Clearance of liquid from lungs of newborn rabbits.

Apparent inconsistencies in the results of previous studies led us to reexamine the process by which fluid leaves the lungs of newborn rabbits after birth. We measured pulmonary blood volume, extravascular lung water, and the size of perivascular cuffs of fluid in frozen sections of lung obtained from 166 full-term rabbits (31 days gestation) born vaginally or by cesarean section. We killed the rabbits by giving them barbiturate intraperitoneally and immersing them in liquid nitrogen before they breathed or at randomly predetermined intervals from 5 min to 24 h after birth. We found that a) pulmonary blood volume of both groups of rabbits increased soon after birth, b) extravascular lung water per gram of dry lung tissue was greater at birth in rabbits born by cesarean section than in those born vaginally, c) extravascular lung water did not begin to decrease in either group of animals until 30-60 min postnatally, after which it decreased progressively for 24 h, and d) the rate of fluid clearance and pattern of puddling around pulmonary vessels was similar in both groups of rabbits, with maximal perivascular cuffs 30 min after birth, followed by diminution of cuff size as the lungs shed water.