Maternal Physiology

and the Anesthetized

Pregnant Patient

Kimberly Babiash, MD, MBA

Oct 7, 2015

Overview Neuraxial Anesthesia

Epidurals vs spinals

How they work

Physiologic alterations

Contraindications

Patient factors

Febrile patient

Pre-eclampsia

Anesthetic choices for Cesarean Sections

Neuraxial vs General Anesthesia

Physiologic alterations and why they matter

Gastric emptying

Airway

Hemodynamic control

Epidural and Spinals

Neuraxial Anatomy

Neuraxial Anatomy

Understanding Neuraxial

Anesthesia

Epidurals

Slow onset

Long duration

VOLUME dependent

Better hemodynamic control

Spinals

Rapid onset

Short duration

DOSE (mass) dependent

Poor hemodynamic control

Understanding Neuraxial

anesthesia

How do local anesthetics work?

Understanding Neuraxial

Anesthesia Differential Blockade

clinical phenomenon that nerve fibers with different

functions have different sensitivities to local anesthetic

blockade.

Length of each nerve in the thecal space

Depth of the nerve fiber

Distribution of Na+ and K+ channels on each nerve type

Understanding Neuraxial

Anesthesia

Understanding Neuraxial

Anesthesia

Differential Blockade

Sympathetics: most sensitive to local anesthetic

agents (2-4 levels beyond motor)

Pain/touch: moderately sensitive (2-3 levels beyond

motor)

Motor fibers: least sensitive

Dermatome goals Labor (T10 usually achieved with 10-15 ml)

Stage I = T10 – L1

Stage II = S2-S4

Cesarean Section

T2-S4 (higher for exteriorization of uterus)

Pain sensations from pelvic organs and visceral pain fibers

from other abdominal structures including the peritoneum

enter spinal cord at T10-L1; however, some pelvic nerves

accompany sympathetic fibers to reach the spinal cord as

far as T2

Traction on uterosacral ligaments and bladder require

anesthesia as low as S4

Achieved with dose of 12 mg bupivacaine

Why add neuraxial opioids?

Factors affecting block height

Dose

Site of injection

Baricity

Dextrose added for hyperbaric (sinks)

Position of patient

Extremes of height (minor)

Lumbosacral CSF volume (interindividual variability)

Explains variability in spinal with similar doses

Physiologic Disturbances with Neuraxial

Anesthesia

Cardiovascular disturbances take the cake

Hyotension and bradycardia (33% and 13%)

Primary Cause = Sympathectomy

SNS arises from the middle of the SC in the interomedial nucleus of the

lateral grey column beginning at T1 and extends to L2 (thoracolumbar

outflow)

Extends approx 2 dermatomes above the sensory level

Venous and arterial dilation

Venodilation predominates (75% of TBV)

Redistributes central blood volume to splanchnics and lower extremities

Vascular smooth muscle on the arterial side retains considerable tone

If normal cardiac output is maintained, PVR should only

decrease by 15% in the normovolemic patient

Physiologic Disturbances with Neuraxial Anesthesia

Modulation of vasomotor tone

A complex process that relies on much more than

just simple augmentation or attenuation of SNS

Simple thought: hypotension should lead to reflex

tachycardia and vasoconstriction….

But bradycardia is more commom….

Shift towards vagal predominance

Level of block? Cardioaccelerator fibers (t1-t4)

Bezold Jarisch Reflex (a cardioinhibitory reflex)

Mechano/chemosensitive receptors located in the ventricles

Involved in the restorative response to reduced cardiac filling

A sudden decrease in VR and PVR triggers bradycardia to

preserve cardiac filling

Physiologic Disturbances with

Neuraxial Anesthesia Respiratory Effects

Decrease in Vital Capacity (IRV+ VT +ERV) due to

decrease in ERV from paralysis of abdominal

muscles necessary for forced expiration

Nothing to do with diaphragm or phrenic nerve

Therefore, expiration >inspiration

Consideration for severe asthmatic

High Spinal and respiratory arrest

Unrelated to phrenic nerve or respiratory function

Hypoperfusion of the respiratory centers of the brainstem

Respiratory Volumes in Pregnancy

Contraindications

Absolute

Patient refusal

Obstructive intracranial hypertension

Infection at the site

Frank coagulopathy

Regional Anesthesia in the

infected febrile patient

Despite the apparent low risk of central nervous

system infection after regional anesthesia,

anesthesiologists have long considered sepsis to

be a relative contraindication to the

administration of spinal or epidural anesthesia.

Sepsis = SIRS + suspected or documented infection

There is no evidence to suggest that neuraxial

anesthesia is contraindicated in chorioamnionitis

alone

Recommendations:

Serious central neuraxial infections such as arachnoiditis, meningitis, and abscess

after spinal or epidural anesthesia are rare (Grade B).

The decision to perform a regional anesthetic technique must be made on an

individual basis considering the anesthetic alternatives, the benefits of regional

anesthesia, and the risk of CNS infection (which may theoretically occur in any

bacteremic patient) (Grade C).

Despite conflicting results, many experts suggest that, except in the most

extraordinary circumstances, central neuronal block should not be performed in

patients with untreated systemic infection (Grade C).

Available data suggest that patients with evidence of systemic infection may safely

undergo spinal anesthesia, provided appropriate antibiotic therapy is initiated before

dural puncture and the patient has shown a response to therapy, such as a decrease

in fever (placement of an indwelling epidural (or intrathecal) catheter in this group of patients remains controversial) (Grade A).

Anesthetic Choices for Cesarean

Delivery

Neuraxial vs General Anesthesia (GA)

Considerations

GA fastest induction to delivery time

GA lower APGAR scores

Neuraxial is preferred to GA in most cases

study done by Mancuso et al. (Spinal vs GA)

179 healthy elective

Umbilical cord artery pH, Apgar score and need for assisted

ventilation were evaluated and found spinal anesthesia superior

to general in fetal outcome.

Anesthetic Choices for Cesarean Delivery

The relative risk of fatality during GA has increased to more

than 8 times that for regional anesthesia

Failed intubation

incidence of failed intubation in OB patient is 1:300

whereas incidence in general population is 1:2,230 (8 fold

increased risk)

Increased difficulty (vascular engorgement, obesity, breast size,

preeclampsia)

Rapid time to desaturation

Increased oxygen consumption and decreased FRC

Pulmonary aspiration

*see next slide

Maternal awareness

Neonatal depression

Gastric Changes in Pregnancy

Decreased tone and motility progesterone

possibly due to decreased levels of motility

Conflicting info about delayed gastric emptying

Reduced tone of the gastroesophageal junction sphincter Increased intraabdominal pressure leads to acid reflux

Preeclampsia and Neuraxial

1950s evidence that preeclampsia actually attenuates spinal-anesthesia

hypotension

1990s clinical trials demonstrate safety of spinals in this population

3 prospective trials

Less severe and less frequent hypotension and smaller doses of

vasopressors

Early epidural placement in laboring preeclamptic parturients is ideal

Complications of GA

Hypertensive crisis (common with RSI)

Stroke (difficult to recognize under GA; conflict in management

between RSI and deep plane of anesthesia/stable induction to

maintain CBF)

Difficult airway management (pharyngeal and subglottic edema along

with traumatic laryngoscopy and further bleeding)

Preeclampsia and Neuraxial

Spinal compared to GA in severe preeclampsia

with nonreassuring FHT

Dyer et al:

70 parturients prospectively compared

Spinal greater mean neonatal umbilical artery base deficit (7.1 vs

4.7) and lower median umbilical artery pH (7.2 vs 7.23)

Spinal group had higher ephedrine use (fetal pH)

1 min APGAR significantly lower in GA (but at 5 min no

statistical difference)

No significant intergroup differences in other markers of

neonatal compromise

Need for resuscitation, APGAR <7, pH <7.2, need for PPV

Anesthetic Choices for Cesarean

Delivery GA

APGAR scores are lower at 1 and 5 min

Equivocal regarding differences in umbilical artery pH values

Reduces the time to skin incision

Greater maternal complications

Epidural

Increases time to skin incision

Reduces the quality of anesthesia compared to spinal

CSE vs epidural

No difference in frequency of hypotension or 1 minute APGAR

Better anesthesia

Faster time to skin incision

Failed Epidural, Now What?

Epidural anesthesia is less reliable that spinal

Concern with spinal after a failed epidural

High spinal from compression of intrathecal space

Especially when >20 ml in epidural space less than 30

minutes before spinal

Reduction in spinal dose of 20-30%

Anesthetic Medications

Volatile anesthetics

Dose dependent decrease in uterine smooth muscle contractility and

blood flow

Only modest at 0.5 MAC

Rapidly cross placenta but quickly exhaled in neonate

Propofol

GABA agonist

Potent myocardial depressant and inhibits sympathetic tone

Distributes quickly to the vessel rich placenta

Studies with barbiturates indicate optimal timing of fetal delivery 4-8

minutes post induction

However, in a poorly perfused placenta, time is of the essence

Pre-induction Fentanyl?

Peak effect 3-5 minutes

Rapid placental transfer

Though not associated with lower umbilical artery

pH or APGAR scores after 1mcg/kg on induction

Nitrous?

Peak effect lags the start of its administration by 50 sec

Uterine contractions typically peak 30 sec after they start

Reductions in pain scores seem similar to that of systemic opioids, which some

authors suggest have little effect on labor pain

Its use does not seem to appreciably affect the rates of maternal nausea or emesis

during labor

The direct respiratory depressant effect along with maternal hypocapnea may increases

the rate of maternal oxygen desaturation between contractions

It does NOT affect uterine contractility

The effects of the fetus exposed to nitrous in utero is unknown

With rising concern about the subtle long-term effects of perinatally administered

anesthetics, the role of nitrous certainly demands judiciuos scrutiny

Thank You!