Upper airway resistances in fetal sheep: the influence of breathing activity

Abstract

Fetal breathing movements (FBM) and lung liquid volume are known to affect lung development, but little is known about mechanisms controlling movement of liquid through the upper respiratory tract (URT). Therefore we measured resistances of the URT in 8 unanesthetized fetal sheep during late gestation while FBM were monitored from pressures in the lower trachea or from electromyogram of respiratory muscles. URT resistance to liquid flow toward the amniotic sac increased from 3.5 +/- 1.9 Torr X ml-1 X min during episodes of FBM to 21.1 +/- 5.7 Torr X ml-1 X min during apnea. Laryngeal resistance during apnea was greater (P less than 0.001) than supralaryngeal resistance in each of six fetuses in which URT resistance was partitioned. Fetal paralysis abolished the increase in laryngeal resistance to efflux that was previously related to the high-voltage electrocortical state and apnea. We were unable to quantify URT resistance to fluid movement toward the lungs because the larynx acted as a valve, permitting flow toward the lungs only in the presence of FBM. The supralaryngeal portion of the URT also apparently acts as a valve, normally preventing the entry of amniotic fluid into the pharynx. These findings help to explain our earlier observations that efflux of liquid from the fetal lungs is greater during episodes of FBM than during apnea.

Lung Hypoplasia Can Be Reversed by Short-Term Obstruction of the Trachea in Fetal Sheep

“…In contr ast, after tracheal obstruction lung liquid volumes progressively increased to control values after 6 d, and associated with this increase in volume was an increase in tracheal pressure; the values measured were similar to the values we recorded in fetuses exposed to 7 d of tracheal occlusion (4). Consequently, we sugges t that the generation of an intraluminal pressur e that will expand the hypoplastic fetal lung to a level that will significantly stimulate lung growth, must be dependent upon the presence of a resistance to lung liquid efflux that is greater than that which norm ally occurs during fetal apnea as a result of glottic narrowing (15,16). Indeed, the mean tracheal pressure measured in one fetus after reconnection of the tracheal circuit (drain and reconnect, 3.4 ± 0.4 mm Hg) was simil ar to that measured in control fetuses at the same age.…”

“…Furthermore, we sugges t that the failure of the hypoplastic lung to expand, in the absence of tracheal obstruction, is most probably the consequence of a relatively incompli ant lung and, perhaps more importantly, chest wall (23). Under these conditions, the normal resistance to lung liquid efflux offered by the upper airway (15,16) is prob ably insufficient to promote fluid accumulation and, therefore, increase lung expansion to the level required to stimulate lung grow th. In contrast, obstruction of the trachea in fetu ses with hypopl astic lung s quickl y increases the degree of lung expansion to normal levels, accelerat es lung growth and rever ses the lung growth deficit in as little as 6 d. Our findin gs support the recent observati ons that occlusion of the trach ea increases lung growth in fetal sheep with hypopl astic lung s indu ced by either herniation of the fetal diaph ragm (24) or nephrectomy (5) .…”

Abolition of Fetal Breathing Movements by Spinal Cord Transection Leads to Reductions in Fetal Lung Liquid Volume, Lung Growth, and IGF-II Gene Expression

“…During periods of apnea, the elastic recoil is opposed by narrowing of the glottis (32) and an increased resistance of the upper airway ( 3 9 , which result in a reduced emux of lung liquid and a supraamniotic pressure in the trachea (36). During episodes of FBM, rhythmic laryngeal dilation and an absence of active laryngeal constriction (32) result in a reduced upper-airway resistance (35), enabling the pulmonary elastic recoil to cause liquid to flow from the lungs at an increased rate (22). We propose that the diaphragmatic component of FBM has the effect of retarding the emux of liquid from the lungs in the presence of laryngeal dilation, thereby helping to maintain lung liquid volume and the degree of lung distension.…”

Identification of Cellular Processes That Are Rapidly Modulated in Response to Tracheal Occlusion Within Mice Lungs

“…Indeed, fetal cervical cord transection, which results in a lack of coordination between larynx opening and diaphragmatic movements, thus allowing excessive loss of lung liquid during fetal breathing movements, leads to lung hypoplasia (4). Similarly, loss of larynx tonus (5) and bypass of the larynx by experimental tracheotomy (6) during fetal life are both associated with excessive lung liquid loss and severe lung hypoplasia. In contrast, stimulation of lung growth and development can be achieved by using tracheal obstruction techniques at an appropriate developmental stage (7).…”