Pathophysiology
Pneumonia that becomes clinically evident within 24 hours of birth may originate at 3 different times. The 3 categories of congenital pneumonia are as follows:
True congenital pneumonia
Intrapartum pneumonia
Postnatal pneumonia
True congenital pneumonia
True congenital pneumonia is already established at birth. It may become established long before birth or relatively shortly before birth. Transmission of congenital pneumonia usually occurs via 1 of 3 routes:
Hematogenous
Ascending
Aspiration
If the mother has a bloodstream infection, the microorganism can readily cross the few cell layers that separate the maternal from the fetal circulation at the villous pools of the placenta. The mother may be febrile or have other signs of infection, depending on the integrity of her host defenses, the responsible organism, and other considerations.
Transient bacteremia following daily activities, such as brushing teeth, defecating, and other potential disruptions of colonized mucoepithelial surfaces, is a well known phenomenon and may result in hematogenous transmission without significant maternal illness. However, the likelihood of hematogenous transmission is increased if the mother has continuous bloodstream infection with a relatively large quantity of microorganisms. In this case, the mother is more likely to have suggestive signs and symptoms.
Because host defenses are limited in fetuses, dissemination and illness may result. The fetus is likely to have systemic disease.
Ascending infection from the birth canal and aspiration of infected or inflamed amniotic fluid have significant common features. Infection of amniotic fluid often involves ascending pathogens from the birth canal but may result from hematogenous seeding or direct introduction during pelvic examination, amniocentesis, placement of intrauterine catheters, or other invasive procedures. Ascension may occur with or without ruptured amniotic membranes.
Most bacterial infections produce clinical signs of infection in the mother, but infections may not be evident if the membranes rupture shortly after inoculation, similar to drainage of an abscess. Some nonbacterial organisms, such as Ureaplasma species (U urealyticum or Uparvum), may be present in the amniotic cavity for long periods yet cause minimal symptoms in the mother.
If the fetus aspirates infected fluid prior to delivery, organisms that reach the distal airways or alveoli may need to cross only 2 cell layers (alveolar epithelium and capillary endothelium) to enter the bloodstream. Typically, these infants present with more pulmonary than systemic signs, but this is not always the case.
Intrapartum pneumonia
Intrapartum pneumonia is acquired during passage through the birth canal. It may be acquired via hematogenous or ascending transmission, from aspiration of infected or contaminated maternal fluids, or from mechanical or ischemic disruption of a mucosal surface that has been freshly colonized with a maternal organism of appropriate invasive potential and virulence.
Postnatal pneumonia
Postnatal pneumonia in the first 24 hours of life originates after the infant has left the birth canal. It may result from some of the same processes described above, but infection occurs after the birth process. Colonization of a mucoepithelial surface with an appropriate pathogen from a maternal or environmental source and subsequent disruption allows the organism to enter the bloodstream, lymphatics, or deep parenchymal structures.
The frequent use of broad-spectrum antibiotics in many obstetrical services and neonatal intensive care units (NICUs) often results in predisposition of an infant to colonization by resistant organisms of unusual pathogenicity. Invasive therapies typically required in these infants often allow microbes accelerated entry into deep structures that ordinarily are not easily accessible.
Enteral feedings may result in aspiration events of significant inflammatory potential. Indwelling feeding tubes may further predispose infants to gastroesophageal reflux and other aspiration events.
Pathogenesis
In neonatal pneumonia, pulmonary and extrapulmonary injuries are caused directly and indirectly by invading microorganisms or foreign material and by poorly targeted or inappropriate responses by the host defense system that may damage healthy host tissues as badly or worse than the invading agent. Direct injury by the invading agent usually results from synthesis and secretion of microbial enzymes, proteins, toxic lipids, and toxins that disrupt host cell membranes, metabolic machinery, and the extracellular matrix that usually inhibits microbial migration.[6, 7]
Indirect injury is mediated by structural or secreted molecules, such as endotoxin, leukocidin, and toxic shock syndrome toxin-1, which may alter local vasomotor tone and integrity, change the characteristics of the tissue perfusate, and generally interfere with the delivery of oxygen and nutrients and removal of waste products from local tissues.
The activated inflammatory response often results in targeted migration of phagocytes, with the release of toxic substances from granules and other microbicidal packages and the initiation of poorly regulated cascades (eg, complement, coagulation, cytokines). These cascades may directly injure host tissues and adversely alter endothelial and epithelial integrity, vasomotor tone, intravascular hemostasis, and the activation state of fixed and migratory phagocytes at the inflammatory focus. The role of apoptosis (noninflammatory programmed cell death) in pneumonia is poorly understood.
On a macroscopic level, the invading agents and the host defenses both tend to increase airway smooth muscle tone and resistance, mucous secretion, and the presence of inflammatory cells and debris in these secretions. These materials may further increase airway resistance and obstruct the airways, partially or totally, causing airtrapping, atelectasis, and ventilatory dead space. In addition, disruption of endothelial and alveolar epithelial integrity may allow surfactant to be inactivated by proteinaceous exudate, a process that may be exacerbated further by the direct effects of meconium or pathogenic microorganisms.
In the end, conducting airways offer much more resistance and may become obstructed, alveoli may be atelectatic or hyperexpanded, alveolar perfusion may be markedly altered, and multiple tissues and cell populations in the lung and elsewhere sustain injury that increases the basal requirements for oxygen uptake and excretory gas removal at a time when the lungs are less able to accomplish these tasks.
Alveolar diffusion barriers may increase, intrapulmonary shunts may worsen, and ventilation-perfusion mismatch may further impair gas exchange despite endogenous homeostatic attempts to improve matching by regional airway and vascular constriction or dilatation. Because the myocardium has to work harder to overcome the alterations in pulmonary vascular resistance that accompany the above changes of pneumonia, the lungs may be less able to add oxygen and remove carbon dioxide from mixed venous blood for delivery to end organs. The spread of infection or inflammatory response, either systemically or to other focal sites, further exacerbates the situation.
PathophysiologyPneumonia that becomes clinically evident within 24 hours of birth may originate at 3 different times. The 3 categories of congenital pneumonia are as follows:True congenital pneumoniaIntrapartum pneumoniaPostnatal pneumoniaTrue congenital pneumoniaTrue congenital pneumonia is already established at birth. It may become established long before birth or relatively shortly before birth. Transmission of congenital pneumonia usually occurs via 1 of 3 routes:HematogenousAscendingAspirationIf the mother has a bloodstream infection, the microorganism can readily cross the few cell layers that separate the maternal from the fetal circulation at the villous pools of the placenta. The mother may be febrile or have other signs of infection, depending on the integrity of her host defenses, the responsible organism, and other considerations.Transient bacteremia following daily activities, such as brushing teeth, defecating, and other potential disruptions of colonized mucoepithelial surfaces, is a well known phenomenon and may result in hematogenous transmission without significant maternal illness. However, the likelihood of hematogenous transmission is increased if the mother has continuous bloodstream infection with a relatively large quantity of microorganisms. In this case, the mother is more likely to have suggestive signs and symptoms.Because host defenses are limited in fetuses, dissemination and illness may result. The fetus is likely to have systemic disease.Ascending infection from the birth canal and aspiration of infected or inflamed amniotic fluid have significant common features. Infection of amniotic fluid often involves ascending pathogens from the birth canal but may result from hematogenous seeding or direct introduction during pelvic examination, amniocentesis, placement of intrauterine catheters, or other invasive procedures. Ascension may occur with or without ruptured amniotic membranes.Most bacterial infections produce clinical signs of infection in the mother, but infections may not be evident if the membranes rupture shortly after inoculation, similar to drainage of an abscess. Some nonbacterial organisms, such as Ureaplasma species (U urealyticum or Uparvum), may be present in the amniotic cavity for long periods yet cause minimal symptoms in the mother.If the fetus aspirates infected fluid prior to delivery, organisms that reach the distal airways or alveoli may need to cross only 2 cell layers (alveolar epithelium and capillary endothelium) to enter the bloodstream. Typically, these infants present with more pulmonary than systemic signs, but this is not always the case.Intrapartum pneumoniaIntrapartum pneumonia is acquired during passage through the birth canal. It may be acquired via hematogenous or ascending transmission, from aspiration of infected or contaminated maternal fluids, or from mechanical or ischemic disruption of a mucosal surface that has been freshly colonized with a maternal organism of appropriate invasive potential and virulence.Postnatal pneumoniaPostnatal pneumonia in the first 24 hours of life originates after the infant has left the birth canal. It may result from some of the same processes described above, but infection occurs after the birth process. Colonization of a mucoepithelial surface with an appropriate pathogen from a maternal or environmental source and subsequent disruption allows the organism to enter the bloodstream, lymphatics, or deep parenchymal structures.The frequent use of broad-spectrum antibiotics in many obstetrical services and neonatal intensive care units (NICUs) often results in predisposition of an infant to colonization by resistant organisms of unusual pathogenicity. Invasive therapies typically required in these infants often allow microbes accelerated entry into deep structures that ordinarily are not easily accessible.Enteral feedings may result in aspiration events of significant inflammatory potential. Indwelling feeding tubes may further predispose infants to gastroesophageal reflux and other aspiration events.PathogenesisIn neonatal pneumonia, pulmonary and extrapulmonary injuries are caused directly and indirectly by invading microorganisms or foreign material and by poorly targeted or inappropriate responses by the host defense system that may damage healthy host tissues as badly or worse than the invading agent. Direct injury by the invading agent usually results from synthesis and secretion of microbial enzymes, proteins, toxic lipids, and toxins that disrupt host cell membranes, metabolic machinery, and the extracellular matrix that usually inhibits microbial migration.[6, 7]Indirect injury is mediated by structural or secreted molecules, such as endotoxin, leukocidin, and toxic shock syndrome toxin-1, which may alter local vasomotor tone and integrity, change the characteristics of the tissue perfusate, and generally interfere with the delivery of oxygen and nutrients and removal of waste products from local tissues.The activated inflammatory response often results in targeted migration of phagocytes, with the release of toxic substances from granules and other microbicidal packages and the initiation of poorly regulated cascades (eg, complement, coagulation, cytokines). These cascades may directly injure host tissues and adversely alter endothelial and epithelial integrity, vasomotor tone, intravascular hemostasis, and the activation state of fixed and migratory phagocytes at the inflammatory focus. The role of apoptosis (noninflammatory programmed cell death) in pneumonia is poorly understood.On a macroscopic level, the invading agents and the host defenses both tend to increase airway smooth muscle tone and resistance, mucous secretion, and the presence of inflammatory cells and debris in these secretions. These materials may further increase airway resistance and obstruct the airways, partially or totally, causing airtrapping, atelectasis, and ventilatory dead space. In addition, disruption of endothelial and alveolar epithelial integrity may allow surfactant to be inactivated by proteinaceous exudate, a process that may be exacerbated further by the direct effects of meconium or pathogenic microorganisms.In the end, conducting airways offer much more resistance and may become obstructed, alveoli may be atelectatic or hyperexpanded, alveolar perfusion may be markedly altered, and multiple tissues and cell populations in the lung and elsewhere sustain injury that increases the basal requirements for oxygen uptake and excretory gas removal at a time when the lungs are less able to accomplish these tasks.Alveolar diffusion barriers may increase, intrapulmonary shunts may worsen, and ventilation-perfusion mismatch may further impair gas exchange despite endogenous homeostatic attempts to improve matching by regional airway and vascular constriction or dilatation. Because the myocardium has to work harder to overcome the alterations in pulmonary vascular resistance that accompany the above changes of pneumonia, the lungs may be less able to add oxygen and remove carbon dioxide from mixed venous blood for delivery to end organs. The spread of infection or inflammatory response, either systemically or to other focal sites, further exacerbates the situation.
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