Role of regional anesthesia in obstetric practice: uterine and lower genital tract innervations, drugs used, types of blocks.

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.

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.