Pulmonary mechanics and energetics in preterm infants who had respiratory distress syndrome treated with synthetic surfactant

Physiological basis of non-invasive ventilation in the newborn

Non-invasive ventilation (NIV) is a form of respiratory support provided primarily to preterm born infants in an effort to avoid any endotracheal intubation or as a weaning step following invasive ventilation. In the context of the respiratory distress syndrome of the newborn, NIV could target and partially reverse specific pathophysiological phenomena, by improving alveolar recruitment and establishing adequate functional residual capacity. It can also assist in minimizing lung injury by avoiding excessive pressure delivery, which can be harmful for the developing lung. Non-invasive ventilation can unload the respiratory muscles and decrease the work of breathing as reported by studies that compare the measured work of breathing at increasing levels of non-invasive support. NIV can also be beneficial in moderating the frequency and intensity of apnea of prematurity. Unintended physiological effects of NIV include gaseous distension of the gastrointestinal tract and increased incidence of air-leak complications. During NIV there is also a lack of direct access to the trachea for suctioning and pulmonary toiletry. Insufficient non-invasive respiratory support could be associated with inadequate oxygenation and ventilation, insufficient gas exchange and atelectotrauma. Excessive provision of non-invasive support could be inefficient or harmful, as overdistention can be associated with decreased compliance of the respiratory system, impaired gas exchange and abnormal diaphragmatic function. An individualized physiological approach could, thus, aim to optimize the beneficial effects of non-invasive ventilation while avoiding inadequate or excessive levels of support.

The Effect of Minimally Invasive Surfactant Therapy on Diaphragmatic Activity

BACKGROUND

Minimally invasive surfactant therapy (MIST) is increasingly used to treat preterm infants with respiratory distress syndrome (RDS). However, the effect of MIST on breathing effort is poorly studied.

OBJECTIVES

To describe the effect of MIST on neural breathing effort assessed with transcutaneous electromyography of the diaphragm (dEMG) in preterm infants with RDS.

METHODS

Preterm infants with a gestational age < 37 weeks treated with MIST for RDS were included. dEMG measurements were done from 15 min before to 1 h after MIST. The percentage change in dEMG activity after MIST and the clinical response were analyzed.

RESULTS

Twenty preterm infants (mean gestational age 29.3 [SD 2.1] weeks; mean birth weight 1,230 [SD 391] g) were included. Seventeen infants did complete the 1-h measurement. Eleven (65%) infants had a decrease in their peakdEMG activity (median change -11.8% [IQR -26.8 to 5.8, p = 0.08]) 1 h after MIST. TonicdEMG activity decreased in 12 (71%) infants, with a median reduction of 6.3% (IQR -29.2 to 9.0, p = 0.07). FiO2 showed a rapid decrease following MIST (before, 0.47 [IQR 0.38-0.84]; 1 h after, 0.25 [IQR 0.21-0.30], p < 0.001).

CONCLUSION

In addition to improved oxygenation, MIST results in a decrease in neural breathing effort measured by dEMG activity in the majority of preterm infants with RDS.

An Official American Thoracic Society/European Respiratory Society Workshop Report: Evaluation of Respiratory Mechanics and Function in the Pediatric and Neonatal Intensive Care Units

Ready access to physiologic measures, including respiratory mechanics, lung volumes, and ventilation/perfusion inhomogeneity, could optimize the clinical management of the critically ill pediatric or neonatal patient and minimize lung injury. There are many techniques for measuring respiratory function in infants and children but very limited information on the technical ease and applicability of these tests in the pediatric and neonatal intensive care unit (PICU, NICU) environments. This report summarizes the proceedings of a 2011 American Thoracic Society Workshop critically reviewing techniques available for ventilated and spontaneously breathing infants and children in the ICU. It outlines for each test how readily it is performed at the bedside and how it may impact patient management as well as indicating future areas of potential research collaboration. From expert panel discussions and literature reviews, we conclude that many of the techniques can aid in optimizing respiratory support in the PICU and NICU, quantifying the effect of therapeutic interventions, and guiding ventilator weaning and extubation. Most techniques now have commercially available equipment for the PICU and NICU, and many can generate continuous data points to help with ventilator weaning and other interventions. Technical and validation studies in the PICU and NICU are published for the majority of techniques; some have been used as outcome measures in clinical trials, but few have been assessed specifically for their ability to improve clinical outcomes. Although they show considerable promise, these techniques still require further study in the PICU and NICU together with increased availability of commercial equipment before wider incorporation into daily clinical practice.

Physiology of non-invasive respiratory support

Non-invasive ventilation (NIV) is used in neonates to treat extrathoracic and intrathoracic airway obstruction, parenchymal lung disease and disorders of control of breathing. Avoidance of airway intubation is associated with a reduction in the incidence of chronic lung disease among preterm infants with respiratory distress syndrome. Use of nasal continuous positive airway pressure (nCPAP) may help establish and maintain functional residual capacity (FRC), decrease respiratory work, and improve gas exchange. Other modes of non-invasive ventilation, which include heated humidified high-flow nasal cannula therapy (HHHFNC), nasal intermittent mandatory ventilation (NIMV), non-invasive pressure support ventilation (NI-PSV), and bi-level CPAP (SiPAP™), have also been shown to provide additional benefit in improving breathing patterns, reducing work of breathing, and increasing gas exchange when compared with nCPAP. Newer modes, such as neurally adjusted ventilatory assist (NAVA), hold the promise of improving patient-ventilator synchrony and so might ultimately improve outcomes for preterm infants with respiratory distress.

Factors influencing delivered mean airway pressure during nasal CPAP with the RAM cannula

OBJECTIVE

To measure mean airway pressure (MAP) delivered through the RAM Cannula® when used with a ventilator in CPAP mode as a function of percent nares occlusion in a simulated nasal interface/test lung model and to compare the results to MAPs using a nasal continuous positive airway pressure (NCPAP) interface with nares fully occluded.

STUDY DESIGN

An artificial airway model was connected to a spontaneous breathing lung model in which MAP was measured at set NCPAP levels between 4 and 8 cmH2 O provided by a Dräger Evita XL® ventilator and delivered through three sizes of RAM cannulae. Measurements were performed with varying leakage at the nasal interface by decreasing occlusion from 100% to 29%, half-way prong insertion, and simulated mouth leakage. Comparison measurements were made using the Dräger BabyFlow® NCPAP interface with a full nasal seal.

RESULTS

With simulated mouth closed, the Dräger interface delivered MAPs within 0.5 cmH2 O of set CPAP levels. For the RAM cannula, with 60-80% nares occlusion, overall delivered MAPs were 60 ± 17% less than set CPAP levels (P < 0.001). Further, MAP decreased progressively with decreasing percent nares occlusion. The simulated open mouth condition resulted in significantly lower MAPs to <1.7 cmH2 O. The one-half prong insertion depth condition, with closed mouth, yielded MAPs approximately 35 ± 9% less than full insertion pressures (P < 0.001).

CONCLUSIONS

In our bench tests, the RAM interface connected to a ventilator in NCPAP mode failed to deliver set CPAP levels when applied using the manufacturer recommended 60-80% nares occlusion, even with closed mouth and full nasal prong insertion conditions.

Risk factor profile of massive pulmonary haemorrhage in neonates: the impact on survival studied in a tertiary care centre

OBJECTIVE

Pulmonary haemorrhage (PH) in neonates is a fatal event leading to hazardous complications and even death. The aim of this study was to elucidate influential factors of the ultimate disease course that affect death or survival.

METHODS

Infants treated for PH in our institution from March 2009 to December 2013 were retrospectively reviewed. Infants transferred from other hospitals were excluded. Infants were grouped into two categories, deceased or survived at neonatal intensive care unit discharge. Information regarding perinatal history, initial management and laboratory results were obtained and analysed for each group.

RESULTS

Seventy infants fulfilled the inclusion criteria, 41 infants in the deceased group and 29 infants in the survived group. Overall, the infants in the deceased group displayed lower gestational age (27 and 1/7 ± 3.610 versus 29 and 3/7 ± 3.530 weeks, p = 0.009) and lower one-minute (2.342 ± 1.493 versus 4.035 ± 2.079, p < 0.001) and five-minute Apgar scores (2.342 ± 1.493 versus 4.035 ± 2.079, p < 0.001) and required aggressive resuscitation (p = 0.003) and a greater number of inotropes (2.195 ± 1.346 versus 1.069 ± 0.704, p < 0.001). Deceased infants were administered increased amounts of fluid during the first 24 h after birth (117.783 ± 32.325 versus 99.379 ± 17.728 mL/kg, p = 0.004). A relatively short prothrombin time impacted survival (p = 0.01), whereas platelet count was the only factor that significantly affected the time length from the onset of PH to death (p = 0.01).

CONCLUSION

Infants with a lower gestational age in a compromised state are prone to die once PH develops. The initial management of fluid intake not to exceed the adequate limit is especially important in order to prevent PH-related deaths when correcting hypoalbuminemia and coagulopathy.

Effect of HFNC flow rate, cannula size, and nares diameter on generated airway pressures: an in vitro study

Increased use of non-invasive forms of respiratory support such as CPAP and HFNC in premature infants has generated a need for further investigation of the pulmonary effects of such therapies. In a series of in vitro tests, we measured delivered proximal airway pressures from a HFNC system while varying both the cannula flow and the ratio of nasal prong to simulated nares diameters. Neonatal and infant sized nasal prongs (3.0 and 3.7 mm O.D.) were inserted into seven sizes of simulated nares (range: 3-7 mm I.D. from anatomical measurements in 1-3 kg infants) for nasal prong-to-nares ratios ranging from 0.43 to 1.06. The nares were connected to an active test lung set at: TV 10 ml, 60 breaths/min, Ti 0.35 sec, compliance 1.6 ml/cm H₂O and airway resistance 70 cm H₂O/(L/sec), simulating a 1-3 kg infant with moderately affected lungs. A Fisher & Paykel Healthcare HFNC system with integrated pressure relief valve was set to flow rates of 1-6 L/min while cannula and airway pressures and cannula and mouth leak flows were measured during simulated mouth open, partially closed and fully closed conditions. Airway pressure progressively increased with both increasing HFNC flow rate and nasal prong-to-nares ratio. At 6 L/min HFNC flow with mouth open, airway pressures remained <1.7 cm H₂O for all ratios; and <10 cm H₂O with mouth closed for ratios <0.9. For ratios >0.9 and 50% mouth leak, airway pressures rapidly increased to 18 cm H₂O at 2 L/min HFNC flow followed by a pressure relief valve limited increase to 24 cm H₂O at 6 L/min. Safe and effective use of HFNC requires careful selection of an appropriate nasal prong-to-nares ratio even with an integrated pressure relief valve.

Lung function tests in neonates and infants with chronic lung disease: lung and chest-wall mechanics

This is the fifth paper in a review series that summarizes available data and critically discusses the potential role of lung function testing in infants and young children with acute neonatal respiratory disorders and chronic lung disease of infancy (CLDI). This review focuses on respiratory mechanics, including chest-wall and tissue mechanics, obtained in the intensive care setting and in infants during unassisted breathing. Following orientation of the reader to the subject area, we focused comments on areas of enquiry proposed in the introductory paper to this series. The quality of the published literature is reviewed critically with respect to relevant methods, equipment and study design, limitations and strengths of different techniques, and availability and appropriateness of reference data. Recommendations to guide future investigations in this field are provided. Numerous different methods have been used to assess respiratory mechanics with the aims of describing pulmonary status in preterm infants and assessing the effect of therapeutic interventions such as surfactant treatment, antenatal or postnatal steroids, or bronchodilator treatment. Interpretation of many of these studies is limited because lung volume was not measured simultaneously. In addition, populations are not comparable, and the number of infants studied has generally been small. Nevertheless, results appear to support the pathophysiological concept that immaturity of the lung leads to impaired lung function, which may improve with growth and development, irrespective of the diagnosis of chronic lung disease. To fully understand the impact of immaturity on the developing lung, it is unlikely that a single parameter such as respiratory compliance or resistance will accurately describe underlying changes. Assessment of respiratory mechanics will have to be supplemented by assessment of lung volume and airway function. New methods such as the low-frequency forced oscillation technique, which differentiate the tissue and airway components of respiratory mechanics, are likely to require further development before they can be of clinical significance.

Postnatal Changes in Pulmonary Mechanics and Energetics of Infants with Respiratory Distress Syndrome following Surfactant Treatment

BACKGROUND

Postnatal alterations in pulmonary mechanics, energetics and functional residual capacity (FRC) describe the structural maturation of the preterm respiratory system.

OBJECTIVE

To evaluate longitudinal changes in pulmonary function in infants with respiratory distress syndrome (RDS) treated with oxygen, positive pressure ventilation and synthetic surfactant (Exosurf).

METHODS

Serial pulmonary function tests were performed in surfactant-treated infants [mean +/- SD birth weight (BW) = 1,112 +/- 276 g, gestational age (GA) = 29 +/- 3 weeks] at postnatal ages: <3 days, 1, 2, 3, 4 and 6-8 weeks until term postmenstrual age (PMA). Tidal volume, pulmonary compliance (C(L)), pulmonary resistance (R(T)) and flow-resistive work were analyzed following simultaneous measurements of airflow and transpulmonary pressure signals. Serial FRC measurements were made in a randomly selected group.

RESULTS

Prior to 28 weeks' PMA, C(L) was unchanged irrespective of GA. At age 1 week the likelihood ratio (LR) for bronchopulmonary dysplasia (BPD) based on C(L), R(T) and GA was predicted to be >90% for those with BW <750 g (LR >100) as compared to <10% probability (LR = 0.3) for infants >1,500 g. Significant linear increase in C(L) to PMA was evident >28 weeks' PMA (r = 0.86, p < 0.01) at 0.17 ml/cm H2O/kg/week. By term PMA, mean C(L) was 2.60 +/- 0.07 ml/cm H2O. Improvements in FRC of preterm infants with RDS who recovered occur at a more rapid rate ( approximately 25 ml/kg) compared to those who developed BPD ( approximately 20 ml/kg).

CONCLUSIONS

Slow but incremental postnatal pulmonary improvement, minimal <28 weeks' PMA, were comparable for all infants. Along with diminished FRC, these changes reflect persistent deleterious effects of positive pressure ventilation, alveolar hyperoxia and unrecognized pulmonary overdistension.

Overview of surfactant replacement trials

Clinical trials have evaluated the overall efficacy of surfactant therapy, as well as the relative efficacy of different surfactant preparations, the optimal timing of administration and the optimal dosage. Surfactant therapy leads to significant clinical improvement in infants at risk for, or having, respiratory distress syndrome (RDS). Clinical trials that compared the effects of synthetic or animal-derived surfactant preparations to placebo or no therapy demonstrate that surfactant therapy lead to rapid improvement in oxygenation, decreased ventilator support, decreased risk of pneumothorax, and mortality. Earlier treatment, prophylactic treatment of infants at high risk of developing RDS, and selective re-treatment leads to improved clinical outcome as well. Currently available animal-derived surfactants are superior to non-protein-containing synthetic surfactants. Ongoing evaluation will determine if important differences in animal-derived products are noted. Future trials will evaluate third-generation surfactant products and further refine what constitutes optimal use of surfactant.

Predictors of successful extubation of preterm low-birth-weight infants with respiratory distress syndrome

OBJECTIVE

The aim of the study was to measure pulmonary mechanics in infants with respiratory distress syndrome before extubation and to correlate pulmonary function values with successful extubation.

DESIGN

Clinical study.

SETTING

Neonatal intensive care unit.

PATIENTS

Fifty-one infants (birth weight, 1158.6 +/- 150.6 g; gestational age, 29.1 +/- 2.0 wks).

INTERVENTIONS

Ventilation and daily ventilatory management.

MEASUREMENTS AND MAIN RESULTS

Of the 51 infants studied, 35 (60.8%) were successfully extubated, whereas 16 (39.2%) required reintubation and mechanical ventilation within 72 hrs after extubation. All patients met the clinical and biochemical criteria for extubation. Variables of artificial ventilation before extubation were minimal in all the studied cases (Fio(2) </=0.4, inspiratory pressure </=20 cm H(2)O, ventilatory rate, </=10/min). Pulmonary mechanics were measured before extubation using a noninvasive, mobile VenTrak measuring station.

RESULTS

Significant differences in pulmonary function values between the groups were found. Lower resistance of airways and work of breathing and higher dynamic compliance, tidal volume, and minute ventilation before extubation were associated with successful extubation.

CONCLUSION

On the average, tidal volume values of >6 mL/kg, minute ventilation of >309 mL/kg/min, work of breathing of <0.172 J/L, dynamic compliance of >/=1 mL/cm H(2)O/kg, and resistance of airways of </=176 cm H(2)O/L/sec predicted successful extubation. We recommend measurement of pulmonary function as an assessment tool in determining readiness for extubation.

Immediate changes in lung compliance following natural surfactant administration in premature infants with respiratory distress syndrome: a controlled trial

OBJECTIVE

To compare immediate changes in lung compliance following the administration of two commercially available natural surfactants.

METHOD

We conducted a prospective, randomized study of 40 preterm infants with respiratory distress syndrome requiring surfactant. Infants received either Infasurf or Survanta. The primary outcome measure was the change in compliance assessed by bedside pulmonary monitoring.

RESULTS

There were no significant changes in dynamic lung compliance within or between the two groups 1 hour after surfactant administration. However, infants given Survanta required more doses per patient (4 vs 2, p=0.05) and were more likely to require >2 doses (57 vs 26%, p=0.05). Infants requiring >1 dose of surfactant had a greater change in airway pressure and improved oxygenation just before the second dose when treated with Infasurf.

CONCLUSIONS

We found no significant difference in acute changes in lung compliance. However, treatment with Infasurf seems to be more long lasting than Survanta.

A randomized trial of moderately early low-dose dexamethasone therapy in very low birth weight infants: dynamic pulmonary mechanics, oxygenation, and ventilation

OBJECTIVE

Dexamethasone is used in very low birth weight (VLBW) ventilator-dependent infants to prevent or decrease the severity of chronic lung disease. We reported a significant increase in respiratory compliance during a 7-day weaning course of moderately early dexamethasone therapy (0.5 mg/kg/d) in VLBW infants, along with a shorter duration of mechanical ventilation and O2 supplementation. Although 0.5 mg/kg/d has been the most commonly used dose in preterm infants, the use of a lower dose of dexamethasone may reduce potential adverse effects of steroid therapy. Quantification of dynamic pulmonary mechanics in VLBW infants who receive low-dose dexamethasone has not been reported. The objective of this study was to compare the effect of 2 dose regimens of dexamethasone on dynamic pulmonary mechanics, mean airway pressure (MAP), and fractional inspired oxygen concentration (Fio2) in intubated VLBW infants who were at risk for chronic lung disease.

METHODS

We studied 47 VLBW (birth weight: 550-1290 g; gestational age: 24-30 weeks) ventilator-dependent infants at 7 to 14 days of age. Twenty-three infants were randomized to receive dexamethasone at 0.5 mg/kg/d intravenously for 3 days (high dose), 0.25 mg/kg/d for 3 days, and 0.1 mg/kg/d during the 7th day; 24 infants received low-dose dexamethasone as 0.2 mg/kg/d for 3 days and 0.1 mg/kg/d for 4 days. Respiratory compliance (Crs) and resistance were measured before and on days 2, 5, and 7 of dexamethasone therapy. We recorded airway pressure, flow, and tidal volume, and mechanical breaths were analyzed.

RESULTS

Crs significantly increased during dexamethasone therapy in both groups of infants when compared with baseline (74% increase in the high-dose group and 66% increase in the low-dose group). Dexamethasone increased tidal volume and significantly reduced Fio2 and MAP in both groups of infants. A transient increase in blood pressure was noted in both groups.

CONCLUSIONS

Our findings indicate that 1) comparable significant increases in Crs are present in the low-dose dexamethasone as well as the high-dose dexamethasone groups on days 2, 5, and 7 of steroid therapy; and 2) MAP and Fio2 are significantly decreased during dexamethasone therapy in both groups of infants. We conclude that low-dose and high-dose dexamethasone, as used in this study, have comparable beneficial effects on dynamic pulmonary mechanics and subsequently on oxygen requirement and applied ventilatory support in VLBW infants.

Current surfactant use in premature infants

Exogenous surfactant therapy has been a significant advance in the management of preterm infants with RDS. It has become established as a standard part of the management of such infants. Both natural and synthetic surfactants lead to clinical improvement and decreased mortality, with natural surfactants having additional advantages over currently available synthetic surfactants. The use of prophylactic surfactant administered after initial stabilization at birth to infants at risk for RDS has benefits compared with rescue surfactant given to treat infants with established RDS. In infants who do not receive prophylaxis, earlier treatment (before 2 hours) has benefits over later treatment. The use of multiple doses of surfactant is a superior strategy to the use of a single dose, whereas the use of a higher threshold for retreatment seems to be as effective as a low threshold. Adverse effects of surfactant therapy are infrequent and usually not serious. Long-term follow-up of infants treated with surfactant in the neonatal period is reassuring. In the future we are likely to see the development of new types of surfactants. Further research is required to determine the optimal use of surfactant in conjunction with other respiratory interventions.

Alveolar recruitment promotes homogeneous surfactant distribution in a piglet model of lung injury

Uneven distribution of exogenous surfactant contributes to a poor clinical response in animal models of respiratory distress syndrome. Alveolar recruitment at the time of surfactant administration may lead to more homogeneous distribution within the lungs and result in a superior clinical response. To investigate the effects of three different volume recruitment maneuvers on gas exchange, lung function, and homogeneity of surfactant distribution, we studied 35 newborn piglets made surfactant deficient by repeated airway lavage with warm saline. Volume recruitment was achieved by either a temporal increase in tidal volume or an increase in end-expiratory pressure during surfactant administration, yielding an increase in dynamic compliance of the respiratory system of 77% in the first group and an increase in functional residual capacity of 108% in the second group. A third group of piglets (all n = 7) received a combination of both volume recruitment maneuvers, with increases in dynamic compliance of the respiratory system of 100% and in functional residual capacity of 192%. Those animals subjected to increased tidal volume showed an improved surfactant response in terms of oxygenation, ventilation, lung volumes, lung mechanics, and homogeneity of surfactant distribution. Increased end-expiratory volume augmented the surfactant effect only to some extent. The combination of both volume recruitment maneuvers, however, needed lung volumes beyond total lung capacity (approximately 56 mL/kg), thus probably inducing early sequelae of ventilator-induced lung injury. We conclude that volume recruitment by means of increased tidal volumes at the time of surfactant administration leads to a superior surfactant effect owing to more homogeneous surfactant distribution within a collapsed lung.

Pulmonary outcome in extremely low birth weight infants

OBJECTIVE

To determine whether infants with hyaline membrane disease (HMD) superimposed on immature lung disease (ILD) have more abnormal lung function and respiratory drive during the evolution of chronic neonatal lung disease (CNLD) in extremely low birth weight infants (ELBW; <1000 g).

METHODS

We measured lung mechanics (respiratory frequency, tidal volume, minute ventilation, lung resistance, lung compliance, lung impedance, and work of breathing per minute) and respiratory drive (airway opening pressure 100 milliseconds after initiation of breath [P(0.1)] and maximal inspiratory pressure generated during airway occlusion) on 3 occasions before term in 24 ELBW infants.

RESULTS

Ten infants with ILD (mean [95% CI] gestation: 24.3 weeks [23.1,25.4]; birth weight: 675 g [553,798]) were studied at 27, 31, and 35 weeks of postconceptional age and 14 infants with HMD superimposed on ILD (gestation: 25.1 weeks [24.4,25.9]; birth weight: 687 g [601,773]) were studied at 28, 32, and 35 weeks of postconceptional age. There were no statistically significant differences between the groups for respiratory frequency, tidal volume, minute ventilation, lung resistance, lung compliance, lung impedance, work of breathing per minute, P(0.1), and maximal inspiratory pressure generated during airway occlusion. With increasing age, both groups demonstrated increased respiratory drive as measured by P(0.1) without significant changes in respiratory frequency or CO(2). Work of breathing per minute increased in the HMD group with age and was higher in extubated subjects. A similar trend with age was demonstrated in ILD infants. Regardless of whether the initial lung disease was ILD alone or HMD + ILD, ELBW infants developed a mildly reduced lung compliance/kg (.8-1.1 mL/cm.H(2)O/kg) and high lung resistance (75-125 cm.H(2)O/L/second) pattern of CNLD, which changed little after 3 weeks of age. Survival to 6 months was 23/24 (96%). Oxygen dependency was 16/24 (67%) at 35 weeks, yet only 5/23 (22%) survivors required oxygen at discharge from the neonatal unit (43 weeks).

CONCLUSIONS

The visco-elastic and flow-resistive properties of the lungs in ELBW infants with CNLD remain only mildly abnormal, suggesting a more favorable prognosis for lung function in later years than previously reported.

Improvement in respiratory compliance after surfactant therapy evaluated by a new method

Descriptions of the effects of intratracheally applied surfactant on respiratory system compliance (C(rs)) have been somewhat controversial because the commonly used methods for assessing pulmonary function were designed for a linear pressure/volume (P/V) relation of the respiratory system. In infants with lung disease a linear P/V relation cannot be expected. Therefore, a new method (APVNL) was employed which enabled us to calculate respiratory system compliance (C(rs)) and resistance (R(rs)) based on changes in volume (V). This method is independent of the P/V relation, and was used to assess the effects of intratracheal instillation of surfactant. Fourteen infants (gestational age, 24 to 30 weeks) with respiratory distress syndrome were treated with bovine surfactant intratracheally while the fractional inspired oxygen concentration (FiO(2)) exceeded 50%. C(rs) was evaluated for the infants using the APVNL method and the method of linear regression (LR) based on the equation of motion designed for linear P/V relationships. Two hours after surfactant treatment, the median reduction of FiO(2) was 33% (95% CI: 20-50%; P < 0.01). There was no correlation between the change in FiO(2) and the change in C(rs), using either the APVNL method or the LR method. Two hours after surfactant treatment, the median improvement in C(rs) was 0.37 mL/cmH(2)O/kg (95% CI: 0.07-1. 16 mL/cmH(2)O) at a change in V of 1 mL/kg (P < 0.02) and 0.23 mL/cmH(2)O/kg (95% CI: 0-0.57 mL/cmH(2)O) at a change in V of 2 mL/kg (P < 0.05) when the APVNL method was used. The LR method could not show a significant change in C(rs) after surfactant treatment. Further, R(rs) did not show significant changes 2 hr after surfactant administration. We conclude that the APVNL method is more appropriate for evaluating changes of C(rs) elicited by surfactant treatment than the LR method. The APVNL method demonstrated significant initial improvements in compliance as lung volumes were increased; there were no significant further decreases in C(rs) as peak inspiratory pressures and the upper limits of tidal volume were approached.

Lung resistance and elastance in spontaneously breathing preterm infants: effects of breathing pattern and demographics

Reported values of lung resistance (RL) and elastance (EL) in spontaneously breathing preterm neonates vary widely. We hypothesized that this variability in lung properties can be largely explained by both inter- and intrasubject variability in breathing pattern and demographics. Thirty-three neonates receiving nasal continuous positive airway pressure [weight 606-1,792 g, gestational age (GA) of 25-33 wk, 2-49 days old] were studied. Transpulmonary pressure was measured by esophageal manometry and airway flow by face mask pneumotachography. Breath-to-breath changes in RL and EL in each infant were estimated by Fourier analysis of impedance (Z) and by multiple linear regression (MLR). RL(MLR) (RL(MLR) = 0.85 x RL(Z) -0.43; r(2) = 0.95) and EL(MLR) (EL(MLR) = 0.97 x EL(Z) + 8.4; r(2) = 0.98) were highly correlated to RL(Z) and EL(Z), respectively. Both RL (mean +/- SD; RL(Z) = 70 +/- 38, RL(MLR) = 59 +/- 36 cm H(2)O x s x l(-1)) and EL (EL(Z) = 434 +/- 212, EL(MLR) = 436 +/- 210 cm H(2)O/l) exhibited wide intra- and intersubject variability. Regardless of computation method, RL was found to decrease as a function of weight, age, respiratory rate (RR), and tidal volume (VT) whereas it increased as a function of RR. VT and inspiratory-to-expiratory time ratio (TI/TE). EL decreased with increasing weight, age, VT and female gender and increased as RR and TI/TE increased. We conclude that accounting for the effects of breathing pattern variability and demographic parameters on estimates of RL and EL is essential if they are to be of clinical value. Multivariate statistical models of RL and EL may facilitate the interpretation of lung mechanics measurements in spontaneously breathing infants.

Tracheal aspirate surface tension in babies with hyaline membrane disease: effects of synthetic surfactant replacement

Our objective was to determine changes in surface tension of tracheal aspirate over the first 4-5 days of life in babies with hyaline membrane disease, with and without synthetic surfactant replacement. Tracheal aspirates were collected prior to and for 96-108 hr after initiation of a randomized double-blind trial of synthetic surfactant (EXOSURF Neonatal) or air-treated control patients. Using the captive bubble technique, we measured minimum surface tension (initial adsorption, first quasi-static compression, dynamic cycling at 30 cpm, second quasi-static compression and 5 min after quasi-static compressions) in 39 surfactant-treated and 44 control babies. We also compared minimum surface tension with the respiratory support provided. Twelve hours after one dose of synthetic surfactant, minimum surface tension on first quasistatic compression decreased significantly from 20.9+/-1.4 to 17.6+/-1.3 mN/m compared to air-treated babies, who did not show any change. Reduction in minimum tracheal aspirate surface tension on first quasi-static compression and during dynamic cycling over 48-60 hr occurred more rapidly in surfactant-treated babies. Ventilator support did not correlate with minimum tracheal aspirate surface tension. We conclude that treatment of babies with synthetic surfactant improved tracheal aspirate minimum surface tension within 12 hr of the first dose and for the next 48-60 hr.

Changes in pulmonary artery pressure during the acute phase of respiratory distress syndrome treated with three different types of surfactant

We studied the changes in acceleration time/right ventricular ejection time ratio (AT/RVET; indicative of changes in pulmonary artery pressure) calculated from Doppler ultrasound examinations performed before and 1, 6, and 12 h after the first and second doses of surfactant following the administration of each of three different surfactants during the acute phase of the respiratory distress syndrome. Maximum fractional inspired oxygen concentration (F(I,O2)) and peak inspiratory pressure (PIP) were recorded during each 4 h period from birth for the first 24 h and subsequently every 24 h until 72 h. Eighty-three infants were studied. Fifty patients weighing > 1 kg received Exosurf (n = 29) or ALEC (n = 21) and 33 weighing < or = 1 kg received Exosurf (n = 22) or Survanta (n = 11). The AT/RVET rose rapidly after administration of all three surfactants. There was no significant difference in the change in AT/RVET between those > 1 kg who received Exosurf and those who received ALEC (a synthetic surfactant). Similarly, there was no difference between those infants < or = 1 kg who received Exosurf and those who received Survanta. The F(I,O2) requirements, but not PIP, were lower in those infants who received Survanta at 12 and 20 h compared with those who received Exosurf. There was no significant difference in the F(I,O2) or PIP requirements between infants > 1 kg who received Exosurf compared with those who received ALEC. The rise in AT/RVET found in this study after administration of ALEC, Exosurf, or Survanta suggests that similar and rapid falls in pulmonary artery pressure occur after all three surfactant administrations, despite the difference in clinical response demonstrated between Exosurf and Survanta.

Effect of neonatal surfactant therapy on lung function at school age in children born very preterm

Our aim was to evaluate long-term effects of exogenous surfactant therapy on pulmonary functional outcome in children born very preterm. We examined 40 children aged 7-12 years who were born before 30 weeks of gestation with an immature surfactant system, and were randomized to one of three treatment groups: human surfactant given at birth (prophylactic), human surfactant given after development of neonatal respiratory distress syndrome (rescue), and placebo (air) treatment. Spirometric parameters of preterm born children were compared with those of 20 children born at term. In addition, spirometric parameters were monitored twice daily for 4 weeks using a home spirometer. All spirometric parameters were significantly lower in the preterm groups than in the controls, except for the forced vital capacity (FVC) in the prophylactically treated group. Bronchial obstruction was found in 53% of the prophylactically treated group, in 36% of the rescue group, in 67% of the placebo group, and in 0% of the control group. Peak expiratory flow (PEF) and FVC values were higher in those children who received surfactant compared with the placebo group (P < 0.05). In 16 children (40%) born preterm, a beta2-agonist induced an increase in PEF > or = 15% at least three times during 2 weeks of home monitoring; eight children (20%) had abnormal diurnal PEF variation. Multiple regression analysis indicated that the independent variables associated with favorable outcomes in spirometric parameters were surfactant therapy (P = 0.012-0.045) and short intubation time after birth (P = 0.0009-0.0044). Bronchial obstruction, responsiveness to a beta2-agonist, and high diurnal PEF variation are common in children born before 30 gestational weeks. Surfactant supplementation reducing the need for mechanical ventilation or supplementary oxygen after birth may decrease the severity of immaturity related bronchial obstruction in childhood.

Effect of volume recruitment on response to surfactant treatment in rabbits with lung injury

We determined if surfactant treatment effect can be enhanced by mechanical volume recruitment during surfactant administration by measuring functional residual capacity, tidal volume, the alveolar portion of tidal volume, dynamic compliance of the respiratory system, a/A ratio, and PaCO2 by measuring before and after surfactant administration to rabbits with lung injury induced by airway lavage. There was improvement in all lung function indices when surfactant was given with volume recruitment, but when surfactant was given without volume recruitment, the only index to show significant improvement was a/A ratio of oxygenation. These results support the hypothesis that mechanical recruitment of terminal airspaces from a previously unventilated compartment will enhance the effectiveness of surfactant replacement by facilitating the distribution of instilled surfactant to this compartment.

Spiking Survanta with synthetic surfactant peptides improves oxygenation in surfactant-deficient rats

The hypothesis that the in vivo function of Survanta (Beractant) can be improved by supplementation with synthetic surfactant peptides B and C was tested in a surfactant-deficient rat model. Full length surfactant protein-B (SP-B1-78) (B) and palmitoylated surfactant protein-C (SP-C1-35) (C), and synthetic KL4 peptide were added to Survanta after extraction, creating extracted Survanta (ES) with 1% B, 2% B, and 2% B plus 1% C, or mixed with Survanta without extraction, creating modified Survanta (S) with 2% B, 2% B plus 1% C, and 2% KL4. Adult rats were ventilated with 100% oxygen, tidal volumes (VT) of 7.5 ml/kg and a rate of 60/min, and were lavaged until the PaO2 dropped below 80 mm Hg, when 100 mg/kg of surfactant was instilled. After 15 to 60 min of ventilation, pressure-volume (P-V) curves were generated in situ. Instillation of ES or S with 2% B plus 1% C led to the greatest increase in oxygenation, closely followed by ES and S with 2% B, and more distantly by S plus 2% KL4. TLC was comparable among the ES and S groups, but greater than that of air-placebo controls. These data suggest that spiking of Survanta with synthetic SP-B and SP-C increased oxygenation more effectively than B or KL4 alone in this surfactant-deficient rat model.

Respiratory mechanics before and after late artificial surfactant rescue

OBJECTIVE

To assess the effect of late administration of synthetic surfactant (Exosurf) on the ventilatory function of premature infants with hyaline membrane disease (HMD).

METHODOLOGY

Prospective non-randomized study in the Neonatal Intensive Care Unit (NICU) of a major referral hospital. The patients included two groups of premature infants with a birthweight between 750 and 2000 g who developed HMD. In group 1 with moderate to severe HMD, 2 x 5 mL/kg doses of Exosurf were given 12 h apart (first dose given at a mean age of 18.7 +/- 3.4 h [mean +/- s.e.m.]). In group 2 with milder HMD, no surfactant was given.

RESULTS

Significant reductions (P < 0.05) in the fraction of inspired oxygen (FIO2) occurred 6 h after surfactant administration (24 h of life) and by 48 h (64 h of life) in group 2. These improvements in gas exchange preceded improvements in passive respiratory compliance which occurred 24 h after surfactant (42 h of life) and by 72 h (88 h of life) in group 2 (P < 0.01). In both groups pulmonary resistance increased and was significant (P < 0.05) by 48 h (66 h of life) in group 1.

CONCLUSIONS

Synthetic surfactant given as late as a mean age 18.7 +/- 3.4 h still improves gas exchange but these early improvements cannot be completely explained by modifications of respiratory compliance.

Exosurf rescue surfactant improves high ventilation-perfusion mismatch in respiratory distress syndrome

OBJECTIVE

To assess ventilation/perfusion (VA/Q) mismatch of the high type, following rescue surfactant therapy for respiratory distress syndrome.

HYPOTHESIS

Surfactant therapy reduces such mismatch.

DESIGN

Randomized, double-blind, placebo-controlled study, assessing VA/Q with the arterial-alveolar difference of CO2 tension (P(a-A)CO2). This difference was determined with capnometry and arterial blood gases, using the equation: P(a-A)CO2 equals arterial CO2 minus alveolar CO2 partial pressure.

SETTING

A level III nursery.

PATIENTS

Ten intubated infants with respiratory distress syndrome.

INTERVENTION

Infants were randomized to each receive two doses of surfactant or two doses of air placebo.

RESULTS

P(a-A)CO2 improved after surfactant and worsened after placebo (P = 0.0021), comparing slopes of 12-hr regression lines. A similar pattern occurred with oxygenation. These changes in P(a-A)CO2 and in oxygenation were minimally correlated within the surfactant group.

CONCLUSION

Exosurf rescue surfactant reduced VA/Q mismatch of the high type, over several hours.

Early changes in lung function and response to surfactant replacement therapy

Dynamic respiratory system compliance (Cdyn) was measured in 44 preterm babies before, immediately after, and for 96 h following administration of artificial surfactant (Exosurf). There was no significant change in Cdyn for the whole group over the entire study period. Subdivision into three groups on the basis of Cdyn prior to surfactant revealed a significant and sustained deterioration in lung function in those babies with the highest starting compliance and a significant and sustained improvement in those with the lowest compliance. Inspired oxygen and alveolar/arterial oxygen gradient also exhibited significant differences with least improvement in the babies with the best initial lung function and most improvement in the babies with worst initial lung function and most improvement in the babies with worst initial lung function. Despite clear initial differences in clinical status, neither long-term oxygen requirements nor the incidence of chronic lung disease differed significantly between the three groups. We conclude that the response of an individual baby to the administration of surfactant is, in part, determined by the lung function before surfactant is administered. Babies with higher initial lung compliance are more likely to deteriorate after administration and caution should be used before selection of such babies for surfactant treatment.

Effects of morphine and pancuronium on lung volume and oxygenation in premature infants with hyaline membrane disease

To determine the effect of analgesia and paralysis on lung volume and oxygenation in premature infants supported by mechanical ventilation because of hyaline membrane disease, functional residual capacity (FRC), and arterial/alveolar oxygen tension ratio were measured in nine premature infants with hyaline membrane disease before and after the administration of morphine sulfate and pancuronium bromide. Without a change of positive end-expiratory pressure, ventilator rate and peak inspiratory pressure were increased before the first set of measurements to minimize the contribution of the infants' own respiratory effort to total ventilation. These ventilator settings were then held constant (except fraction of inspired oxygen) before and after the administration of the drugs. The FRC was measured with a multiple-breath N2 washout technique by means of whole-body plethysmography to measure airway flow. The FRC and the ratio of arterial to alveolar oxygen tension decreased in seven of nine patients after treatment with morphine and pancuronium. The decrease in FRC for all patients was significant (2.4 +/- 2.9 ml/kg; p < 0.05), and a significant correlation was demonstrated between the change in the arterial/alveolar oxygen tension ratio and the change in FRC (r = 0.82; p < 0.01). Gestational age, birth weight, postnatal age, severity of lung disease, and time after the administration of morphine and pancuronium were not significantly correlated with the change in FRC. We believe that a decrease in oxygenation caused by alveolar derecruitment occurred even though the ventilator settings had been increased before the first set of measurements. The decrease in FRC in these infants, who are thought to have alveolar instability because of surfactant deficiency, may have resulted from the loss of expiratory braking mechanisms. We conclude that analgesia and paralysis should be used with caution under these circumstances.

Extensive intraalveolar pulmonary hemorrhage in infants dying after surfactant therapy

To assess the possible relationship between exogenous surfactant therapy and pulmonary hemorrhage in premature infants, we compared autopsy findings in 15 infants treated with exogenous surfactant and in 29 who died before the introduction of surfactant therapy. Infants who met the following criteria were included: birth weight 501 to 1500 gm, survival 4 hours to 7 days, and no congenital anomalies. Average birth weight, gestational age, and age at death were equivalent for the two groups. High rates of pulmonary hemorrhage were present in both groups (treated 80% vs untreated 83%). The untreated group had higher incidences of interstitial hemorrhage and lung hematomas and significantly more large interstitial hemorrhages: 31% untreated versus 0% treated (p < 0.05). The overall rate of intraalveolar hemorrhage was similar in the two groups, but surfactant-treated infants were more likely to have extensive intraalveolar hemorrhage: 53% versus 14% (p < 0.05). Most surfactant-treated infants who survived more than 24 hours had extensive intraalveolar hemorrhage (8/9). Patients who had extensive intraalveolar hemorrhage, with or without prior surfactant therapy, frequently had clinically significant pulmonary hemorrhage (7/12). These findings indicate that infants who die after surfactant therapy have higher rates of a specific type of pulmonary hemorrhage--extensive intraalveolar hemorrhage.

Static respiratory compliance in the newborn. III: Early changes after exogenous surfactant treatment

Static respiratory system compliance (Crs) was measured by a single breath passive expiratory flow technique in 73 newborn infants treated with exogenous surfactant. The first 39 received Curosurf, a natural porcine surfactant. The other 34 received Exosurf Neonatal, a synthetic surfactant. All had a diagnosis of respiratory distress syndrome with an arterial/alveolar oxygen ratio < 0.22. Static Crs and arterial blood gases were measured shortly before, and at three and 12 hours after the first dose of surfactant. In 32 infants treated with Curosurf with initial static Crs < 1.8 ml/cm H2O/m body length, which is consistent with surfactant deficiency, static Crs improved by 18% at three hours and by 39% at 12 hours along with a median reduction in fractional inspired oxygen (FIO2) at three hours by 0.32. In 26 infants treated with Exosurf with initial Crs < 1.8 ml/cm H2O/m, Crs did not improve three and 12 hours after treatment and oxygenation improved less than after Curosurf, with a median reduction in FIO2 at three hours of 0.11. Fifteen of the 73 (21%) infants had initial static Crs of > or = 1.8 ml/cm H2O/m, not consistent with surfactant deficiency. Thirteen of these 15 infants showed a fall in static Crs after surfactant treatment, raising the question whether exogenous surfactant did them more harm than good. Initial static Crs and surfactant type both appear to determine the early response to the first dose of surfactant. Only a considerably larger, randomised study can show which surfactant is more effective in reducing adverse clinical outcome.

Incidence and prediction of bronchopulmonary dysplasia in a cohort of premature infants

A prospective study on the incidence of bronchopulmonary dysplasia (BPD) in premature infants is reported. A cohort of premature infants with gestational ages < or = 32 weeks, treated during 1989, was followed for one year. Of a total study population of 117 infants, 23 (19.6%) developed BPD, defined as oxygen dependence at 28 postnatal days. However, only 15 infants (12.8%) needed supplementary oxygen at the age of 36 gestational weeks and 5 infants (4.2%) needed supplementary oxygen periodically at one year of age. BPD was found to account for a significant part of both the total and late mortality in the cohort. Measurements of pulmonary mechanics were performed at 3 +/- 1 and 12(13) +/- 1 days of life in a subgroup of 26 infants with RDS who required assisted ventilation for 4 days or longer. No significant difference in lung compliance or resistance could be found during the first examination between infants who later developed BPD and infants with RDS only. At the second examination, infants who later developed BPD had significantly lower lung compliance (0.48 +/- 0.23 ml/cmH2O) than infants in the RDS group (1.50 +/- 0.72 ml/cmH2O) (p < 0.001). Measurements of pulmonary mechanics could be of importance for early prediction of infants at risk of BPD.

Respiratory function at follow-up after neonatal surfactant replacement therapy

Respiratory function was assessed at a median of 7 months (range 6-12) in 17 preterm infants who, in the neonatal period, had been entered into a multi-centre randomized placebo-controlled trial of prophylactic surfactant replacement therapy. Seven infants (median gestational age 28 weeks) received surfactant and the remaining ten infants (median gestational age 27 weeks) placebo. Respiratory function was assessed by measuring functional residual capacity (FRC), thoracic gas volume (TGV) and airways resistance (RAW). Specific conductance (SGAW) was calculated from RAW and TGV. There was no significant difference in FRC or TGV between the two groups. RAW, however, was significantly lower in the surfactant (median 41, range 21-48 cmH2O l-1 s-1) compared to the placebo group (median 57, range 40-68 cmH2O l-1 s-1), P < 0.05 and SGAW significantly higher in the surfactant (median 0.136, range 0.063-0.289 l cmH2O-1 s-1) compared to the placebo group (median 0.081, range 0.062-0.134 l cmH2O-1 s-1), P < 0.05. These results suggest that surfactant replacement therapy improves lung function at follow-up.

Pulmonary follow-up 2.5 years after a randomized, controlled, multiple dose bovine surfactant study of preterm newborn infants

Forty-seven preterm infants, who were previously enrolled in a prospective, randomized, blinded study at birth to assess the effects of multiple doses of exogenous bovine surfactant to prevent respiratory distress syndrome, underwent lung function evaluation and review of their medical histories at 2 1/2 years of age. During their initial hospitalization there were no differences between the 17 control infants and the 30 surfactant-treated infants in the duration of ventilator or oxygen therapy and the incidence of bronchopulmonary dysplasia. At the follow-up both groups were similar in chronological and corrected ages, weights, lengths, and sex ratios and there were no differences in the occurrence of allergy, asthma, bronchiolitis, eczema, pneumonia, and wheezing. In addition, there was no significant difference regarding the incidence of chest illnesses lasting either 3 or 7 days and in the total number of required rehospitalizations. Functional residual capacity (FRC), tidal volume (VT/kg), compliance (Crs/kg), resistance (Rrs), and time constant of the respiratory system were not significantly different between the two groups at 2 1/2 years of age. We conclude that bovine surfactant, when given during the neonatal period, has little long-term effect on lung function. Neonatal bovine surfactant therapy neither improves nor produces any adverse effects on the developing respiratory system.

Long-term pulmonary consequences of respiratory distress syndrome in preterm infants treated with exogenous surfactant

The pulmonary outcome for preterm infants 1 year after synthetic surfactant replacement for respiratory distress syndrome was assessed by examining their pulmonary status and the results of pulmonary function tests. A total of 47 infants were followed: 13 infants mean +/- SD: birth weight, 1960 +/- 616 gm; gestation, 32 +/- 1.1 weeks) had been assigned to the placebo group and 34 (birth weight = 1890 +/- 530 gm; gestation = 32 +/- 2.5 weeks) to surfactant treatment. The infants were examined at 3 to 6 months of age (n = 45) and at 9 to 12 months of age (n = 36). There were no significant differences between the two groups in predisposing clinical conditions that would lead to chronic lung disease. The infants had similar patterns of growth, respiratory-related illness, and need for theophylline therapy, diuretic therapy, or both. None had hypoxemia by pulse oximetry. Mean (+/- SEM) values for pulmonary mechanics and energetics in surfactant-treated infants were significantly (p < 0.01) lower for total pulmonary resistance in late infancy (57.7 +/- 11.7 vs 35.3 +/- 4.6 cm H2O/L per second). Lower values (mean +/- SEM) of resistive work of breathing were also measured in the surfactant-treated group (60.7 +/- 12.0 vs 38.2 +/- 3.6 gm-cm/kg per breath). The dynamic pulmonary compliance values were in the low-normal range for both groups, and the mean (+/- SEM) peak-to-peak esophageal pressure values were elevated (11.47 +/- 2.26 cm H2O in the placebo group; 9.24 +/- 0.69 cm H2O in the surfactant group). Forced expiratory flow measurements in late infancy demonstrated significant (p < 0.01) improvement in expiratory reserves and reduced evidence of airflow obstruction in the surfactant-treated infants (peak flow (mean +/- SEM): 287.1 +/- 69 vs 396.9 +/- 27 ml/sec; forced expiratory flow (mean +/- SEM) at functional residual capacity: 56.3 +/- 7.5 vs 83.4 +/- 19.5 ml/sec). No significant differences in pulmonary functions were noted in early infancy. These data suggest that surfactant replacement for respiratory distress syndrome may be associated with beneficial long-term effects on the resistive airflow properties of larger preterm infants.

Rapid improvement of static compliance after surfactant treatment in preterm infants with respiratory distress syndrome

Respiratory mechanics were measured in 20 preterm infants before and in the 24-hr period after treatment with surfactant. All infants were enrolled in the rescue clinical trial with Curosurf carried out in the Neonatal Intensive Care Unit. They received a dose of 200 mg/kg lipid surfactant intratracheally after birth. Static compliance of the respiratory system (Crs) was measured by the single breath occlusion technique during both spontaneous and mechanical ventilation. Resistance of the respiratory system (Rrs) and expiratory time constant (Trs) were also measured. As early as 3 hr after surfactant administration a significant improvement of 45% in Crs measured during mechanical ventilation (CrsV) was noted (0.40 +/- 0.14 vs 0.58 +/- 0.17 mL/cm H2O/kg, P < 0.001), together with a significant improvement of the arterial/alveolar O2 tension ratio (Pa/AO2) (0.12 +/- 0.03 vs 0.30 +/- 0.16, P < 0.01). The improvement of CrsV and Pa/AO2 was confirmed 24 hr later (0.55 +/- 0.15 mL/cm H2O/kg and 0.33 +/- 0.18, respectively). A significant correlation was found between Crs and Pa/AO2 ratio (r = 0.56, P < 0.001). Time constant values were significantly higher after surfactant treatment (0.15 +/- 0.07 vs 0.09 +/- 0.03 sec; P < 0.01). Rrs remained unchanged. These data indicate that Curosurf given intratracheally after birth determines a rapid improvement of respiratory mechanics as soon as 3 hr after dosing, together with the improvement of oxygenation. From the findings obtained with the present study we show evidence that respiratory system mechanics may be a useful physiological measure to guide ventilatory strategy following surfactant therapy.

Delayed compliance increase in infants with respiratory distress syndrome following synthetic surfactant

Recent research has demonstrated that Exosurf (EXSF), a newly synthesized artificial surfactant, increases survival when administered endotracheally to premature infants with RDS. This study examines the effects of EXSF on static respiratory system compliance (Crs). Thirty-four patients received two doses of EXSF in this rescue protocol. Crs (mL/cmH2O/kg) did not significantly change within the first 4 hours after either dose. However, Crs values did increase significantly (paired Student's t-test, P = 0.005) when data collected after the second dose (0.36 +/- 0.13 mL/cmH2O/kg) were compared to first week follow-up data (0.51 +/- 0.21 mL/cmH2O/kg). Crs data collected between 2 and 4 weeks after treatments were again not significantly different from non-concurrent control data collected at 3-4 weeks of life. The measurement of Crs in infants receiving EXSF may have been affected by an increase in lung inflation, which could mask an increase in Crs. We speculate that improved lung inflation may occur with less barotrauma in the first week of life due to surfactant replacement treatment and may in part explain the improved Crs seen at 1 week of age. Many investigators using different surfactants, dosing schedules, and pulmonary function methodologies to evaluate lung mechanics have reported that the improvement in compliance after surfactant treatment usually follows the clinical improvement in gas exchange. Additional studies are needed to explain the mechanism of early improvement following surfactant replacement in infants with RDS.

Energy expenditure during synthetic surfactant replacement therapy for neonatal respiratory distress syndrome

Little information is available on the energy expenditure of infants with increased work of breathing from respiratory distress syndrome (RDS). A study was carried out to determine whether surfactant replacement therapy modifies respiratory gas exchange in newborn infants with RDS and an arterial-alveolar oxygen tension ratio of less than 0.22. In a double-blind, placebo-controlled, rescue trial, infants received either two 5 ml/kg doses of a synthetic surfactant, Exosurf Neonatal, or air placebo. Of 23 infants ventilated for RDS, 11 were randomly assigned to receive air and 12 to receive surfactant. Oxygen consumption, carbon dioxide production, respiratory quotient, and metabolic rate were measured by computerized, closed-circuit, indirect calorimetry. Concomitantly, transcutaneous oxygen and carbon dioxide tension were continuously recorded. Oxygen consumption and carbon dioxide production remained constant during the period infants received surfactant. In patients randomly assigned to surfactant, a decrease in respiratory quotient was observed after the first (p less than 0.025) but not the second dose. This decrease was possibly related to a change in substrate utilization. The improved clinical outcomes reported among infants receiving surfactant were not accompanied by changes in energy expenditure.

Elastase activity and surfactant protein concentration in tracheal aspirates from neonates receiving synthetic surfactant

Neutrophil elastase activity and the concentration of surfactant proteins A and B (SP-A, B) were measured in tracheal aspirate fluid from preterm neonates who were treated with the synthetic surfactant Exosurf Neonatal or air placebo in randomized, placebo-controlled, clinical trials. Elastase activity was transiently reduced in surfactant-treated infants on the second day of life, but the reduction was not sustained. In placebo-treated infants with established respiratory distress syndrome, tracheal aspirate SP-A was low on the first day of life and increased with time as respiratory distress syndrome resolved. In infants with respiratory distress syndrome treated with surfactant, significantly higher levels of SP-A were observed by 2 days after treatment and were maintained through at least the sixth day of life. These data suggest that lung inflammation is not increased and that endogenous surfactant secretion may be stimulated, not suppressed, by treatment with synthetic surfactant.

Pulmonary mechanics in premature infants one month after treatment with synthetic surfactant

During the double-blind, multicenter trials of the synthetic surfactant Exosurf Neonatal, we measured pulmonary mechanics at 28 days of age in 30 surfactant- or placebo-treated infants. In the 20 surfactant-treated and 10 air-treated infants studied, there were no differences in lung compliance or resistance at 28 days of age. These observations suggest that improvements in pulmonary function reported early in the neonatal course after the administration of exogenous surfactant are not detectable at 28 days of age.