anatomical and physiological considerations in pregnancy · 2012-07-27 · maternal physiological...

Anatomical and physiological considerations in pregnancy

Edward Waters

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Maternal physiological changes

Most changes are adaptive and help the mother sustain the pregnancy & tolerate childbirth. First trimester changes are primarily due to hormonal changes, third trimester changes are due to anatomical changes (gravid uterus).

Maternal physiological changes

The clinician should understand the physiological changes of pregnancy (as compared to the non pregnant state) as well as the impact of these changes on anesthetic care and complications. Changes of the pulmonary, C.V., nervous & G.I. systems are extremely important to the anesthetist. Renal and hepatic changes are also of importance.

Nervous system

What is the physiology of pain in the 1st stage of labor?*

First stage labor pain is visceral pain arising from uterine contractions and dilation of the cervix.First stage labor pain is carried through spinal segments T10-L1.

What is the physiology of pain in the 2nd stage of labor?*

Pain is carried to the S2-S4 spinal segments by the pudendal nerve.2nd stage labor pain is somatic pain caused by stretching of the vagina and perineum.

Nervous system

Pregnancy induced analgesia.Parturients have increased pain tolerance Probably a result of elevated maternal progesterone and endorphins

Important clinical ramifications.

Nervous system

Decreased volume in epidural and subarachnoid spaces.

A result of engorged epidural veins and epidural fat. Decreased CSF volume is secondary to the decreased volume in epidural and subarachnoid spaces.

Nervous system

Regulation of blood volume in the venous capacitance system is more dependent on the SNS in parturients than non pregnant women.

Pulmonary system

Pulmonary system

The thoracic cage increases 5 to 7 cm in circumference and diaphragm elevates. Capillary engorgement of the respiratory tract leads to swelling of the nasal and oral pharynx.

Pulmonary system: Ventilation

Tidal volume increases 45%.Respiratory rate is slightly increased.Therefore minute ventilation and alveolar ventilation is increased ~45-50%.Increased ventilation.

Pulmonary system: Labor and delivery

Minute ventilation can increase by as much as 140% in the first stage of labor and 200% during the second stage.Hyperventilation during L&D can lead to a PaCO2 as low as 10 or 15 mm/Hg.

Pulmonary system: FRC

Functional residual capacity (residual vol. and expiratory reserve vol.) is reduced 20% at term.Increased size of uterus leading to elevation of diaphragm decreased FRC.

Why do parturients desaturate so rapidly when apneic?*

FRCO2 consumption

V/Q mismatch (indicated by increased PAO2-PaO2 gradient).

Pulmonary system: ABGs

Increased alveolar ventilation leads to:Slightly increased PO2.PaCO2 declining to ~30mm/Hg by 12 weeks gestation, bicarb drops to ~20 mEq/L resulting in a compensated respiratory alkalosis.

Which respiratory parameters don’t change during pregnancy?*

Dead space (Vd)Lung complianceArterial blood pH Vital capacityForced expiratory vol in 1 sec (FEV-1)Diffusing capacity

Hematologic system

Hematologic system *

Blood volume increases 35% in pregnancy but plasma volume increases more (50%) relative to RBC volume (20%) this leads to a dilutional anemia.Normal Hgb 11-12.Normal Hct ~35%.Colloid osmotic pressure decreases.

Hematologic system

Platelet turnover, clotting and fibrinolysis are enhanced in pregnancy.Coagulation in pregnancy can be characterized as accelerated, but compensated.

Hematologic system

Most coagulation factors are elevated in pregnancy. The elevated levels of coagulation factors is opposed by increased fibrinolytic activity (e.g. increased plasminogen).

Hematologic system

WBCs typically increase from 6,000 per cubic mm to 9,000 or 11,000per cubic mm. Polymorphonuclear leukocyte function is impaired in pregnancy leading to increased incidence of infection and reduced symptoms of autoimmune disease.

Hematologic system

Total amount of protein in circulation increases but plasma protein levels are low as a result of dilution. Total protein in the circulation decreases to < 6 g/dL.A/G ratio decreases due to relatively greater decrease in albumin concentration (~12% decrease).Pseudo cholinesterase decreased by 25%.

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Cardiovascular system

Cardiovascular system*

CO increases 30-40% in the first trimester.From second trimester on CO 50% above nonpregnant level. Increase in CO secondary to increases in heart rate and stroke volume.

Cardiovascular system*

During labor CO increases 45% above prelabor values.Highest CO observed the first 4 hours post delivery & can be 80% above prelabor levels.CO falls to prelabor values 24 hrs after delivery and prepregnant levels 12 to 24 weeks post partum.

Cardiovascular system

Increased stroke volume results in myocardial hypertrophy and increased myocardial contractility.Myocardial hypertrophy can lead to:

3rd & 4th heart sounds and SEMECG changes

Cardiovascular system

SBP, DBP and MAP decrease during mid pregnancy and return to baseline near term.DBP falls to the greatest degree.The decrease in blood pressure is due to decreased SVR which is lowest at 20 weeks gestation (a 35% reduction over pre-pregnant SVR). SVR remains 20% below baseline at term.

Cardiovascular system

Aortocaval compression (aka supine hypotensive syndrome).Compression of vena cava by gravid uterus impairs maternal venous return compression of the aorta reduces LE blood flow and uteroplacental blood flow.

Cardiovascular system: Symptoms of aortocaval compression

ApprehensionVertigo

TachycardiaDizziness

Changes in mentationVomiting

NauseaDiaphoresis

Cardiovascular system : Prevention of aortocaval compression

Left uterine displacement. Elevate right hip 10-15 cm while patient supine.

G.I./metabolic system

G.I./metabolic system

Body compositionMean 17% increase in body wt. (~12kg).Wt. gain from increased size of uterus and contents, increased blood volume and increased deposition of fat and protein. Most weight gain in last 2 trimesters.

G.I./metabolic system

The gravid uterus compresses, elevates & rotates the stomach increasing intragastric pressure.Gastroesophageal sphincter tone is decreased.Barrier pressure is decreased to about 25% of normal.

G.I./metabolic system

Gastric emptying and pH is not altered during pregnancy.Peristalsis in the esophagus and intestines is slowed by pregnancy.During L&D gastric empting is slowed, gastric acid secretions may decrease.Gastric volumes and pH normalize 18 hours after delivery.

G.I./metabolic system

Pregnant women demonstrate a state of insulin resistance.The glucose demands of the fetus and placenta may result in lower than normal maternal blood glucose levels in the third trimester.

G.I./metabolic system

The thyroid gland enlarges 50 -70% during pregnancy.T3 and T4 concentrations increase by ~ 50%, but free fractions of the hormones are unchanged due to an increased level of thyroid–binding globulin.TSH levels transiently decrease during the first trimester.

Hepatic & renal systems

Hepatic & renal systems

Renal plasma flow increases by 75%. GFR increases by 50% by the end of the first trimester. The increased vascular volume of the kidneys in pregnancy leads to the organ enlarging by as much as 30%.

Hepatic & renal systems

Increased GFR tends to increase water and electrolye excretion.Tubular reabsorptive capacity for Na, Cl, and H2O is increased by 50%.Net effect is a accumulation of ~3 kg of water and salt in a healthy parturient.

Hepatic & renal systems

The increased GFR leads to the BUN decreasing by 8-9mg/dL and serum creatinine decreasing by 0.5-0.6 mg/dL.

Hepatic & renal systems

In spite of expanded blood volume and increased CO the size of the liver doesn’t increase and blood flow to the liver doesn’t increase in absolute terms and as a percentage of CO decreases 35%.

Musculoskeletal system

Musculoskeletal concerns

Back pain during pregnancy is common and is attributed to:

Hormones, in particular relaxin.Exaggerated lumbar lordosis caused by the enlarged uterus.

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