Lactation and neonatal physiology Flashcards
describe fetal breathing movements
These periodic movements start at 10 weeks, but peak 2-3 weeks before delivery.
They strengthen the respiratory muscles before birth.
Phrenectomy studies showed that their absence prevents appropriate growth and maturation of the lungs.
describe the first breath a baby takes
First breath. Baby grunts against a closed glottis in order to create a high trans-pulmonary pressure to establish FRC.
The stimuli to the first breath are?
generalised arousal (due to increased sensory input), cold (stimulates C fibres) and tying/breaking the umbilical cord leads to progressive hypoxia and hypercapnia in the neonate.
All of these are thought to activate the reticular formation in brainstem, where the nuclei controlling respiration lie
are light nad sound required for the first breath?
Light and sound are not required, as congenitally blind and deaf babies still breathe.
Maintenance of lung ventilation after birth requires which 2 processes?
Maintenance of lung ventilation after birth requires appropriate lung liquid reabsorption and the maintained synthesis of surfactant.
describe fetal lung liquid turn over?
Lung liquid. Fetal lungs secrete a chloride rich lung liquid (LL) at rates of 4-5 ml.kg-1.h-1. i.e. term fetus (3.5 kg) produces 330 to 450 ml.day-1.
Volume of LL is 30 ml.kg-1 i.e. 100ml.
LL production is vital for normal lung growth and contributes between 1/3 to a 1/2 of daily amniotic fluid turnover.
describe why at birth fetal lung liquid is reabsorbed?
Cortisol increases beta-adrenoreceptor expression in the lung epithelium, it increases epithelial Na+ pump expression, increasing Na+ and thus fluid reabsorption.
The Cl- pump that accumulates ions and thus fluid into the lung is inhibited by catecholamines released close to delivery.
Cortisol also increases the expression of PNMT, phenyl-N-methyl-transferase, which converts noradrenaline to adrenaline in the medulla of the fetal adrenal gland.
Cortisol also increases the activity of deiodinase enzymes that promote the conversion of T4 to T3.
fetal adrenaline, which opens Na+ channels in the lung epithelium.
maturation of the cellular mechanisms respon- sible for the absorptive processes is under the synergistic control of fetal thyroid hormone and fetal cortisol.
where does reabsorbed LL go?
About 30% of the lung fluid is reabsorbed in labour e.g. via the mouth as a result of the rise in intra-thoracic pressure during vaginal delivery. The rest is taken up into the pulmonary interstitium and then removed via the lymphatics, circulation and the kidney (~12 hours, increased diuresis).
problems for LL reabsorbtion in C section babies
Caesarean section prevents some of the hormonal changes associated with lung fluid reabsorption and the ‘squeezing out’ of fluid during delivery - increased fluid retention may lead to transient tachypnoea of the newborn (TTN).
when can pulmonary surfactant first be measured in amniotic fluid?
28 weeks
_______ induces pulmonary surfactant production by type II pneumocytes
Cortisol induces pulmonary surfactant production by type II pneumocytes
what receptor expression is essentia lfor pulmonary surfactant releasE?
beta adreno receptor
Synthesis of pulmonary surfactant is by…
Synthesis is by osmiophillic lamellar bodies (OLBs)
why is surfactant needed?
Surfactant is needed to reduce surface tension and increase compliance of the lung - reducing work of breathing and preventing atelectasis and pulmonary oedema.
The function of pulmonary surfactant is to stabilise the alveoli on expiration thus producing a gas volume at end expiration at low trans- pulmonary pressure gradients.
whast la places equation?
Please remember La Place’s equation which states that the pressure inside a bubble (alveolus) = 2T/r. T, surface tension; r, radius. The action of surfactant allows subsequent breaths to be taken in an “opened” lung and so reduces the work of breathing tremendously.
Some experts say that surfactant on expiration produces a solid monolayer of pure DPPC, as its melting point is over 42 oC. This is an alternative theory on what keeps alveoli opened and patent.
is heat loss increased at birth?
yes - tremendously
Heat loss is increased through evaporation (they are wet at birth), and their limited insulation (as they have little hair and subcutaneous fat). Parental behaviour can help reduce heat loss - e.g. giving infant a wooly hat.
do neonates shiver?
Shivering is limited (as neonates have little voluntary muscle),
how do neonates produce heat?
they can also produce heat through non-shivering thermogenesis (NST). \
NST depends on brown fat, which accounts for 2-6% of body weight in human neonates and has a characteristic distribution round the scapula and kidney (positioned to warm peripheral blood as it enters into the main veins).
how does brown fat produce heat?
In brown fat aerobic respiration (oxygen/ nutrient consumption) is uncoupled from ATP production, so energy is released as heat instead. Brown fat activity may be stimulated by circulating thyroid hormones, catecholamines and stimulation of the sympathetic nerves.
how do babies born in hot condition skeep cool?
Prevention of heat gain in hot environments is even harder.
The high SA:V ratio leads to increased heat gain in neonates.
Additionally, they have a higher threshold for sweating and poorly developed sweating mechanisms.
Again, parental behaviour is crucial e.g. tepid sponging, fans, shade, cover (loose dark clothes- surface heats up and radiates outwards, due to air insulation inwards), drink isotonic solutions, such as breast milk or flat cola.
describe how cardiovascular reflex set points are reset?
The fetal arterial blood pressure is much lower than the postnatal arterial blood pressure, as the placental high capacitance circulation is lost.
Therefore, the set-point and sensitivity of the arterial baroreflex must be changed from fetal to postnatal life to allow a greater resting arterial blood pressure without promoting bradycardia and peripheral vasodilatation.
Similarly, the fetal arterial PO2 is much lower than in postnatal life.
Therefore, the arterial chemoreceptors, principally the carotid body, must change its set-point to begin discharge at a higher PO2 threshold.
Failure of chemoreceptor resetting from fetal to postnatal life may trigger SIDS.
why does the foramen ovale close?
Foramen ovale closes as the pressure in the left heart (from the lungs) now exceeds that of the right, pushing the flap closed.
problems with a patent foramen ovale ?
A patent foramen ovale (PFO) can lead to paradoxical stroke. A paradoxical stroke, also called a crossed embolism, refers to an embolus, which is carried from the venous side of circulation to the arterial side, or vice versa.
why does the ductus arteriosus close?
Ductus arteriosus closes as increased blood flow through it clears out PGE2 and PGI2, which are required to keep it patent.
Evidence for this is that indomethacin (COX inhibitor) causes contraction and arachidonic acid (PG precursor) blocks contraction.
The increase in PO2 is also thought to stimulate closure - in an artificial lung/heart preparation, increasing ventilation decreases diameter of duct.
Closure of the ductus arteriosus leads it to become …..
Closure of the ductus arteriosus leads it to become the ligamentum arteriosum.
how does the ductus venosus close?
what does it bevcome?
Gradual closure of ductus venosus prevents bypass of fetal liver, and the previous duct becomes the ligamentum venosum.
does overall circulatory ressitance increase after birth?
Overall systemic resistance: increases.
due to loss of low resistance placental shunt (~55% of fetal CO goes to placenta). Total cardiac output per unit tissue mass in the neonate is about half that of the fetus (but because PO2 is increased there is still sufficient O2 delivery).
