2 ObjectivesState the major developmental events of the respiratory system in the womb.Describe the key elements of normal fetal circulation.State what happens to the respiratory system at birth.Describe the developmental events in the respiratory system that continue after birth.
6 Fetal Lung Development At end of canalicular phase, primitive acini form, covered with type I and II pneumocytes.Life viable if airway, MV, surfactant providedDuring terminal saccular stage more acini form.There is thinning of the type I cellsType II cells mature and produce surfactant.Alveolar stage begins at about 32 weeks.Mature alveoli/capillary membranes appear.At birth, about 50 million alveoli are present.By age 8, there will be about 300 million.MV – Mechanical Ventilation See Table 8-1 for weeks associated with each stage and level of maturation
12 The Fetal Lung Lung maturation is determined by surfactant. Phosphatidylcholine levels predictiveLecithin/sphingomyelin ratio (L/S ratio)Phosphatidylglycerol (PG) concentrationFetal lung fluid is constantly producedSlight positive pressure keeps lungs inflated.Promotes normal lung developmentAt birth, lungs hold about 40 ml of fluid.If deficient, can result in hypoplastic lung
13 Uterine LifeIn utero life depends on placental structure, which provides, among many things:Gas exchangeNutrients and waste removalDefense against diseaseFetal circulation incorporates the placenta by the umbilicus and use of three special shunts:Ductus venosus, ductus arteriosus, and foramen ovale
16 Fetal CirculationPlacenta large volume, low resistance system, so fetal SVR is lowUmbilical vein returns oxygenated blood from the placenta to fetus via the ductus venosus.Flows into the IVC and on to the RAOxygenated blood is preferentially shunted through the foramen ovale from the right to the left atrium.Provides oxygenated blood to systemic circulation
17 Fetal Circulation (cont.) In utero fetal lungs have high PVR due to low PAO2.Ductus arteriosus shunts blood from high-resistance pulmonary artery to low-resistance aorta.PAO2 – Alveolar PO2
18 Cardiopulmonary Events at Birth Fetal lung fluidPrior to birth, production stops and absorption starts.One third of fluid is expelled by vaginal squeeze.Pulmonary lymphatics absorb remaining fluid.Tactile and thermal stimuli initiate first breath.Initial breath requires transpulmonary pressures >40 cm H2O.Subsequent breaths require progressively less pressure as lung volume increases.
19 Cardiopulmonary Events at Birth (cont.) Air in lung increases PO2 and pH, while PCO2 decreases, which results in:Pulmonary vasodilation and decreased PVRDuctus arteriosus constriction/closureIncreased pulmonary blood flowAt the same time, placenta removal results in:Sudden increase in SVRNet results:LAP > RAP, so foramen ovale closesTransition to extrauterine circulation completeSee Figure 8-9. Persistance of fetal circulation can lead to cardiopulmonary failure
22 Postnatal Upper Airway Head flexion can cause airway obstruction.Factors contributing to airway obstructionTongue is relatively larger compared with adults.Nasal passages are relatively smaller.Most infants nose breathe exclusively.At 4 to 5 months, most infants can breathe orally.Infections or Intubations can cause obstruction at the cricoid cartilage (narrowest point) or the epiglottis, which is relatively longer and less flexible than that of an adult.
24 Vascular Development Basic structure is in place at birth. Subsequent vascular growth involves increased smooth muscle growth and increased density of arterioles and capillaries in distal regions.Lungs are unique as blood from RV and LV provide flow to alveoli microcirculation.Pulmonary circulation from RVBronchial circulation from LVProvides greater stability and resistance against the impact of disease processes
25 Lymphatic and Nervous Development Lymph nodes and vessels are located in connective tissues beside pulmonary structuresProvide fluid control and defenseAbsorbed fluid travels to hilar lymph nodesNervous tissue developmentBrainstem centers for automatic controlPhrenic and intercostal nerves form to carry motor signals to diaphragm and intercostal musclesAutonomic fibers form for smooth muscle control
26 Chest Wall Development, Diaphragm and Lung Volume Infant thorax is more compliant than that of an adult.FRC is established by the equal and opposing forces of chest wall to expand against the lungs tendency to collapse.The infant’s more compliant thorax results in lower FRC and TLC.Predisposes infant to early airway closure, atelectasis, V/Q mismatch, and hypoxemiaCombated by ending expiration early, results in gas trapping, narrowing the glottis, back pressurePatient will make a grunting sound..FRC = Functional Residual Capacity, TLC = Total Lung Capacity