Positive end expiratory pressure in acute and chronic respiratory distress

Parâmetros hemodinâmicos e ventilatórios em coelhos anestesiados com isoflurano, submetidos à ventilação controlada à pressão com ou sem PEEP

RESUMO Foi comparada a ventilação controlada à pressão com ou sem pressão positiva expiratória final (PEEP), em coelhos, distribuídos em três grupos, denominados GP (grupo ventilação ciclada à pressão), GPP (grupo ventilação ciclada à pressão com PEEP) e GE (grupo ventilação espontânea - grupo controle). Os animais foram anestesiados com isoflurano, em circuito com reinalação de gases, durante duas horas. As médias de pressão arterial média (PAM) e pressão arterial sistólica (PAS) permaneceram discretamente abaixo dos valores normais em todos os grupos. Houve diminuição significativa da PAM e da PAS no grupo submetido à PEEP (GPP) ao longo do tempo. A pressão parcial de dióxido de carbono arterial (PaCO2) foi maior no GPP quando comparado aos outros grupos no último momento, gerando acidemia respiratória após uma hora de procedimento. A concentração de dióxido de carbono ao final da expiração (ETCO2) apresentou médias discretamente elevadas no grupo não tratado com PEEP (GP) e no grupo controle, enquanto o GPP apresentou maiores médias, possivelmente, relacionadas à diminuição do volume corrente neste grupo. Com base nesses resultados, foi possível concluir que a utilização da PEEP levou à acidemia, que se agravou ao longo do tempo anestésico. Ademais, a anestesia prolongada com isoflurano promove depressão cardiorrespiratória, independentemente do modo ventilatório empregado.

Positive end-expiratory pressure for preterm infants requiring conventional mechanical ventilation for respiratory distress syndrome or bronchopulmonary dysplasia

BACKGROUND

Conventional mechanical ventilation (CMV) is a common therapy for neonatal respiratory failure. While CMV facilitates gas exchange, it may simultaneously injure the lungs. Positive end-expiratory pressure (PEEP) has received less attention than other ventilation parameters when considering this benefit-risk balance. While an appropriate PEEP level may result in clinical benefits, both inappropriately low or high levels may cause harm. An appropriate PEEP level may also be best achieved by an individualized approach.

OBJECTIVES

1. To compare the effects of PEEP levels in preterm infants requiring CMV for respiratory distress syndrome (RDS). We compare both: zero end-expiratory pressure (ZEEP) (0 cm H₂O) versus any PEEP and low (< 5 cm H₂O) vs high (≥ 5 cm H₂O) PEEP.2. To compare the effects of PEEP levels in preterm infants requiring CMV for bronchopulmonary dysplasia (BPD). We compare both: ZEEP (0 cm H₂O) vs any PEEP and low (< 5 cm H₂O) versus high (≥ 5 cm H₂O) PEEP.3. To compare the effects of different methods for individualizing PEEP to an optimal level in preterm newborn infants requiring CMV for RDS.

SEARCH METHODS

We used the standard search strategy of Cochrane Neonatal to search the Cochrane Central Register of Controlled Trials, MEDLINE via PubMed, Embase, and CINAHL to 14 February 2018. We also searched clinical trials databases, conference proceedings, and the reference lists of retrieved articles for randomized controlled trials and quasi-randomized trials.

SELECTION CRITERIA

We included all randomized or quasi-randomized controlled trials studying preterm infants born at less than 37 weeks' gestational age, requiring CMV and undergoing randomization to either different PEEP levels (RDS or BPD); or, two or more alternative methods for individualizing PEEP levels (RDS only). We included cross-over trials but limited outcomes to those from the first cross-over period.

DATA COLLECTION AND ANALYSIS

We performed data collection and analysis according to the recommendations of the Cochrane Neonatal Review Group. We used the GRADE approach to assess the quality of evidence for prespecified key clinically relevant outcomes.

MAIN RESULTS

Four trials met the inclusion criteria. Two cross-over trials with 28 participants compared different PEEP levels in infants with RDS. Meta-analysis was limited to short-term measures of pulmonary gas exchange and showed no differences between low and high PEEP.We identified no trials comparing PEEP levels in infants with BPD.Two trials enrolling 44 participants compared different methods for individualizing PEEP in infants with RDS. Both trials compared an oxygenation-guided lung-recruitment maneuver (LRM) with gradual PEEP level titrations for individualizing PEEP to routine care (control). Meta-analysis showed no difference between LRM and control on mortality by hospital discharge (risk ratio (RR) 1.00, 95% confidence interval (CI) 0.17 to 5.77); there was no statistically significant difference on BPD, with an effect estimate favoring LRM (RR 0.25, 95% CI 0.03 to 2.07); and a statistically significant difference favoring LRM for the outcome of duration of ventilatory support (mean difference -1.06 days, 95% CI -1.85 to -0.26; moderate heterogeneity, I² = 67%). Short-term oxygenation measures also favored LRM. We graded the quality of the evidence as low for all key outcomes due to risk of bias and imprecision of the effect estimates.

AUTHORS' CONCLUSIONS

There continues to be insufficient evidence to guide PEEP level selection for preterm infants on CMV for RDS or BPD. Low-quality data suggests that selecting PEEP levels through the application of an oxygenation-guided LRM may result in clinical benefit. Well-conducted randomized trials, particularly to further evaluate the potential benefits of oxygenation-guided LRMs, are needed.

Bronchopulmonary dysplasia: A review of pathogenesis and pathophysiology

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of primarily premature infants that results from an imbalance between lung injury and repair in the developing lung. BPD is the most common respiratory morbidity in preterm infants, which affects nearly 10, 000 neonates each year in the United States. Over the last two decades, the incidence of BPD has largely been unchanged; however, the pathophysiology has changed with the substantial improvement in the respiratory management of extremely low birth weight (ELBW) infants. Here we have attempted to comprehensively review and summarize the current literature on the pathogenesis and pathophysiology of BPD. Our goal is to provide insight to help further progress in preventing and managing severe BPD in the ELBW infants.

Matching ventilatory support strategies to respiratory pathophysiology

Neonates can suffer from various diseases that impact differently on lung function according to the specific pulmonary pathophysiology. As a consequence, the optimal respiratory support will vary according to disorder. Most randomized trials have only included prematurely born infants who have respiratory distress syndrome (RDS) or infants who have severe respiratory failure. Meta-analysis of the results has demonstrated that for the prematurely born infant who has RDS, prophylactic high-frequency oscillatory ventilation only results in a modest reduction in bronchopulmonary dysplasia, and patient-triggered ventilation (assist/control or synchronized intermittent mandatory ventilation) reduces the duration of ventilation if started in the recovery phase. Whether the newer triggered modes are more efficacious remains to be appropriately tested. In term infants who have severe respiratory failure, extracorporeal membrane oxygenation increases survival, but inhaled nitric oxide only reduces the need for extracorporeal membrane oxygenation. Research is required to identify the optimum respiratory strategy for infants who have other respiratory disorders, particularly bronchopulmonary dysplasia.

Acute effects of PEEP on tidal volume and respiratory center output during synchronized ventilation in preterm infants

BACKGROUND

Positive end expiratory pressure (PEEP) is routinely used in mechanically ventilated preterm infants to maintain lung volume. An acute increase in PEEP can affect lung mechanics and tidal volume, but it is unknown if these effects elicit compensatory changes in respiratory center output.

OBJECTIVES

To investigate the acute effects of changes in PEEP on tidal volume (V(T)), lung compliance (C(L)), and respiratory center output (RCO) during synchronized intermittent mandatory ventilation (SIMV) in preterm infants at different levels of basal respiratory drive.

METHODS

Preterm infants were studied during SIMV at three levels of PEEP (2, 4, and 6 cm H(2)O for 2-3 min each) and at two levels of inspired CO(2). Peak inspiratory pressure (PIP) was adjusted to maintain the same delta pressure at the airway. RCO was assessed by measuring total diaphragmatic electrical activity. The level of inspired CO(2) was adjusted by modifying the instrumental dead space.

RESULTS

Sixteen preterm infants GA: 25 +/- 2 weeks, BW: 786 +/- 242 g, age: 18 +/- 15 days, SIMV: rate 14 +/- 3 b/min, Ti: 0.35 +/- 0.01 s, PIP: 16 +/- 1 cm H(2)O, and FiO(2): 0.31 +/- 0.06 were studied. At both levels of inspired CO(2), C(L), V(T), and V'(E) from spontaneous and mechanical breaths decreased significantly with higher PEEP. RCO did not change, but at lower respiratory drive, there was a trend towards an increase in RCO with higher PEEP.

CONCLUSION

Higher PEEP levels can have acute negative effects on lung mechanics and ventilation in preterm infants without a sufficient compensatory increase in RCO.

Ventilatory strategies in the prevention and management of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) leads to considerable mortality and morbidity in premature infants. Although mechanical ventilation is lifesaving in infants with respiratory distress syndrome (RDS), it may contribute to lung injury and subsequently to BPD. Appropriate ventilatory strategies for reducing BPD include redefining the goals for "adequate gas exchange," using less mechanical ventilation support, refining the methods of mechanical ventilation, and using alternative techniques. Permissive hypercapnia, permissive hypoxemia, minimal peak pressures, rapid rates, early therapeutic continuous positive airway pressure (CPAP), and rapid extubation may help reduce mechanical ventilation-induced lung injury and possibly reduce BPD. Newer techniques of ventilation such as volume-targeted ventilation are also promising. High frequency ventilation has not been proven to reduce BPD. There is a lack of evidence-based guidelines on management of infants with established BPD. Optimization of clinical care practices and ancillary therapies need to be combined with ventilatory strategies to prevent and manage BPD.

Optimal strategies for newborn ventilation--a synthesis of the evidence

A variety of ventilation modes are available for the newborn. Although, there have been randomised trials assessing certain modes, these have generally only included prematurely born infants with RDS or infants with severe respiratory failure. Meta-analysis of the results of those trials has demonstrated that neither patient triggered nor high frequency oscillatory ventilation is advantageous for the prematurely born infant with RDS, but extracorporeal membrane oxygenation increases survival in infants with severe respiratory failure. Appropriately designed studies are required to determine the role of newer ventilation modes and whether forms of respiratory support avoiding intubation are less injurious to the lungs. Research should also focus on infants with other respiratory disorders, particularly BPD. Prior to further randomised trials being undertaken, it is essential that the optimum method of applying each ventilator mode is identified and it is clearly understood whether differences in ventilator/oscillator performance influence outcome.

PEEP--a "cheap" and effective lung protection

Mechanical ventilation is a complex therapy with several different parameters which can be altered. In preterm and term infants, more attention has been paid to the levels of peak inspiratory pressure than to the positive end-expiratory pressure (PEEP). An awareness that lung protection can be conferred by an appropriate level of PEEP has increasingly stimulated a renewed interest in achieving the "best PEEP" strategy. We review the history of the introduction of PEEP therapy, some of the early demonstrations of its potential for mischief, the evidence on what levels of PEEP are appropriate in infants, some data concerning the lung-protective value of PEEP and finally some recent efforts at defining measures to determine the so-called "best PEEP". Some of this work has been performed in adults with the acute respiratory distress syndrome. In newborns, we are regrettably forced to conclude that there is, for the immediate present, no easy substitute for sensible clinical observations coupled with a judicious and cautious adjustment of PEEP. We anticipate that a more logical application of PEEP with individualisation of therapy, based on a pressure-volume relationship, will in future enable targeted tests of PEEP as a lung-protection strategy.

Comparison of respiratory function and fluid balance in very low birthweight infants given artificial or natural surfactant or no surfactant treatment

Exogenous surfactant administration improves respiratory function. The speed of improvement appears greater if a natural rather than an artificial surfactant is used, our aim was to determine if that effect was explained by differences in fluid balance, evidenced by the timing of the diuresis onset (i.e. output greater than input). Thirty infants (median gestational age 29 weeks), 10 given an artificial surfactant (Exosurf), 10 a natural surfactant (Survanta) and 10 no surfactant (controls) were studied. During the first three days, compliance and functional residual capacity were measured daily, arginine vasopressin (AVP) levels estimated on days 1, 3 and 5 and, in 8-hourly intervals, the median arterial/alveolar ratio was calculated for each individual and urine output and fluid input recorded. Throughout the three-day period, the median arterial/alveolar ratio was always significantly higher in the control compared to the two surfactant groups (p < 0.05). On day 3 the Exosurf-treated babies had lower compliance and functional residual capacity (p < 0.05) than the other two groups. Neither the timing of diuresis onset, timing of the maximum diuresis nor the AVP levels, however, differed significantly between the groups. Only surfactant treatment and type of surfactant, but not the timing of the onset nor of the maximum diuresis, related significantly to changes in lung function. These results do not support the hypothesis that differences in fluid balance explain differences in the lung function improvement rate following natural and artificial surfactant.

Early measurement of lung volume--a useful discriminator of neonatal respiratory failure severity

Respiratory distress syndrome (RDS) is characterized by lungs having collapsed alveoli (atelectasis) which reduces the volume of the gas-containing spaces of the lung. It seems likely, therefore, that measurement of lung volume might discriminate between infants with severe respiratory failure due to RDS and those with minimal respiratory distress. To test this hypothesis, lung volume was measured at end expiration, that is functional residual capacity (FRC), in 40 infants (median gestational age 29 weeks, range 24-35) all mechanically ventilated from birth. FRC was measured using a helium gas dilution technique at a median of 3 h of age. The infants were divided into two groups according to their FRC results: group A (n = 29) low FRC (FRC < 24 ml kg-1) and group B (n = 11) normal FRC (FRC > or = 24 ml kg-1). The clinicians were unaware of the FRC results. There was no significant difference in the gestational age or birthweight of the two groups, but group A were characterized by a significantly greater proportion requiring surfactant replacement therapy (p < 0.01), a higher maximum peak inspiratory pressure (p < 0.01) and inspired oxygen requirement (p < 0.01). A low FRC had 79% sensitivity and 91% specificity in predicting a requirement for surfactant replacement therapy. We conclude that measurement of FRC in the first hours of life does allow discrimination of disease severity.

Influence of end expiratory pressure on cerebral blood flow in preterm infants

The effect of interruption of positive and expiratory pressure (PEEP) on cerebral blood flow velocity (CBFV) and CBF fluctuation (CBFF) in the internal carotid arteries and on heart rate, restlessness and wakefulness has been studied in 17 mechanically ventilated neonates with RDS. A decrease in CBFV was found, but no significant change in CBFF. Multiple regression analysis showed that the decrease in CBFV is less pronounced if the PEEP interruption is accompanied by restlessness. It further appeared that the decrease in CBFV is more pronounced if CBFV is high, the ductus arteriosus is patent, or RDS follows a complicated course. These findings indicate that PEEP supports CBF, probably by a decrease in ductal stealing from the brain. Therewith PEEP protects against cerebral hypoperfusion which is one of the major risks in RDS and immaturity. Furthermore, our findings suggest that the decrease in CBF during PEEP interruption is moderated by restlessness and accentuated by brain damage.

Effect of positive end expiratory pressure and mean airway pressure on respiratory compliance and gas exchange in children with liver disease

The effect of positive end expiratory pressure (PEEP) and mean airway pressure (MAP) on respiratory compliance and gas exchange was assessed in children with liver disease. In the first study of 12 patients, PEEP was decreased either by 3 cmH2O below the baseline level (the child's original level) or to 0 cmH2O and then increased to 3 cmH2O above the baseline. Decreasing PEEP impaired compliance (P < 0.01), and oxygenation (P < 0.05), whereas increasing PEEP improved compliance (P < 0.05) and oxygenation (P < 0.05). Neither increasing nor decreasing PEEP caused significant changes in the carbon dioxide levels. In the second study, 24 children were studied at their baseline settings and then after increasing the PEEP by 3 cm H2O while simultaneously lowering the peak inspiratory pressure (PIP) to maintain MAP constant (12 children had lung function measurements). In the group overall increasing PEEP while decreasing PIP resulted in an insignificant change in paO2, but a significant increase paCO2 (P < 0.01) and reduction in tidal volume (P < 0.01), the change in compliance was not significant. After a second period at the baseline settings, in 12 children inspiratory time (TI) was increased while keeping MAP constant by reducing PIP. No significant change in paO2 or compliance was experienced, but paCO2 increased (P < 0.05) and tidal volume decreased (P < 0.01). In the other 12 children MAP was increased by prolonging TI. Increasing MAP had a variable effect and the changes in paO2 and paCO2 were not significant. No critical MAP level with regard to oxygenation was demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of variations in positive end expiratory pressure on gas exchange in ventilated children with liver disease

The effect of varying the positive and expiratory pressure (PEEP) level during mechanical ventilation has been assessed in ten children with liver disease, mean age 3.8 years. PEEP was increased 3 cmH2O above the child's original (baseline) PEEP level and then decreased either by 3 cmH2O below the baseline or to 0 cmH2O. In all ten children increasing the PEEP above the baseline improved oxygenation; in the group overall the median PaO2 increased from 90 mmHg to 97 mmHg (P < 0.01). In eight of ten children decreasing the PEEP level below the baseline resulted in a deterioration in oxygenation; in the group overall the median PaO2 decreased from 91 mmHg to 82 mmHg (P < 0.05). Changes in PEEP levels, however, did not result in clinically significant alterations in PaCO2, heart rate or blood pressure. We conclude that modest increases in PEEP are well tolerated in children with liver disease and result in an improvement in oxygenation.

Randomised trial of methods of extubation in acute and chronic respiratory distress

Sixty infants (median gestational age 29 weeks) with acute and 60 infants (median gestational age 25 weeks) with chronic respiratory distress were randomised to be extubated either directly into a headbox or onto 3 cm H2O nasal continuous positive airway pressure (CPAP). Our aim was to test the hypothesis that extubation onto nasal CPAP rather than directly into a headbox was more likely to be associated with successful extubation in infants with acute rather than chronic respiratory distress. Overall the failure rate of extubation was approximately 33%, with no significant difference between the infants with acute and chronic respiratory distress. There was no significant difference in the failure rate of extubation among infants randomised to receive nasal CPAP or headbox oxygen in either the acute or chronic respiratory distress groups.