PHYSIOLOGIC AND ANATOMIC CHANGES DURING PREGNANCY AND LABOR-ANESTHETIC IMPLICATIONS

Change Anesthetic Implications

Circulation  

Hyperdynamic: increased reliance on sympathetic nervous system

Increase in incidence and severity of hypotension after regional analgesia/anaesthesia

Capillary engorgement, increase in airway edema

Increased incidence of difficult airway

Aortocaval compression More profound hypotension with parturient in the supine position

Metabolism and Respiration  

Increase in O2 requirement and CO2 production

Greater risk of desaturation after induction of general anesthesia

Decrease in FRC Greater risk of desaturation after induction of general anesthesia

Gastrointestinal System  

Reduced oesophageal---stomach barrier pressure

Increased risk of aspiration pneumonitis

Endocrine  

Increased progesterone levels Increased pain threshold

Nervous System  

Increase in ß-endorphin concentration Increased tolerance to pain

Increased susceptibility to local anesthetics Decrease in local anesthetic dose requirements

Anatomic changes in the spinal column Decrease in local anesthetic dose requirements

Increased susceptibility to CNS depressants Decrease in dose requirements for general anesthetics and adjuvants

Myometrial hypoxia

Stretching of the cervix

Pressure on the nerve ganglia adjacent to the cervix and vagina

Traction on the tubes, ovaries and peritoneum

Traction and stretching of the supporting ligaments

Pressure on the urethra, bladder and rectum

Distention of the muscles of the pelvic floor and perineum

MOTHER:

Relief of pain

By relieving pain the changes of ventilation, circulation, hormonal function that ordinarily accompany pain can be controlled

Freedom from fear

Safe and comfortable delivery

INFANT:

To be given a favorable physiologic milieu for delivery

To use techniques not associated with fetal depression or long term poor outcome

OBSTETRICIAN:

Reduction of pressure from patient & relatives to do something prematurely

Optimum conditions at delivery

The method must ensure that:

The health of the mother is not endangered

The newborn should not be depressed at delivery. Drugs cross the placenta

The technique effectively controls pain

The efficiency of uterine contractions is not decreased

The ability of the patient to cooperate intelligently with the medical and nursing staff is maintained

There is no need for operative interference because of anesthesia

The method is relatively simple to use

“to prepare a woman for labor and delivery so that she approaches the end of her pregnancy with knowledge, understanding and confidence rather than apprehension and fear”

Aims of perinatal training:

Counteract apprehension of young women caused by exaggerated tales of horror

The patient is given an opportunity to gain confidence

Exercises are taught which strengthen certain muscles and relaxes others

The patient is trained in breath control

The patient told about labor pains and analgesic options

Patient to choose the method most suitable for her away from others experiences

Emphasis on the fact that most labors are normal

Simple analgesia:

Aspirin

Paracetamol

Non-steroidal Anti-Inflammatory Drugs

Early in pregnancy

Third trimester

Opioid analgesics:

Codeine

Pethidine/morphine

Non-pharmacological:

Breathing exercise

TENS machine

Inhalation anesthesia: “Nitrous Oxide”

Insoluble in blood, rapid induction and recovery

Rapidly transported to maternal tissue, placenta and fetus

Effective when taken during contractions

Nontoxic

NO 50:50 O2, decreasing the chance of maternal hypoxia

Prolonged use may lead to neonatal depression

No/little effect on labor

Systemic medications “Narcotics”… alleviate pain. If given in large doses in the latent phase < contractions & cx dilatation. When labor established, relief of pain and anxiety make the uterine contractions more efficient

S.Effect: resp. depression, ortho hypotenb, <gastric motility, nausea & vomitting. Affects neonatal neurobehavior

Morphine: 0.1mg/kg 3-4 hrs peak effect1-2 hrs (I.m.)/20 min (I.v.). Duration 4-6 hrs. if given< 3 hrs before delivery…. Fetus affected

Demerol(meperidine, pethidine):

synthetic narcotic with atropine-like action.

1mg/kg 3-4 hrs peak effect 40-50 min(I.m.)/5-10 min (I.v.). Duration 3-4 hrs. the greatest effect on the fetus reached within 1.5 hrs after I.m.

Narcotic effects on the newborn are best antagonised with Naloxone 5-10 micg/kg

Paracervical block: XXXXXXXXXX

Lumber Epidural block:

Injecting of Marcaine 0.25-0.5% or others in a continuous infusion

Advantages:

Almost pain free labor

Can be kept as long as desired

Level of analgesia can be controlled

Mother is alert and cooperative. Retains ability to bear down. If forceps req.

Can be used when converting to C/S

Minimal effect on the fetus!!!!!!!!!!!!!!!!!!!

Disadvantages:

Might mask the strength of contractions esp. with syntocinone

10% significant hypotension if lie supine. Epidural anesthesia>>20mm Hg drop in syst or diast in 30% of pnt. Due to:

Sympathetic vasomotor blockade >>> 20% drop in arteriolar resistance

Increased venous capacitation and pooling>>>decreased in venous return and COP

Uterine pressure on aorta and vena cava

To prevent hypotension:

Infuse 1 L RL or Saline

Wedge under right hip to displace the uterus to the left and reduce vascular pressure

Use Ephedrine as vasopressor if required

Intravascular injection: convulsion and hypotension

If dura is punctured>> headache. Subarachnoid >>>massive motor block, hypotension and respiratory distress

Disadvantages:

Fetal heart rate patterns may be affected

Infection at site of injection,,rare

Backache

Neurological side effect debatable

Might affect uterine contractions needing oxytocin

If large dose, would paralyze pelvic floor>> failure to rotate and mother not bearing down due to lack of sensation>>> instrumental delivery

Contraindications:

Drug allergy

Skin infection

Coagulopathy(severe PET,active Hg)

Severe supine hypotension

Certain cardiopulmonary diseases

Inhalation Anesthesia:

Halothane, Enflorane, Isoflorane

Intravenous Anesthesia:

Thiopentone, Ketamine

Not routinely used in every days work. Mainly in private sector. Consider side effect of general anesthesia and need to be administered by anesthetist.

Local Anesthesia:

Advantages

Disadvantages

1. Direct infiltration >>>>>>>

2. Pudendal Nerve Block

1. Topped up ongoing epidural

2. Spinal block anesthesia

3. General anesthesia

>90% of C/s in UK

  Epidural Spinal

Onset 10---30 min 5---10 min

Duration & effect Continuous Single shot. effect lasts for 2 hours

Success rate Higher incidence of patchy, one-sided blocks  

Block quality Less-dense sensory block More dense sensory block

  Less motor block More motor block

Hypotension Same incidence, slower onset Same incidence, more rapid onset

Risk of PDPH Approximately 1% Approximately 1%

Risk of systemic local anesthetic toxicity

Inadvertent intravenous injection may cause systemic toxicity

Dose too small to cause systemic toxicity if inadvertently injected intravascular

Risk of total spinal Possible with inadvertent subarachnoid injection or “overdose” epidural injection

Less likely because of small drug dose

Post---cesarean delivery analgesia

Continuous or single-shot Single-shot only

Effects on the fetus

Greater drug exposure Minimal drug exposure

Block Clinical Use Advantage Disadvantages/Side Effects/Complications/

Spinal (saddle block)

Instrumental vaginal delivery anesthesia

Rapid onset analgesia with perineal motor block

Single shot, not continuous

Caudal block Labor and delivery analgesia Another access to epidural space

Technically more difficult than lumbar epidural

    Useful for patients with lumbar spine fusion

Requires large volume of local anesthetic to provide labor analgesia to T10 level

Paracervical block

Early---mid 1st stage labor analgesia

No motor block Not continuous

      Risk of fetal bradycardia

Lumbar sympathetic block

Early---mid 1st stage labor analgesia

No motor block. Speeds labor

Not continuous. Requires bilateral injections

    Useful for patients with lumbar spine fusion

Technically more difficult to learn

Pudendal block

2nd stage analgesia; instrumental vaginal delivery anesthesia

Performed by obstetrician before delivery

Not continuous

      Complications rare

Perineal infiltration

Episiotomy or repair anesthesia Technically simple No motor relaxation

    Performed by the obstetrician as needed

Complications rare

Inhalation Anesthesia:

Halothane, Enflorane, Isoflorane

Intravenous Anesthesia:

Thiopentone, Ketamine

Aspiration of vomitus during anesthesia. Commonest morbidity

Methods to prevent its occurrence

Failed intubation

Mendelson Syndrome

Can operate in relaxed state in difficult cases. If >3 min before delivering baby >>drowsy