly used nerve block anesthetics, along with their usual concentrations, doses, and durations of action, are summarized in Table 25-4. he dose of each agent varies widely and is dependent on the particular nerve block and physical status of the woman. he onset, duration, and quality of analgesia can be enhanced by raising the volume and/or concentration. his can be done safely only by incrementally administering small-volume boluses of the agent and by careully monitoring early warning signs of toxicity. Administration of these agents must be followed by appropriate monitoring for adverse reactions. Equipment and personnel to manage these reactions must be immediately available.
Most often, serious toxicity follows inadvertent intravenous injection. Systemic toxicity from local anesthetics typically manifests in the central nervous and cardiovascular systems. For this reason, when epidural analgesia is initiated, dilute epinephrine is sometimes added and given as a test dose. A sudden significant rise in the maternal heart rate or blood pressure immediately after administration suggests intravenous catheter placement. his should halt further injection and should prompt catheter repositioning. Local anesthetic agents are manufactured in more than one concentration and ampule size, which raises the potential for dosing errors.
aWithout epinephrine.
bEsters are hydrolyzed by plasma cholinesterases and amides by hepatic clearance. D&C = dilatation and curettage.
Data from Liu SS, Lin Y: Local anesthetics. In Barash P, Cullen B, Stoeling R, et al (eds): Clinical Anesthesia, 6th ed. Philadelphia, Lippincott Williams & Wilkins, 2009.