Obstetric, Physiology

Outline the cardiovascular,
respiratory, haematological and genitourinary changes that occur during
pregnancy

Cardiovascular: these include effects on cardiac output and
mechanical effects

• Cardiac output
  • Stroke volume:  increases by 20-30%, mainly in the first
    trimester
  • Heart rate: increases as early
    as 4 weeks after conception and increases by 17% at the end of first trimester
    and 25% by the middle of the third trimester with no further subsequent increase.
  • Preload: Increase in plasma and
    blood volume mainly occurs during the 1^st and 2^nd
    trimester. The ECF increases by 45% due to sodium and water retention as a
    result of increased oestrogen and activation of the renin-angiotensin system.
    Blood volume begins to increase in the first trimester and increases by about
    35-40% by the end of pregnancy.
  • Afterload: Total peripheral
    resistance decreases by 30% at the end of the first trimester and 35% by the 20^th
    week and remains at 30% below non-pregnant values due to the effect of
    increased progesterone and prostaglandins causing vasodilatation and smooth
    muscle relaxation. This results in decreased blood pressure with systolic and
    diastolic pressures decreased by about 10% and reaching their lowest values at
    about 20 weeks. The vascular system also becomes more refractory to the effects
    of vasoconstrictors.
  • As a result of the above
    factors cardiac output increases during pregnancy to 40-45% greater by 12-28
    weeks and a peak of 50% during weeks 32-36 followed by a slight decrease to 47%
    greater at term. This is mainly due to increased venous return secondary to
    venodilatation and increased vascular volume. A large proportion of this
    increased cardiac output is directed to the uteroplacental circulation which
    receives up to 10 times increased blood flow to about 750ml/min at term.
• Mechanical effects:
  • The heart is more rotated to
    the left and can result in q waves and TWI in the inferior leads on ECG.
  • Oedema occurs due to a 14% decrease
    in colloid oncotic pressure and the mechanical effects of the enlarging uterus
    compressing lymphatics
  • The uterus can also cause
    aortocaval compression when the patient is lying on her back resulting in
    hypotension and nausea.

Respiratory: these include changes to the airways, changes
to ventilation, changes to lung volumes and changes in respiratory mechanics

• Airway
  • Hyperaemia and oedema of the
    mucosal surfaces in the airway which can result in difficult intubation
  • Dilatation of the large airways
    which decreases airway resistance by about 35%
  • Anatomical dead space increases
    by 45%
• Ventilation
  • Minute ventilation increases by
    20 – 50% at due to an increase in respiratory rate and increase in tidal volume
    due to the effects of progesterone which stimulates the respiratory centre.
    This results in chronic respiratory alkalosis on ABG with a baseline pH of 7.47
    a paco2 of 30 – 32 and renal compensation with a HCO3 of 18-21
  • Oxygen consumption increases by
    about 20%
• Lung volumes
  • Tidal volume increases in the
    first trimester and rises to about 30% at term
  • Functional residual capacity
    decreases by 20% due to the elevation of the diaphragm and increase in
    pulmonary blood volume. In the supine position at term the FRC decreases to 70%
    of the FRC in the sitting position.
  • Inspiratory capacity increases
    by 10% at term while the expiratory capacity decreases by 5%
  • Total lung capacity decreases
    by approximately 5%
• Respiratory mechanics
  • Diaphragm is pushed upwards by
    about 4cm by end of term
  • Chest wall compliance decreases
    whilst lung compliance does not change
  • AP and transverse diameters of
    thoracic cage increase 2-3%, lower ribs flare out, subcostal angle increases
  • Circumference of thoracic cage
    increased by 5-7cm due to the effects of relaxin produced by the corpus luteum

Haematological: include changes to coagulation and changes
to full blood count

• Coagulation factors
  • Significant increase in factors
    5, 7, 8, 9, 10, 12, von Willebrand factor and fibrinogen
  • Decrease in factor XI to 60-70%
    of non-pregnant value
  • Protein S levels decrease
    during pregnancy
  • Plasminogen markedly raised.
    This is offset by plasminogen activator inhibitors produced by placenta
• Full blood count
  • Physiological anaemia of
    pregnancy due to dilutional decrease in Hb
  • Relative leukocytosis
  • Greater platelet production but
    overall platelet count slightly reduced due to increased destruction in the
    uteroplacental circulation
  • RBC volume increases by 20% due
    to increased erythropoietin. However, there is a slower rate of rise in red
    cell mass compared with plasma volume resulting in a decreased
    haematocrit 

Genitourinary: mechanical changes and changes to blood flow

• Mechanical
  • At 12/40 bladder becomes an
    abdominal structure and compression by the enlarging uterus can result in
    urinary frequency
  • Dilatation of renal pelvis,
    calcyces and ureters begin within 2-3 months resulting in physiological
    hydronephrosis
• Blood flow
  • Gfr increases by up to 80%
    resulting in decreased urea and creatinine
  • Renal blood flow increases by
    80% during first trimester and there is decreased tubular resorption of urate
    and glucose

List the functions of
the placenta

The placenta is a fetomaternal organ which consists of the
fetal part that develops from the chorionic sac and the maternal part which
develops from the decidua basalis of the endometrium. It connects the
developing fetus to the uterine wall. Placental development begins at 6 weeks
and is completed by the 12^th week. The placenta has four main
functions; transport, immunological, metabolic and endocrinological.

• Transport: this is one of the
  main functions of the placenta. It transports the following substances
  • Gases: delivers oxygen to the
    fetus and removes carbon dioxide from the fetal circulation
  • Nutrition: delivers water and
    nutrients including glucose, amino acids, fatty acids, calcium, iron, folate
    and vitamins A and C.
  • Drugs
  • Heat
  • Removes waste products such as
    urea and bilirubin
• Immunological
  • Provides passive immunity to
    the fetus as the placenta is selectively permeable to IgG, but not IgA or IgM
  • Provides a barrier between the fetal
    blood in the chorionic villi and the maternal blood in the intervillous spaces.
    There is little HLA and blood group antigens on the trophoblast surface so
    there is poor antigenic stimulus.
  • Protects fetus from some
    substances: those with large molecular weight or size such as heparin are not
    transferred
• Metabolic:
  • Synthesis of glycogen from
    maternal glucose and then stored
  • Synthesis of cholesterol which
    is a precursor to placental progesterone and estrogen
  • Synthesis of protein: up to
    7.5g/day
  • Synthesis of fatty acids
• Endocrine
  • Synthesis of multiple hormones
    including:
    • BHCG: a glycoprotein which supports the corpus luteum in the first
      10 weeks of pregnancy. It also suppresses maternal immune function.
    • Oestrogens: has multiple effects including stimulation of growth of
      the myometrium and mammary gland development. It also suppresses gonadotropin
      secretion from the maternal pituitary gland.
    • Progesterone: supports the endometrium and suppresses uterine
      contractility. Both oestrogen and progesterone are responsible for the majority
      of the maternal changes in pregnancy.
    • Human placental lactogen: involved in regulation of nutrient supply
      to the fetus. It inhibits gluconeogenesis, is lipolytic thus increases free
      fatty acids, promotes growth and has a lactogenic effect. It is structurally
      similar to growth hormone
    • Hypothalamic and pituitary like hormones: include chorionic
      thyrotropin, chorionic adenocorticotropin, gonadotrophin releasing hormone,
    • Others such as inhibin, relaxin, beta endorphins

Outline the determinants
of placental blood flow

Placental blood flow consists of a maternal and fetal part.

Maternal or utero-placental circulation:

The maternal blood supply to the uterus is via the uterine
and ovarian arteries which then form the arcuate arteries from which radial
arteries divide and penetrate the myometrium. The radial arteries then divide
into spiral arteries which supply the intervillous space.

Fetal-placental circulation:

The basic structural unit of the placenta is the choronic
villus which are vascular projections of fetal tissue surrounded by chorion.
The chorion is composed of an outer syncytiotrophoblast in direct contact with
maternal blood within the intervillous space, and the inner cytotrophoblast.
The villous capillaries are branches of the chorionic arteries which are
branches of the two umbilical arteries which bring blood from the fetus to the
placenta. Oxygenated blood then leaves the placenta via the umbilical vein.

Determinants of utero-placental
circulation:

• Blood pressure: Flow is passive and pressure dependent not
  auto-regulated: pressure is about 80-100mmHg in uterine arteries, 70mmHg in
  spiral arteries and just 10mmHg within the intervillous space. This
  low-resistance and pressure gradient allows efficient maternal blood perfusion.
  Consequently, hypotension will decrease the utero-placental circulation
• Vasoconstrictors: utero-placental circulation exist in a state of
  vasodilatation so vasodilator stimuli have minimal effect. Vasoconstrictors
  such as alpha adrenergic stimuli, sympathomimetic agents and severe hypoxemia
  can reduce placental blood flow
• Myometrial contraction: decreases placental blood flow as the
  intervillous space lies within the uterine cavity and contraction will result
  in increased pressure within this space and therefore decreased perfusion
  pressure.

Determinants of fetal-placental
circulation:

• Gestational age: umbilical blood flow increases as the fetus grows
• Umbilical blood flow is increased during fetal movements
• Placental vascular resistance: increased vascular resistance
  decreases blood flow therefore vasoconstrictors can decrease placental blood
  flow
• Fetal cardiac output

List the mechanisms
of transport across the placenta

Mechanism of transport across placenta:

• Diffusion
  • Governed by Fick’s law of
    diffusion where rate of diffusion is proportional to surface area and
    concentration gradient and inversely proportional to membrane thickness
  • Examples include water, electrolytes
    and gases
• Facilitated diffusion
  • Transport by specific carriers
    down a concentration gradient and does not require energy expenditure – glucose
    is transported in this manner via hexose transporters.
• Active transport
  • Mediated by specific protein
    carrier, occurs against a concentration or electrochemical gradient, involves
    energy expenditure
  • Examples include amino acids,
    calcium and iron
• Piniocytosis
  • Only way to transport molecules
    which are large, non-lipid soluble and/or do not have a carrier. The substance
    becomes completely enveloped into invaginations of the membrane and is then
    released on the other side of the cell.
  • Examples include globulins,
    phospholipids, lipoproteins, IgG antibodies

What is the Double
Bohr and Double Haldane effect?

Oxygen transfer from the maternal to the fetal circulation
is via simple diffusion down a partial pressure gradient of 30mmHg. This oxygen
transfer is facilitated by fetal haemoglobin which has a higher affinity for
oxygen compared to adult haemoglobin and has a left shifted oxygen haemoglobin
dissociation curve. Furthermore, the haemoglobin concentration of fetal blood
is approximately 50% greater than that of maternal blood, thus increasing
oxygen transport. The Bohr effect states that increases in the partial pressure
of carbon dioxide of blood or a decrease in pH will lead to a rightward shift
of the oxygen-haemoglobin dissociation curve thus decreasing the affinity of
haemoglobin for oxygen. The Double Bohr effect describes this happening in opposite
directions in the maternal and fetal circulations. Within the feto-placental
circulation, blood is releasing co2 down its concentration gradient from fetal
to maternal blood. The fetal blood is thus becoming more alkaline leading to a
leftward shift in the o2-hb dissociation curve and increasing the fetal
haemoglobin’s affinity for oxygen. Conversely, within the utero-placental
circulation the maternal blood is gaining co2 leading to a right shift of the
o2-hb curve and encouraging oxygen release from maternal haemoglobin.

The Haldane effect describes the effect of oxygen on co2
carriage with deoxygenated haemoglobin having a greater affinity for carbon
dioxide. The Double Haldane effect refers to the fact this happens in opposite
directions within the fetal and maternal interface. Within the utero-placental
circulation oxygen is lost thus increasing haemoglobin’s affinity for carbon
dioxide. Conversely within the feto-placental circulation, oxygen is gained
thus decreasing haemoglobin’s affinity for c02.

List the differences
in the respiratory system between newborns and adults

The differences can be divided into anatomical, lung
mechanics, lung volumes and respiratory control.

Anatomical: can be sub-divided into upper and lower
airways.

Upper airways:

• Narrow nasal passage
• Large tongue
• Obligate nasal breathers à obstruction can cause respiratory distress
• Epiglottis is stiff, relatively
  larger, U shaped and angled back at about 45 degrees – C1 in children C3 in
  adults
• Glottis is high: C3-4 (C5-6 in adults)
  with anterior angulation
• Cricoid is usually narrowest
  part until age 8 (then becomes vocal cords) – subglottic oedema can cause
  severe respiratory distress
• Cricoid opposite C4 (C6 in
  adults)
• Thyroid cartilage flat and
  overlying cricoid cartilage and hyoid bone overlies it à space
  difficult to discern

Lower airway

• Trachea is short 4-5cm (12cm in
  adults), diameter 6mm (2cm in adults)
• Right main bronchus is more
  directly in line with trachea than left side à
  increased risk of right main intubation
• Peripheral airways with
  diameter less than 2mm contribute 50% of airway resistance (contribute only 20%
  in adults) à can cause severe resp distress in disease
• Less bronchial muscle present à bronchospasm is uncommon

Lung mechanics:

• Chest wall is highly compliant
  (necessary to allow passage through birth canal) à ribs
  and sternum primarily cartilage so poor chest wall stabilisation
• Ribs are horizontal – no bucket
  handle/pump movements during breathing à
  minimal thoracic component to breathing
• Breathing is largely
  diaphragmatic but horizontal insertion of diaphragm and low proportion of high
  oxidative capacity fibres mean it is susceptible to fatigue à abdominal distension causes resp distress
• Alveolar ventilation is
  120-140ml/kg/min (twice that of adult) and have short time constants – work of
  breathing is thus minimised at high respiratory rates rather than large tidal
  volumes
• Ratio of inspiration:
  expiration is 1:1

Lung volumes and gas exchange:

• FRC is 30ml/kg (same as adult)
  – but less stable and more prone to atelectasis
• FRC is less than closing
  capacity – gas trapping due to closure of small airways occurs during normal
  ventilation à alveolar-arterial po2 gradient is increased (30mmHg compared to
  5mmHg) due to venous admixture
• Oxygen consumption is higher on
  weight basis 6-7ml/kg/min compared to 3-3.5m/kg/min in adults à increased metabolic rate and consequent increased o2 demand is
  aided by high alveolar ventilation, high cardiac output and distribution of
  high percent of CO to metabolically active tissues

Respiratory control:

• Less developed especially in
  premature babies
• Periodic respirations are
  common – pauses 5-10 seconds during sleep up to 6x/night normal

Tagged as: Intensive care, Pregnancy, Physiology.