To give pain relief to the woman in labor neuronal innervations of reproductive system will be blocked with drugs this procedure is called regional analgesia.
Sensory innervation of the female genital tract:
In
female reproductive system is having high neuronal innervation. The organs mainly involved in the process of
labor are uterus, cervix, vagina and surrounding muscles.
Uterine innervation:
Uterus is the main organ of pregnancy where the growth of the fetus occurs. The pregnant
woman experiences tightening of uterus throughout the pregnancy which
is painless and gradually merges into painful uterine contractions.
During the first stage of labor pain is mainly generated from the uterus. On just lateral side of to the cervix the Frankenhauser ganglion lies, through which the visceral sensory fibers from the uterus, cervix, and upper vagina traverse into the pelvic plexus, and then to the middle and superior internal iliac plexuses.
From these middle and superior internal iliac plexuses the fibers travel in the lumbar and lower thoracic sympathetic chains and enter the spinal cord through the white rami communicants in association with the T10 through T12 and L1 nerves.
The pain due to uterine contractions is transmitted mainly through the T11 and T12 nerves in the early labor.
Whereas the motor pathways to the uterus leave the spinal cord at the level of the T7 and T8 vertebrae.
So,
when block is given to extend up to T11 and T12 nerves, T7 and T8
nerves will be spared and only sensory innervations will be affected
relieving the pain but motor innervations is spared and action of the uterus will continue. This principle is used in giving regional anesthesia.
Lower genital tract innervation:
As labor progresses lower genital tract also involves and pain of vaginal delivery arises from the lower vagina, other parts of external genitalia and perineal muscles also.
The
pudendal nerve with its branches supplies the lower genital tract. The
peripheral branches of the pudendal nerve provide sensory innervation to
the perineum, anus, and the more medial and inferior parts of the vulva
and clitoris.
The
pathway of the pudendal nerve is, it passes beneath the posterior
surface of the sacrospinous ligament just as the ligament attaches to
the ischial spine.
The sensory nerve fibers of the pudendal nerve are derived from the ventral branches of the S2 through S4 nerves.
Anesthetic agents used in regional block:
The anesthetic agents used in regional block called local anesthetics are of two groups.
The amino-esters:
There are two drugs in amino-ester group which are used in regional anesthesia.
a) 2-Chloroprocaine:
It is having rapid onset of action.
1 to 2% concentrated 400 to 600 mg dose in 20 to 30 ml solution can be used in local or pudendal block with average duration of action for 15 to 30 min.
And 2
to 3% concentrated 300 to 750 mg dose in 15 to 25 ml solution used in
epidural block for caesarean delivery with average duration of action
for 30 to 60 min but not used in spinal block.
b) Tetracaine:
It is having slow onset of action.
0.2% concentrated 4 mg dose can be used in low spinal block with average duration of action for 75 to 150 min.
And 0.5% concentrated 7 to 10 mg dose is used in spinal block for caesarean delivery.
The amino-amides:
There are three drugs in amino-amide group which are used in regional anesthesia.
a) Lidocaine:
It is having rapid onset of action.
1 % concentrated 200 to 300 mg dose in 20 to 30 ml solution can be used in local or pudendal block with average duration of action for 30 to 60 min.
2
% concentrated 300 to 450 mg dose in 15 to 30 ml solution used in
epidural block for caesarean delivery with average duration of action
for 60 to 90 min.
5%
concentrated 50 to 75 mg dose in 1 to 1.5 ml solution used in spinal
block for caesarean delivery with average duration of action for 50 to
75 min.
And
5% concentrated 25 to 50 mg dose in 0.5 to 1 ml solution used in spinal
block for vaginal delivery with average duration of action for 25 to 50
min.
b) Bupivacaine:
It is having slow onset of action.
0.5 % concentrated 50 to 100 mg dose in 15 to 20 ml solution can be used in epidural block for caesarean delivery with average duration of action for 90 to 150 min.
0.25
% concentrated 50 to 100 mg dose in 8 to 10 ml solution used in
epidural block for vaginal delivery with average duration of action for
60 to 90 min.
0.75%
concentrated 7.5 to 11 mg dose in 1 to 1.5 ml solution used in spinal
block for caesarean delivery with average duration of action for 50 to
75 min.
c) Ropivacaine:
It is having slow onset of action.
0.5 % concentrated 75 to 100 mg dose in 15 to 20 ml solution can be used in epidural block for caesarean delivery with average duration of action for 90 to 150 min.
0.25
% concentrated 20 to 25 mg dose in 8 to 10 ml solution used in epidural
block for vaginal delivery with average duration of action for 60 to 90
min.
Diluted epinephrine is added sometimes to prolong the action of the anesthetic. This may cause symptoms when a test dose is inadvertently given intravenously.
There are wide variations in the doses of agents because they dependent on the type of nerve block to be given and on the physical status of the woman.
If we want to enhance the onset, duration or quality of analgesia, it can be done by increasing the dose but only incrementally administering small-volume boluses of the agent. Strict vigilance to be maintained for early warning signs of toxicity along with readily available equipment and personnel to manage these reactions.
Serious toxicity normally occurs either after inadvertent intravenous injection or after administration of excessive amounts.
The
anesthesiologist should have thorough knowledge about preparations
concentration and dosage etc, as many of these agents are manufactured
in more than one concentration and ampoule size.
Toxicity of local anesthetics:
The systems mainly affected by the toxicity local anesthetics are typically the central nervous system and the cardiovascular systems.
Central nervous system:
Stimulation of central nervous system occurs in the beginning of toxicity followed by depression as serum levels increases.
The
symptoms of central nervous system toxicity are light-headedness,
dizziness, tinnitus, metallic taste and numbness of the tongue and
mouth.
Patients present with bizarre
behavior, slurred speech, muscle fasciculation and excitation and
ultimately turns into generalized convulsions which is followed by loss
of consciousness. Immediate steps to be taken are controlling the convulsions, establishing the airway and delivering the oxygen.
To abolish the peripheral manifestations of the convulsions and to allow tracheal intubation succinylcholine is used. To inhibit convulsions centrally thiopental or diazepam can be used. Magnesium sulphate may be used in dosage used for eclampsia to control convulsions.
Convulsions
can induce maternal hypoxia and lactic acidosis leading to abnormal
fetal heart rate patterns, such as late decelerations or persistent
bradycardia etc.
In
this case two options remain one taking resuscitative measures which
will treat mother and baby or going for caesarean section. As per the
records arresting the convulsions,
administration of oxygen, and application of other supportive measures will recover the fetus more
quickly in utero than following immediate cesarean delivery. That too
first treating the hypoxia and the metabolic acidosis is important for
maternal well-being.
Cardiovascular system:
The manifestations of the cardiovascular system generally develop after the cerebral toxicity.
They may not occur always because they manifest at higher drug levels.
But in case of bupivacaine, neurotoxicity and cardiotoxicity will develop at almost identical serum drug levels. The use of 0.75-percent solution of bupivacaine for epidural
injection was stopped because of this risk of systemic toxicity by the Food and Drug Administration in 1984.
Cardiovascular toxicity also starts with stimulation followed by depression as in case of neurotoxicity.
In
stimulated state there will be hypertension and tachycardia followed by
hypotension and cardiac arrhythmias in depression state.
Because of hypotension impaired uteroplacental perfusion becomes impaired leading to fetal distress.
The steps taken to manage the hypotension are:
Turning the woman onto either side to avoid aortocaval compression.
Rapidly
infusing a crystalloid solution along with intravenously administered
ephedrine. Emergency cesarean delivery should be considered if maternal
vital signs have not been restored within 5 minutes of cardiac arrest.
As
in case of convulsions, in hypotension also the fetus is likely to
recover more quickly in utero than after caesarean section once maternal
cardiac output is reestablished.
Pudendal block:
This is use in vaginal delivery. It is a relatively safe and simple method of providing analgesia for spontaneous delivery.
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The procedure:
To guide the needle a tubular introducer that allows 1.0 to 1.5 cm of a 15-cm 22-gauge needle to protrude beyond its tip is used into position over the pudendal nerve.
The end of the introducer is placed against the vaginal mucosa just beneath the tip of the ischial spine.
The needle to be pushed beyond the tip of the director into the mucosa and a mucosal wheal is made either with 1 mL of 1-percent lidocaine solution or an equivalent dose of another local anesthetic.
To guard against intravascular infusion, aspiration is attempted before this and all subsequent injections.
The needle is then advanced until it touches the sacrospinous ligament, which is infiltrated with 3 mL of lidocaine.
The needle is advanced
farther through the ligament, and as it pierces the loose areolar
tissue behind the ligament which can be noted as the resistance of the plunger decreases. Another 3 mL of the anesthetic solution is injected into this region.
After this, the needle is withdrawn into the introducer, and it is moved to just above the ischial spine.
The needle is inserted through the mucosa and the rest of 10 mL of solution is deposited. The procedure to be repeated on the other side also.
If successful pudendal block occurred it can
be tested within 3 to 4 minutes of the time of injection by pinching of
the lower vagina and posterior vulva bilaterally without pain.
It is often of benefit before pudendal block to infiltrate the fourchette, perineum, and
adjacent vagina with 5 to 10 mL of 1-percent lidocaine solution directly at the site where the episiotomy is to be made.
So,
episiotomy can be made without pain if delivery occurs before pudendal
block becomes effective and by the time of the repair of episiotomy the
pudendal block usually has become effective which will relieve from
pain.
Drawbacks:
In
case of extensive obstetrical manipulation like instrumental delivery,
need to give extended episiotomy pudendal block usually does not provide
adequate analgesia.
Also
it is inadequate for women in whom complete visualization of the cervix
and upper vagina, or manual exploration of the uterine cavity are
indicated.
Complications:
If accidentally local anesthetic goes into intravascular space it may cause serious
systemic toxicity.
Perforation of a blood vessel may lead to hematoma formation. This is more likely
when there is a coagulopathy as in case of placental abruption where defective coagulation is seen.
Severe infection though rarely may originate at the injection site and it may spread posterior to the hip joint, into the gluteal musculature or into the retropsoas space.
Paracervical block:
This is also one of the widely practiced blocks in vaginal delivery and it usually provides satisfactory pain relief during the first stage of labor.
But additional analgesia is required as labor progresses because the pudendal nerves are not blocked.
Procedure:
To give paracervical block lidocaine or chloroprocaine, 5 to 10 mL of a 1-percent solution, is injected into the cervix laterally at 3 and 9 o'clock.
Bupivacaine is contraindicated because of an increased risk of cardiotoxicity.
This block may have to repeated as these anesthetics are relatively short acting.
Complications:
In approximately 15 percent of paracervical blocks fetal bradycardia is seen.
Bradycardia usually develops within 10 minutes and may last up to 30 minutes.
As the bradycardis is usually transient and the newborns are normal at birth, it can be taken that this bradycardia may not be a sign of fetal asphyxia.
But
according to some reports, fetal scalp blood pH and apgar scores were
sometimes low, and a few foetuses have died after paracervical block.
This
may occurred because of the trans placental transfer of the anesthetic
agent or its metabolites causing a depressant effect on the fetal heart.
Another
thought is that fetal bradycardia may resulted from decreased placental
perfusion as the consequence of drug-induced uterine artery
vasoconstriction and myometrial hypertonus.
Doppler studies showing
an increase in the pulsatility index of the uterine arteries following
paracervical block also support the hypothesis of drug-induced
vasospasm.
Because of these reasons, paracervical block better to be avoided in situations of potential fetal compromise.
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