Inhalational Analgesia
Inhalational analgesia can be defined as the administration of
subanesthetic concentrations of inhaled anesthetics to relieve pain during labor.
This pain relief technique should not be confused with inhalational anesthesia that
produces unconsciousness and loss of protective laryngeal reflexes. Although inhalational
analgesia provides a limited amount of pain relief, it is not adequate to provide
sufficient pain relief for most mothers. It may, however, have a place as an adjunct
to neuraxial techniques or in parturients in whom regional anesthesia is not possible.
Inhaled analgesics can be administered either intermittently (during contractions)
or continuously. They can be self-administered, but the patient should have a health
care provider present to ensure an adequate level of consciousness and proper use
of the equipment. Although inhaled analgesics continue to be used in parts of Europe
as well as developing countries, they are seldom used for labor analgesia in the
United States.
Entonox (50:50 N2
O/O2
mixture) has been
used for many years as both a sole analgesic and an adjuvant to systemic and regional
techniques for labor. Associated side effects include dizziness, nausea, dysphoria,
and lack of cooperation. The maximum analgesic effect occurs after 45 to 60 seconds,
and it is therefore important that the parturient use entonox at the early onset
of her contractions and discontinue its use after the peak of the contraction. The
lack of scavenging systems in labor rooms may theoretically put staff at risk of
exposure to excessive levels over a prolonged period. The administration of nitrous
oxide and oxygen in a 50:50 combination appears to have no effect on hepatic, renal,
cardiac, or pulmonary functions. Though safe and economic, it is unfortunate that
reliable analgesia with 50% nitrous oxide has not been demonstrated to be reliable.
[100]
Desflurane (0.2%), enflurane, and isoflurane (0.2% to 0.25%) have
also been used successfully to provide labor analgesia, but their effectiveness appears
to be comparable to that of nitrous oxide. Although Abboud and coworkers found that
enflurane and nitrous oxide were equipotent, others have suggested that 1% enflurane
provides better pain relief.[101]
[102]
[103]
The use of these volatile analgesics, however,
is limited by drowsiness, unpleasant smell, and high cost. The major risk when using
volatile analgesics is accidental overdose resulting in unconsciousness and loss
of protective reflexes.
