HYALINE MEMBRANE DISEASE RESPIRATORY DISTRESS SYNDROME
ObjectivesTo understand the risk factors, pathogenesis , pathology,and clinical features of respiratory distress syndrome To list the differential diagnosis of respiratory distress in newborn baby To recognise how to to investigate and manage a newborn baby with respiratory distress syndrome To understand the features and management of patent ducutus arteriosus To recognise the pathogenesis and clinical features of bronchopulmonary dysplasia and retinopathy of prematurity
Respiratory distress in the newborn is defined by the presence of one or more of the following: tachypnea, retractions, nasal flaring, grunting, and cyanosis.
INCIDENCE HMD occurs primarily in premature infants, and its incidence inversely proportional to the gestational age and birth weight. 60-80% of infants less than 28wk of gestation 15-30% of infants between 32&36 wk rarely in those >37 wk of gestational age.
The risk of developing RDS increases with
maternal diabetes multiple births cesarean section delivery precipitous delivery asphyxia cold stress history of previously affected infants. The incidence is highest in preterm male or white infants.The risk of RDS is reduced in
pregnancies with chronic or pregnancy-associated hypertension maternal heroin use prolonged rupture of membranes antenatal corticosteroid prophylaxisETIOLOGY &PATHOPHISOLOGY Surfactant deficiency [decreased production&secretion] is the primary cause of HMD. Surfactant Is phospholipid protein ,its major constituents: 1-Dipalmitoyl phostidylcholine [Lecithine] 2-Phosphatidyle glycerol 3-Apoproteins 4-Cholesterol Surfactants are synthesized and stored in type2 alveolar cells.
Deficiency of surfactant leads to: -alveolar collapse -decreased lung volume &compliance -ventilation-perfusion abnormalities -right to left shunt -persistent hypoxemia[<30mm Hg]causes metabolic acidosis -respiratory acidosis also present because alveolar hypoventilation
Decreased myocardial contractility, decreased cardiac out put&arterial blood pressure -Perfusion of kidneys,GIT,muscle,&skin is reduced leading to edema & electrolytes disorders.
PATHOLOGY The lungs appear deep purplish red,&liver like in consistency. Microscopically: A number of alveolar ducts, alveoli,& resp. bronchiole are lined with acidophilic homogenous, or granular membrane.
Clinical manifestations Signs of HMD usually appear within minutes of birth, but may be delayed for several hours in large premature infants. Early clinical signs of HMD: 1-Tachypnea[>60/min] 2-Expiratory grunting 3-Sternal&intercostal recession 4-Cyanosis in room air 5-Delayed onset of respiration in very immature babies
Late clinical signs in severe HMD 1-Decrease blood pressure 2-Fatigue 3-Cyanosis 4-Pallor increase 5-Grunting decrease or disappears 6-Apnea& irregular respiration[ominous sign] Other signs: -Mixed resp.& metabolic acidosis -Edema,ileus,oliguria In most cases symptoms&signs reach peak within 3 days after which improvement is gradual.
Cyanosis. This critically ill infant exhibits cyanosis and poor skin perfusion.
Flaring. Reflexive widening of the nares may be seen in infants with respiratory distress.Retractions. The inward collapse of the lower anterior chest wall can be seen in this premature infant with RDS.
INVESTIGATIONS Chest x.ray -Grade-1-fine reticular granular mottling, good lung expansion -Grade-2-mottling with air bronchogram -Grade-3-diffuse mottling,heart border just discernable,prominent air bronchogram -Grade-4-bilateral confluent opacification of lungs[white out] BD gas analysis 1-Initially hypoxemia 2-Later progressive hypoxemia ,hypercapnia, &metabolic acidosis
RDS. Note the ground-glass appearance and the presence of air bronchograms
Diffrential diagnosis of respiratory distressPrevention 1-Prevention of prematurity, including : -avoidance of unnecessary or poorly timed c.s -appropriate management of high risk pregnancy& labour. -Estimation of fetal head circumferance by ultrasound& determination of lecithin concentration in the amniotic fluid by [L/S]ratio decrease like hood of delivering premature infants 2-Prevention of asphyxia may decrease the incidence and severity of RDS, because asphyxia leads to hypoxemia and acidosis, which reduce surfactant synthesis.
3-Adminstration of betamethasone to women 48hr before delivery of fetuses between 24-34wk of gestation significantly reduce the incidence&mortality&morbidity of HMD.One course of corticosteroid required. ACOG recommends a single course of antenatal corticosteroids for pregnant women between 34 and 36 6/7 wk gestation at risk for preterm birth within 7 days, who have not received a previous course of antenatal corticosteroids. 4-Adminstration of first dose of surfactant in to the trachea of symptomatic premature infants immediately after birth[prophylactic] reduce air leak&mortality from HMD
Treatment
Nasal Continuous Positive Airway PressureWarm, humidified oxygen should be provided at a concentration sufficient to keep PaO 2 between 50 and 70 mm Hg (91–95% SaO 2 ) to maintain normal tissue oxygenation while minimizing the risk of O2 toxicity. If there is significant respiratory distress (severe retractions and expiratory grunting) or if SaO 2 cannot be kept >90% at FIO 2 of ≥40–70%, applying nCPAP at 5-10 cm H2 O is indicated and usually produces a rapid improvement in oxygenation. Nasal CPAP reduces collapse of surfactant-deficient alveoli and improves both FRC and ventilation-perfusion matching. Early use of nCPAP for stabilization of at risk preterm infants beginning early (in the delivery room) reduces the need for mechanical ventilation.
Maechanical Ventilation
Infants with respiratory failure or persistent apnea require assisted mechanical ventilation.(1)arterial blood pH <7.20, (2) PaCO 2 ≥60 mm Hg, (3) SaO 2 <90% at O2 concentration of 40–70% and nCPAP of 5-10 cm H2 O, (4) persistent or severe apnea.Modes of Mechanical Ventilation
Synchronized intermittent mechanical ventilation (SIMV) High-frequency oscillatory ventilation (HFOV) and highfrequency jet ventilation (HFJV)Surfactant therapy: Synthetic &natural surfactants[from calf,pig,&cow lungs].Multidose endotracheal instillation of surfactant Metabolic acidosis: in RDS may be a result from perinatal asphyxia &hypotension. The aim to keep pH above 7.25. It is treated by sodium bicarbonate 1-2meq/kg administered over 15-20min through peripheral or umbilical vein.
Complications of HMD 1-Patent ductus arteroisus 2-Interventricular hemorrhage 3-pulmonary: A-air leak: pneumothorax,pneumomediastinum, P.I.E,pneumopericardium,pneumoperitonium,air embolism, subcutanous emphesema. B-bronchopulmonary dysplasia. C-pneumonia: aspiration,bacterial. 4-Complication of mechanical ventilation including subglottic stenosis(causing stridor) 5-Long term neurological sequele.
Patent Ductus Arteriosus
PDA is a common complication that occurs in many low birth weight infants who have RDS. The incidence of PDA is inversely related to the maturity of the infant. In term newborns, the ductus closes within 24-48 hours after birth. However, in preterm newborns, the ductus frequently fails to close, requiring medical or surgical closure. The ductus arteriosus in a preterm infant is less responsive to vasoconstrictive stimuli, which, when complicated with hypoxemia during RDS, may lead to a persistent PDA that creates a shunt between the pulmonary and systemic circulations.Clinical manifestations of a PDA usually become apparent on day 2-4 of life. Because the left-to-right shunt directs flow to a low-pressure circulation from one of high pressure, the pulse pressure widens; a previously inactive precordium shows an extremely active precordial impulse, and peripheral pulses become easily palpable and bounding. The murmur of a PDA may be continuous in systole and diastole, but usually only the systolic component is auscultated. Heart failure and pulmonary edema result in rales and hepatomegaly. A chest radiograph shows cardiomegaly and pulmonary edema; a two-dimensional echocardiogram shows ductal patency; and Doppler studies show markedly increased left-to-right flow through the ductus.
Treatment of a PDA during RDS involves initial fluid restriction and diuretic administration. If there is no improvement after 24-48 hours, a prostaglandin synthetase inhibitor,indomethacin or ibuprofen, is administered. Because 20-30% of infants do not respond initially to indomethacin and because the PDA reopens in 10-20% of infants, a repeat course of indomethacin or surgical ligation is required in some patients
BRNCHOPULMONARY DYSPLASIA Oxygen concentration above 40% are toxic to the neonatal lung. Oxygen mediated lung injury results from generation of super oxides, hydrogen peroxides[H2O2],&Oxygen free radicals which disrupt membrane lipids. Mechanical ventilation with high peak pressure produces barotroma. Definition: Failure of RDS to improve after 2 weeks& need for prolonged mechanical ventilation,&oxygen therapy required at 36 weeks post conception age.
Clinical feature: Oxygen dependence, hypercapnia , compensatory metabolic alkalosis,pulmonary hypertension, poor growth,& development of right sided heart failure.Increase air way resistance with reactive air way constriction. Treatment: 1-Bronchodilator 2-Fluid restriction& diuretics 3-Mechanical ventilation 4-Dexamethazone 5-They are at risk for severe respiratory syncytial virus pneumonia and as infants should receive prophylaxis against respiratory syncytial virus.
Bronchopulmonary dysplasia. Note the alternating areas of hyperinflation and atelectasis.
Retinopathy of prematurity [ROP]Retrolental fibroplasia It is caused by acute and chronic effects of oxygen toxicity on the developing blood vessels of premature retina. The completely vascularized retina of term infant is not susceptible to ROP. ROP is a leading cause of blindness for VLBW infant[<15oogm]. Excessive arterial oxygen tensions produce vasoconstriction of retinal vessels this followed by vaso obliteration,then proliferative stages[extraretinal fibrovascular proliferation]. Severe cases leads to retinal detachment, leukokoria, glucomaThe incidence of ROP may be reduced by careful monitoring of arterial blood gases& to keep arterial PaO2 50-70mm Hg.Infant <1500gm, or born before 28 weeks gestation should be screened when they are older than 4 weeks of age or more than 34 weeks’ corrected gestational age, whichever comes first.Laser therapy &less often cryotherapy may be used for viterous hemorrhage& for severe progressive proliferation.Surgery indicated for retinal detachment.