The effect of tracheal smooth muscle tone on neonatal airway collapsibility

The effect of smooth muscle tone on the functional characteristics of neonatal airways was studied in six newborn lambs. Tracheal mechanics, resistance, and collapsibility were determined in tracheal segments of uniform length which were surgically isolated just caudal to the cricoid cartilage. Resistance to airflow through uncompressed (zero transmural pressure) and compressed (increased transmural pressure) trachea was evaluated over a range of physiological flows both before and after induced contraction of airway smooth muscle. Results demonstrate that neonatal tracheal smooth muscle does respond to cholinergic stimulation and that there was a significant decrease in airway compliance. Furthermore, the consequent increase in airway rigidity may play a role in resisting collapse or compression of the intrathoracic airway during expiration.

Acquired tracheomegaly in very preterm neonates

Proximal airways are compliant structures at early gestational ages and may be susceptible to pressure-induced deformation following prolonged ventilatory support. Sixteen neonates (mean +/- SD gestational age, 27.0 +/- 0.6 weeks; mean +/- SD birth weight, 847 +/- 68 g) were studied to assess tracheal volume deformation. The neonates received ventilatory support for a mean +/- SD duration of 25.4 +/- 4.9 days. During this period the maximum peak inspiratory pressures ranged from 15 to 25 cm H2O, and respirations ranged from 20/min to 60/min. These neonates were studied at seven days postextubation and were individually matched for body weight with 16 nonventilated neonates. The width of the tracheal air column was measured at the lower border of the first thoracic (T-1) and third thoracic (T-3) vertebrae. The average tracheal width (average of T-1 and T-3) was significantly (38%) wider in the ventilated group, and the mean +/- SD tracheal width values were 3.79 +/- 0.29 mm, as compared with the control values of 2.74 +/- 0.31 mm. Based on these data it was estimated that the tracheal volume was 91% greater in the ventilated group. These observations demonstrate tracheal volume deformation and acquired tracheomegaly in neonates who have received mechanical ventilatory support. In addition to increased dead space ventilation, these findings also indicate underlying mechanical deformation of the tracheal wall.

A new approach to induced hypothermia

A variety of methods have been employed for the induction of hypothermia; however, there are still some inherent problems that remain with current techniques. Liquid ventilation, a process used in several other environmental and clinical research areas, may be a feasible method since it takes advantage of the effectiveness of the pulmonary architecture as a heat exchanger. Hypothermia induced by liquid ventilation was studied in 8 newborn lambs, mean age = 10 +/- 8 SEM days. Each lamb was anesthetized with sodium pentobarbitol (20 mg/kg) and intubated. Cardiopulmonary measurements were taken during a control period prior to induced hypothermia. Liquid temperatures of 20 and 30 degrees C were used in cooling the animal while monitoring rectal and surface temperatures. Temperatures decreased producing rectal cooling rates of 8.4 and 4.8 degrees C/hr, respectively. Blood gas analysis showed adequate physiological gas exchange for all lambs during the liquid ventilation period. Based on the data, the process of liquid ventilation offers a unique potential both in experimental and clinical areas as a new approach to the technique of induced hypothermia.

Gallbladder mechanics in newborn piglets

The mechanical properties of the newborn piglet gallbladder were evaluated in both the stimulated and unstimulated states. The pressure-volume relationships, compliance, and the estimated active tension of the gallbladder were determined in 10 newborn piglets (2-7 days of age). Agonist stimulation was achieved by administration of histamine (25 micrograms/kg/h) and cholecystokinin (CCK) (60 ng/kg/h). Both histamine and CCK increased the intracholecystic pressure at the 50% resting volume from 12.4 cm H2O to 18.9 and 15.5 cm H2O, respectively. This resulted in a significant (p less than 0.05) increase in the active tension. However, no significant changes were observed in the gallbladder compliance after stimulation. These findings characterize the mechanical properties of the normal neonatal gallbladder. The low magnitude of intracholecystic pressure response to agonist stimulation, when compared to adult data, may explain the occurrence of decreased neonatal choledochal bile flow.

Mechanics and energetics of breathing helium in infants with bronchopulmonary dysplasia

The mechanics and energetics of breathing were studied in preterm infants with bronchopulmonary dysplasia while spontaneously breathing control gas and helium-oxygen (Heliox) gas mixtures. During Heliox breathing, there was a significant decrease in pulmonary resistance, resistive work of breathing, and mechanical power of breathing, whereas ventilation remained unchanged. Breathing a lower density gas mixture (Heliox) may have therapeutic value by decreasing the demands on the respiratory muscles and the caloric requirements for breathing. Therefore, this modality may reduce potential respiratory muscle fatigue and avail additional calories for growth and recovery in the preterm infant with bronchopulmonary dysplasia.

Liquid ventilation: effects on pulmonary function in distressed meconium-stained lambs

Seven lambs (0.93 term gestation) were delivered by cesarean section with evidence of meconium in the amniotic fluid, meconium staining, and respiratory distress. The initial arterial blood gas and acid-base status indicated severe hypoxemia and acidosis. Three of these lambs developed pneumothoraces and died on control gas ventilation with positive end expiratory pressure. During the control period (90 min) with ventilatory support, there were no significant alterations in mean arterial oxygen tension (PaO2) and alveolar-arterial oxygen gradient (A-aDO2). The initial hypercarbia and acidosis were effectively controlled and corrected using mechanical ventilation and bicarbonate infusion. Fifteen min after the onset of fluorocarbon ventilation mean PaO2 significantly increased and A-aDo2 decreased. After 90 min of fluorocarbon ventilation, lambs were returned to gas ventilation. During this recovery period, PaO2 and A-aDo2 remained significantly improved compared with control gas values. Dynamic lung compliance increased, alveolar and peak tracheal pressure decreased and inspiratory elastic work of breathing decreased during liquid ventilation.

Tracheal volume deformation in a developmental rabbit model

Alterations in tracheal volume were evaluated in a developmental in vivo rabbit model during and following the application of continuous positive pressure (CPP). Sequential volume changes were recorded during 60 min of CPP to a bypassed tracheal segment. The distal trachea was utilized for spontaneous ventilation. CPP of 10 cm H2O was applied to three developmental groups: group I, 5 term newborn pups; group II, 5, 7-day-old pups, and group III, 5 adult female rabbits of 18 +/- 6 months old. Changes in tracheal volume were measured by a micropipette system. Following 60 min of CPP, tracheal volumes increased (p less than 0.01) by 39.8% in group I; 36.1% in group II and 5.1% in group III. During a recovery period (60 min), return towards initial resting volume was observed in all groups, though maximal persistent volume deformation was observed in groups I and II. Thus, these data indicate tracheal barotrauma in the form of persistent dimensional deformation at early stages of development in an in vivo rabbit model.

Cardiopulmonary function in very preterm lambs during liquid ventilation

Cardiopulmonary function was evaluated in very preterm lambs (106 +/- 0.7 S.E. days gestation, 1.66 +/- 0.12 S.E. kg birth weight) during fluorocarbon ventilation. Lambs were delivered by cesarean section after epidural anesthesia. Indwelling arterial, venous, and tracheal cannulae were placed before clamping the cord. Lambs were then mechanically ventilated with oxygenated fluorocarbon for approximately 2 h. During this period it was possible to maintain adequate gas exchange and stable cardiac function. Transpulmonary pressure, liquid flow, and tidal volume tracings enabled determination of lung compliance, CL = 0.58 +/- 0.12 S.E. ml X cmH2O-1 X kg-1, inspiratory resistance, RI = 3600 +/- 604 S.E. cmH2O X liter-1 X sec-1, and expiratory resistance, RE = 4034 +/- 2183 S.E. cmH2O/liter/sec. Lung compliance of the 106-day-old fluorocarbon-filled lung is similar to the more mature 138-143-day-old air-filled lung in preterm lambs. Based on the data presented herein we have extended the viability of the preterm lamb to the limit of pulmonary capillary development rather than that of the pulmonary surfactant system.

The effects of liquid ventilation on cardiopulmonary function in preterm lambs

The effects of fluorocarbon ventilation on cardiopulmonary function were studied in 8 preterm lambs, 132-136 days gestation. After mechanical ventilation with 100% oxygen (control period), the lambs were ventilated with fluorocarbon (PIO2 = 622 torr). The liquid was then removed from their lungs and gas ventilation resumed (recovery period). During normothermic liquid breathing the alveolar-arterial O2 gradient (A-a DO2) decreased (P less than 0.01) from control by 154 torr and remained decreased (P less than 0.05) by 85 torr during recovery. Dynamic lung compliance (CL) increased 50% (P less than 0.05); PaO2 increased 50% (P less than 0.05); and PaCO2 decreased 29% (P less than 0.01) as compared to control values. The change in A-a DO2 and PaO2 before and after liquid ventilation was correlated (r = 0.79 and P less than 0.01) with control CL. There was a gradual decrease (P less than 0.01) in mean arterial pressure from 62 +/- 5.4 torr (control) to 53,1 +/- 9,3 torr (recovery); however, there were no significant alterations in mean central venous pressure, heart rate, or mean electrical axis.

Effect of liquid ventilation on preterm lamb tracheal mechanics

Liquid ventilation with fluorocarbon has been demonstrated as a viable mode of ventilatory support to preterm or distressed lambs. The effect of liquid ventilation and its high inflation pressures in proximal airways was evaluated on the mechanical behavior of the preterm trachea. 7 preterm lambs (134 +/- 1.5 SE days of gestation; 90% term) were ventilated with fluorocarbon (Rimar) for 58.6 +/- 8 min. The tracheae were dissected and the ventilated, distal segments were compared to nonventilated proximal segments. Pressure-volume curves were determined by plethysmography and specific tracheal compliance was computed. Tracheal dimensions increased 13.6 and 25.6% in diameter and volume (p less than 0.05), respectively, and the compliance decreased 29.9% (p less than 0.05). The magnitude of these changes is comparable to that observed during gaseous ventilation of term animals.

Time-dependent tracheal deformation in fetal, neonatal, and adult rabbits

Sequential magnitude and the rate of change in tracheal mechanics after application of intermittent positive pressure (IPP), and the time constant, tau, of this deformation were determined in fetal (21 and 27 days gestation), neonatal (term, 31 days gestation) and adult rabbits. In vitro tracheal mechanics were determined by liquid plethysmography before and after IPP, (25 cm H2O at 30 cycles/min) of 20, 40, 60, and 120 min duration. A sequential decrease in tracheal compliance was documented at all ages. This was associated with increased values of tau as the maturational age advanced. Further, the rate of tracheal deformation was 0.008 cm H2O min-1 at 21 days gestation compared to 0.00025 cm H2O min-1 at adulthood. The decrease in rate of deformation was related to the gestational maturity and duration of IPP. The rate of deformation, as predicted by calculated deformation time constants, appears to be a function of structural elastic and viscoelastic properties of the trachea as well as the duration and magnitude of applied pressure. The deformation time constant may be defined as a useful index to predict the rate of airway deformation. Utilizing the rabbit model, this provides a measure of comparing barotrauma with different modes of ventilation in preterms.

Respiratory muscle function, assessment, and training

This article presents a brief overview of respiratory muscle mechanics and the effects of lung disease and neuromuscular disease on pulmonary function. A variety of current specific and general muscle training techniques are described and discussed. Also presented is a current review of training studies and the effects of muscle training on cardiopulmonary function, muscle strength, endurance and fatigue, and exercise tolerance.

Pressure-induced deformation in immature airways

The effect of positive pressure ventilation on the highly compliant fetal airways was evaluated utilizing fetal rabbit tracheal segments. The degree of mechanical and dimensional deformation was quantified to the pressure applied intermittently (IPP) or continuously (CPAP) for 60 min. Excised tracheal segments were obtained at 21, 27, and 31 days fetal rabbit gestation (term = 31 days) and from 18 +/- 6-month-old adults. Internal diameter, resting length, and volume of the tracheal segments were measured, and their pressure-volume relationships were determined by plethysmography. Tracheal specific compliance at deflation pressure of 0 to 10 cm H2O was calculated from these data. In all fetal groups, the application of positive pressure resulted in a decrease of both tracheal distensibility and the slope of the pressure-volume curves; in addition, a highly significant decrease in tracheal specific compliance was documented. At 21 days gestation, the tracheal specific compliance decreased from 0.089 to 0.034 cm H20-1 (P less than 0.001) after CPAP and to 0.025 cm H2O (P less than 0.001) after IPP. For the same group, the resting tracheal volume increased by a dramatic 89% (P less than 0.001) after CPAP and by 124% (P less than 0.001) after IPP. The magnitude of these alterations decreased as gestational age advanced; the changes were not significant by adulthood. These data indicate that significant pressure-induced deformation (barotrauma) is sustained by susceptible immature airways.

Pressure--volume relationships of tracheae in fetal newborn and adult rabbits

The physical properties of the trachea in fetal, newborn, and adult rabbits change with age. Tracheal dimensions and pressure--volume relationships were determined in excised tracheae at a wide developmental age range (from early gestational age to infancy and adulthood). At intraluminal pressures of 0 to 10 cm H2O, the tracheal specific compliance was 0.089 (cm H2O)(-1) for a 21 day old fetus. By adulthood, the tracheal specific compliance decreased five fold to 0.017 (cm H2O)(-1); a dramatic proportion of this reduction occurred during fetal life itself, followed by a steady, progressive decline to adult age. The volume of the tracheal segment, at intraluminal pressure of 20 cm H2O, increased by 29% (adult), 41.5% (31 day fetus) and 113% (21 day fetus). Developmental alterations in the specific airway compliance suggest significant changes in anatomical dead space would occur as a function of maturation and airway transmural pressure.

Effect of intermittent positive pressure application on the bulk modulus of the developing rabbit trachea

Tracheal dimension, pressure-volume relationships and bulk modulus (K) were determined in excised fetal and adult rabbit tracheal segments before and after application of intermittent positive pressure (IPP) which was 0--25 cm H2O pressure, at 30 cycles/min, and for a duration of 60 min. Both tracheal elastic behavior and dimensions underwent dramatic changes following IPP. At 21, 27, 31 days gestation age, the bulk modulus, K, of the rabbit trachea approximated those of the adult control values. However, the adult values of K did not vary significantly after IPP. The associated increase in tracheal dimensions were also related to the maturity of the animal. Significant structural and functional barotrauma is thus sustained by immature airways, and may account for the tracheal deformation frequently observed as a result of IPP.

Alterations in bulk modulus of tracheae during development

Tracheal dimensions, pressure-volume relationships, and bulk moduli, K, were determined in excised tracheae at different developmental ages: 21-, 27-1, and 31-day rabbit fetuses; 7-day pups; and adult rabbits 18 +/- 6 months of age. Alterations in tracheal dimensions paralleled the growth of the animal. At low segmental pressures such as 10 cm H2O, the tracheal segments became stiffer with age (K = 20.4 cm H2O for the 21-day fetus to K = 58.0 cm H2O for the adult). However, at segmental pressures greater than 15 cm H2O, the overdistended immature trachea became stiffer than even those of the adults. Developmental alterations in bulk modulus suggest significant changes in the material and mechanical properties of the wirways as a function of both pressure and developmental age.