Changes in Obstetric Physiology Flashcards
Plasma Volume
Increases by 45% due to progesterone/oestrogen causing renin release and therefore activation of renin-angiotensin axis causing sodium retention and increase in total body water
Red cell mass
Increases by 20% due to increase in renal erythropoietin
Haemoglobin concentration
Falls to around 120 g/L - physiological anaemia of pregnancy (the increase in plasma volume is more than the increase in red cell mass)
White cell count
Increases throughout pregnancy
Systemic vascular resistance
Reduces by 20% due to increased levels of oestrogen and progesterone
Systolic and diastolic blood pressure
Reduced due to lower SVR
Heart rate
Increases by 25% as a response to reduced SVR (by 20%)
Stroke volume
Increases by 25%
Cardiac output
Increases by 50% by third trimester
Aortocaval compression: compensation
Occurs typically after 20 weeks, may cause maternal hypotension - compensation is an increase in sympathetic tone causing vasoconstriction and tachycardia and utilisation of azygos veins and vertebral plexus.
Aortocaval compression: complications
In 10% of mothers, causes inadequate blood pressure in the supine position (and may cause mother to lose consciousness)
However, may cause foetal hypoxia in the asymptomatic mother
Coagulation: Factors and Fibrinogen
Gradually increase therefore pregnancy is a hypercoagulable state
Platelets
Function remains normal. Increased turnover.
Anatomical changes of the respiratory system
Capillary engorgement and increased oedema of the upper airway (therefore bleeding is more likely and intubation may be more difficult)
Minute ventilation
Increased
Functional Residual Capacity
Reduced by 20%
Residual volume and expiratory reserve volume
Decreased
Inspiratory reserve volume
Increased
FEV1, Vital Capacity, Total Lung Capacity
Unchanged
Pre-oxygenation
Less effective due to reduced FRC
Respiratory Rate
Increased by 10% (progesterone mediated hypersensitivity to CO2)
Renal function
Increased (therefore reduced serum urea, creatinine and bicarbonate)
Propensity to UTI
Increased due to progesterone mediated uterine smooth muscle relaxation causing urinary stasis
Acid-base regulation
Increased RR causes reduced PCO2 causing leftward shift in oxyhaemoglobin dissociation curve
2,3-DPG concentration
Increased by 30%
Lower oesophageal sphincter tone
Reduced, increasing propensity to heartburn (worsened by displacement by gravid uterus) - progesterone induced smooth muscle relaxation
Upper oesophageal sphincter tone
Not affected
Gastric emptying
Pregnancy does not affect gastric emptying
However, labour increases gastric volume and decreases gastric emptying
Insulin production
Rises
Insulin resistance
Rises
Does insulin cross the placenta?
No
Superior boundary of the epidural space
Foramen magnum
Inferior boundary of the epidural space
Sacrococcygeal membrane
Anterior boundary of the epidural space
Posterior longitudinal ligaments, vertebral bodies
Posterior boundaries of the vertebral space
Vertebral laminae, ligamenta flava
Lateral boundaries of the vertebral space
Open, pedicles and intervertebral foraminae
Layers of ligament to the epidural space
Supraspinous ligament (crunchy, approx 2cm) Interspinous ligament (spongy, up to 6cm) Ligamentum flavum (variable thickness, tough, loss of resistance technique here)
Epidural test dose
Small enough that it will do no harm if in the wrong place e.g. subarachnoid
