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PHYSIOLOGIC CHANGES OF PREGNANCY

...

What is the most important anesthetic goal during the care of a pregnant patient?

Avoidance of decreased uteroplacental blood flow.

What is the effect of pregnancy on MAC?

Pregnancy decreases MAC by about 40%.

What is the effect of pregnancy on spinal and epidural dose requirements?

Local anesthetic dose requirements are reduced because of 1) decreased volume of epidural and spinal space, and 2) increased sensitivity of nerves to block.

Pregnant patients demonstrate more rapid induction, emergence, and change in the depth of anesthesia because of...

INCREASED MV:FRC RATIO

What is the normal PaCO2 in pregnancy?

32 mmHg

What is the normal HCO3- in pregnancy?

21 mEq/L

By how much is minute ventilation changed at term pregnancy?

50% increase - due to increased tidal volume

Which two factors predispose the pregnant patient to the development of rapid hypoxemia upon development of apnea?

1) Increased VO2
2) Decreased FRC

What is the common relationship between Closing Capacity and FRC in pregnant patients when supine? How often?

Closing Capacity exceeds FRC in 50% of supine pregnant patients.

What is the effect of progesterone on the airway?

Progesterone leads to decreased airway resistance.

What is the effect of pregnancy on vital capacity?

Vital capacity is unchanged in pregnancy.

What accounts for the change in FRC in pregnancy?

Decreased RV accounts for the decrease in FRC seen in pregnancy.

What happens to PaO2 in pregnancy? To what can this change be attributed?

PaO2 increases in pregnancy. This can be accounted for by the decrease in PaCO2.

When is cardiac output greatest in the setting of pregnancy?

Immediately postpartum (60 - 80% increase over the nonpregnant state)

What is the change in cardiac output observed during pregnancy?

Cardiac output increases by 40% at term.

What happens to CVP and PA pressures in pregnancy?

They are likely to remain unchanged.

What change is seen in cardiac output during labor?

Labor increases CO by 45%.

When, after delivery, does cardiac output return to normal?

2 weeks postpartum.

What happens to plasma volume and RBC mass during pregnancy?

Plasma volume increases 40%
Red cell volume increases 20%

What is considered true anemia during pregnancy?

Hct < 33%
OR
Hb < 11 g/dL

What is typical blood loss during vaginal delivery?

500 mL

What is typical blood loss during C-section?

800 - 1000 mL

What is normal SVR?

Normal SVR = 800 - 1200 dynes x sec / cm^5

What is normal PVR?

Normal PVR < 250 dynes x sec / cm^5

What is normal CVP?

About 5 mmHg

What is normal stroke volume?

60 - 100 mL/beat

What is normal mixed venous O2 content?

15 mL/dL

What happens to SVR during pregnancy?

It decreases.

What happens to BP during pregnancy?

First, BP should never rise above non-pregnant levels at any time. BP usually decreases during the 2nd trimester, but tends to return to normal during the 3re trimester.

What are the two principal GI changes associated with pregnancy?

Cephalad displacement of the stomach
Decreased LES tone

When do pregnant women exhibit delayed gastric emptying?

During labor

What is the syndrome associated with chemical pneumonitis secondary to aspiration of gastric acid?

Mendelson's Syndrome

What happens to renal function during pregnancy?

Renal blood flow and GFR are both increased by about 50% during pregnancy.

What happens to serum creatinine levels during pregnancy?

Serum creatinine is reduced

What happens to pseudocholinesterase levels in pregnancy? What about succinylcholine dosing?

They are reduced. Succinylcholine effects, however, are not prolonged.

What happens to serum albumin during pregnancy?

It is decreased, resulting in higher free blood levels of all drugs which rely on protein binding.

What happens to PT and PTT during pregnancy?

They are unchanged.

UTEROPLACENTAL BLOOD FLOW

...

What is normal uterine blood flow?

500 - 700 mL/min

By how much must uterine blood flow decrease before fetal distress is detected?

50%

What feature of the uteroplacental circulation is especially important in the management of uteroplacental blood flow?

It is NOT autoregulated.

What is the umbilical artery/vein structure?

Two umbilical arteries. One umbilical vein, which carries oxygenated blood to the fetus.

Where does uterine blood supply come from?

Two uterine arteries.

What is the PO2 of umbilical venous blood? What is the SO2?

30 mmHg; 70%

What is the PCO2 of umbilical venous blood?

40 mmHg

What is the pH of umbilical venous blood?

7.35

What is the PO2 of umbilical arterial blood (blood which has traveled through the fetus and returns to the placenta)? What is the SO2?

20 mmHg; 40%

What is the PCO2 of umbilical arterial blood?

50 mmHg

What is the pH of umbilical arterial blood?

7.28

What is the impact of elevated uterine venous pressure?

Increased uterine venous pressure reduces uterine perfusion pressure.

Under what circumstances does uterine venous pressure rise?

IVC compression
Placental abruption

What is the umbilical ABG at 60 minutes?

7.30-7.35/30/60

What is the umbilical ABG at 24 hours?

7.30-7.35/30/70

What is the effect of epinephrine on uterine blood flow?

Epinephrine decreases uterine blood flow.

What are the determinants of transplacental diffusion of drugs?

Concentration gradient
Protein binding
Molecular weight
Lipid solubility
Ionization
Fetal pH

What are five important drugs which DO NOT CROSS the placenta?

Heparin
Insulin
Glycopyrrolate
Nondepolarizing NMBs
Succinylcholine

What is the mnemonic for recalling the drugs which do not cross the placenta?

"He Is Going Nowhere Soon."

What is the percent protein binding of lidocaine?

50%

What is the percent protein binding of bupivacaine?

95%

Why do nondepolarizers not cross the placenta?

They are too large

Why does succinylcholine not cross the placenta?

It is too highly ionized

TESTED POINT: What is the beneficial effect of fetal-to-maternal CO2 transfer?

It enhances maternal-to-fetal oxygen transfer.

MEDICATIONS DURING LABOR

...

Which four anesthetic drugs are "probably safe" during labor?

Small doses of:
Benzodiazepines
Opioids
Thiopental (4 mg/kg)
Ketamine

Why is thiopental usually safe in labor?

Thiopental is usually safe because the fetus is removed before peak serum concentrations reaches the placenta.

What are the stages of labor?

1) Cervical dilation
2) Delivery of the fetus
3) Delivery of the placenta

Which nerves transmit pain in the first stage of labor?

T10 - L1

Which nerves transmit pain in the second stage of labor?

S2 - 4 (The Pudendal Nerve)

Which block can relieve the pain of the first stage of labor but not the second stage?

Paracervical Block

Which block can relieve the pain of the second stage of labor but not the first stage?

Pudendal Nerve Block

What is a major concern related to paracervical block?

10 - 40% incidence of fetal bradycardia.

Which two blocks are not effective during the second stage of labor?

Paracervical block
Lumbar sympathetic block

Which crosses the placenta more effectively, bupivacaine or lidocaine?

Lidocaine; it is less protein-bound than bupivacaine.

What is a risk of epidural test dose in the laboring patient?

In the case of accidental intravenous injection, the epinephrine WILL decrease uterine blood flow. However, the benefit of the test dose is believed to outweigh the risk.

What are advantages of GA in the parturient?

Controlled airway
Absence of hypotension seen with regional

What are disadvantages of GA in the parturient?

Risk of aspiration
Risk of failed intubation

What dose of volatile agent can be administered without a decrease in uterine blood flow or depression of the fetus?

2/3 MAC

What commonly-used drugs for epidural analgesia are NOT used for spinal?

Chloroprocaine
Lidocaine

What is the risk associated with chloroprocaine spinal? And lidocaine spinal?

ADHESIVE ARACHNOIDITIS; TNS

What is the advantage of caudal anesthesia in the parturient?

Excellent perineal anesthesia

What are the disadvantages of caudal anesthesia in the parturient?

Accidental subarachnoid or IV injection, infection, injection into the fetus, and inability to raise the level of the block should C-section become necessary.

What is an appropriate medication and dose for caudal anesthesia in the parturient?

10 - 12 mL of 0.25% bupivacaine

Which condition should be first on the differential diagnosis of hypotension after the first trimester of pregnancy?

Aortocaval compression

What is the appropriate course for the treatment of hypotension in a pregnant patient?

1) LUD
2) Fluids
3) Phenylephrine or Ephedrine

What are the two most common complications of regional for parturients?

Accidental subarachnoid or IV injection.

What steps should be taken prior to induction of anesthesia in an emergency C-section?

1) Sodium citrate
2) Placement of monitors
3) Eval fluid status
4) FINALLY... recheck fetus

What are five indications for C-section with GENERAL ANESTHESIA?

Fetal distress
Maternal hemorrhage
Cephalopelvic disproportion
Uterine dystocia
Breech presentation

What is the most important factor determining fetal outcome following C-section?

Time from uterine incision to delivery of the fetus

ABNORMAL PRESENTATION

...

What are the different types of breech presentation?

Frank Breech
Complete Breech
Incomplete Breech

What is a Frank Breech presentation?

Frank Breech means the buttocks is presenting. Feet are against the face.

What is a Complete Breech presentation?

Buttocks and feet both present.

What is an Incomplete Breech presentation?

One or both feet are presenting.

What are the fetal risks associated with breech delivery? Maternal?

Fetal risks include asphyxia due to cord compression and IVH due to trauma. Maternal risks include hemorrhage, infection, and retained placenta.

MULTIPLE GESTATIONS

...

What is the maternal mortality associated with multiple gestations?

2-3 fold elevated

What are the specific risks leading to elevated maternal mortality in association with multiple gestations?

Anemia
Hemorrhage
Toxemia
Prolonged labor

Multiple gestation is associated with increased incidence of...

supine hypotension syndrome.

What are the risks associated with twin-twin transfusion?

Twin A: CHF and polycythemia
Twin B: Anemia and hypovolemia

What are the umbilical cord problems associated with multiple gestation?

Occlusion
Intertwining

What are the risks to the second and subsequent neonates following birth of the first baby?

Second twin depression can occur, due to prolonged contractions and early placental separation.

How much longer is the labor in multiple gestations?

It is not longer; labor is shorter with multiple gestations.

Which precautions should be undertaken prior to delivery of multiple gestations?

1) Blood cross-matched
2) 2 IVs
3) MDs available for resuscitation

PREECLAMPSIA AND ECLAMPSIA

...

What are the cardinal manifestations of preeclampsia?

HTN
Proteinuria
Edema

In what population is preeclampsia most common?

Primigravidas

What is the window for onset of preeclampsia?

20 weeks gestation - 6 weeks postpartum

What is the diagnosis of preeclampsia?

Two BPs of 140/90 or greater in the presence of proteinuria 2g/day

What is the diagnosis of severe preeclampsia?

Two BPs of 160/110 or greater in the presence of proteinuria.

What do we call preeclampsia without proteinuria?

Pregnancy-induced HTN

A woman with preeclampsia and abdominal pain should be suspected of having...

HELLP Syndrome. Abdominal pain could be caused by hepatic inflammation.

What are complications of severe preeclampsia?

CHF
Pulmonary edema
Renal failure
DIC
Eclampsia

Incidence of preeclampsia is greatest among mothers who undergo rapid uterine enlargement, such as...

Diabetes
Multiple gestations
Polyhydramnios
(also common in past Preeclampsia)

How should magnesium administration begin?

Deliver a 4 g IV loading dose as a 20% solution.

Magnesium effects are VASST...

Vasodilation
Anticonvulsant
Sedative
Skeletal muscle relaxant
Tocolytic

What are the two effects of magnesium on the skeletal muscle?

1) Decreases ACh release
2) Decreases endplate sensitivity to ACh

What is the therapeutic level of magnesium?

5 mEq/L

What is the range of magnesium over which EKG changes are seen?

5 - 10 mEq/L

What is the range of magnesium over which loss of DTRs occurs?

> 10 mEq/L

What is the concentration of magnesium at which respiratory paralysis is seen?

15 mEq/L

What is the concentration of magnesium at which cardiac arrest is seen?

20 mEq/L

What happens upon succinylcholine dosing in a patient with therapeutic magnesium level?

Fasciculations are absent.
Sux dose should be 50% of normal dose.

Why is captopril avoided in the treatment of HTN in pregnant women?

Captopril has been associated with fetal death, IUGR, and renal problems.

What constitutes ideal anesthetic management of the patient with preeclampsia?

Epidural is a great choice, unless emergent or extremely severe.

What should be done prior to placement of an epidural in a patient with severe preeclampsia?

Check coags
Give aspiration prophylaxis

What constitutes ideal anesthetic management of the patient with eclampsia?

Secure the airway with thiopental and succinylcholine, then deliver the child.

What are 6 causes of maternal hemorrhage?

Placental abruption
Placenta previa
Placenta accreta
Placental retention
Uterine atony
Uterine rupture

What is the incidence of placenta previa? What is a known risk factor?

0.1 - 1%. Increasing age is a risk factor.

What is the typical presentation of placenta previa? What should be the first step in management?

Painless vaginal bleeding. The first step is to obtain an emergency sonogram.

What is a good choice for anesthetic in a parturient with severe and active bleeding?

GA with ketamine.

Which cause of hemorrhage in the parturient is the leading cause of maternal mortality worldwide?

Uterine atony.

What is the approach to non-obstetric surgery for pregnant patients, based on urgency?

Elective surgery should be postponed until delivery, AFTER physiology returns to normal. Urgent surgery should be postponed until after the first trimester.

Why is the first trimester the riskiest time to undergo non-obstetric surgery? (two reasons)

1) Organogenesis
2) Highest risk of miscarriage

Which monitors and preparations are necessary for non-obstetric surgery in the parturient?

FHR monitoring after 20 weeks
Uterine tocometry
LUD after first trimester

What is the best approach to anesthetic medications for pregnant patients undergoing non-obstetric surgery?

Opioids are great because they can reduce the need for volatile anesthetics, thereby attenuating BP reduction. Also, opioids are very useful for the pregnant patient with cardiac disease. BE SURE TO INFORM THE PEDIATRICIAN OF OPIOID DOSING IF DELIVERY IS TO BE PERFORMED.

Which drugs should be avoided in pregnant patients undergoing non-obstetric surgery?

Midazolam and diazepam

Is nitrous oxide flammable?

No, but it supports combustion.

What is the MAC of nitrous oxide?

104%

What is diffusion hypoxia?

When delivery of N2O is stopped, the high concentration in blood leads to rapid transfer to the alveoli, potentially diluting the O2 content of the alveolus. For this reason, delivery of a high concentration of O2 is indicated upon cessation of N2O delivery.

Why is it important to avoid hyperventilation in the pregnant patient during anesthesia?

1) Left-shift of O2-Hb curve
2) Umbilical arterial constriction
3) May reduce venous return and, thereby, maternal CO.

What are the keys to anesthetic management (not including drugs) of the pregnant patient for non-obstetric surgery?

Avoid hypoxemia
Avoid hypotension
Avoid hyperventilation
Strongly consider regional
Consider monitoring of PaCO2 rather than PETCO2
Have a plan of action if there is a persistent nonreassuring FHT.

Which is better for the prevention of maternal hypotension: ephedrine or phenylephrine? Why?

Phenylephrine may be preferable because ephedrine is associated with lower umbilical arterial pH.

What about NMBs and reversal agents?

Just be sure to administer reversal slowly. Glycopyrrolate does not cross the placenta.

What is the published risk of preterm labor related to non-obstetric surgery? When is it lowest?

The risk appears to be around 22%, with lowest risk in the second trimester.

Is pregnancy a contraindication to ECT?

NO.

When does FHR variability develop?

Between weeks 25 - 27 of gestation. This is a reassuring sign of fetal well-being.

Which type of regional is not recommended for non-obstetric surgery in the pregnant patient?

Paracervical block is not recommended. Bear in mind that most abdominal surgeries during pregnancy will require GA with definitive airway control.

What is the latest consensus related to the use of nitrous oxide in pregnant patients?

Scientific evidence does NOT support avoidance of nitrous oxide during pregnancy, particularly after the 6th week of gestation. A cautious approach would restriction N2O to concentrations of 50% or less, and limit use in prolonged anesthetics.

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