Mod VII: Physiologic Changes of Pregnancy

Question 1

_{Maternal & Fetal Physiology and Anesthesia}

In order to care for a pregnant patient we need to understand their altered physiology and the implications for our analgesia/anesthesia. Therefore, objectives of this lecture include:

Answer 1

Discuss physiologic adaptations during pregnancy

Review the maternal-placental-fetal unit

Placental transfer and fetal exposure of anesthetic drug

Review intrapartum fetal assessment

Question 2

_{Maternal & Fetal Physiology and Anesthesia}

Physiologic adaptations during pregnancy involve which body systems?

Answer 2

Cardiovascular

Respiratory

Gastrointestinal

Hematological

Central nervous system

Renal

Hepatic

Question 3

_{Maternal & Fetal Physiology and Anesthesia}

Review the maternal-placental-fetal unit means review of:

Answer 3

Placental component

Umbilical-placental circulation (fetal component)

Uteroplacental circulation (maternal component)

Question 4

_{Maternal & Fetal Physiology and Anesthesia}

Placental transfer and fetal exposure of anesthetic drug

Answer 4

…

Question 5

_{Maternal & Fetal Physiology and Anesthesia}

Review intrapartum fetal assessment means review of:

Answer 5

Fetal heart rate monitoring (biophysical)

Fetal scalp monitoring (biochemical)

Fetal pulse oximetry

Question 6

_{Cardiovascular (CV) Changes of pregnancy}

What are the Four major changes to the cardiovascular system during pregnancy

Answer 6

Increase in intravascular volume and hematologic changes

Increased cardiac output (CO)

Decreased systemic vascular resistance (SVR)

Supine hypotension d/t Aortocaval compression

Question 7

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

Changes to which system is responsible for increase in maternal fluid volume starting in the First trimester?

Answer 7

Renin-Angiotensin-Aldosterone System

=> Na+ absorption + Water retention

_{[Likely from increased Progesterone from gestational sac]}

Question 8

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

Which changes to plasma volume and red blood cells at full term produces a dilutional anemia and decreases blood viscosity?

Answer 8

Plasma volume increase by 55%

Red blood cells increase by 25%

[Plasma volume increase is greater than RBC increase]

Question 9

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

Why is there a slight decrease in Hgb at full term?

Answer 9

Hgb decreses from 12 g/dL to 11 g/dL

D/t smaller increase in RBC compared to increase in plasma volume

_{<strong>[</strong>The relatively smaller increase in red blood cell volume (20%) accounts for a reduction in hemoglobin concentration (from 12 g/dL to 11 g/dL) and hematocrit (to 35%)]}

Question 10

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

At full term, O2 delivery isn’t decreased despite the anemia - Why not?

Answer 10

Increased CO

^{The increase in CO allows for redistribution of blood flow}

The uterus recieves an in increase in blood flow from 50 mL/min to 700 mL/min

The skin, liver and breast have an increase flow that accounts for an increase of 1.5 to 2.0 mL/min in CO

Question 11

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

What’s the anticipated blood loss during labor for vaginal delivery?

Answer 11

300-500 mL

Question 12

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

What’s the anticipated blood loss during labor for cesarean section?

Answer 12

800-1000 mL

Question 13

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

What helps compensate for the estimated blood loss of 300 to 500 mL typically associated with vaginal delivery and the estimated blood loss of 800 to 1000 mL that accompanies a standard cesarean section?

Answer 13

The additional intravascular fluid volume of approximately

1000-1500 mL

above prepregnancy volume

_{[After delivery, uterine contraction creates an autotransfusion of blood often in excess of 500 mL that offsets the blood loss from delivery]}

Question 14

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

Pregnancy in a hypercaoguable state - which factors are increased during pregnancy? How does this affect PT and PTT?

Answer 14

I (Fibrinogen), VII, VIII, X, XII, and Von Willebrand Factor

=> Decrease PT and PTT by 20%

[Increase in coags means it takes less time for blood to clot]

Question 15

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

Graphical representation of the coagulation cascade

Answer 15

Graphical representation of the coagulation cascade

Question 16

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

Pregnancy in a hypercaoguable state - which factors are decreased during pregnancy?

Answer 16

XI, XIII, antithrombin III, and tPA

Question 17

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

8% of otherwise healthy women have a platelet count 70,000/mm3 - 150,000/mm (gestational thrombocytopenia) - However, this syndrome is not associated with abnormal bleeding - Why not?

Answer 17

Gestational thrombocytopenia is Not a/w abnormal bleeding

Gestational thrombocytopenia is due to a combination of hemodilution and more rapid platelet turnover and is a diagnosis of exclusion

Other more consequential diagnoses such as idiopathic thrombocytopenic purpura and HELLP syndrome (hemolysis, elevated liver enzyme levels, and low platelet count) must be excluded (see section on maternal comorbidities and coagulopathies)

Question 18

_{CV Changes of pregnancy - Intravascular Volumes & Hematology}

TBV returns to normal approximately how long after delivery?

Answer 18

2 weeks after delivery

Question 19

_{CV Changes of pregnancy - Cardiac Output}

Increased CO during pregnancy is due to increases in:

Answer 19

Heart rate: by 15-25%

Stroke volume: by 25-35%

[CO = HR x SV]

Question 20

_{CV Changes of pregnancy - Cardiac Output}

When and by how much does CO increase during pregnancy?

Answer 20

1st Trimester: 35% above prepregnancy

2nd Trimester: 40-50% above prepregnancy values

Question 21

_{CV Changes of pregnancy - Cardiac Output}

When and by how much does CO increase during the stages of labor and after delivery?

Answer 21

1st stage – 10-25%

2nd stage – 40%

After delivery – 80-100%

Question 22

_{CV Changes of pregnancy - Cardiac Output}

How soon after delivery does CO return to prelabor values?

Answer 22

24 hours

Question 23

_{CV Changes of pregnancy - Cardiac Output}

How soon does CO return to nonpregnant levels?

Answer 23

between 12-24 weeks

Question 24

_{CV Changes of pregnancy - Systemic Vascular Resistance & Aortocaval Compression}

Why is there Decreased SVR despite increases in TBV and CO?

Answer 24

SBP, DBP, and MAP are decreased up to 20% until week 20

but increased towards normal by full-term

Question 25

_{CV Changes of pregnancy - Systemic Vascular Resistance & Aortocaval Compression}

The hypotension that results from compression of the abdominal aorta and inferior vena cava by the gravid uterus when a pregnant woman lies in the supine position is also known as:

Answer 25

Supine Hypotension Syndrome, or

Aortocaval compression

Question 26

_{CV Changes of pregnancy - Systemic Vascular Resistance & Aortocaval Compression}

How can Supine Hypotension Syndrome, or Aortocaval compression be prevented?

Answer 26

Left uterine displacement (LUD)

Place wedge >15˚ under right hip

Question 27

_{CV Changes of pregnancy - Systemic Vascular Resistance & Aortocaval Compression}

Which position must be avoided w/ Supine Hypotension Syndrome or Aortocaval compression?

Answer 27

Supine position

^{(No supine position w/o LUD)}

Question 28

_{CV Changes of pregnancy - Physical Exam}

How is the heart expected to appear on CXR during pregnancy?

Answer 28

Enlarged heart

^{(Especially on parturients)}

elevation of the diaphragm shifts the heart’s position in the chest, resulting in the appearance of an enlarged heart on a plain chest film

Question 29

_{CV Changes of pregnancy - Physical Exam}

What are expected EKG findings in pregnancy?

Answer 29

Left axis deviation

Nonspecific downsloping ST /T wave changes

T-wave inversion lead III

_{(Elevation of the diaphragm shifts the heart’s position in the chest, resulting in left axis deviation and T wave changes on the electrocardiogram)}

Question 30

_{CV Changes of pregnancy - Physical Exam}

What are possible Heart sounds in pregnancy?

Answer 30

Systolic ejection murmur (grade I or II) common

^{Functional flow murmur due to hyperdynamic state}

Splitting S1 - Audible S3

_{[Physical examination often reveals a systolic ejection flow murmur (grade I or II) and exaggerated splitting of the first heart sound (S1); a third heart sound (S3) may be audible]}

Question 31

_{CV Changes of pregnancy - Physical Exam}

Why is a Systolic ejection murmur (I/II) commonly heard during pregnancy?

Answer 31

It’s Functional flow murmur

due to hyperdynamic state

Question 32

_{CV Changes of pregnancy - Physical Exam}

T/F: A Pericardial effusion is an expected finding in some pregnancies

Answer 32

True

[A few patients develop small, asymptomatic pericardial effusion]

Question 33

_{Respiratory Changes of pregnancy}

What are the Three major respiratory changes during pregnancy?

Answer 33

Upper airway & breathing restriction changes

Lung volumes and minute ventilation changes

Oxygen consumption and metabolic rate changes

Question 34

_{Respiratory Changes of pregnancy - Upper Airway}

What causes airway and vocal cord edema during pregnancy? when does is begin? when does it worsen?

Answer 34

Capillary engorgement of the mucosa

Begins in 1st trimester → ↑’s throughout pregnancy

May worsen during active labor, preeclamptic parturient, T-burg position and use of tocolytics

Question 35

_{Respiratory Changes of pregnancy - Upper Airway​}

Intubation may prove more difficult - What size ETT is recommended?

Answer 35

Smaller (6.0-6.5) ETT recommended

Question 36

_{Respiratory Changes of pregnancy - Upper Airway​}

Intubation may prove more difficult - Why should nasal ETT be avoided during pregnancy, if possible?

Answer 36

Epistaxis

(Acute hemorrhage from the nostril, nasal cavity, or nasopharynx)

Question 37

_{Respiratory Changes of pregnancy - Upper Airway​}

Why must NGT/nasal trumpet airways be used cautiously in pregnancy?

Answer 37

Capillary engorgement of the mucosa => increased risk of bleeding

Question 38

_{Respiratory Changes of pregnancy - Upper Airway​}

Which hormone causes a 50% ↓↓ in Airway resistance during pregnancy? How is this accomplished?

Answer 38

Progesterone causes relaxation bronchial smooth

which leads to decreased airway resistance

Question 39

_{Respiratory Changes of pregnancy - Upper Airway​}

T/F: Lung compliance is unchanged in pregnancy

Answer 39

True

Question 40

_{Respiratory Changes of pregnancy - Mechanics of Breathing}

Respiratory Changes a/w Expanding uterus include:

Answer 40

Diaphragm

Cephalad displacement of the diaphragm

Thoracic cage

Compensatory subcostal widening &↑ AP and transverse diameters of thoracic cage

Diaphragmatic motion

Diaphragmatic motion itself is not restricted

However, Chest wall compliance: ↓↓ Restrictive pattern because of the Cephalad displacement of the diaphragm

Question 41

_{Respiratory Changes of pregnancy - Mechanics of Breathing}

How does the expanding uterus affect Lung Volumes?

Answer 41

FRC → ↓↓ 20% (starts at 5 mos.)

ERV (↓ 20%) & RV (↓ 15%)

IRV (↑ 5%) & Vt (↑↑ 40%)

TLC/VC/IC → unchanged

Closing volumes→ unchanged

^{(however, exceed FRC in 50% pregnant women when supine)}

Question 42

_{Effects of expanding uterus on Lung Volumes}

When and why is FRC ↓↓ by 20%?

Answer 42

Starts at 5 mos.

D/t Size of the featus

Question 43

_{Effects of expanding uterus on Lung Volumes}

Which lung volume/capacities are unchanged in pregnancy?

Answer 43

TLC/VC/IC → unchanged

Closing volumes→ unchanged

_{[During pregnancy, the expanding uterus forces the diaphragm cephalad and creates a 20% decrease in functional residual capacity (FRC) by term (see Table 77-2). This reduction is made up of nearly equal reductions in both the expiratory reserve volume (ERV) and residual lung volume (RV). However, closing capacity (CC) remains unchanged and creates a reduced FRC/CC ratio. This creates more rapid small airway closure with reduced lung volumes, and in the supine position FRC can be less than CC for many small airways, giving rise to atelectasis. Vital capacity does not change with pregnancy. The combination of increased minute ventilation and decreased FRC results in a more rapid rate at which changes in the alveolar concentration of inhaled anesthetics can be achieved]}

Question 44

_{Effects of expanding uterus on Lung Volumes}

T/F: Closing volumes are unchanged during pregnancy - However they exceed FRC in 50% pregnant women when supine

Answer 44

True

The supine position is to be avoided in the pregnant pt

Question 45

_{Respiratory Changes of pregnancy - Gas Exchange}

Why is Minute ventilation ↑↑↑ by 50 % during pregnancy?

Answer 45

B/c of increase in both Vt and RR

^{VT (↑↑ 40%) + RR (↑ 15%)}

^{Beginning of pregnancy to max at term}

Question 46

_{Respiratory Changes of pregnancy - Gas Exchange}

How is Alveolar ventilation affected by pregnancy?

Answer 46

↑↑↑↑ 70%

Question 47

_{Respiratory Changes of pregnancy - Gas Exchange}

What are some Predicted Blood gases values in pregnancy (PaCO2, PaO2, HCO3+, pH)?

Answer 47

PaCO2: ↓ 10% (28-32 mmHg)

PaO2: ↑ 10% (106 mmHg)

^{Hyperventilation may also increase PaO2 slightly}

HCO3+: ↓ (compensatory renal excretion)

pH: near normal to slightly alkalotic

Significant respiratory alkalosis is compensated by decrease plasma HCO3+ concentration

Question 48

_{Respiratory Changes of pregnancy - Gas Exchange}

Why is Maternal PaCO2 decreases from 40 mm Hg to approximately 30 mm Hg during the first trimester?

Answer 48

PaCO2: ↓ 10% (28-32 mmHg)

[Maternal PaCO2 decreases from 40 mm Hg to approximately 30 mm Hg during the first trimester as a reflection of the increased minute ventilation]

Question 49

_{Respiratory Changes of pregnancy - Gas Exchange}

Why is maternal room air PaO2 increased Early in gestation?

Answer 49

Hyperventilation + decrease in alveolar CO2

Hyperventilation may also increase PaO2 slightly to PaO2: ↑ 10% (106 mmHg)

Because of the presence of hyperventilation and the associated decrease in alveolar CO2

Later, PaO2 becomes normal or even slightly decreased, most likely reflecting small airway closure with normal tidal volume ventilation and intrapulmonary shunt

Question 50

_{Respiratory Changes of pregnancy - Gas Exchange}

Which patient position can significantly improve Arterial oxygenation?

Answer 50

Lateral position

Arterial oxygenation can be significantly improved by moving the patient from the supine to the lateral position

Question 51

_{Respiratory Changes of pregnancy - Gas Exchange}

Withe the decrease in PaCO2 caused by hyperventilation, what prevents significant respiratory alkalosis and keeps pH (typically 7.42 to 7.44) near normal to slightly alkalotic?

Answer 51

Increased renal excretion of bicarbonate ions

Compensatory decrease in plasma bicarbonate concentration, or

Metabolic compensation with increased renal excretion of bicarbonate ions (HCO3− is typically 20 or 21 mEq/L at term)

Question 52

_{Respiratory Changes of pregnancy - Gas Exchange}

From a metabolic standpoint, ↑↑ O2 consumption (20-50%) is compensated for by:

Answer 52

↑ Alveolar ventilation

↑ C.O

Question 53

_{Respiratory Changes of pregnancy - Gas Exchange}

How is the Oxyhemoglobin Dissociation Curve shifted in pregnancy for the mother and why?

Answer 53

Right shift in Oxyhemoglobin Dissociation Curve

D/t increase in O2 demand

P50: 30 mmHg

Allows enhanced release of O2 to the fetus

Due to ↑ 2,3 DPG levels

^{Offset effect of alkalosis}

Maternal Hb is right shifted, with the P50 (partial pressure of O2 at which Hb is 50% saturated with oxygen) increasing from 27 to approximately 30 mm Hgb

The higher P50 in the mother and lower P50 in the fetus mean that the fetal blood has higher affinity for O2 and offloading of O2 across the placenta is facilitated

Question 54

_{Respiratory Changes of pregnancy - Gas Exchange}

Which offsets the effect of hyperventilation on hemoglobin’s affinity for oxygen?

Answer 54

Elevated levels of 2,3-diphosphoglycerate

The P50 for hemoglobin increases from 27 to 30 mm Hg; the combination of the latter with an increase in cardiac output (see next section on Cardiovascular Effects) enhances oxygen delivery to tissues.

Question 55

_{Respiratory Changes of pregnancy - Gas Exchange}

What the P50 of maternal Hgb during pregnancy?

Answer 55

P50: 30 mmHg

Question 56

_{Respiratory Changes of pregnancy - Gas Exchange}

During pregnancy, there is a Right shift in Oxyhemoglobin Dissociation Curve. How does this affect release of O2 to the fetus?

Answer 56

Allows enhanced release of O2 to the fetus

Due to ↑ 2,3 DPG levels

^{Offset effect of alkalosis}

Question 57

_{Gastrointestinal Changes of pregnancy}

Increaseed levels of which hormone may decrease Gastric emptying during 1st trimester of pregancy?

Answer 57

Progesterone

Question 58

_{Gastrointestinal Changes of pregnancy}

What do Conflicting data regarding Gastric emptying during pregnancy suggest?

Answer 58

Gastric emptying

NOT delayed during most of pregnancy but

Delayed at Term & during labor

Question 59

_{Gastrointestinal Changes of pregnancy}

What are two known risk factors for aspiration pneumonitis during pregnancy?

Answer 59

Gastric pH <2.5 + Gastric volume >25mL

^{(But this is more likely at term, and not necessarily early on in the pregnancy)}

Question 60

_{Gastrointestinal Changes of pregnancy}

Why are all pregnant women considered “Full stomach”?

Answer 60

Decreased Esophageal peristalsis

Increased I_ntestinal transit time_

Slowed Gastric emptying during labor

^{(and opioid may further exacerbate that effect)}

Question 61

_{Gastrointestinal Changes of pregnancy}

How does an Enlarged uterus affect Intragastric pressures?

Answer 61

Increased Intragastric pressures

Question 62

_{Gastrointestinal Changes of pregnancy}

How does an Enlarged uterus affect the angle of the gastroesophageal junction aka Angle of His?

Answer 62

Decrseased Angle of His

The angle of His is the acute angle created between the cardia at the entrance to the stomach, and the esophagus. It forms a valve, preventing reflux of duodenal bile, enzymes and stomach acid from entering the esophagus, where they can cause inflammation.

Question 63

_{Hepatic Changes of pregnancy}

T/F: Overall hepatic function and blood flow are unchanged

Answer 63

True

Question 64

_{Hepatic Changes of pregnancy}

How does pregnancy affect Plasma cholinesterase activity?

Answer 64

Plasma cholinesterase activity ↓↓ 25-30%

Question 65

_{Hepatic Changes of pregnancy}

Although Plasma cholinesterase activity ↓↓ 25-30% during pregnancy, why should you not ↓ intubating dose of succinylcholine?

Answer 65

May produce inadequate conditions for intubation

They are already a difficult airway

So you need ideal intubation conditions

However, expect slightly prolonged duration

Question 66

_{Hepatic Changes of pregnancy}

When does Plasma cholinesterase activity reaches its lowest level? When does it return to normal?

Answer 66

Reaches lowest level in postpartum period

May not return to normal until 6 weeks postpartum

Question 67

_{Hepatic Changes of pregnancy}

Why is he risk for gallbladder disease increased during pregnancy?

Answer 67

Incomplete gallbladder emptying

^{High progesterone levels appear to inhibit the release of cholecystokinin, resulting in incomplete emptying of the gallbladder}

Changes in bile composition

^{Pregnancy is a/w altered bile acid composition, which, coupled with Incomplete gallbladder emptying can predispose to the formation of cholesterol gallstones during pregnancy}

Cholecystectomy

^{Cholecystectomy during pregnancy is required in between 1 in 1600 to 1 in 5000 pregnancies}

Question 68

_{Renal Changes of pregnancy}

How are GFR & RBF affected by pregnancy?

Answer 68

GFR & RBF ↑↑ 50%

_{[Renal plasma flow and the glomerular filtration rate increase during pregnancy; as a result, serum creatinine and blood urea nitrogen may decrease to 0.5 to 0.6 mg/dL and 8 to 9 mg/dL, respectively]}

Question 69

_{Renal Changes of pregnancy}

GFR & RBF ↑↑ 50% during pregnancy - What are the consequences of this on Creatinine clearance, BUN and creatinine levels?

Answer 69

↑ Creatinine clearance

^{(which results in decreased serum creatinine)}

↓ BUN and creatinine levels

_{[Renal plasma flow and the glomerular filtration rate increase during pregnancy; as a result, serum creatinine and blood urea nitrogen may decrease to 0.5 to 0.6 mg/dL and 8 to 9 mg/dL, respectively]}

Question 70

_{Renal Changes of pregnancy}

GFR & RBF ↑↑ 50% during pregnancy - When does this begin? When does it return to normal?

Answer 70

Begins 16th week

Returns to normal 1-3 weeks postpartum

Question 71

_{Renal Changes of pregnancy}

Why are levels of urine protein and glucose (Glycosuria/Proteinuria) commonly increased during pregnancy?

Answer 71

Decreased renal tubular reabsorptive capacity

^{The upper limits of normal in pregnancy in a 24-hour urine collection are 300 mg protein and 10 g glucose}

Lower renal threshold for glucose excretion

^{Glycosuria is common during pregnancy because of the lowering of renal threshold for glucose excretion}

↑ glucose load due to ↑ GFR​

^{The increase in the GFR delivers an overwhelming glucose load to the renal tubule. Glucose absortion, which is normally complete is compromised}

Question 72

_{Renal Changes of pregnancy}

Increased levels of which hormones levels → Na+ & H20 retention?

Answer 72

Renin & Aldosterone

This results in ↑ TBV & Edema