Unique structures of fetal cardiovascular system
- Oval foramen - between right and left atrium
- Arterial duct - between pulmonary artery and descending aorta
- venous duct - umbilical vein and inferior vena cava
Flow of blood in fetus
- systemic and pulmonary circulation are parallel to each other
- total fetal cardiac output is 450 ml/kg/min (compared to 75 mg/kg/min in adult)
- Pulmonary vascular resistance > systemic vascular resistance
Preferential steaming
The blood from the inferior Vena cava is preferentially diverted to left atrium through crista dividens, so that the developing brain receives the most oxygen
Transitional circulation
Changes at birth
| Changes | Effect |
|---|---|
| increase in arterial PO2 | 1. rapid decrease in pulmonary resistance 2. closes arterial duct to form arterial ligament (occurs over days) |
| removal of placenta - loss of low resistance placental circulation | 1. increased systemic vasular resistance 2. closure of venous duct |
| PVR < SVR | entire right ventricular output flows into pulmonary circulation |
| increased flow to left atrium from pulmonary circulation | closes oval foramen functionally anatomical closure occurs years later |
Consequences
- increase in systemic vascular resistance
- systemic and pulmonary circulation are coupled in series
- left ventricle now needs to pump blood to the entire body instead of just head and brain
- increase in left ventricular output by 200%
- This effect is achieved through combination of hormonal and metabolic signals
- increased catecholamines and their receptors play an important role in this process
Consequences in congenital heart disease
- oval Forman and arterial duct may not close completely at birth
- this can be life-saving (PDA in pulmonary atresia or coarctation of aorta, PFO in TGA) or detrimental (PDA in prematurity, PPHN)
- drugs can either help in closure (indomethacin) or maintaining patency (PG E1) of these structures depending on need
Neonatal circulation
- some of the above-mentioned changes can be instantaneous or some can occur after a long time over hour or even weeks.
- Largest decline in the PVR occurs at day 2-3 of life (can occur till day 7)
- decrease in PVR and increase in SVR is an ongoing process, so minor cardiovascular defects like VSD may not present early in life due to PVR still higher than SVR
Differences between neonatal and adult circulation
- RTL or LTR shunting may still persist through PFO, PDA
- neonatal pulmonary vasculature closes vigorously to hypoxemia, hypercapnia and acidosis
- wall thickness of left and right ventricles are almost equal
- newborn have relatively higher oxygen consumption
- newborn cardiac output is 350 ml/kg/min at birth, decreases to 150 ml/kg/min by 1 to 2 months and further reduces gradually to 75 ml/kg/min
Timing of closure of neonatal structures
| Structure | Functional closure | Anatomical closure |
|---|---|---|
| Oval foramen | about 3 months of age | over years |
| Arterial duct | 10-15 hours | 1-3 months |
| Venous duct | few minutes after removal of placenta | 3-7 days |
Hemodynamic Values: PaO2 and SaO2
| Location | PaO2 (mmHg) | SaO2 (%) |
|---|---|---|
| Umbilical Vein | 30 - 35 | 80% |
| Left Ventricle / Ascending Aorta (Brain/Heart) | 26 - 28 | 65% |
| Descending Aorta (Lower Body) | 20 - 22 | 55 - 60% |
| Pulmonary Artery / SVC | 12 - 15 | 40% |
| Umbilical Arteries | 18 - 20 | 50 - 55% |