Physiology 22 Flashcards
Pregnancy
How does cardiac output change throughout pregnancy and delivery?
Increases due to Increased heart rate (+25% by term) and stroke volume (+30% by term)
CO increases by 40% at the end of the first trimester and plateaus at +50% by the end of the second trimester until delivery
During labour, CO further increases by 45% and during the 3rd stage rises to 80% above 3rd trimester levels (partly due to uteroplacental transfusion), this can result in increased CO immediately post-delivery
How does SVR change during pregnancy?
Decreases to 60% of non-pregnant SVR due to:
- Development of a low-resistance vascular bed (intervillious space)
- Vasodilatory effect of progesterone, oestrogens and prostacyclin
How does blood pressure change during pregnancy?
Falls to trough in mid-pregnancy, returning to normal by term
SBP reaches trough of -8%
DBP reaches trough of -25%
(thus MAP reduced to trough ~-20%)
In the supine position, 70% of mothers have a drop in BP of >10%. 8% have a drop of 30-50% (supine hypotension syndrome)
How does blood flow change during pregnancy?
Uterine flow:
-Increases to 12% of CO (700ml/min)
Renal flow:
-Increases
Skin flow:
-Increases
Hepatic and cerebral flow is unchanged
What factors affect aortocaval compression?
- Position
- Gestation
- BP
- Presence of sympathetic block
IVC compression starts from 13 weeks gestation and is maximal at 36-38 weeks. May decline after this due to descent of foetal head.
How is CVP affected by pregnancy / labour?
CVP is normal in pregnancy (except during IVC compression)
During contractions, CVP may increase by 5cmH2O
During delivery, CVP may increase by 50cmH2O
IV ergometrine can increase CVP by around 8mcH2O, lasting around an hour
How does pregnancy affect the blood volume?
- Plasma volume increased by +50% at term, continuing to increase in the first 24h after delivery, then falling to normal by day 6 postpartum
- Plasma expansion occurs due to oestrogen and progesterone stimulating the RAAS
- RBC volume falls in the first 8 weeks, then returns to normal by 16 weeks and rises to +30% by term, due to increased EPO production from week 12
- [Hb] and haematocrit is reduced by around 15% by term
- Total blood volume is increased by 10% (end of 1st trim), 30% (end of 2nd trim) and 45% by term
How does pregnancy affect WCC?
Increased to around 9-11 x10^9
Further increased during labour to ~15x10^9
Primarily polymorphonuclear cell proliferation
How does pregnancy affect coagulation?
Increases in:
Platelet turnover:
- thrombocytopaenia in up to 1% of women
- Increased platelet factor
- Increased β thromboglobulin
Clotting:
- Increase in all coagulation factors except:
- XI, XIII - decreased
- II, V - unchanged
- AT-III reduced
Fibrinolysis:
-Increased fibrinogen degradation products and plasminogen
PT, APTT and bleeding time fall slightly
How does pregnancy affect plasma proteins?
Reduced:
- Albumin
- α acid glycoprotein
- Pseudocholinesterase (-25%)
Increased:
- Globulin
- Fibrinogen
Overall conc. drops by 65-70g/L leading to:
- Decreased colloid osmotic pressure
- Altered drug binding
- Increased ESR (decreased blood viscosity)
Outline anatomical changes in the respiratory system during pregnancy
Mediated by increased progesterone
- Upper airway capillary engorgement -> tissue oedema
- Flaring of ribcage increases thoracic circumference by 5-7cm, reducing chest wall movement
- Bronchial SM relaxation maintains lung compliance by reducing airway resistance. FEV1, FVC and flow-volume loops are unchanged.
- Enlarging uterus displaced diaphragm upward later in pregnancy
Volumes/capacities:
- TV ↑ by 45% at term
- RR ↑ by 10%, plateauing at 20 weeks
- Thus MV ↑ by 50% and AMV ↑ by 70% at term
- FRC ↓ by 20-30% at term due to decreases in both ERV and RV
- Closing capacity encroaches on FRC, leading to potential V/Q mismatch and hypoxia
How are ABG values affected by pregnancy?
- PaCO2 reduced to 3.7-4.2 kPa by increased AMV
- Compensation by reduction in plasma HCO3- to 19-21 mmol/L
- Incomplete compensation results in increase in blood pH of ~0.04 units
- PaO2 is higher in erect pregnant women due to lower PaCO2 levels
- PaO2 reduces approaching term due to increased O2 consumption not fully compensated for by increased CO and DO2
What is the effect of pregnancy on the HbO2 dissociation curve?
pCO2 decreased -> left pressure
Increased 2,3-DPG -> right pressure
Net movement of HbODC to right
P50 non-pregnant: 3.5 kPa
P50 pregnant: 4.0 kPa
How is PVR affected by pregnancy?
Reduced by around 1/3 at term, increasing pulmonary blood flow and volume.
No effect on RV/PA/PC pressures in health
How does pregnancy affect a woman’s risk of aspiration of stomach contents?
Increased, due to:
- Reduced barrier pressure (altered stomach position and displacement of lower oesophagus from abdomen into thorax, reducing LOS pressure
- Relaxant effect of progesterone
- Gastric pressure elevation in 3rd trimester (supine and standing) and even further in lithotomy (+5.6 cmH2O)/Trendelenburg (+8.8cmH2O) position
- LOS pressure returns to normal by 48h post-delivery
- Delayed gastric emptying during labour (esp. if opioids given), returning to normal 48h post-delivery in absence of opioids
How common is heartburn in pregnancy?
up to 80% of women may complain of dyspepsia, which may commence earlier than 20 weeks gestation
What is the effect of pregnancy on the epidural space?
Aortocaval compression engorges epidural veins -> reduced volume of epidural space
This increases spread of solutions injected epidurally
Pressure in the epidural space is slightly positive in the pregnant patient. During contractions is can rise to 8cmH2O and during expulsion can rise to 60cmH2O
What is the effect of pregnancy on the subarachnoid space?
Aortocaval compression -> increased CSF pressure
At term CSF pressure is 28cmH2O. Contractions and expulsion may increase this to 70cmH2O
The constituents of CSF do not change
What effect does pregnancy have on the sympathetic ANS?
Increases throughout pregnancy, mainly acting to limit lower limb venous capacitance and counteracting IVC compression
Sympathetic block can result in marked decrease in blood pressure for pregnant women.
How are doses of local anaesthetic for spinal/epidural anaesthesia affected by pregnancy?
- Reduced epidural/subarachnoid volumes
- Increased nerve fibre sensitivity to LAs
- Hypocapnia leads to reduced buffering of LAs so they last longer as free bases
How does pregnancy affect MAC?
Reduced by 40%, possibly due to increased progesterone levels
What happens to β-endorphin levels during pregnancy?
Increased throughout pregnancy, labour and delivery
What is the effect of pregnancy on the thyroid?
- Increased size and vascularity
- Increased iodine uptake and possible goitre development
- Increased TBG levels but free T3/T4 remains constant or fall
What is the effect of pregnancy on adrenal function?
- Increased corticosteroid production (plasma levels increased up to 5x)
- t/12 of cortisol doubled due to decreased clearance
