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Hypoxia and hypovolemia result in similar ultrastructural changes. These changes are characterized by aggregation of platelets and leukocytes in the vascular space, focal endothelial cell disruption, and appearance of increased amounts of perivascular edema, and loss of ultrastructural organization of the mucosal cell. These changes appear to relate to variations in blood flow to the mucosa of different areas of the bowel and to follow the distribution observed for necrotizing enterocolitis in the human infant.

Six children have undergone lung biopsy for evaluation of fine structure. Four children demonstrated increased DAVO2 preoperatively and had marked ultrastructural changes in their lungs. Such changes appear to result from embolization of microthrombi or microaggregates to the lung from under-oxygenated tissue.

The effect of systemic hypoxemia and hypovolemia on pulmonary fine structure was studied and compared in 12 Labrador puppies 6 days to 3 weeks old. Central venous and arterial pressures were monitored and arterial pH, PCO2 and PO2 recorded serially. In 4 puppies lung biopsies were taken after a two hour period of breathing 7% oxygen. Four other puppies were made hypovolemic by acute hemorrhage of 50% of the calculated blood volume and lung biopsies taken after one hour. Four control puppies were maintained under anesthesia for two hours, breathing an atmosphere of 20% oxygen before lung biopsies were taken. Both hypoxemia and hypovolemia resulted in similar pulmonary ultrastructural changes. These changes which occurred more frequently in areas where the air spaces and blood vessels were expanded, were characterized by aggregation of platelets and leucocytes in the vascular space, focal endothelial cell disruption, interstitial edema and type I epithelial cell discontinuities with exudation of plasma into the alveolar air spaces.

The present study demonstrates that a 2 hr period of sustained hypoxia results in a focally destructive process, a process which resolves by 5 days. That the changes described in earlier work are not overwhelming to the animal suggest that an early, focal, and reversible stage of the development of acute enterocolitis in the puppy can be described with the aid of electron microsocpy.