Cardio pulmonary events at birth full details for md exam answer 15 marks crisp and consise information only for marks
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Cardiopulmonary Events at Birth — 15 Marks
FETAL CIRCULATION (Prerequisite — 2 marks)
In fetal life, the right and left ventricles pump in parallel (not in series). Three key shunts allow this:
| Shunt | Course |
|---|---|
| Ductus venosus | Umbilical vein → bypasses liver → IVC |
| Foramen ovale | RA → LA (right-to-left atrial shunt) |
| Ductus arteriosus | Pulmonary trunk → descending aorta |
Pulmonary vascular resistance (PVR) is very high in utero (hypoxic vasoconstriction, collapsed lungs). Only 8–10% of RV output goes to lungs. The fetal PO₂ through the ductus is only 15–20 mmHg.
TRIGGER FOR FIRST BREATH (1 mark)
The stimulus to breathe is not primarily hypoxia during a normal labor. Key triggers are:
- Sudden exposure to cold, light, sound, and tactile stimuli
- Mild hypercapnia / mild acidosis during labor
- Compression-decompression of thorax during delivery (chest recoil)
PULMONARY EVENTS AT FIRST BREATH (3 marks)
- Lung expansion physically decompresses pulmonary vessels → immediate fall in PVR (to <20% of intrauterine value)
- Rising alveolar PO₂ → nitric oxide (NO) production → pulmonary vasodilation
- Clearance of fetal lung fluid (previously held in alveoli by high surface tension): fluid is absorbed into pulmonary lymphatics and capillaries
- Surfactant (produced from ~24 weeks by type II pneumocytes) lowers surface tension → prevents alveolar collapse after first breath
- First inspiratory pressure is −30 to −50 mmHg (intrathoracic), necessary to overcome surface tension of fluid-filled alveoli
- PVR falls → pulmonary blood flow increases 8-fold → oxygenated blood returns to LA → LA pressure rises
Net effect: PVR ↓↓↓, Pulmonary blood flow ↑↑↑, LA pressure ↑
CARDIOVASCULAR EVENTS AT BIRTH (5 marks)
1. Closure of Foramen Ovale (Functional → Anatomical)
| Phase | Mechanism | Timing |
|---|---|---|
| Functional | LA pressure > RA pressure → septum primum pushed against septum secundum → valve effect closes foramen | Minutes after birth |
| Anatomical | Fibrous fusion of septa | ~1 year (complete) |
- LA pressure rises because: ↑ pulmonary venous return
- RA pressure falls because: ↓ placental venous return (cord cut), ↓ PVR
- In 20% of adults, anatomical closure never completes → probe-patent foramen ovale — hemodynamically insignificant as long as LA > RA pressure
- Crying → transient RA > LA → temporary right-to-left shunt → physiological cyanosis in newborn
2. Closure of Ductus Arteriosus
| Phase | Mechanism | Timing |
|---|---|---|
| Functional | ↑ PO₂ (100 mmHg vs. 15–20 mmHg fetal) → smooth muscle contraction; ↓ PGE₂ (placental source removed, cyclo-oxygenase inhibited at birth) | 30 min – 8 hours |
| Anatomical | Intimal proliferation, fibrous obliteration → ligamentum arteriosum | 1–3 months |
- Bradykinin (released from lungs on initial inflation) also acts as a vasoconstrictor on ductus
- PGE₂ (and PGF₂α) maintained ductus open in fetal life; removal of placenta + COX inhibition at birth = closure
- In premature infants: ductus may fail to close → Patent Ductus Arteriosus (PDA) → treat with indomethacin (COX inhibitor) to promote closure
- Reversal of flow: fetal — PA → Aorta; after birth — Aorta → PA (until ductus closes)
3. Hemodynamic Reversal
| Parameter | Fetal | At Birth |
|---|---|---|
| Systemic vascular resistance (SVR) | Low (placenta = low-resistance circuit) | Doubles (cord cut) |
| PVR | High | Falls 5-fold |
| Aortic pressure | Low | Rises |
| PA pressure | High | Falls |
| RV pressure | High | Falls |
| LV pressure | Low | Rises |
Cord clamping removes the large, low-resistance placental circulation → SVR doubles → aortic and LV pressure ↑
CLOSURE OF UMBILICAL VESSELS (2 marks)
Umbilical Arteries
- Close first, within 3–5 minutes of birth
- Mechanism: thermal stimulus (cold), mechanical handling, ↑ PO₂ → smooth muscle contraction
- Functional closure: minutes; anatomical obliteration: 2–3 months
- Remnant: medial umbilical ligaments (distal); proximal segments remain as superior vesical arteries
Umbilical Vein
- Closes shortly after umbilical arteries
- Blood from placenta may still enter newborn for some time after birth ("placental transfusion")
- Remnant: ligamentum teres hepatis (round ligament of liver) in falciform ligament
Ductus Venosus
- Closes 1–3 hours after birth (flow stops as umbilical vein closes → ↓ pressure in ductus)
- Functional closure: 1–3 hours; anatomical closure: 2–18 days
- Causes portal pressure to rise from ~0 → 6–10 mmHg → forces blood through liver sinusoids
- Remnant: ligamentum venosum
SUMMARY TABLE — SHUNTS (2 marks)
| Fetal Shunt | Closes (Functional) | Closes (Anatomical) | Adult Remnant |
|---|---|---|---|
| Foramen ovale | Minutes (first breath) | ~1 year | Fossa ovalis |
| Ductus arteriosus | 30 min – 8 hours | 1–3 months | Ligamentum arteriosum |
| Ductus venosus | 1–3 hours | 2–18 days | Ligamentum venosum |
| Umbilical arteries | 3–5 minutes | 2–3 months | Medial umbilical ligaments + sup. vesical arteries |
| Umbilical vein | Shortly after arteries | 2–3 months | Ligamentum teres hepatis |
CLINICAL SIGNIFICANCE / FAILURE OF TRANSITION (bonus)
| Condition | Mechanism |
|---|---|
| Persistent Pulmonary Hypertension of Newborn (PPHN) | PVR fails to fall → continued R→L shunting through FO and DA → cyanosis |
| Patent Ductus Arteriosus (PDA) | DA fails to close; common in prematurity (↑ PGE₂ sensitivity); L→R shunt → pulmonary overcirculation |
| Patent Foramen Ovale (PFO) | 20% adults; R→L shunt if RA > LA (e.g. PE, Valsalva) |
| Neonatal asphyxia | Hypoxia → persistent pulmonary vasoconstriction → failure to establish serial circulation |
Sources: Guyton & Hall Medical Physiology; Ganong's Review of Medical Physiology (26e); Langman's Medical Embryology; Bailey & Love's Surgery (28e); Color Atlas of Human Anatomy Vol. 2
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