Establishing lung gas volumes at birth: interaction between positive end-expiratory pressures and tidal volumes in preterm rabbits

Efficacy of perinatal pharmacotherapeutic actions for survival of very preterm newborn rabbits at 26-day gestation

Investigation of the pathophysiology of lung impairment and protection in very preterm neonates at birth requires adequate experimental models. This study aimed to elucidate the efficacy and mechanism of perinatal pharmacotherapeutic action in postnatal survival of very preterm rabbits. Pregnant New Zealand White rabbits on 25-day gestation (term 31 days) were given dexamethasone (D), or sham injection as control (C), and cesarean delivered 24 hours later on day 26. Newborns were anesthetized, intratracheally intubated, randomly received either saline or porcine surfactant (S), allocated to four groups (C, S, D, and DS), and ventilated with low tidal volume. Under the identical protocol, another four groups were added with nitric oxide (N) inhalation (CN, SN, DN, and DSN). Survival length, lung mechanics, histopathology, and pathobiology of lung tissue were measured for benefits and injury patterns. DSN had the longest median survival time (ST50, 10.3 h), whereas C had the shortest (3.5 h), with remaining groups in-between. The survival was mainly benefited by S, when additive effects with D and/or N were discernible, by improved lung mechanics and alveolar aeration, ameliorated lung injury severity and pneumothorax, and augmented lung phospholipid pools, with DSN being the most optimal. Variable mRNA expression profiles of alveolar epithelia-associated cytokines and inflammatory mediators further characterized injury and response patterns as phenotyping conditioned in pharmacotherapeutic actions. In conclusion, the combined regimens of perinatal medications achieved remarkable survival in very preterm rabbits with lung protective ventilation strategy, offering a unique model in investigation of very preterm birth-associated respiratory physiology and morbidities.NEW & NOTEWORTHY By establishing a very preterm rabbit model with 26-day gestation (term 31 days), optimal survival length for 50% of animals in groups was achieved by comparing regimens of combined antenatal glucocorticoids, postnatal surfactant and inhaled nitric oxide, with a low tidal volume ventilation strategy. The efficacies of pharmacotherapeutic action were associated with significantly improved lung mechanics, ameliorated lung injury and pneumothorax, and enhanced surfactant phospholipid metabolism, along with variable mRNA expression profiles characterizing the response patterns.

Respiratory function monitoring to improve the outcomes following neonatal resuscitation: a systematic review and meta-analysis

IMPORTANCE

Animal and observational human studies report that delivery of excessive tidal volume (V_T) at birth is associated with lung and brain injury. Using a respiratory function monitor (RFM) to guide V_T delivery might reduce injury and improve outcomes.

OBJECTIVE

To determine whether use of an RFM in addition to clinical assessment versus clinical assessment alone during mask ventilation in the delivery room reduces in-hospital mortality and morbidity of infants <37 weeks' gestation.

STUDY SELECTION

Randomised controlled trials (RCTs) comparing RFM in addition to clinical assessment versus clinical assessment alone during mask ventilation in the delivery room of infants born <37 weeks' gestation.

DATA ANALYSIS

Risk of bias was assessed using Covidence Collaboration tool and pooled into a meta-analysis using a random-effects model. The primary outcome was death prior to discharge.

MAIN OUTCOME

Death before hospital discharge.

RESULTS

Three RCTs enrolling 443 infants were combined in a meta-analysis. The pooled analysis showed no difference in rates of death before discharge with an RFM versus no RFM, relative risk (RR) 95% (CI) 0.98 (0.64 to 1.48). The pooled analysis suggested a significant reduction for brain injury (a combination of intraventricular haemorrhage and periventricular leucomalacia) (RR 0.65 (0.48 to 0.89), p=0.006) and for intraventricular haemorrhage (RR 0.69 (0.50 to 0.96), p=0.03) in infants receiving positive pressure ventilation with an RFM versus no RFM.

CONCLUSION

In infants <37 weeks, an RFM in addition to clinical assessment compared with clinical assessment during mask ventilation resulted in similar in-hospital mortality, significant reduction for any brain injury and intraventricular haemorrhage. Further trials are required to determine whether RFMs should be routinely available for neonatal resuscitation.

Increased end-expiratory pressures improve lung function in near-term newborn rabbits with elevated airway liquid volume at birth

Approximately 53% of near-term newborns admitted to intensive care experience respiratory distress. These newborns are commonly delivered by cesarean section and have elevated airway liquid volumes at birth, which can cause respiratory morbidity. We investigated the effect of providing respiratory support with a positive end-expiratory pressure (PEEP) of 8 cmH₂O on lung function in newborn rabbit kittens with elevated airway liquid volumes at birth. Near-term rabbits (30 days; term = 32 days) with airway liquid volumes that corresponded to vaginal delivery (∼7 mL/kg, control, n = 11) or cesarean section [∼37 mL/kg; elevated liquid (EL), n = 11] were mechanically ventilated (tidal volume = 8 mL/kg). The PEEP was changed after lung aeration from 0 to 8 to 0 cmH₂O (control, n = 6; EL, n = 6), and in a separate group of kittens, PEEP was changed after lung aeration from 8 to 0 to 8 cmH₂O (control, n = 5; EL, n = 5). Lung function (ventilator parameters, compliance, lung gas volumes, and distribution of gas within the lung) was evaluated using plethysmography and synchrotron-based phase-contrast X-ray imaging. EL kittens initially receiving 0 cmH₂O PEEP had reduced functional residual capacities and lung compliance, requiring higher inflation pressures to aerate the lung compared with control kittens. Commencing ventilation with 8 cmH₂O PEEP mitigated the adverse effects of EL, increasing lung compliance, functional residual capacity, and the uniformity and distribution of lung aeration, but did not normalize aeration of the distal airways. Respiratory support with PEEP supports lung function in near-term newborn rabbits with elevated airway liquid volumes at birth who are at a greater risk of suffering respiratory distress.NEW & NOTEWORTHY Term babies born by cesarean section have elevated airway liquid volumes, which predisposes them to respiratory distress. Treatments targeting molecular mechanisms to clear lung liquid are ineffective for term newborn respiratory distress. We showed that respiratory support with an end-expiratory pressure supports lung function in near-term rabbits with elevated airway liquid volumes at birth. This study provides further physiological understanding of lung function in newborns with elevated airway liquid volumes at risk of respiratory distress.

Positive End-Expiratory Pressure in Newborn Resuscitation Around Term: A Randomized Controlled Trial

BACKGROUND

International guidelines for resuscitation recommend using positive end-expiratory pressure (PEEP) during ventilation of preterm newborns. Reliable PEEP-valves for self-inflating bags have been lacking, and effects of PEEP during resuscitation of term newborns are insufficiently studied. The objective was to determine if adding a new PEEP valve to the bag-mask during resuscitation of term and near-term newborns could improve heart rate response.

METHODS

This randomized controlled trial was performed at Haydom Lutheran Hospital in Tanzania (September 2016 to June 2018). Helping Babies Breathe-trained midwives performed newborn resuscitation using self-inflating bags with or without a new, integrated PEEP valve. All live-born newborns who received bag-mask ventilation at birth were eligible. Heart rate response measured by ECG was the primary outcome, and clinical outcome and ventilation data were recorded.

RESULTS

Among 417 included newborns (median birth weight 3200 g), 206 were ventilated without and 211 with PEEP. We found no difference in heart rate response. Median (interquartile range) measured PEEP in the PEEP group was 4.7 (2.0-5.6) millibar. The PEEP group received lower tidal volumes (4.9 [1.9-8.2] vs 6.3 [3.9-10.5] mL/kg; P = .02) and had borderline lower expired CO₂ (2.9 [1.5-4.3] vs 3.3 [1.9-5.0] %; P = .05). Twenty four-hour mortality was 9% in both groups.

CONCLUSIONS

We found no evidence for improved heart rate response during bag-mask ventilation with PEEP compared with no PEEP. The PEEP valve delivered a median PEEP within the intended range. The findings do not support routine use of PEEP during resuscitation of newborns around term.

Elevated airway liquid volumes at birth: a potential cause of transient tachypnea of the newborn

Excessive liquid in airways and/or distal lung tissue may underpin the respiratory morbidity associated with transient tachypnea of the newborn (TTN). However, its effects on lung aeration and respiratory function following birth are unknown. We investigated the effect of elevated airway liquid volumes on newborn respiratory function. Near-term rabbit kittens (30 days gestation; term ~32 days) were delivered, had their lung liquid-drained, and either had no liquid replaced (control; n = 7) or 30 ml/kg of liquid re-added to the airways [liquid added (LA); n = 7]. Kittens were mechanically ventilated in a plethysmograph. Measures of chest and lung parameters, uniformity of lung aeration, and airway size were analyzed using phase contrast X-ray imaging. The maximum peak inflation pressure required to recruit a tidal volume of 8 ml/kg was significantly greater in LA compared with control kittens (35.0 ± 0.7 vs. 26.8 ± 0.4 cmH2O, P < 0.001). LA kittens required greater time to achieve lung aeration (106 ± 14 vs. 60 ± 6 inflations, P = 0.03) and had expanded chest walls, as evidenced by an increased total chest area (32 ± 9%, P < 0.0001), lung height (17 ± 6%, P = 0.02), and curvature of the diaphragm (19 ± 8%, P = 0.04). LA kittens had lower functional residual capacity during stepwise changes in positive end-expiratory pressures (5, 3, 0, and 5 cmH20). Elevated lung liquid volumes had marked adverse effects on lung structure and function in the immediate neonatal period and reduced the ability of the lung to aerate efficiently. We speculate that elevated airway liquid volumes may underlie the initial morbidity in near-term babies with TTN after birth.

NEW & NOTEWORTHY Transient tachypnea of the newborn reduces respiratory function in newborns and is thought to result due to elevated airway liquid volumes following birth. However, the effect of elevated airway liquid volumes on neonatal respiratory function is unknown. Using phase contrast X-ray imaging, we show that elevated airway liquid volumes have adverse effects on lung structure and function in the immediate newborn period, which may underlie the pathology of TTN in near-term babies after birth.

Effect of betamethasone, surfactant, and positive end-expiratory pressures on lung aeration at birth in preterm rabbits

Antenatal glucocorticoids, exogenous surfactant, and positive end-expiratory pressure (PEEP) ventilation are commonly provided to preterm infants to enhance respiratory function after birth. It is unclear how these treatments interact to improve the transition to air-breathing at birth. We investigated the relative contribution of antenatal betamethasone, prophylactic surfactant, and PEEP (3 cmH₂O) on functional residual capacity (FRC) and dynamic lung compliance (C_DL) in preterm (28 day GA) rabbit kittens at birth. Kittens were delivered by cesarean section and mechanically ventilated. FRC was calculated from X-ray images, and C_DL was measured using plethysmography. Without betamethasone, PEEP increased FRC recruitment and C_DL Surfactant did not further increase FRC, but significantly increased C_DL Betamethasone abolished the benefit of PEEP on FRC, but surfactant counteracted this effect of betamethasone. These findings indicate that low PEEP levels are insufficient to establish FRC at birth following betamethasone treatment. However, surfactant reversed the effect of betamethasone and when combined, these two treatments enhanced FRC recruitment irrespective of PEEP level.

The interrelationship of recruitment maneuver at birth, antenatal steroids, and exogenous surfactant on compliance and oxygenation in preterm lambs

BACKGROUND

To describe the interrelationship between antenatal steroids, exogenous surfactant, and two approaches to lung recruitment at birth on oxygenation and respiratory system compliance (Cdyn) in preterm lambs.

METHODS

Lambs (n = 63; gestational age 127 ± 1 d) received either surfactant at 10-min life (Surfactant), antenatal corticosteroids (Steroid), or neither (Control). Within each epoch lambs were randomly assigned to a 30-s 40 cmH2O sustained inflation (SI) or an initial stepwise positive end-expiratory pressure (PEEP) open lung ventilation (OLV) maneuver at birth. All lambs then received the same management for 60-min with alveolar-arterial oxygen difference (AaDO2) and Cdyn measured at regular time points.

RESULTS

Overall, the OLV strategy improved Cdyn and AaDO2 (all epochs except Surfactant) compared to SI (all P < 0.05; two-way ANOVA). Irrespective of strategy, Cdyn was better in the Steroid group in the first 10 min (all P < 0.05). Thereafter, Cdyn was similar to Steroid epoch in the OLV + Surfactant, but not SI + Surfactant group. OLV influenced the effect of steroid and surfactant (P = 0.005) on AaDO2 more than SI (P = 0.235).

CONCLUSIONS

The antenatal state of the lung influences the type and impact of a recruitment maneuver at birth. The effectiveness of surfactant maybe enhanced using PEEP-based time-dependent recruitment strategies rather than approaches solely aimed at initial lung liquid clearance.

Measurement of absolute regional lung air volumes from near-field x-ray speckles

Propagation-based phase contrast x-ray (PBX) imaging yields high contrast images of the lung where airways that overlap in projection coherently scatter the x-rays, giving rise to a speckled intensity due to interference effects. Our previous works have shown that total and regional changes in lung air volumes can be accurately measured from two-dimensional (2D) absorption or phase contrast images when the subject is immersed in a water-filled container. In this paper we demonstrate how the phase contrast speckle patterns can be used to directly measure absolute regional lung air volumes from 2D PBX images without the need for a water-filled container. We justify this technique analytically and via simulation using the transport-of-intensity equation and calibrate the technique using our existing methods for measuring lung air volume. Finally, we show the full capabilities of this technique for measuring regional differences in lung aeration.

Sustained inflation at birth did not protect preterm fetal sheep from lung injury

Sustained lung inflations (SI) at birth may recruit functional residual capacity (FRC). Clinically, SI increase oxygenation and decrease need for intubation in preterm infants. We tested whether a SI to recruit FRC would decrease lung injury from subsequent ventilation of fetal, preterm lambs. The preterm fetus (128±1 day gestation) was exteriorized from the uterus, a tracheostomy was performed, and fetal lung fluid was removed. While maintaining placental circulation, fetuses were randomized to one of four 15-min interventions: 1) positive end-expiratory pressure (PEEP) 8 cmH2O (n=4), 2) 20 s SI to 50 cmH2O then PEEP 8 cmH2O (n=10), 3) mechanical ventilation at tidal volume (VT) 7 ml/kg (n=13), or 4) 20 s SI then ventilation at VT 7 ml/kg (n=13). Lambs were ventilated with 95% N2/5% CO2 and PEEP 8 cmH2O. Volume recruitment was measured during SI, and fetal tissues were collected after an additional 30 min on placental support. SI achieved a mean FRC recruitment of 15 ml/kg (range 8-27). Fifty percent of final FRC was achieved by 2 s, 65% by 5 s, and 90% by 15 s, demonstrating prolonged SI times are needed to recruit FRC. SI alone released acute-phase proteins into the fetal lung fluid and increased mRNA expression of proinflammatory cytokines and acute-phase response genes in the lung. Mechanical ventilation further increased all markers of lung injury. SI before ventilation, regardless of the volume of FRC recruited, did not alter the acute-phase and proinflammatory responses to mechanical ventilation at birth.