Obstetric Anesthesia Considerations

Two patients i.e. mother and fetus

Maternal physiologic changes:

1. Cardiac output increases by 50% by 2nd trimester w/ increased HR & SV

2. Blood volume increases by 45% --> dilutional anemia

3. Hypercoagulable state --> increases risks of periop thromboembolic events

4. Aorto-caval compression w/ supine position --> oblique position for left uterine displacement --> prevents decrease in uterine blood flow and maternal hypotension

5. O2 consumption increases by 60%

6. Minute ventilation increases by 45% by increasing TV

7. FRC decreases by 20% (30% in supine) --> decreased safe duration of apnea

8. Delayed gastric emptying & increased intraabdominal pressure --> GERD --> aspiration prophylaxis e.g. 30ml NaCitrate, 10mg metoclopramide IV, 50mg ranitidine IV; RSI w/ cricoid pressure in case of GA

9. Reduced MAC by 30%

10. Reduced LA requirement by 25%

11. Renal blood flow increased by 75% --> GFR increased by 50%

12. Sizeable breasts, airway edema --> potential difficult airway

Semi-urgent to urgent

Risk factors:

1. Family hx of PIH

2. Chronic HTN?

3. Previous hx of preeclampsia?

4. Co-existing vascular / endothelial disease (e.g. chronic renal diseases, lupus, protein S deficiency etc.)

5. Nulliparity, > 40yo, multiple gestations, DM, obesity?

Multisystem involvement:

1. Airway w/ internal and external edema --> potential difficult airway

2. Pulmonary edema - uncommon; reduced colloid oncotic pressure, endothelial dysfunction and diastolic dysfunction predisposed to development of pulmonary edema

3. CNS involvements e.g. cerebral edema, ICH, increased ICP, HA, visual disturbance, altered mentation, seizure

4. Hypertension, labile BP --> Art line prior to GA induction; NTG, phenylephrine readily available during induction; hypertensive crisis, LV dysfunction / cardiomyopathy / CHF

5. Reduced intravascular volume --> cautious with neuraxial blockade; volume expansion prior to spinal/epidural; phenylephrine readily available

6. Coagulopathy --> early insertion of epidural catheter before any coagulopathy, epidural is also partially therapeutic

7. Hemolysis, Elevated Liver enzymes, Low Platelets

   Platelets < 50 neuraxial technique contraindicated

   50-70: risk vs benefit judgement (consider trend, other competing factors e.g. airway, cardiac / neuro status)

   >70: likely safe

8. Renal impairment due to glomerular endotheliosis

Uteroplacental unit; increased chance of abruptio placenta

Risks to baby: IUGR, increased risk of fetal asphyxia, IUD, prematurity

Goals:

1. BP control (sBP <160 mmHg, dBP <110 mmHg)

2. Optimize volume status

3. Maintain adequate uteroplacental perfusion

4. Minimize end-organ issues e.g. seizures, ICH, cardiac ischemia

5. Optimize labour analgesia to minimize exacerbation of BP due to pain

Pharmacological intervention:

1. MgSO4 4g IV over 10-15min, then 1-3g per hour for maintenance; constant monitoring of cardiopulmonary status, knees DTR, urine output; Ca gluconate 1g IV over 3-5min in case of overdose

2. MgSO4 also for neuroprotection in premature babies

3. Hyralazine 5-10mg IV bolus, then 5mg per hour

4. Labetolol 20mg IV repeated q15min prn, 20mg/hr maintenance