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 circulationplacenta ->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)↓NO

oxidation of quanidine N moiety

sGC (soluble guanylate cyclase)

↓activates

↓GTP

cGMP (cyclic-3’,5’-guanosine monophosphate)activates

↓protein kinase

↓GMP

PDE (phosphodiesterase)

Pediatric Fundamentals – Heart and Circulation

Neonatal myocardial function

Contractile elements comprise 30% (vs 60% adult) of newborn myocardiumAlpha isoform of tropomyosin predominates

more efficient binding for faster relaxation at faster heart ratesRelatively disorganized myocytes and myofibrilsMost of postnatal increase in myocardial mass due to

hypertrophy of existing myocytesDiminished role of relatively disorganized sarcomplasmic reticulum (SR)

and greater role of Na-Ca channels in Ca flux sogreater dependence on extracellular Camay explain:

Increased sensitivity to calcium channel blockers (e.g. verapamil)hypocalcemiadigitalis

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) Sys/Dias mean1000 50/25 352000 55/30 403000 60/35 504000 70/40 50

Age (months) Sys/Dias mean 1 85/65 50 3 90/65 50 6 90/65 50 9 90/65 55 12 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) Rate 1-3 100-140 4-7 80-145 8-15 110-165

Age (months) Rate 0-1 100-180 1-3 110-180 3-12 100-180

Age (years) Rate 1-3 100-180 3-5 60-150 5-9 60-130 9-12 50-11012-16 50-100

Pediatric Fundamentals – Heart and Circulation

Newborn myocardial physiology

Type I collagen (relatively rigid) predominates (vs type III in adult)

Neonate AdultCardiac output HR dependent SV & HR dependentStarling response limited normalCompliance less normalAfterload compensation limited effectiveVentricular high relatively low interdependence

So:

Avoid (excessive) vasoconstrictionMaintain heart rateAvoid rapid (excessive) fluid administration