OBGYN - Maternal-Fetal Physiology
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- | *Given this respiratory alkalosis and metabolic | + | *Given this respiratory alkalosis and metabolic compensation, we expect: |
**the pCO2 to be low: 26-32 mmHg (compared to non-pregnancy 38-45) | **the pCO2 to be low: 26-32 mmHg (compared to non-pregnancy 38-45) | ||
**the pH to be high (alkaline): 7.40-7.46 (compared to non-pregnancy 7.38-7.42) | **the pH to be high (alkaline): 7.40-7.46 (compared to non-pregnancy 7.38-7.42) |
Revision as of 14:38, 16 December 2011
Maternal-Fetal Physiology
Objectives
- List pertinent physiologic changes in various maternal systems
- Recognize that signs and symptoms in a pregnant patient are often more difficult to interpret
- Non-pregnant lab values and measurements are frequently abnormal in the pregnant state
Case: First Trimester
- 21 y/o G1P0 at 8w0d by L=6 week US at the WVC (women's visit center at Wishard) presents for her OB registration appointment.
- She denies any LOF (leaking of fluid), VB (vaginal bleeding).
- Pt with C/C of cramping and dark “poop”, nausea and vomiting with heart burn.
- PMH: Denied
- PSocialH: Denied
- OB: G1
- Gyn: Denied
- Social Hx: Denied x 3
- Allergies: NKDA
- Medications: PNV (Prenatal Vitamin)
- VS: BP: 110/80 P: 80 T: 98.6 Weight: 65Kg Height: 5’1
- UA: WNL
- PE: WNL
GI Issues in the First Trimester
- Constipation:
- Iron (ferrous-sulfate) in the pre-natal vitamins slows GI motility and causes constipation.
- Progesterone: a smooth muscle relaxant; causes decreased bowel movement which can lead to increased time for water reabsorption and thus constipation
- Tx: give colace
- Could be as bad a month!
- There is fear of miscarriage throughout pregnancy so anything seeming abnormal (like not pooping) starts a fear of miscarriage.
- Nausea and Vomiting:
- beta-HCG causes n/v
- beta-HCG peaks at 10-12 weeks.
- {rogesterone: a smooth muscle relaxant; causes decreased bowel movement which can lead to n / v.
- beta-HCG causes n/v
- Weight gain:
- can occur early or late, ideal is a progression.
- vomitting can lead to weight loss over the first weeks, but mom and baby will be ok
- Normal Weight for Height: about 20 lbs
- Underweight Women: about 30 lbs
- Overweight Women: about 16 lbs
- Only an addition 300 calories / day are necessary to feed a fetus.
- 500 cal for twins
Case: Second Trimester
- 21 y/o is now 20 weeks pregnant
- Presents to triage at Wishard with horrible heartburn after eating a deep fried twinkie at the state fair.
- She threw-up so much in early pregnancy that she is making up for it now.
- She has gained 25 lbs (from 8 weeks to second trimester).
- That is a lot of weight gain.
- She denied any LOF (leaking of fluid), denied VB (vaginal bleeding) or cramping.
- She stated that she had some of the chalkie things and it didn’t make it better this time.
- Tums.
- She was given a GI cocktail in triage and she was still miserable.
- Differential Diagnosis:
- Constipation
- Small bowel obstruction
- Ask when they last deficated.
- Gallstones
- Increased estrogen and progesterone
- Slow emptying of the stomach
- Often already at high risk (four "F"s: fat, forty, fertile, female)
- Can look really, really, bad.
- Pancreatitis
- Appendicitis
- Appendix moves from right lower quadrant to midquadrant
- Can get tucked up behind the uterus
- Constipation
First / Second Trimester GI Presentations
- High levels of estrogen / progesterone in pregnancy lend themselves to gallstone formation
- Especially when woman gets pregnant a second time without having gallstones from a previous pregnancy removed.
- Recall that progesterone is a smooth muscle relaxant.
- Recall that progesterone is at high levels throughout pregnancy to maintain the uterine environment.
- Progesterone is really rampinig up during the first trimester.
- High circulating levels of progesterone can lead to:
- Increased GERD from relaxation of GE sphincter
- Constipation from delayed transit through large bowel
- Increased N/V from delayed gastric emptying
- Stomach can still be full of content 8 hours later.
- “Full stomach” sensation
- Tx is often to keep pt from eating for 8 to 12 hours and then to switch to a little bit, throughout the day type consumption pattern.
Case: Second / Third Trimester
- 21 y/o G1P0 at 28w0d
- Presents to triage at St. Francis Beech Grove.
- Patient has been brought in by her family “Doc I feel horrible. I am dizzy. I was sitting at the high-school football game, I stood up to cheer, my heart started racing and I felt like I was going to pass-out! My ankles are swollen and I feel horrible."
- Denies LOF (leaking of fluid), VB (vaginal bleeding), CTXs (contractions).
- Endorses +FM (fetal movement).
- ED doctor noticed that she is breathing deeply.
- Pertinent Vitals:
- BP: 100/60
- P: 90
- T: 98.6
- RR: 16
- O2: 96% RA
- Weight: 80kgs
- Labs ordered:
- CBC w/platelets
- CXR (chest xray)
- ABG (arterial blood gas)
- UA (urinary analytes)
- CMP (complete metabolic panel)
- Coags (coagulation studies)
- PE:
- Lungs: CTA B
- CV: Systolic murmur
- Abd: Gravid, NT
- Extrem: +1 pitting edema
- Lab results:
- WBC: 10,000 (slightly elevated)
- Hgb / HCT: 10.5/31.7% (decrease)
- Platelets: 200,000 (slight decrease)
- Fibrinogen: 600 (increased)
- Bun / Cr: 3 / 0.6 (decreased)
- pH: 7.44
- pCO2: 30
- BiCarb: 21
- paO2: 103
Physiological Changes in Pregnancy
Blood pressure in pregnancy
- Pt blood pressure: 100/60
- Both systolic and diastolic BP decrease in 2nd trimester
- Systalic decreases 5-10 mmHg
- Diastolic decreases 10-15 mmHg
- We even take severe hypertensive pts off their meds.
- Normal
- Monitor from baseline!
- Athletes will have low baseline BP and therefore it will be a challenge to keep their BP up during pregnancy.
Pulse in pregnancy
- Pt's pulse: 90
- Increases 20 beats (peak at 32 weeks, middle of 3rd trimester)
- Normal
Cardiac output in pregnanc
- Recall that cardiac output (CO) is stroke volume (SV) multiplied by the heart rate (HR).
- CO increases at 8 weeks and peaks at 20 weeks.
- CO increases by 30-50%
- CO rises to 4.5-6.0 L / min
How do we know her stroke volume?
- Systolic ejection murmur is common (90% of women).
- S3 gallop is common (90% of women).
- Peripheral vascular resistance falls during pregnancy, too.
- Hence the swelling.
- Wear flipflops, wear compression socks
- Especially medical residents.
Respiratory rate in pregnancy
- Recall your standard respiratory volumes (see image below).
- Note that O2 demand (consumption) is increased by 15-20% in pregnancy.
- These volumes are affected by the displacement of the diaphragm rostrally.
- Note that respiratory rate should remain unchanged in normal pregnancy.
- This is possible because the tidal volume increases (because the minute ventilation increases).
- Functional residual capacity decreases by 20% because of displacement the uterus.
- So dyspnea is normal with pregnancy.
Volume (mL) | Definition | Non-pregnant | Pregnant | Change |
---|---|---|---|---|
Total lung capacity | Vital capacity + residual volume | 4200 | 4000 | -4% |
Vital Capacity | Total lung capacity - residual volume | 3200 | 3200 | 0% |
Inspiratory Capacity | Vital capacity - expiratory reserve volume | 2500 | 2650 | +6% |
Tidal volume | Volume moved in and out with each normal breath | 450 | 550 | +33% |
Expiratory Reserve Volume | Vital capacity - inspiratory capacity | 700 | 550 | -20% |
Inspiratory Reserve Volume | Inspiratory capacity - total volume | 2050 | 2050 | 0% |
Residual volume | Total volume - vital capacity | 1000 | 800 | -20% |
Functional Residual Capacity | Residual volume + expiratory reserve volume | 1700 | 1350 | -20% |
Arterial blood gas (ABG) in pregnancy
- NB: ABG should never be "normal" in pregnancy!
- Should have a respiratory alkalosis from blowing off their CO2 at higher rates.
- Should have a metabolic acidosis (compensatory).
- Many pregnant women will hyperventalate.
- Give them a bag in which to breath.
- Very hard to settle them down so important to give the bag quickly and pt will often become syncopous.
- Pt's ABG was:
- pH: 7.44 (
- pCO2: 30
- BiCarb: 21
- paO2: 103
- Recall that O2 consumption increases in pregnancy.
- 15-20%
- Recall that minute ventilation (the amount of air breathed in over the course of one minute) is increased relatively more than the O2 consumption.
- This means that more air is breathed in and out than is necessary for the rate of oxygen consumption.
- This is the definition of hyperventilation!
- Recall the chemical equation: CO2 + H20 <=> H2CO3 <=> HCO3 + H
- Recall that hyperventilation gives off CO2, dragging the equation to the left.
- That is, pCO2 will be decreased.
- Therefore pregnant, hyperventilating women become alkalotic (decreased H+).
- That is, pH will be high (less H+).
- This is a respiratory alkalosis.
- Recall that the kidneys can excrete HCO3 to compensate for loss of CO2 (because of hyperventilation) and thus draw the equation back to the right.
- Thus we expect the serum HCO3 to be low.
- Recall that in pregnancy there is an increased oxygen carrying capacity (facilitated by RBC volume increased by 50%).
- Thus we expect the pO2 to be high.
- Given this respiratory alkalosis and metabolic compensation, we expect:
- the pCO2 to be low: 26-32 mmHg (compared to non-pregnancy 38-45)
- the pH to be high (alkaline): 7.40-7.46 (compared to non-pregnancy 7.38-7.42)
- the HCO3 to be low: 18-21 mEq / L (compared to non-pregnancy 24-31)
- the pO2 to be high: 106-108 mmHg (compared to non-pregnancy 70-100)
- A high pCO2 is cause for alarm in the asthmatic pregnant woman.
- Also, pCO2 should be monitored carefully in the acutely infected pregnant woman because they are immune compromised and have very little respiratory reserve.
Measure | Non-pregnant | Pregnant | Change | Reason |
---|---|---|---|---|
pH | 7.38 - 7.42 | 7.4 - 7.46 | Higher | Because of the balance of respiratory alkalosis and (compensatory) metabolic acidosis. |
pCO2 (mmHg) | 38 - 45 | 26 - 32 | Lower | Because minute ventilation increases more than oxygen consumption (CO2 breathed off faster than oxygen consumed). |
pO2 (mmHg) | 70 - 100 | 101 - 106 | Lower | Because RBC volume is increased. |
HCO3- (mEq / L) | 24 - 31 | 18 - 21 | Lower | Because the kidneys are excreting HCO3- in an attempt to compensate for respiratory alkalosis. |
O2 saturation (%) | 95 - 100 | 95 - 100 | None |
- Importance:
- A normal pregnant woman has a compensated respiratory alkalosis (with a compensatory metabloic acidosis) and a diminished pulmonary reserve.
- Cause:
- Progesterone: relaxes the smooth muscle
- Diaphragm raised 4cm
CBC in pregnancy
- Plasma Volume and RBC Mass
- Plasma volume increases by about 50%
- RBC volume increases by about 30%
- Result:
- “Physiologic anemia of pregnancy”
- The mean Hgb is about 11.5 g/dl
- At 28 weeks, get new labs.
- Will likely need to start iron.
- Start at or below 10.
- Yucky tasting, makes woman feel horrible.
- Necessary for proper oxygenation.
Coagulation in pregnancy
- In pregnancy the pt is hypercoaguable
- This is the body's way to not die from bleeding when the parasite is ejected / ripped from the internal surface of the uterus.
- DIC is the number 1 cause of death in laboring women around the world.
- Not so much here in the US.
- Procoagulant’s are increased: I, VII, VIII, IX, X and Fibrinogen.
- Prevents Peripartum Hemorrhage
- Can lose 1500mL without blinking.
- Over 2000 mL lost, start worrying b/c coag factors are becoming deficient.
Renal System in pregnancy
- Bun & Serum Creatinine decreases by about 25%
- Low because of increased blood volume, increased GFR, increased blood flow to the kidneys
- Plasma osmolarity decreases by about 10 mOsm / kg H20
- Increase in tubal reabsorption of sodium
- Marked increase in renin and angiotensin levels
- But markedly reduced vascular sensitivity to their hypertensive effects
- Increased glucose excretion
- Fine during labor, a coping mechanism.
- Labs at 28 weeks detect gestational diabetes.
- Anatomic:
- Kidney size increases
- Kidney weight increases
- Ureteral dilation (right side greater than left side)
- Because of the angle of the uterus
- Bladder becomes an intra-abdominal organ
- Hemodynamic:
- GFR increase by 50%
- Renal plasma increased by flow by 75%
- Creatinine Clearance increases to 150-200 cc / min
- Most important kidney function test in pregnancy
- Swollen Ankles:
- Recall that the Plasma Volume Increases 50% in pregnancy.
- Recall that there is decreased Systemic Vascular resistance in pregnancy.
- Recall that there is renal retention of Na and H20 in pregnancy.
- Leads to a total increase of 6 – 8 L in total body H20!
- 2 / 3rd extracellular
- 1 / 3rd intravascular
- Intravascular increase is put in the "dependent" (affected by gravity) areas like the lower venous system.
- Hard to move fluids through the venous system.
Case: Caring Family
- The patient’s husband lies her flat in an effort to keep her comfortable.
- The fetus’s FHTs (fetus heart tones) drop to 80.
- But, the ROCK STAR medical student wedges the patient quickly on her left side and the FHT’s recover quickly!
- Turns her off the vena cava.
- Aids in venous return to the heart and therefore to oxygenation of blood for mom and baby.
- What happened to the cared-for mother?
- Gravid Uterus placed pressure on the IVC which resulted in decreased “utero-placental blood flow.”
- Cardiac output falls
- Blood is shunted to the brain and
- Blood is shunted away from the uterus and placenta
- This causes a reflex fetal bradycardia
Importance of Physiology
- All can be abnormal in the Non-Pregnant state:
- Decreased BP
- Swollen ankles
- CXR: cardiomegaly, leftward deviation
- Systolic Ejection murmur
- Dyspnea
- Anemia
- Hypercoaguable
- NORMAL for Pregnancy
Maternal Physiology Review