pregnancy Flashcards
describe the phyiological changes that occur in pregnancy
can be described by system and in time order. ALL occur to support developing fetus and prepare mum for birth and breast feeding.
governed by hormonal changes - bHCG, progesterone, oestrogen, human placental lactogen.
and also the mechanical effects of the fetus…
AIRWAY
* upper airway engorgement for tissue oedema (progesterone) - may need smaller tube
* breasts pushed up and shorted neck may make positioning hard and difficult aiway risk
BREATHING
* reduced FRC from gravid uterus and larger breasts
* closing capacity may encroach FRC - shunting, hypoxaemia
* bronchial smooth muscle relaxation (progesterone) - reduced resistance, increased deadspace.
* increase in TV and RR by term and hence MV
CARDIO
* progesterone dilates vessels - drop in SVR - both systolic BP and diastolic drop but systolic more - widened pulse pressure.
* increased CO - due to increase HR, SV and drop in SVR. and increased blood volume increases preload. increases all way up to delivery.
* aortocaval compression - compression of IVC and aorta by gravid uterus can reduce preload/ cardiac output - left lateral tilt to avoid this.
NEURO
* reduced epidural and intrathecal space - due to venous engorgement from aortocaval compression from gravid uterus - hence need less anaesthetic needed and higher risk of catheter becoming misplaced.
* higher sympathetic tone in pregnancy helps to counter effects of progesterone. however when sympathetic blocked in spinal, can see a big drop in BP
* MAC reduced
RENAL:
* increased blood flow and therefore GFR (150ml/hr)
* increased urine volume - drop in plasma urea and creat
* hence T max redcued for glucose - glucosuria is normal.
* progesterone relaxes ureters - increased stasis and infection risk.
GASTRO:
* reduced LOS tone and increased gastric pressure - more risk of reflux and aspiration
* hence commonly use antacids in labour and RSI
* returns to normal 48 hours after labour
LIVER:
* reduced production of plasma cholinesterases and albumin - affects pharmacokinetics
* increased gallstone risk
HAEM
* increased plasma volume - secondary to RAAS simulation and fluid retention
* red cell volume falls initially then increases back to normal by 16 weeks and then above normal by term. due to EPO increase
* however the above are disproportionate i.e. red cell volume increases less than plasma hence drop in haematocrit and physiological anaemia
* neutrophilia is seen
* hypercoaguable state - increased fibrinogen and clotting factors.
ENDO:
increased levels of corticosteroids
risk of sheehans syndrome at birth
changes to insulin production to counter anti-insulin hormones of fetus - risk of gestational diabetes.
what happens to compliance of lungs in pregnacy?
chest wall compliance reduced
lung compliance no change
overall reduced.
what happens to blood gas in pregnancy?
fall in PaCO2 due to alveolar hyperventilation - resp alkalosis
drop in bicarb to compensate
PaO2 may be higher as well due to lower PaCO2 - as per alveolar gas equation.
what happens to pulmonary vascular resistance in pregnancy?
drops
progesterone
what is vena cava compression syndrome seen in pregnancy?
compression of vena cava
drops preload and hence CO
sweating , pallor, drop in BP , nausea
can also cause fetal distress if cardiac output drops
describe change in CO throughout pregnancy and at delivery
even at birth there is an increase
as the uterine contraction and delivery of placenta results in return of 500ml of blood to systemic circulation - hence increased preload.
what murmur may be normal in pregnancy?
systolic murmurs
LV hypertrophy
can you describe some hormonal changes seen in pregnancy?
bHCG - early secreted from placenta, maintains corpus luteum for progesterone secretion
human placental lactogen - growth of fetus and breasts, decreases insulin sensitivity to promote hyperglycaemia for baby
oestrogen - made by placenta - increases uteroplacental blood flow, stimulates uterine growth and sensitises the endometrium to oxytocin. increase VTE risk
progesterone - wide variety of roles. uterine relaxation, breast development, vasodilation
what changes are seen to skin and MSK system in pregnancy?
increased melanocyte stimulating hormone (MSH) - hyperpigmentation - linea nigra
increase in weight - fetus, blood volume, placenta etc , breast enlargmnet
MSK - relaxation of ligaments ready for birth, widening of pubic symphysis
describe the development of the placenta.
derived from materal and fetal parts
fetus - oocyte divides and starts to differentiate.
eventually becomes a blastocyst with inner and outer cell mass.
the throphoblasts of fetal part invade the materal part (inner lining of endometium - deducia)
the deducia contains spiral arteries which drain into intervilous spaces
the fetal cells orgnaise to form a villous tree of fetal blood which sits between these spaces - increases S.A and contact between maternal and fetal blood for gas exchange
describe the structure of the placenta
placenta consists of both maternal and fetal material.
the maternal endometrium has been invaded by fetal tissue and cells and organised to give the following..
- The maternal uterine artery supplies spiral arteries within the placenta.
- spiral arteries these drain into the intervillous space - low resistance, low pressure vascular bed.
- the 2 fetal umbilical artery (arise from internal iliac arteries) supply the chorionic villous which descend into the intervillous space and come into close contact with maternal blood
- allows gas exchange, nutrient exchange and other exchange.
what are the functions of the placenta?
- gas exchange - O2 and CO2
- nutrient and waste exchange - glucose, urea etc
- metabolic functions - contains some enzymes for metabolism of drugs
- Endocrine functions - hCG maintains corpus luteum for progesterone in early preg. progesterone secreted later responsible for maternal adaptations to support pregnancy. oestrogens also support fetal growth and prepare endometrium for birth. human placental lactogen - insulin insensitivity for growth
- transfer of immune complexes - igG gives passive immunity to fetus
describe the gas exchange seen across placenta
deoxygenated blood via 2 fetal umbilical arteies (from internal iliac artery) supplies blood to chorionic villous tree
this is is close contact with the intervillous space where blood is supplied from materal spiral artery (oxygenated)
hence diffusion of CO2 and O2 occurs
oxygenated blood leaves fetal chorionic villous tree and enters the umbilical vein to circulate fetal circulation.
describe the layers of the uterine wall
what is the equation for uteroplacental blood flow?
UBF = difference in uterine arterial and venous pressure / uterine vascular resistance
V=IR
what factors influence uterine blood flow?
V= IR
hence
pressure difference - increase in uterine arterial pressure or reduced venous pressure. OR reduced resistance…
increased venous pressure - from contractions, IVC compression and valsalva during labour - can reduce flow
maternal arterial hypotension - e.g. bleeding, aorta compression, sympathetic block can reduce flow
increased vascular resistance e.g. pre-eclampsia, vasoconstrictors - can reduce flow
what is the double bohr effect?
bohr effect describes the right shift of O2 dissciation curve seen with acidosis
across placenta this is seen as a 2 fold effect
as CO2 crosses placenta to maternal circulation, it causes drop in pH and right shift of materanl Hb dissociation curve - mum therefore gives up O2.
as CO2 leaves the fetus - shift fetal Hb to the left - higher affinity for O2
how is fetal blood adapted to take O2 from mum?
fetal Hb subunits have naturally a higher affinity for O2
double bohr effect
increased 2,3 DPG in maternal blood - shifts to right
how does haldane effect play a role in transfer of gases across placenta
as O2 enters fetal circulation and Hb becomes oxygenated, it has less affintiy for CO2 and hence less carbamino Hb produced.
as O2 leaves maternal circulation, deoxygenated Hb has more addintiy for CO2 so more CO2 can be carried as carbamino compounds
= 2 fold
describe the mechanisms of transport across placenta..
simple diffusion - CO2, O2
fasciliated diffusion - glucose, aa
primary active transport - iron, calcium
secondary AT - amino acids
ionic transport - channels
osmosis
pinocytosis - IgG
draw a graph to show how hormones change throughout pregnancy..
hCG - down at 12 weeks.
progesterone - sharp rise and levels out around 15 weeks
oestrogen steady rise from 12 weeks
hpl - steady rise from 5 weeks
what factors affect transport of drugs across placenta?
ficks principle describes the rate of diffusion as being equal to
Kx S.A x Conc difference / distance
K = diffusion contant for a specific drug - depends on molecular weight, ionisation, lipid solubilty, protein binding etc
therefore increasing S.A, conc difference and reducing S.A
inturn
grahams law - diffusion proportional to 1/root MW - hence larger molecules slower diffusion
ionisation - will also slow down diffusion, hence pH and pKA will influence this.
lipid solubility will increase diffusion.
protein binding - the more free portion, the more can diffuse (in pregancy PB is reduced)
on top of this placental blood flow!
describe the significance of transfer of local anaesthetic agents across the placenta…
LA are weak bases
ionise below their pKa
fetal blood is more acidic than materal
unionised LA will cross placenta - small and lipid soluble.
then become ionised at fetal pH
ion trapping can occur
can result in toxic levels in fetus
describe the transfer of common anaesthetic agents across the placenta
volatiles - all cross - small and lipid soluble hence rapid transfer
opioids and benzos - lipid soluble and cross placenta freely.
muscle relaxants - large and charged - do not cross placenta easily.
