A Brief Synopsis of Obstetric Anesthesia for Medical Students

Of all the subspecialties of Anesthesia, obstetric anesthesia is perhaps is the most gratifying to the anesthesia care provider because he/she is able to relieve the pain of labor which can often be excruciating. Some women rate labor pain as being the next worst thing to having a severe tooth ache or an amputation of the digit. We as anesthesia care providers are able to create a pleasant atmosphere for the baby to be born. The key to success of our activities is to be able to produce pain relief in a safe and effective manner. The services of the anesthesia care provider are required for performing cesarean section (C.S) or delivering babies by forceps. The purpose of this summary is to give you an idea of the various anesthetic procedures on the labor ward. Please read this synopsis before you start your OB anesthesia rotation at MWH.

What are the bad effects of labor pains?

The responses to pain may be classified into three categories: cortical, suprasegmental and segmental. Cortical responses arise from the connection of the neospinothalamic tract to the cerebral cortex, suprasegmental responses are mediated by the connection of paleospinothalamic tract mainly to the reticular formation and the segmental responses to pain arise as a result of pain impulses stimulating the actual segments of the spinal cord that receive the pain fibers directly from the uterus and cervix. The following table summarizes the bad physiologic and endocrine responses to pain. Please note all responses to pain are mediated by neural connections. The anesthesia care provider can effectively block these responses by preventing the pain impulses reaching the brain and the spinal cord by judicious administration of regional blocks.

Cortical Responses	Suprasegmental responses	Segmental responses
Pain perception	Hyperventilation	Increased skeletal muscle tension or spasm
Emotional arousal	Increased ACTH, cortisol, aldosterone	Increased sympathetic tone
Anxiety, fear (Long term sequelae)	Increased catecholamine	Decreased GI motility, delayed gastric emptying
Motivational/ affective	Increased blood pressure	Nausea, vomiting
Cognitive/conceptual/judgmental	Tachycardia	Decreased bladder function
	Increased O2 consumption	Possible decreased uterine activity
	Increased lactate production

Epidural analgesia is the most effective and practical way of relieving labor pains and more than 80% of women receive epidural analgesia at MWH. In the next section we describe the anatomy of the epidural and subarachnoid spaces.

Anatomy of the Epidural Space:

The epidural space lies superficial to the dura mater and deep to the ligamentum flavum (Fig). The epidural space extends from the foramen magnum to the sacral hiatus. The epidural space is usually entered in the lumber area for labor analgesia. The space is located using a #17 or # 16 G needle and a catheter is inserted into the space so that repeated needle sticks are not necessary.

To find the epidural space, the anesthesiologist uses a technique known as “loss of resistance” to air or saline. The epidural needle advances through

Skin , superficial fascia, deep fascia

Supraspinous Ligament

Interspinous Ligament

Ligamentum Flavum.

As soon as the needle tip enters the epidural space, you will be able to inject air or saline into the space without resistance. However, when the tip of the needle is still located in the interspinous or ligamentum, there will be resistance when you try inject air or saline. After the space is located, an epidural catheter is inserted. Note in the illustration, the following:

The spinous process, Interspinous Ligament, Ligamentum Flavum, Epidural catheter

The cauda equine, Conus Medullaris, The spinal nerve and the sympathetic chain.

Once the epidural space is identified, the anesthesiologist inserts an epidural catheter into the space. The catheter is inserted 6-7 cm beyond the tip of the needle and the needle is withdrawn. The catheter is pulled back such that 4-5 cm of the catheter is left in the space. The catheter must then be aspirated to make sure that it has not entered the subarachnoid space or an epidural vein. The aspiration should be negative for CSF or blood. Aspiration per se does not rule out an inadvertent placement into an epidural vein or the subarachnoid space. A test dose must be administered to rule out these complications:

Test Dose: Usually lidocaine 1.5% with 1:200,000 epinephrine (5ug/ml) 3 ml is injected into the epidural catheter. If the catheter has inadvertently been placed into the subarachnoid space the patient will complain of severe numbness in the lower extremities and her blood pressure will decrease. If the catheter tip has been placed in an epidural vein, the epinephrine contained in the test dose will cause palpitations, tachycardia, “jumpy heart” and hypetension. Before the test dose is administered the patient must be told what untoward reactions to expect and report to you immediately. If the catheter has been placed correctly into the epidurals space, none of the untoward symptoms should occur. A test dose is a vital step in establishing proper placement of the epidural catheter.

Once the correct placement of the catheter has been verified, the epidural block may be induced by injecting more anesthetic. The local anesthetic commonly used are bupivacaine, ropivacaine and charoaccaine. The anesthetics are used in dilute concentrations 0.06% to 0.25% in 5 ml increments to a total of 10 ml. Once the patient feels adequate pain relief, she is connected to an infusion pump which delivers a dilute solution at 10-15 ml/hour. The patients are usually instructed to lay on their side to avoid aorto caval compression by the gravid uterus. They are also instructed to switch sides every hour to facilitate the spread of the local anesthetic evenly. Many anesthesiologists use narcotics with the initial bolus (fentanyl 100 ug) which speeds up the onset of action and reduces the need for concentrated local anesthetic solution. Several drug regimens are used at MWH and the local anesthetic may be delivered by different methods. If you are interested further, you can use this hyperlink to access the drug Regimen Protocol.

The following table lists the dermatomes involved in pain conduction during various stages of labor. To determinethe blocked dermatomes, one can use light pinprick method.

Stage	Description	Spinal Dermatomes
Stage I	Onset of labor to 10 cm cervical dilation	T10 – L1 (Sympathetic fibers)
Stage II	10 cm cervical dilatation to the birth of the baby	S 2- S4 (Pudendal nerves, somatic)
Stage III	Delivery of the Placenta	T10 – L1 (Sympathetic fibers)

The above illustration shows how the pain impulses are conducted to the spinal cord during first and second stage and it also depicts where exactly the patient fells labor pains during first and second stage. Note the dermatome at which the sympathetic fibers arising of the uterus terminate at the spinal cord (T10-L1) and also note the distribution of pain transmitted by the pudendal nerve during second stage.

Complications of epidural anesthesia:

1) Wet tap: wet taps occur when the epidural needle inadvertently enters the subarachnoid space. Usually, (but not always) there will be a gush of CSF from the needle. Remove the needle and reinsert at a different space. A wet tap results in rapid leakage of CSF and results in headache. The post dural puncture headache (PDPH) is usually postural. The most definitive treatment for a PDPH is an epidural blood Patch (EBP). 15-20 ml of patient’s own blood will be injected into the epidural catheter after the patient has delivered and epidural block had worn off completely. This is called a prophylactic blood patch. Conversely, you can remove the epidural catheter and discharge the patient home and perform an EBP if she develops a headache. This is called a therapeutic blood patch. An untreated PDPH may be incapacitating and lead to cranial nerve palsies.

2) Total spinal anesthesia: Injecting large volumes of local anesthetic solution inadvertently into the intrathecal space may result in total spinal anesthesia. This may cause respiratory and cardiac arrest which requires endotracheal ventilation and support of the mother and the baby.

3) The commonest complication of epidural analgesia is hypotension due to sympathectomy and it can be treated with increased administration of fluids and administration of vasopressors. The vasopressor that is currently used in ephedrine 5 –10 mg increments and in refractory cases, phenylephrine 50-100 ug may be used. Ephedrine is a mixed alpha-beta agonist and acts indirectly by liberating catecholamines from the nerve endings. Phenylephrine is a pure alpha agent and produces an intense vasoconstriction of the capacitance bed. Maternal blood pressure maintenance is important to ensure fetal well-being.

4) there are other very rare complications such as permanent neurologic damage, epidural abscess, epidural hematoma (patients with coagulopathy or anticoagulants) and cauda equina syndrome.

Spinal anesthesia:

Spinal anesthesia is produced by injecting local a local anesthetics into the subarachnoid space (see the Fig below). Since the nerve roots are not protected by the dura , it takes only a small amount of local anesthetic to produce anesthesia (~1/10^th the usual epidural dose). The anesthesiologist uses a small gauge #24G - #27G to perform the spinal tap. One of the problems with spinal anesthesia is the possibility of a spinal headache. The incidence of headache may be reduced by using smaller gauge pencil point needles (Sprotte needle). The use of cutting needles (Quincke type) predisposes to a higher incidence of headache.

Note that in the illustration to the left that a pencil point needle has been passed through the dura and arachnoid membranes into the subarachnoid space.

Advantages of spinal anesthesia: Quick onset, reliable anesthesia, small amount of drug used.

Disadvantages: Unpredictable sensory level and duration of action. Most importantly, spinal anesthesia is associated sometimes with a drastic reduction in blood pressure, which requires immediate treatment with vasoopressors such as ephedrine and phenylephrine. It is customary to hydrate the patient with crystalloid solution 1200-1500 ml to minimize the incidence of hypotension although recent literature has not produced conclusive evidence to support this practice. Occasionally, the patient may faint or sustain a respiratory arrest either because of a high block or failing medullary perfusion due to hypotension. The blood pressure must be restored quickly to avoid a drastic reduction in placental perfusion. Use this hyperlink to get an idea about what medications are used to produce spinal anesthesia. The local anesthetics are usually mixed with dextrose solutions to make them heavier than CSF (hyperbaric) to prevent an exaggerated cephalad spread.

Intraspinal Opiates:

Ever since the discovery of endogenous opiates such as b-endorphin and opiate receptors in the spinal cord and the brain, investigators started using opioids in the epidural and subarachnoid spaces. Morphine, which has a low lipid solubility, can produce long lasting post operative analgesia when injected into the epidural or subarachnoid space. As in the case of the local anesthetic, the spinal dose is much smaller than the epidural dose. Opioid receptors are present throughout the nervous system. However, the ones that concern us the most are the ones in the substantia gelatinosa of the dorsal horn of the spinal cord. Several types of opioid receptors are described including m, k,s,d,e types. m receptor activation by exogenously administered narcotics produce intense analgesia, respiratory depression, miosis and bradycardia. Anesthesiologists combine a narcotic with a dilute solution of local anesthetic to produce excellent analgesia. The narcotic-local anesthetic mixture may be injected into the epidural space (10 ml 0.125% bupivacaine+ 100 ug fentanyl) or into the intrathecal space (1 ml of 0.25% bupivacaine+25 ug fentanyl) to produce good analgesia.

Route	Narcotic	Local anesthetic	Total volume	Purpose
Epidural	Fentanyl 100 ug 2 ml	Bupivacaine 0.125% -10 ml	12 ml	Labor analgesia
Spinal	Fentanyl 25 ug	0.25% -1 ml	1.5 ml	Labor analgesia
Epidural	5 mg			Postop analgesia after C.S.
Spinal	0.25 mg			Postop analgesia after C.S.

Combined-spinal Epidural Anesthesia (CSE)

Anesthesiologists sometimes inject a mixture of narcotic and local anesthetics (see Table above) into the spinal fluid. They would first insert the epidural needle into the epidural space and insert a longer spinal needle (120 mm) through the epidural needle (see the photo on the left) into the spinal fluid and inject the narcotic+local anesthetic mixture into the spinal needle. The spinal needle will then be withdrawn and an epidural catheter will be inserted into the epidural space. The narcotic+local anesthetic mixture usually works for 70-90 min and the epidural catheter will be activated to produce pain relief. It is a called combined spinal epidural technique because the patient receives both spinal and epidural blocks. The advantage of CSE mainly lies in its ability to produce immediate analgesia and the complications of CSE technique include headache and the possibility of fetal bradycardia.

General Anesthesia for Cesaran Section:

General anesthesia may become necessary if the patient refuses regional anesthesia or the regional block is difficult to perform or fails to work adequately. General anesthesia is also needed to deliver the baby if it is in severe distress or if there is umbilical cord prolapse. Problems with general anesthesia include 1) depressed neonate and 2) inability to intubate the maternal trachea, 3) risk of aspiration. A proper evaluation of the patient’s airway must be done before undertaking general endotracheal anesthesia. No mask anesthesia must be used in pregnant women for fear of aspiration.

Technique of General Anesthesia

1. Patient evaluation: NPO status; airway Mallampatti classification.

2. Proper patient positioning on the table

3. Sodium Citrate prophylaxis.

4. Apply cricoid pressure

5. Induction with pentothal 3-4 mg/Kg

6. Paralyze with sucinylcholine 1.5-2 mg/Kg

7. Apply cricoid Pressure and pass the endotracheal tube.

8. Maintain with N2O, Oxygen 50:50 with isoflurane 0.75 to 1 MAC concentration.

9. At the end of the procedure. Make sure that the patient satisfies all the extubation criteria.

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