Transcript Pediatric Cardiovascular Physiology

Pediatric Fundamentals – Heart and Circulation
Embryology
1. Cardiovascular system begins forming at 3 weeks
(diffusion no longer adequate)
2. Angiogenetic cell cluster and blood islands ->
intraamniotic blood vessels
3. Heart tube
4. Heart begins to beat 22 – 23 days
5. Heart looping -> 4 chambers, 27 – 37 days
6. Valves 6 – 9 weeks
Pediatric Fundamentals - Growth and Development
Cardiovascular system
In utero circulation
placenta ->
umbilical vein (UV)->
ductus venosus (50%) ->
IVC ->
RA ->
foramen ovale (FO) ->
LA ->
Ascending Ao ->
SVC ->
RA ->
tricuspid valve ->
RV (2/3rds of CO) ->
main pulmonary artery (MPA) ->
ductus arteriosus (DA) (90%) ->
descending Ao ->
umbilical arteries (UAs)->
Pediatric Fundamentals – Heart and Circulation
Transitional circulation
Placenta Out and Lungs In
PVR drops dramatically
(endothelial-derived NO and prostacyclin)
FO closes
DA closes
10-12 hours to 3 days to few weeks
prematures: closes in 4-12 months
PFO potential route for systemic emboli
DA and PFO routes for R -> L shunt in PPHN
Pediatric Fundamentals – Heart and Circulation
Persistent pulmonary hypertension of the newborn (PPHN)
Old PFC misnomer
Primary
Secondary
meconium aspiration
sepsis
birth asphyxia
Treatment
cardiopulmonary support
inhaled NO
ECMO
Pediatric Fundamentals – Heart and Circulation
Nitric oxide (NO) – cGMP transduction pathway
l-arginine
↓
eNOS (endothelial NO synthetase)
oxidation of quanidine N moiety
NO
activates
↓
GTP
↓
sGC (soluble guanylate cyclase)
cGMP (cyclic-3’,5’-guanosine monophosphate)
activates
↓
protein kinase
PDE (phosphodiesterase)
GMP
Pediatric Fundamentals – Heart and Circulation
Neonatal myocardial function
Contractile elements comprise 30% (vs 60% adult) of newborn myocardium
Alpha isoform of tropomyosin predominates
more efficient binding for faster relaxation at faster heart rates
Relatively disorganized myocytes and myofibrils
Most of postnatal increase in myocardial mass due to
hypertrophy of existing myocytes
Diminished role of relatively disorganized sarcomplasmic reticulum (SR)
and greater role of Na-Ca channels in Ca flux so
greater dependence on extracellular Ca
may explain:
Increased sensitivity to
calcium channel blockers (e.g. verapamil)
hypocalcemia
digitalis
Pediatric Fundamentals – Heart and Circulation
Myocardial energy metabolism
Young infant heart
lactate: primary metabolite
later: glucose oxidation and amino acids (aa’s)
metabolize glucose and aa’s under hypoxic conditions
(may lead to greater tolerance of ischemic insults)
Gradual transition to adult:
fatty acid primary metabolite by 1-2 years
Pediatric Fundamentals – Heart and Circulation
Normal aortic pressures
Wt (Gm)
1000
2000
3000
4000
Age (months)
1
3
6
9
12
Sys/Dias
50/25
55/30
60/35
70/40
mean
35
40
50
50
Sys/Dias mean
85/65
50
90/65
50
90/65
50
90/65
55
90/65
55
Pediatric Fundamentals – Heart and Circulation
Adrenergic receptors
Sympathetic receptor system
Tachycardic response to isoproterenol and epinephrine
by 6 weeks gestation
Myocyte β-adrenergic receptor density
peaks at birth then
decreases postnatally
but coupling mechanism is immature
Parasympathetic, vagally-mediated responses are mature at birth
(e.g. to hypoxia)
Babies are vagotonic
Pediatric Fundamentals – Heart and Circulation
Normal heart rate
Age (days)
1-3
4-7
8-15
Rate
100-140
80-145
110-165
Age (months) Rate
0-1
100-180
1-3
110-180
3-12
100-180
Age (years)
1-3
3-5
5-9
9-12
12-16
Rate
100-180
60-150
60-130
50-110
50-100
Pediatric Fundamentals – Heart and Circulation
Newborn myocardial physiology
Type I collagen (relatively rigid) predominates (vs type III in adult)
Cardiac output
Starling response
Compliance
Afterload compensation
Ventricular
interdependence
Neonate
HR dependent
limited
less
limited
high
Adult
SV & HR dependent
normal
normal
effective
relatively low
So:
Avoid (excessive) vasoconstriction
Maintain heart rate
Avoid rapid (excessive) fluid administration