Mod VII: Physiologic Changes of Pregnancy Flashcards
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:
Discuss physiologic adaptations during pregnancy
Review the maternal-placental-fetal unit
Placental transfer and fetal exposure of anesthetic drug
Review intrapartum fetal assessment
Maternal & Fetal Physiology and Anesthesia
Physiologic adaptations during pregnancy involve which body systems?
Cardiovascular
Respiratory
Gastrointestinal
Hematological
Central nervous system
Renal
Hepatic
Maternal & Fetal Physiology and Anesthesia
Review the maternal-placental-fetal unit means review of:
Placental component
Umbilical-placental circulation (fetal component)
Uteroplacental circulation (maternal component)
Maternal & Fetal Physiology and Anesthesia
Placental transfer and fetal exposure of anesthetic drug
…
Maternal & Fetal Physiology and Anesthesia
Review intrapartum fetal assessment means review of:
Fetal heart rate monitoring (biophysical)
Fetal scalp monitoring (biochemical)
Fetal pulse oximetry
Cardiovascular (CV) Changes of pregnancy
What are the Four major changes to the cardiovascular system during pregnancy
Increase in intravascular volume and hematologic changes
Increased cardiac output (CO)
Decreased systemic vascular resistance (SVR)
Supine hypotension d/t Aortocaval compression
CV Changes of pregnancy - Intravascular Volumes & Hematology
Changes to which system is responsible for increase in maternal fluid volume starting in the First trimester?
Renin-Angiotensin-Aldosterone System
=> Na+ absorption + Water retention
[Likely from increased Progesterone from gestational sac]
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?
Plasma volume increase by 55%
Red blood cells increase by 25%
[Plasma volume increase is greater than RBC increase]
CV Changes of pregnancy - Intravascular Volumes & Hematology
Why is there a slight decrease in Hgb at full term?
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%)]
CV Changes of pregnancy - Intravascular Volumes & Hematology
At full term, O2 delivery isn’t decreased despite the anemia - Why not?
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
CV Changes of pregnancy - Intravascular Volumes & Hematology
What’s the anticipated blood loss during labor for vaginal delivery?
300-500 mL
CV Changes of pregnancy - Intravascular Volumes & Hematology
What’s the anticipated blood loss during labor for cesarean section?
800-1000 mL
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?
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]
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?
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]
CV Changes of pregnancy - Intravascular Volumes & Hematology
Graphical representation of the coagulation cascade
Graphical representation of the coagulation cascade
CV Changes of pregnancy - Intravascular Volumes & Hematology
Pregnancy in a hypercaoguable state - which factors are decreased during pregnancy?
XI, XIII, antithrombin III, and tPA
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?
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)
CV Changes of pregnancy - Intravascular Volumes & Hematology
TBV returns to normal approximately how long after delivery?
2 weeks after delivery
CV Changes of pregnancy - Cardiac Output
Increased CO during pregnancy is due to increases in:
Heart rate: by 15-25%
Stroke volume: by 25-35%
[CO = HR x SV]
CV Changes of pregnancy - Cardiac Output
When and by how much does CO increase during pregnancy?
1st Trimester: 35% above prepregnancy
2nd Trimester: 40-50% above prepregnancy values
CV Changes of pregnancy - Cardiac Output
When and by how much does CO increase during the stages of labor and after delivery?
1st stage – 10-25%
2nd stage – 40%
After delivery – 80-100%
CV Changes of pregnancy - Cardiac Output
How soon after delivery does CO return to prelabor values?
24 hours
CV Changes of pregnancy - Cardiac Output
How soon does CO return to nonpregnant levels?
between 12-24 weeks
CV Changes of pregnancy - Systemic Vascular Resistance & Aortocaval Compression
Why is there Decreased SVR despite increases in TBV and CO?
SBP, DBP, and MAP are decreased up to 20% until week 20
but increased towards normal by full-term
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:
Supine Hypotension Syndrome, or
Aortocaval compression
CV Changes of pregnancy - Systemic Vascular Resistance & Aortocaval Compression
How can Supine Hypotension Syndrome, or Aortocaval compression be prevented?
Left uterine displacement (LUD)
Place wedge >15˚ under right hip
CV Changes of pregnancy - Systemic Vascular Resistance & Aortocaval Compression
Which position must be avoided w/ Supine Hypotension Syndrome or Aortocaval compression?
Supine position
(No supine position w/o LUD)
CV Changes of pregnancy - Physical Exam
How is the heart expected to appear on CXR during pregnancy?
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
CV Changes of pregnancy - Physical Exam
What are expected EKG findings in pregnancy?
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)
CV Changes of pregnancy - Physical Exam
What are possible Heart sounds in pregnancy?
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]
CV Changes of pregnancy - Physical Exam
Why is a Systolic ejection murmur (I/II) commonly heard during pregnancy?
It’s Functional flow murmur
due to hyperdynamic state
CV Changes of pregnancy - Physical Exam
T/F: A Pericardial effusion is an expected finding in some pregnancies
True
[A few patients develop small, asymptomatic pericardial effusion]
Respiratory Changes of pregnancy
What are the Three major respiratory changes during pregnancy?
Upper airway & breathing restriction changes
Lung volumes and minute ventilation changes
Oxygen consumption and metabolic rate changes
Respiratory Changes of pregnancy - Upper Airway
What causes airway and vocal cord edema during pregnancy? when does is begin? when does it worsen?
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
Respiratory Changes of pregnancy - Upper Airway
Intubation may prove more difficult - What size ETT is recommended?
Smaller (6.0-6.5) ETT recommended
Respiratory Changes of pregnancy - Upper Airway
Intubation may prove more difficult - Why should nasal ETT be avoided during pregnancy, if possible?
Epistaxis
(Acute hemorrhage from the nostril, nasal cavity, or nasopharynx)
Respiratory Changes of pregnancy - Upper Airway
Why must NGT/nasal trumpet airways be used cautiously in pregnancy?
Capillary engorgement of the mucosa => increased risk of bleeding
Respiratory Changes of pregnancy - Upper Airway
Which hormone causes a 50% ↓↓ in Airway resistance during pregnancy? How is this accomplished?
Progesterone causes relaxation bronchial smooth
which leads to decreased airway resistance
Respiratory Changes of pregnancy - Upper Airway
T/F: Lung compliance is unchanged in pregnancy
True
Respiratory Changes of pregnancy - Mechanics of Breathing
Respiratory Changes a/w Expanding uterus include:
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
Respiratory Changes of pregnancy - Mechanics of Breathing
How does the expanding uterus affect Lung Volumes?
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)
Effects of expanding uterus on Lung Volumes
When and why is FRC ↓↓ by 20%?
Starts at 5 mos.
D/t Size of the featus
Effects of expanding uterus on Lung Volumes
Which lung volume/capacities are unchanged in pregnancy?
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]
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
True
The supine position is to be avoided in the pregnant pt
Respiratory Changes of pregnancy - Gas Exchange
Why is Minute ventilation ↑↑↑ by 50 % during pregnancy?
B/c of increase in both Vt and RR
VT (↑↑ 40%) + RR (↑ 15%)
Beginning of pregnancy to max at term
Respiratory Changes of pregnancy - Gas Exchange
How is Alveolar ventilation affected by pregnancy?
↑↑↑↑ 70%
Respiratory Changes of pregnancy - Gas Exchange
What are some Predicted Blood gases values in pregnancy (PaCO2, PaO2, HCO3+, pH)?
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
Respiratory Changes of pregnancy - Gas Exchange
Why is Maternal PaCO2 decreases from 40 mm Hg to approximately 30 mm Hg during the first trimester?
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]
Respiratory Changes of pregnancy - Gas Exchange
Why is maternal room air PaO2 increased Early in gestation?
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
Respiratory Changes of pregnancy - Gas Exchange
Which patient position can significantly improve Arterial oxygenation?
Lateral position
Arterial oxygenation can be significantly improved by moving the patient from the supine to the lateral position
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?
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)
Respiratory Changes of pregnancy - Gas Exchange
From a metabolic standpoint, ↑↑ O2 consumption (20-50%) is compensated for by:
↑ Alveolar ventilation
↑ C.O
Respiratory Changes of pregnancy - Gas Exchange
How is the Oxyhemoglobin Dissociation Curve shifted in pregnancy for the mother and why?
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
Respiratory Changes of pregnancy - Gas Exchange
Which offsets the effect of hyperventilation on hemoglobin’s affinity for oxygen?
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.
Respiratory Changes of pregnancy - Gas Exchange
What the P50 of maternal Hgb during pregnancy?
P50: 30 mmHg
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?
Allows enhanced release of O2 to the fetus
Due to ↑ 2,3 DPG levels
Offset effect of alkalosis
Gastrointestinal Changes of pregnancy
Increaseed levels of which hormone may decrease Gastric emptying during 1st trimester of pregancy?
Progesterone
Gastrointestinal Changes of pregnancy
What do Conflicting data regarding Gastric emptying during pregnancy suggest?
Gastric emptying
NOT delayed during most of pregnancy but
Delayed at Term & during labor
Gastrointestinal Changes of pregnancy
What are two known risk factors for aspiration pneumonitis during pregnancy?
Gastric pH <2.5 + Gastric volume >25mL
(But this is more likely at term, and not necessarily early on in the pregnancy)
Gastrointestinal Changes of pregnancy
Why are all pregnant women considered “Full stomach”?
Decreased Esophageal peristalsis
Increased I_ntestinal transit time_
Slowed Gastric emptying during labor
(and opioid may further exacerbate that effect)
Gastrointestinal Changes of pregnancy
How does an Enlarged uterus affect Intragastric pressures?
Increased Intragastric pressures
Gastrointestinal Changes of pregnancy
How does an Enlarged uterus affect the angle of the gastroesophageal junction aka Angle of His?
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.
Hepatic Changes of pregnancy
T/F: Overall hepatic function and blood flow are unchanged
True
Hepatic Changes of pregnancy
How does pregnancy affect Plasma cholinesterase activity?
Plasma cholinesterase activity ↓↓ 25-30%
Hepatic Changes of pregnancy
Although Plasma cholinesterase activity ↓↓ 25-30% during pregnancy, why should you not ↓ intubating dose of succinylcholine?
May produce inadequate conditions for intubation
They are already a difficult airway
So you need ideal intubation conditions
However, expect slightly prolonged duration
Hepatic Changes of pregnancy
When does Plasma cholinesterase activity reaches its lowest level? When does it return to normal?
Reaches lowest level in postpartum period
May not return to normal until 6 weeks postpartum
Hepatic Changes of pregnancy
Why is he risk for gallbladder disease increased during pregnancy?
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
Renal Changes of pregnancy
How are GFR & RBF affected by pregnancy?
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]
Renal Changes of pregnancy
GFR & RBF ↑↑ 50% during pregnancy - What are the consequences of this on Creatinine clearance, BUN and creatinine levels?
↑ 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]
Renal Changes of pregnancy
GFR & RBF ↑↑ 50% during pregnancy - When does this begin? When does it return to normal?
Begins 16th week
Returns to normal 1-3 weeks postpartum
Renal Changes of pregnancy
Why are levels of urine protein and glucose (Glycosuria/Proteinuria) commonly increased during pregnancy?
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
Renal Changes of pregnancy
Increased levels of which hormones levels → Na+ & H20 retention?
Renin & Aldosterone
This results in ↑ TBV & Edema
