Pediatric Anesthesiology
Congenital Heart Disease - 2
III. Developmental physiology
A. Perinatal circulatory changes
- Fetal circulation
- Hallmark is presence of a large, low-resistance vascular bed, the placenta.
- Blood flow: placenta -> umbilical vein (pO2=28) -> IVC -> RA -> (via foramen ovale) LA -> LV -> ascending Ao (brain)
-> SVC -> (pO2=22) -> RA -> RV -> ductus arteriosus -> descending Ao -> placenta
- PVR >> SVR
- Combined ventricular output (CVO) = CO
- thicker RV pumps 2/3rds of CVO (10% to lungs)
- LV pumps only 1/3rd of CVO
- Ductus arteriosus is largest intrauterine vessel
- Postnatal circulatory changes
- "Transitional circulation" concept
- First days to weeks of life
- Relatively "unstable" circulation
- Ductus & foramen ovale available for shunting
- Pulmonary vascular resistance relatively higher and "sensitive"
- Ductus arteriosus
- Funtional closure in 24-48 hours
- Constriction:
- O2
- Prostaglandin inhibitors (indomethacin)
- Dilation:
- Anatomic closure 2-3 weeks
- Foramen ovale
- Functional closure first hours as LA > RA pressure
- Septum primum acts as 1-way "flap valve"
- Probe-patent
- 50% for 5 years
- 25% for 20 years
- Source of paradoxical embolus
- Pulmonary vascular resistance
- Marked decrease with the first breath
- Similar to adult by about 6 weeks
- Hypoxic pulmonary vasoconstriction marked
- Persistent pulmonary hypertension of the newborn (PPHN, "PFC") may be stress related:
- Hypoxia
- Acidosis
- Hypothermia
B. Developmental changes - infancy and childhood.
- Newborn "RVH" is normal with RV=LV thickness; gradual change to LV=2RV.
- NB myocardial ultrastructure
- small, rounded, disorganized myofibrils
- only 30% of cell mass = contractile protein (vs 60% in adult)
- Autonomic nervous system
- Parasympathetic system essentially complete at birth
- Sympathetic innervation of heart and vasculature incomplete
- Greater dependence on adrenal-circulating catecholamine system
- Vagal tone predominates
- Functional limitations of NB ventricles
- Relatively noncompliant
- Relatively restricted in ability to change stroke volume
- Plateau of press-vol curve reached early, after which:
- Cardiac output more rate-dependent
- By age 2-3 years, cardiovascular system essentially that of a very fit adult
