Neonatal Circulation Adjustments

• Foetal and Neonatal Physiology

Foetal Circulation

• Pulmonary and systemic circulations operate in parallel due to foramen ovale and ductus arteriosus shunts.
• The left ventricle (LV) and right ventricle (RV) are the same size and thickness.
• Cardiac output depends on heart rate, with foetal distress indicated by bradycardia (HR < 100 bpm) suggesting hypoxemia, or tachycardia (HR > 180 bpm) potentially from skull pressure.

Blood Flow and Oxygenation

• Oxygenated blood returns from placenta via the umbilical vein which leads to the hepatic portal vein
• 60% of blood is shunted into the IVC via the Ductus venosus"
• 40% of blood mixes with circulation of the GIT/liver
• IVC blood goes to the right atrium
• 60% of the blood is directed to the left atrium via the foramen ovale to be supplied to coronary arteries and the brain
• 40% mixes with poorly oxygenated blood draining from the head/neck and passes to into the pulmonary artery
• 10% of this blood enters the lungs
• 90% is shunted via the "Ductus arteriosus" to perfuse the lower body
• This blood returns to the placenta cia the umbilical arteries

Foetal Circulation Note

• The venous return to the heart is 70% via IVC, 20% via SVC, and 10% via lungs/coronary sinus.
• LV afterload has high resistance in cerebral and upper body circulation.
• RV afterload has low resistance in the ductus arteriosus and placenta, and high resistance in the pulmonary vasculature and lower body.

Circulatory Changes at Birth

• The circulatory system transitions from parallel (foetal) to serial (adult) as a result of increased resistances.

Key Adaptations at Birth

• Clamping of umbilical vessels causes an increased systemic vascular resistance and decreased venous return to the right atrium.
• Expansion of lung parenchyma and pulmonary HPV decreases resistance in the lungs
• Closure of ductus venosus occurs and decreases VR via IVC
• Shunt closures result in reversion to adult circulation.
• The foramen ovale closes due to increased left atrial pressure.
• Ductus arteriosus constricts due to higher PaO2 after the first breath and decreased PGE1/E2 levels.

Respiratory Changes at Birth

• Loss of gas exchange through the placenta
• Ventilation of the newborns lungs leads to commencement of pulmonary gas exchange and the start of FRC

Key Reperatory Adaptations at Birth

• Compression of the foetal thorax during vaginal delivery squeezes out 35 mL of lung fluid.
• Clamping the umbilical ensures that gas is no longer exchanged
• Hypoxemia, hypercapnoea, and acidaemia stimulate the first few breaths.
• Environmental sensory stimuli stimulate the reticular system and increase sensitivity in respiratory centres.
• Chemoreceptors show greater responsiveness due to circulation.
• The 1st breath requires high pressure magnitude for subsequent breaths decreases do to presence of surfactant

Neonatal Airways

• The first breath results in rapid establishment of Functional Residual Capacity (FRC), pulmonary gas exchange, and decreased pulmonary vascular resistance (PVR).
• A significant decrease in resistance of the a system can be do to the expansion of lung parenchyma and a removal of pulmonary by increase in PAO2
• Ductus arteriosus closes within 10-15 hours due to contraction from increased PaO2 with first breaths and decreased [PGE1/2].
• Foetal lung fluid is absorbed into pulmonary circulation.

Temperature Regulation

• Thermoneutral Zone (TNZ): The range of ambient temperatures where the core body temperature is kept at normal without increasing the metabolic rate or o2 Consumption

TNZ of a Neonate vs Adult

• Higher and narrower range than adults because neonates have increased evaporative heat losses, requiring higher ambient temperatures.
• Critical temperature limits decrease with maturity and body size.

Neonatal Thermoregulation

• Neonates' limited ability to balance hear production and heat loss results in a risk of heat or cold stress with changes in ambient temperatures.
• There is a large surface area relative to volume which increases environmental heat gain and loss
• There are thin subcutaneous tissues which decreases insulation and higher heat gain/loss
• Higher TNZ results in environmental heart losses at lower ambient temperatures
• Higher BMR requires high heat loss.
• Limited sweating and shivering ability

Mechanisms of Neonatal Thermoregulation

• Within the TNZ, there are little demands on temperature because it is maintained by changes in skin blood flow
• For cold stress, the body can invoke behavioural changes such as crying or shivering. Skin vasoconstriction. Non-shivering thermogenesis. Muscular activity.
• For heat stress, the body invoke behavioural changes removing coverings. Skin vasodilation and sweating.

Aside Thermogenesis

• Non Shivering Thermogenesis is when the SNS-mediated uncoupling of oxidative phosphorylation in brown fat and skeletal muscle causes an increase in metabolic heat production
• Brown fat has SNS control
• The uncoupling of mitochondrial oxidative phosphorylation allows more heat to be produced for the amount of metabolic fuels metabolized.

Hypothermia dangers in neonates

• Hypoxemia do to high MRO2 or respiratory distress
• CVS instability
• Hypoglycaemia
• Delayed recovery from anesthesia
• Coagulopathy
• PK drug alterations for drugs

Hyperthermia dangers in neonates:

• Increased evaporative water loss leading to dehydration
• Apnoeic attacks

Organ-Specific Differences Between Neonates and Adults

• Before: Foetus depends on the mother
• After birth: placenta transitions to neonate's organs, require continuous adaptation until fully mature

Cardiovascular System Differences

• CO is higher at birth
• The blood distribution increases to VRG
• Its dependent on Heart rate
• HR is also high starting at 120-160
• BP starts low
• The SNS system is immature
• High physiological shunting in neonatal circulation

Respiratory System Differences

• In the upper airway, the tongue is large
• There are narrow nasal passage
• New born is an obligate nasal breather
• The epiglottis is stiff and angled
• The glottis sits high at C3/C4
• Trachea is short
• Bronchi branch angle similar for adults
• High risk remains of endobronchial intubation
• Airways are small, there is not much bronchial muscle, chest wall ribs are horizontal
• Lung volume is high, but stability reduced
• Minute ventilation id high
• Lung compliance increases as it expands
• Airway resistance decreases
• There is significant V to Q matching for small airways
• Respiratory controls ares less developed, relies more on chemoreceptors

Flux of Oxygen

• High compared to adults, high levels of Hb and ability to bind
• This allows resting metabolic rates by increasing the O2 flux to tissue

Haematological System

• RBC production begins in the yolk sack of the liver, then the bone marrow
• Red cell survival lasts 30-70 days
• Hb is low, around 110-120 g/L
• Plates at the same levels
• Deficit to Coagulation

Renal System

• Renal blood flow is low do incomplete
• Glomerular is at low function
• Tubular matures slowly

Fluid Compartments

• Total body of water is high, about 75%
• Decreases to adult volumes

Hepatic System

• Poorly developed
• Low amounts of glycogen and albumin

Metabolic System

• Main glucose substrate
• High metabolic function

Nervous System

• High brain, myelation occurs
• Immature blood brain barrio
• Motor muscles need to develop full function

The circulatory system adapts at birth, shifting from parallel to series arrangement due to vascular resistance changes. Umbilical cord clamping decreases SVR and LAP. PVR decreases primarily because of lung constriction. The foramen ovale closes when RAP exceeds LAP.