PHYSIOLOGY OF TRANSITION PERIOD IN NEONATES WITH RESPECT TO CARDIOVASCULAR SYSTEM

Speaker: Dr Bhagirath.S.N

Moderator: Dr Sarika

Understanding Foetal Circulation

• Parallel system pumping into the systemic circulation

• Three shunts are active before birth

Placenta

Foramen ovale

Ductus arteriosus

• Upper body receives better oxygenated blood compared to the lower body

What happens at birth….?

•Three shunts begin to close one by one

•Closure of placenta causes increase in systemic vascular resistance

•Onset of breathing causes the foramen ovale to close

•Increasing O2 tension prompts the closure of ductus arteriosus

•Functional closure: 1st hour of life•Anatomical closure: 1st year of life

•Functional closure: 24-48 hours•Anatomical closure:

A saturation schematic at different points of time

Foetal saturation values

Saturation values at day 1

Saturation values in a neonate

The transition from foetal to neonatal circulation

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Changes in the myocardium

Foetal myocardium differs from the adult heart

•In having fewer myofibril elements

•Less organised contractile elements

•Non-parallel arrangement of myofibrils sacrificing efficiency

•Immature sarcoplasmic reticulum

•Reduced Ca2+ATP activity-reduced contractility.

Implications

•Lesser cardiac reserve

•Poor tolerance to even mildest of hemodynamic

insultsResponse

•Increase in size of myocytes•Increase in number of myocytes. (changes being more pronounced in left ventricle).

Adaptive Mechanisms

Normal mechanisms in the adult

•Sympathetic nervous system

•Parasympathetic nervous system

•Frank-starling mechanism

•Sufficient contractileforce conferringample cardiac reserve

•Mature baroreceptors

•Reduced sensitivityto anesthesia in baroreceptors

What is lacking in the neonate.?

•underdeveloped

•Well developed

•Ineffective due to non-compliant myocardium

•Lacks contractile force (SV)

•Immature baroreceptors

•increased sensitivityto anesthesia in Baroreceptors

Adaptation seen

•Increased glycogen stores

•Increased rate of anaerobic glycolysis

•Advantage being relative resistance to hypoxia

•Increased heart rate as CO=HR X SV

•No adaptation. Implication is that even mild hypotension is poorly tolerated. •Increased depression even at lighter planes of anesthesia

In summary:

• Resting heart rate is much higher compared to body weight because of higher O2 consumption per kg/body weight.

•Even mild bradycardia compromises the cardiac output as stroke volume is limited owing to immature myocardial musculature.

•Cardiac reserve is greatly limited (30%-40%)

•Limited inotropic and chronotropic support from sympathetic nervous system