Non-invasive measurement of reduced ventilation:perfusion ratio and shunt in infants with bronchopulmonary dysplasia: a physiological definition of the disease

Use of ventilation/perfusion mismatch to guide individualised CPAP level selection in preterm infants: a feasibility trial

OBJECTIVE

To measure within-subject changes in ventilation/perfusion (V'/Q') mismatch in response to a protocol of individualised nasal continuous positive airway pressure (CPAP) level selection.

DESIGN

Single-arm, non-randomised, feasibility trial.

SETTING

Three centres in the Children's Hospital of Philadelphia neonatal care network.

PATIENTS

Twelve preterm infants of postmenstrual age 27-35 weeks, postnatal age >24 hours, and receiving a fraction of inspired oxygen (FiO₂) >0.25 on CPAP of 4-7 cm H₂O.

INTERVENTIONS

We applied a protocol of stepwise CPAP level changes, with the overall direction and magnitude guided by individual responses in V'/Q' mismatch, as determined by the degree of right shift (kilopascals, kPa) in a non-invasive gas exchange model. Best CPAP level was defined as the final pressure level at which V'/Q' improved by more than 5%.

MAIN OUTCOME MEASURES

Within-subject change in V'/Q' mismatch between baseline and best CPAP levels.

RESULTS

There was a median (IQR) within-subject reduction in V'/Q' mismatch of 1.2 (0-3.2) kPa between baseline and best CPAP levels, p=0.02. Best CPAP was observed at a median (range) absolute level of 7 (5-8) cm H₂O.

CONCLUSIONS

Non-invasive measures of V'/Q' mismatch may be a useful approach for identifying individualised CPAP levels in preterm infants. The results of our feasibility study should be interpreted cautiously and replication in larger studies evaluating the impact of this approach on clinical outcomes is needed.

TRIAL REGISTRATION NUMBER

NCT02983825.

Pathophysiology of gas exchange impairment in extreme prematurity: Insights from combining volumetric capnography and measurements of ventilation/perfusion ratio

Background

Infants born extremely preterm often suffer from respiratory disease and are invasively ventilated. We aimed to test the hypothesis that gas exchange in ventilated extremely preterm infants occurs both at the level of the alveoli and via mixing of fresh deadspace gas in the airways.

Methods

We measured the normalised slopes of phase II and phase III of volumetric capnography and related them with non-invasive measurements of ventilation to perfusion ratio (V_A/Q) and right-to-left shunt in ventilated extremely preterm infants studied at one week of life. Cardiac right-to-left shunt was excluded by concurrent echocardiography.

Results

We studied 25 infants (15 male) with a median (range) gestational age of 26.0 (22.9-27.9) weeks and birth weight of 795 (515-1,165) grams. The median (IQR) V_A/Q was 0.52 (0.46-0.56) and shunt was 8 (2-13) %. The median (IQR) normalised slope of phase II was 99.6 (82.7-116.1) mmHg and of phase III was 24.6 (16.9-35.0) mmHg. The V_A/Q was significantly related to the normalised slope of phase III (ρ = -0.573, p = 0.016) but not to the slope of phase II (ρ = 0.045, p = 0.770). The right-to-left shunt was not independently associated with either the slope of phase II or the slope of phase III after adjusting for confounding parameters.

Conclusions

Abnormal gas exchange in ventilated extremely preterm infants was associated with lung disease at the alveolar level. Abnormal gas exchange at the level of the airways was not associated with quantified indices of gas exchange impairment.

AZTEC-azithromycin therapy for prevention of chronic lung disease of prematurity: a statistical analysis plan for clinical outcomes

Background

The AZTEC trial is a multi-centre, randomised, placebo-controlled trial of azithromycin to improve survival without development of chronic lung disease of prematurity (CLD) in preterm infants. The statistical analysis plan for the clinical outcomes of the AZTEC trial is described.

Methods and design

A double-blind, randomised, placebo-controlled trial of a 10-day course of intravenous azithromycin (20 mg/kg for 3 days; 10 mg/kg for 7 days) administered to preterm infants born at < 30 weeks’ gestational age across UK tertiary neonatal units. Following parental consent, infants are randomly allocated to azithromycin or placebo, with allocated treatment starting within 72 h of birth. The primary outcome is survival without moderate/severe CLD at 36 weeks’ postmenstrual age (PMA). Serial respiratory fluid and stool samples are being collected up to 21 days of life. The target sample size is 796 infants, which is based on detecting a 12% absolute difference in survival without moderate/severe CLD at 36 weeks’ PMA (90% power, two-sided alpha of 0.05) and includes 10% loss to follow-up.

Results

Baseline demographic and clinical characteristics will be summarised by treatment arm and in total. Categorical data will be summarised by numbers and percentages. Continuous data will be summarised by mean, standard deviation, if data are normal, or median, interquartile range, if data are skewed. Tests of statistical significance will not be undertaken for baseline characteristics. The primary analysis, on the intention to treat (ITT) population, will be analysed using multilevel logistic regression, within a multiple imputation framework. Adjusted odds ratios, 95% confidence intervals, and p-values will be presented. For all other analyses, the analysis population will be based on the complete case population, which is a modified ITT population. All analyses will be adjusted for gestational age and treatment arm and account for any clustering by centre and/or multiple births as a random effect.

Conclusion

We describe the statistical analysis plan for the AZTEC trial, including the analysis principles, definitions of the key clinical outcomes, methods for primary analysis, pre-specified subgroup analysis, sensitivity analysis, and secondary analysis. The plan has been finalised prior to the completion of recruitment.

Trial registration

ISRCTN registry ISRCTN11650227. Registered on 31 July 2018.

Automated control of oxygen titration in preterm infants on non-invasive respiratory support

OBJECTIVE

To evaluate the performance of a rapidly responsive adaptive algorithm (VDL1.1) for automated oxygen control in preterm infants with respiratory insufficiency.

DESIGN

Interventional cross-over study of a 24-hour period of automated oxygen control compared with aggregated data from two flanking periods of manual control (12 hours each).

SETTING

Neonatal intensive care unit.

PARTICIPANTS

Preterm infants receiving non-invasive respiratory support and supplemental oxygen; median birth gestation 27 weeks (IQR 26-28) and postnatal age 17 (12-23) days.

INTERVENTION

Automated oxygen titration with the VDL1.1 algorithm, with the incoming SpO₂ signal derived from a standard oximetry probe, and the computed inspired oxygen concentration (FiO₂) adjustments actuated by a motorised blender. The desired SpO₂ range was 90%-94%, with bedside clinicians able to make corrective manual FiO₂ adjustments at all times.

MAIN OUTCOME MEASURES

Target range (TR) time (SpO₂ 90%-94% or 90%-100% if in air), periods of SpO₂ deviation, number of manual FiO₂ adjustments and oxygen requirement were compared between automated and manual control periods.

RESULTS

In 60 cross-over studies in 35 infants, automated oxygen titration resulted in greater TR time (manual 58 (51-64)% vs automated 81 (72-85)%, p<0.001), less time at both extremes of oxygenation and considerably fewer prolonged hypoxaemic and hyperoxaemic episodes. The algorithm functioned effectively in every infant. Manual FiO₂ adjustments were infrequent during automated control (0.11 adjustments/hour), and oxygen requirements were similar (manual 28 (25-32)% and automated 26 (24-32)%, p=0.13).

CONCLUSION

The VDL1.1 algorithm was safe and effective in SpO₂ targeting in preterm infants on non-invasive respiratory support.

TRIAL REGISTRATION NUMBER

ACTRN12616000300471.

Capillary partial pressure of carbon dioxide for predicting rehospitalization in preterm infants under noninvasive respiratory support with severe bronchopulmonary dysplasia

BACKGROUND

The severity of bronchopulmonary dysplasia (BPD) is an important predictor of prognosis in preterm infants. However, the severity of BPD was determined mainly by the degree of oxygen supplementation and mode of respiratory support.

OBJECTIVES

This retrospective study aimed to examine the role of partial pressure of carbon dioxide (pCO₂ ) in predicting rehospitalization among preterm infants with severe BPD without invasive ventilation at 36 weeks' postmenstrual age (PMA).

METHODS

We assessed preterm infants aged <32 gestational weeks with severe BPD who were receiving noninvasive respiratory support at 36 weeks' PMA. Patients were compared after stratifying them according to the history of rehospitalization owing to respiratory infection before a corrected age (CA) of 1 year and pCO₂ measured by capillary blood gas analysis at 36 weeks' PMA.

RESULTS

Among 54 infants who had severe BPD with noninvasive respiratory support at 36 weeks' PMA, 16 (29.6%) experienced rehospitalization due to respiratory problems. At 36 weeks' PMA, the amount of oxygen supplementation (0.30 vs. 0.28, p = 0.021) and pCO₂ (62.1 vs. 53.6 mmHg, p = 0.006) were higher in the rehospitalization group than in the no rehospitalization group. Multivariate logistic analysis findings indicated that pCO₂  ≥ 57.4 mmHg was the only factor associated with rehospitalization (adjusted odds ratio: 8.017, 95% confidence interval 1.239-51.859).

CONCLUSION

High pCO₂ during noninvasive respiratory support at 36 weeks' PMA in severe BPD was associated with rehospitalization. Consideration of the degree of impairment in ventilatory capacity may improve the prediction of later respiratory outcomes in infants with BPD.

The challenge of accurately describing the epidemiology of bronchopulmonary dysplasia (BPD) based on the various current definitions of BPD

Despite marked improvements in the survival of extremely low birth weight preterm infants, bronchopulmonary dysplasia (BPD) remains a prevalent morbidity. BPD has evolved pathologically and epidemiologically but the definition has failed to keep up. The majority of the definitions of BPD still use the respiratory support provided to the infants at a single timepoint. The lack of a uniform definition of BPD presently reflects the changing BPD pathogenesis and phenotype and limits defining the epidemiology. To address the epidemiology of BPD, the definition should be clarified; even the newer definitions have not been validated entirely. The definition needs to be meaningful clinically and be predictive of long-term respiratory outcomes. We believe the definition should have a composite assessment like a score (quantitative measurement) and include the different phenotypes (qualitative measurements) so that optimally they can be applied to the different phases of BPD and at different timepoints. Furthermore, the definitions need to be easy to measure and assess so that generalizability is enhanced.

Early lung ultrasound affords little to the prediction of bronchopulmonary dysplasia

OBJECTIVE

To test the hypothesis that lung ultrasound (LUS) performed in the first week of life would predict bronchopulmonary dysplasia (BPD). Secondary outcomes included the utility of LUS in predicting interim respiratory interventions.

DESIGN

A prospective observational cohort study in preterm infants born <28 weeks' gestation in the single tertiary statewide neonatal intensive care unit in Western Australia.

METHODS

A rigorous protocol for LUS acquisition on day 1, day 3-4, day 7, day 28 and 36 weeks' postmenstrual age (PMA) was implemented with blinded analysis using a modified, previously validated LUS score. BPD was defined by both recent National Institute of Child Health and Human Development categorical criteria and a continuous physiological variable using a modified Shift test.

RESULTS

Of the 100 infants studies, primary outcome data were available for the 96 infants, surviving to 36 weeks' PMA. In a univariate logistic regression analysis, LUS on days 3-4 and day 7 accurately predicted BPD (day 3-4 OR (95% CI)=1.54 (1.03 to 2.42), p=0.044; day 7 OR (95% CI)=1.66 (1.07 to 2.70), p=0.031). The predictive value of LUS was insignificant in a multivariate model in which gestational age was the dominant predictor. LUS accurately predicted interim respiratory outcomes including surfactant administration, duration of intubation and extubation to non-invasive support at 48 hours.

CONCLUSIONS

LUS in the first week of life predicted BPD. However, LUS offers little additive accuracy to current gestational age-based models.

TRIAL REGISTRATION NUMBER

ACTRN12617000208303.

Simplified bedside assessment of pulmonary gas exchange in very preterm infants at 36 weeks' postmenstrual age

INTRODUCTION

We aimed to develop and validate a prediction table for a simplified measure of rightward shift of the fetal oxyhaemoglobin saturation (SpO₂) versus inspired oxygen pressure (P _IO₂) curve as an objective marker of lung disease severity in very preterm infants, independent of unit altitude or oxygen prescribing policies.

METHODS

Very preterm infants (n=219) had an oxygen reduction test at median (IQR) test age of 35⁴ (34⁵-36⁰) weeks' postmenstrual age (PMA). Shift was derived from at least three paired SpO₂ versus P _IO₂ measurements using a computer algorithm, using the fetal oxyhaemoglobin dissociation curve as the reference. Linear regression of resultant shift values enabled construction of a table to predict shift using a single paired SpO₂ versus P _IO₂ measurement, validated subsequently in a separate infant cohort using Bland-Altman analysis. Receiver operating curve analysis provided threshold values equating to a clinical diagnosis of mild bronchopulmonary dysplasia (BPD) or moderate to severe BPD.

RESULTS

The median (IQR) age of 63 infants in the validation cohort was 36⁰ (35⁶-36²) weeks' PMA. Mean difference (95% CI) between predicted and measured shift was 2.1 (-0.8% to 4.9%) with wide limits of agreement (-20.7% to 24.8%). Predicted shift >10.1 kPa identified mild BPD with 71% sensitivity and 88% specificity while values>13.0 kPa identified moderate to severe BPD with 81% sensitivity and 100% specificity.

DISCUSSION

Shift predicted from a single paired SpO₂ versus P _IO₂ measurement using our validated table enables objective bedside screening of lung disease severity in very preterm infant cohorts at 36 weeks' PMA.

The impact of targeted oxygen reduction test in preterm infants on reducing the oxygen exposure

BACKGROUND

Caring for infants on respiratory support is a challenge in the middle-income countries, applying a protocol of targeted oxygen reduction test (tORT) guided by daily assessment of oxygen histograms is novel and practical approach.

OBJECTIVE

To study the impact of tORT guided by daily assessment of oxygen histograms as a quality improvement project aiming to decrease days on oxygen support, and duration of hospital admission in preterm infants.

STUDY DESIGN

A quality project conducted in neonatal intensive care units (NICU) of two hospitals, from 2017- 2018 (Epochs II). After a period of observation of a cohort of preterm Infants, 2016-2017 (Epoch I). The main aims were to reduce days on oxygen and hospital admission days. All infants in Epoch II underwent daily assessment of oxygen histograms and a trial of oxygen reduction if applicable as per a predefined protocol. Comparison was made between these two Epochs, and the primary outcome was the time to successful discontinuation of oxygen support.

RESULTS

Fifty-nine infants were included; 30 underwent the protocolized tORT (Epoch II) with a median (IQR) of 4 (2-6) tORC per infant. Postanal age at presentation (time of initial tORT assessment was performed at the postnatal age of 8 (5, 13) days. Days on oxygen and total numbers of hospital days were significantly less in Epoch II. Oxygen histograms significantly improved after tORT.

CONCLUSIONS

Applying tORT guided by oxygen histograms may have a significant impact on oxygen exposure and hospitalization days of patients admitted to the NICU.

Enteral Vitamin A for Reducing Severity of Bronchopulmonary Dysplasia: A Randomized Trial

BACKGROUND AND OBJECTIVES

Evidence suggests that intramuscular vitamin A reduces the risk of bronchopulmonary dysplasia (BPD) in preterm infants. Our objective was to compare enteral water-soluble vitamin A with placebo supplementation to reduce the severity of BPD in extremely preterm infants.

METHODS

We conducted a double-blind randomized controlled trial in infants <28 weeks' gestation who were to receive either enteral water-soluble vitamin A (5000 IU per day) or a placebo. Supplementation was started within 24 hours of introduction of feeds and continued until 34 weeks' postmenstrual age (PMA). The primary outcome was the severity of BPD, assessed by using the right shift of the pulse oximeter saturation versus the inspired oxygen pressure curve.

RESULTS

A total of 188 infants were randomly assigned. The mean ± SD birth weight (852 ± 201 vs 852 ± 211 g) and gestation (25.8 ± 1.49 vs 26.0 ± 1.39 weeks) were comparable between the vitamin A and placebo groups. There was no difference in the right shift (median [25th-75th percentiles]) of the pulse oximeter saturation versus inspired oxygen pressure curve (in kilopascals) between the vitamin A (11.1 [9.5-13.7]) and placebo groups (10.7 [9.5-13.1]) (P = .73). Enteral vitamin A did not affect diagnosis of BPD or other clinical outcomes. Plasma retinol levels were significantly higher in the vitamin A group versus the placebo group on day 28 and at 34 weeks' PMA.

CONCLUSIONS

Enteral water-soluble vitamin A supplementation improves plasma retinol levels in extremely preterm infants but does not reduce the severity of BPD.

Study protocol: azithromycin therapy for chronic lung disease of prematurity (AZTEC) - a randomised, placebo-controlled trial of azithromycin for the prevention of chronic lung disease of prematurity in preterm infants

INTRODUCTION

Chronic lung disease of prematurity (CLD), also known as bronchopulmonary dysplasia (BPD), is a cause of significant respiratory morbidity in childhood and beyond. Coupled with lung immaturity, infections (especially by Ureaplasma spp) are implicated in the pathogenesis of CLD through promotion of pulmonary inflammation. Azithromycin, which is a highly effective against Ureaplasma spp also has potent anti-inflammatory properties. Thus, azithromycin therapy may improve respiratory outcomes by targeting infective and inflammatory pathways. Previous trials using macrolides have not been sufficiently powered to definitively assess CLD rates. To address this, the azithromycin therapy for chronic lung disease of prematurity (AZTEC) trial aims to determine if a 10-day early course of intravenous azithromycin improves rates of survival without CLD when compared with placebo with an appropriately powered study.

METHODS AND ANALYSIS

796 infants born at less than 30 weeks' gestational age who require at least 2 hours of continuous respiratory support within the first 72 hours following birth are being enrolled by neonatal units in the UK. They are being randomised to receive a double-blind, once daily dose of intravenous azithromycin (20 mg/kg for 3 days, followed by 10 mg/kg for a further 7 days), or placebo. CLD is being assessed at 36 weeks' PMA. Whether colonisation with Ureaplasma spp prior to randomisation modifies the treatment effect of azithromycin compared with placebo will also be investigated. Secondary outcomes include necrotising enterocolitis, intraventricular/cerebral haemorrhage, retinopathy of prematurity and nosocomial infections, development of antibiotic resistance and adverse reactions will be monitored.

ETHICS AND DISSEMINATION

Ethics permission has been granted by Wales Research Ethics Committee 2 (Ref 18/WA/0199), and regulatory permission by the Medicines and Healthcare Products Regulatory Agency (Clinical Trials Authorisation reference 21323/0050/001-0001). The study is registered on ISRCTN (ISRCTN11650227). The study is overseen by an independent Data Monitoring Committee and an independent Trial Steering Committee. We shall disseminate our findings via national and international peer-reviewed journals, and conferences. A summary of the findings will also be posted on the trial website.

Physiology and Predictors of Impaired Gas Exchange in Infants with Bronchopulmonary Dysplasia

Rationale: A sensitive outcome measure for infants with bronchopulmonary dysplasia would facilitate clinical benchmarking and enhance epidemiologic understanding, evaluation of clinical interventions, and outcome prediction.Objectives: Noninvasive assessment of pulmonary gas exchange in preterm infants with and without bronchopulmonary dysplasia to grade disease severity and to identify determinants of impaired gas exchange.Methods: This is a prospective observational study in very preterm infants. Inspired oxygen partial pressure (Pi_O2) was decreased stepwise to achieve oxygen saturation as measured by pulse oximetry (Sp_O2) that decreased from 95% to 86%. Right shift, V⋅a/Q⋅, and right-left shunt were derived from the resulting Sp_O2 versus Pi_O2 curve and compared with current disease severity classification. Potential determinants of shift, V⋅a/Q⋅, and shunt were identified using principal components analysis and multiple linear regression.Measurements and Main Results: A total of 219 infants with median (interquartile range) gestation of 28 weeks and 0 days (26 weeks and 0 days to 29 weeks and 0 days) had a valid study at 35 weeks and 4 days (34 weeks and 1 day to 39 weeks and 3 days) of postmenstrual age. Shift increased and V⋅a/Q⋅ decreased as severity of bronchopulmonary dysplasia increased. Infants with moderate-severe disease also had increased shunt. Extent of impaired gas exchange overlapped between severity groups. Infants requiring mechanical support but no supplemental oxygen at 36 weeks' postmenstrual age had similar values of shift, V⋅a/Q⋅, and shunt to preterm infants without bronchopulmonary dysplasia. Lower gestation and increased duration of invasive ventilation independently predicted increased shift, decreased V⋅a/Q⋅, and increased shunt. Shift was the most sensitive and specific index of the severity of bronchopulmonary dysplasia.Conclusions: Most infants with bronchopulmonary dysplasia have impaired oxygenation quantified by a simple, sensitive bedside test. Shift of the Sp_O2/Pi_O2 curve may be useful for prediction and measurement of preterm infant respiratory outcomes.

Chronology and Determinants of Respiratory Function Changes Following Administration of Systemic Postnatal Corticosteroids in Extremely Preterm Infants

OBJECTIVE

To describe the effect of systemic corticosteroids administered to treat evolving bronchopulmonary dysplasia on oxygen diffusion and ventilation efficiency.

STUDY DESIGN

This was a retrospective cohort study of ventilated infants who received a 9-day course of dexamethasone in a tertiary neonatal unit. We calculated the transcutaneous oxygen saturation-to-fraction of inspired oxygen (F_iO₂) ratio (SFR), the ventilation perfusion ratio (V_A/Q), and the ventilation efficiency index (VEI) before, during, and after the course of corticosteroids. The response to corticosteroids was calculated as the difference between the F_iO₂ percentage before starting steroids and the lowest F_iO₂ value during the course of steroid treatment.

RESULTS

Seventy infants (38 males) with a median gestational age (GA) of 25.0 weeks (IQR, 24.3-26.0 weeks) and a median birth weight of 0.70 kg (IQR, 0.63-0.82 kg) were studied at a median postnatal age of 39 days (IQR, 29-48 days). The median SFR before treatment was 1.42 (IQR, 1.19-1.72), and the highest SFR was 2.35 (IQR, 1.87-2.83) after 9 days of treatment. The median V_A/Q before treatment was 0.14 (IQR, 0.11-0.18) and was significantly higher at 72 hours after the start of treatment (0.22; IQR, 0.15-0.29; P < .001). The median VEI was 0.06 (IQR, 0.04-0.08) before treatment and was highest, 0.10 (IQR, 0.07-0.13) at 48 hours after starting treatment. The median rate of response to corticosteroids was 28% (IQR, 20%-37%). GA was significantly related to the response to corticosteroids (ρ = 0.283; P = .019).

CONCLUSIONS

Oxygen diffusion continues to improve throughout the entire duration of a 9-day course of systemically administered corticosteroids in ventilated extremely preterm infants. More immature infants are less responsive to corticosteroids.

Bronchopulmonary dysplasia: Rationale for a pathophysiological rather than treatment based approach to diagnosis

Bronchopulmonary dysplasia (BPD), also known as Chronic Lung Disease (CLD), is a chronic respiratory condition of prematurity with potential life-long consequences for respiratory well-being. BPD was first described by Northway in 1967, when the mean gestation of preterm infants with BPD was 34 weeks' postmenstrual age (PMA). Survival of preterm infants at lower gestational ages has increased steadily since 1967 associated with marked improvements in respiratory management of respiratory distress syndrome. Currently, BPD develops in approximately 45 % of all infants born extremely preterm (Stoll et al., 2015). These smaller and more immature babies are born during the late canalicular or early saccular period of lung development. Not surprisingly, the pathophysiology of BPD also evolved since classical BPD was described. As the nature and our understanding of BPD evolved, so too the definitions and classification of BPD changed over time. These differing and ever-changing definitions hamper clinical benchmarking as they are interpreted and applied inconsistently, and define BPD and its severity by non-standardised treatments rather than independent evaluations of structure or function. A standardised, unambiguous definition and classification of BPD is essential for evaluation and improvement in clinical practice, both within an individual unit, as well as across and between neonatal networks. The determination and implementation of diagnostic criteria and severity classification that is standardised, globally applicable, and that has prognostic utility for clinical outcomes and guidance of ongoing respiratory management remain of utmost importance. This review describes the evolution of BPD definitions, evaluates the benefits and limitations of each approach, and discusses alternative approaches that may improve the functional assessment of BPD severity.

Utility of lung ultrasound scanning in neonatology

The utility of point-of-care lung ultrasound in neonatology is rapidly expanding. This review summarises current evidence of a diagnostic, procedural and observational tool valuable in the management of newborns requiring intensive care. Approaching a patient, probe in-hand with focused clinical question is essential, and barriers to implication together with important research questions are explored.

Functional morphometry for the estimation of the alveolar surface area in prematurely-born infants

Conventionally, the alveolar surface area (S_A) has been measured by using post-mortem morphometry. Such studies have highlighted that S_A in prematurely-born infants is markedly smaller when compared to term-born infants as a result of postnatal impairment or arrest of alveolar development. We herein explore how, non-invasive measurements of the ventilation/perfusion ratio (V_A/Q) can be used to estimate S_A in prematurely-born surviving, convalescent infants. We also compare S_A in prematurely-born infants measured at term-corrected age, to term-born infants using previously published datasets of V_A/Q. Fick's first law of diffusion is employed for the conversion of V_A/Q measurements to S_A values after correcting for differences in pulmonary perfusion, thickness of the respiratory membrane and alveolar-arterial gradient. We report that S_A is fivefold smaller in prematurely-born compared to term-born infants. We conclude that non-invasive measurements of V_A/Q can be used for the functional estimation of S_A which could, in turn, be used as a future outcome measure in respiratory studies of prematurely-born infants.

How best to capture the respiratory consequences of prematurity?

Chronic respiratory morbidity is a common complication of premature birth, generally defined by the presence of bronchopulmonary dysplasia, both clinically and in trials of respiratory therapies. However, recent data have highlighted that bronchopulmonary dysplasia does not correlate with chronic respiratory morbidity in older children born preterm. Longitudinally evaluating pulmonary morbidity from early life through to childhood provides a more rational method of defining the continuum of chronic respiratory morbidity of prematurity, and offers new insights into the efficacy of neonatal respiratory interventions. The changing nature of preterm lung disease suggests that a multimodal approach using dynamic lung function assessment will be needed to assess the efficacy of a neonatal respiratory therapy and predict the long-term respiratory consequences of premature birth. Our aim is to review the literature regarding the long-term respiratory outcomes of neonatal respiratory strategies, the difficulties of assessing dynamic lung function in infants, and potential new solutions.

Better measures are needed to predict chronic respiratory morbidity in survivors born prematurely http://ow.ly/1L3n30ihq9C

Enteral vitamin A for reducing severity of bronchopulmonary dysplasia in extremely preterm infants: a randomised controlled trial

Background

Intramuscular vitamin A supplementation decreases the risk of bronchopulmonary dysplasia (BPD) in very-low-birth-weight preterm infants without significant adverse effects. However, intramuscular vitamin A supplementation is not widely accepted because of the discomfort and risk of trauma associated with repeated injections. Enteral vitamin A supplementation has not been studied adequately in the clinical trials. Enterally administered water-soluble vitamin A is absorbed better than the fat-soluble form. We hypothesised that enteral administration of a water-soluble vitamin A preparation will decrease severity of BPD compared with a control group receiving placebo.

Methods

We plan a double-blind randomised placebo-controlled trial at a tertiary neonatal-perinatal intensive care unit. Eligibility criteria include infants born at less than 28 weeks’ gestational age and less than 72 h of life. Infants with major congenital gastrointestinal or respiratory tract abnormalities will be excluded. After parental consent, infants will be randomized to receive either enteral water-soluble vitamin A (5000 IU once a day) or placebo. The intervention will be started within 24 h of introduction of feeds and continued until 34 weeks’ post-menstrual age (PMA).

The primary outcome is severity of BPD at 36 weeks’ PMA. Severity of BPD will be assessed objectively from the right-shift of the peripheral oxyhaemoglobin saturation versus partial pressure of inspired oxygen (SpO₂-PiO₂) curve. We require 188 infants for 80% power and 5% significance level based on an expected 20% decrease in the right shift of the SpO₂-PiO₂ curve in the vitamin A group (primary outcome) compared with control group at 36 weeks’ PMA, and a 20% attrition rate.

Secondary outcomes will be plasma and salivary concentrations of vitamin A on day 28 of the trial (first 30 infants), lung and diaphragm function, clinical outcomes at 36 week’ PMA or before discharge/death, and safety of vitamin A.

Discussion

BPD poses a significant economic burden on the health-care system. If our study shows that enteral supplementation of water-soluble vitamin A is safe and effective for decreasing the severity of BPD, it will provide the opportunity to further evaluate a simple, globally acceptable preventive therapy for BPD.

Trial registration

ANZCTR; ACTRN12616000408482 (30th March 2016).

Electronic supplementary material

The online version of this article (10.1186/s12887-017-0958-x) contains supplementary material, which is available to authorized users.

Development and preclinical testing of an adaptive algorithm for automated control of inspired oxygen in the preterm infant

OBJECTIVE

To assess the performance of a novel algorithm for automated oxygen control using a simulation of oxygenation founded on in vivo data from preterm infants.

METHODS

A proportional-integral-derivative (PID) control algorithm was enhanced by (i) compensation for the non-linear SpO₂-PaO₂ relationship, (ii) adaptation to the severity of lung dysfunction and (iii) error attenuation within the target range. Algorithm function with and without enhancements was evaluated by iterative linking with a computerised simulation of oxygenation. Data for this simulation (FiO₂ and SpO₂ at 1 Hz) were sourced from extant recordings from preterm infants (n=16), and converted to a datastream of values for ventilation:perfusion ratio and shunt. Combination of this datastream second by second with the FiO₂ values from the algorithm under test produced a sequence of novel SpO₂ values, allowing time in the SpO₂ target range (91%-95%) and in various degrees of hypoxaemia and hyperoxaemia to be determined. A PID algorithm with 30 s lockout after each FiO₂ adjustment, and a proportional-derivative (PD) algorithm were also evaluated.

RESULTS

Separate addition of each enhancing feature to the PID algorithm showed a benefit, but not with uniformly positive effects. The fully enhanced algorithm was optimal for the combination of targeting the desired SpO₂ range and avoiding time in, and episodes of, hypoxaemia and hyperoxaemia. This algorithm performed better than one with a 30 s lockout, and considerably better than PD control.

CONCLUSIONS

An enhanced PID algorithm was very effective for automated oxygen control in a simulation of oxygenation, and deserves clinical evaluation.

Ventilation/perfusion ratio and right to left shunt in healthy newborn infants

Oxygenation impairment can be assessed non-invasively by determining the degree of right-to-left shunt and ventilation/perfusion (V_A/Q) inequality. These indices have been used in sick newborn infants, but normative values have not been reported which are essential to determine the magnitude of the abnormality. We, therefore, aimed to measure the shunt and V_A/Q in infants with no history of respiratory conditions and determine if there was any effect of supine or prone position and the reproducibility of the data. Data were analysed from infants who had undergone a hypoxic challenge and in a subset who had been assessed in the supine or prone position. Transcutaneous oxygen saturations (SpO₂) were recorded at fractions of inspired oxygen (F_IO₂) of 0.21 and 0.15. Two independent raters used a computer software algorithm which analysed and fitted paired data for F_IO₂ and SpO₂ and derived a curve which represented the best fit for each infant's data and calculated the shunt and V_A/Q. The raters ability to interpret the SpO₂ value which corresponded to a given F_IO₂ was compared. The downwards displacement of the F_IO₂ versus SpO₂ curve was used to estimate the degree of right-to-left shunt and the rightwards shift of the curve was used to calculate the V_A/Q ratio. The mean (SD) gestational age of the 145 infants was 39 (1.6) weeks, their birth weight was 2990 (578) gms and median (range) postnatal age at measurement 3 (1-8) days. The mean (SD) V_A/Q ratio was 0.95 (0.21). None of the infants had a right-to-left shunt. No significant differences were found in V_A/Q in the supine compared to the prone position. The intraclass correlation coefficient of V_A/Q between two independent raters was 0.968 (95% CI 0.947-0.980), p < 0.001. Right-to-left shunt and V_A/Q ratio in healthy newborn infants were similar in the prone compared to the supine position.

Influence of pulmonary factors on pulse oximeter saturation in preterm infants

AIM

To describe how the stability of oxygen saturation measured by pulse oximetry (SpO2%) varies within and between infants with bronchopulmonary dysplasia (BPD).

METHODS

Clinically stable infants with BPD had SpO2 measured at different inspired oxygen concentrations (FIO2 expressed as %). A computer model of gas exchange, that is, ventilation/perfusion ratio (VA/Q) and shunt, plotted the curve of SpO2 versus FIO2 best fitting these data. The slope of this curve is the change in SpO2 per % change in FIO2, hence SpO2 stability, calculated at each SpO2 from 85% to 95%.

RESULTS

Data from 16 infants with BPD previously described were analysed. The dominant gas exchange impairment was low VA/Q (median 0.35, IQR, 0.16-0.4, normal 0.86). Median shunt was 1% (IQR, 0-10.5; normal <2%). Slope varied markedly between infants, but above 95% SpO2 was always <1.5. In infants with least severe BPD (VA/Q ≈0.4, shunt ≤2%) median slope at 85% SpO2 was 5.1 (IQR, 3.7-5.5). With more severe BPD (VA/Q ≤0.3) slope was flatter throughout the SpO2 range. The highest FIO2 for 90% SpO2 was in infants with the lowest VA/Q values.

CONCLUSIONS

In infants with BPD, there was large variation in the slope of the curve relating SpO2% to inspired oxygen fraction in the SpO2 range 85%-95%. Slopes were considerably steeper at lower than higher SpO2, especially in infants with least severe BPD, meaning that higher SpO2 target values are intrinsically much more stable. Steep slopes below 90% SpO2 may explain why some infants appear dependent on remarkably low oxygen flows.

Reliability of a Noninvasive Measure of V./Q. Mismatch for Bronchopulmonary Dysplasia

RATIONALE

Currently used definitions of bronchopulmonary dysplasia (BPD) lack a continuous measure of disease severity.

OBJECTIVES

To determine if an indirect measure of V./Q. mismatch is reliable when simplified to facilitate more widespread use for grading disease severity in BPD at 36 weeks postmenstrual age.

METHODS

We used prospectively collected data from 32 preterm infants undergoing an oxygen reduction test at 36 weeks postmenstrual age to perform a simplified indirect assessment of V./Q. mismatch for each infant. Independent raters applied the model, and interrater reliability for a quantitative measure of mismatch was measured by intraclass correlation coefficient. A receiver operating characteristic curve evaluated the impact of increasing degrees of V./Q. mismatch on diagnosing BPD as defined by oxygen reduction test failure.

MEASUREMENTS AND MAIN RESULTS

Concordance for the quantitative measure of V./Q. mismatch between independent raters improved from 0.72 (confidence interval [CI], 0.48-0.86) to 0.93 (CI, 0.87-0.96) after refinement of instructions for applying the simplified model. Higher degrees of mismatch were increasingly predictive of oxygen reduction test failure, with a receiver operating characteristic curve analysis area under the curve of 0.83 (CI, 0.68-0.99; P = 0.03).

CONCLUSIONS

A simplified indirect measure of V./Q. mismatch for diagnosing and grading disease severity in BPD has high reliability and can be performed with data obtained during a standard oxygen reduction test. This should facilitate more widespread investigation of this model as a technique for characterizing BPD severity.

Characterisation of the Oxygenation Response to Inspired Oxygen Adjustments in Preterm Infants

BACKGROUND

Oxygen saturation (SpO2) targeting in the preterm infant may be improved with a better understanding of the SpO2 responses to changes in inspired oxygen (FiO2).

OBJECTIVE

We investigated the first-order FiO2-SpO2 relationship, aiming to quantify the parameters governing that relationship, the influences on these parameters and their variability.

METHODS

In recordings of FiO2 and SpO2 from preterm infants on continuous positive airway pressure and supplemental oxygen, we identified unique FiO2 adjustments and mapped the subsequent SpO2 responses. For responses identified as first-order, the delay, time constant and gain parameters were determined. Clinical and physiological predictors of these parameters were sought in regression analysis, and intra- and inter-subject variability was evaluated.

RESULTS

In 3,788 h of available data from 47 infants at 31 (28-33) post-menstrual weeks [median (interquartile range)], we identified 993 unique FiO2 adjustments followed by a first-order SpO2 response. All response parameters differed between FiO2 increments and decrements, with increments having a shorter delay, longer time constant and higher gain [2.9 (1.7-4.8) vs. 1.3 (0.58-2.6), p < 0.05]. Gain was also higher in less mature infants and in the setting of recent SpO2 instability, and was diminished with increasing severity of lung dysfunction. Intra-subject variability in all parameters was prominent.

CONCLUSIONS

First-order SpO2 responses show variable gain, influenced by the direction of FiO2 adjustment and the severity of lung disease, as well as substantial intra-subject parameter variability. These findings should be taken into account in adjustment of FiO2 for SpO2 targeting in preterm infants.

Oxygen Saturation Targeting and Bronchopulmonary Dysplasia

Oxygen saturation targeting is widely used in neonatal intensive care, but the optimal target range in very preterm infants has been uncertain and is the subject of recent debate and research. This review briefly discusses the technology of oxygen monitoring and the role of oxygen toxicity in preterm infants. The background to the recent trials of oxygen saturation targeting in acute and continuing care of very preterm infants is reviewed, and the findings and implications of the recent trials, particularly with respect to bronchopulmonary dysplasia, are discussed.

Correlation of radiographic thoracic area and oxygenation impairment in bronchopulmonary dysplasia

We hypothesized that radiographically-assessed hyperinflation in bronchopulmonary dysplasia (BPD) is related to the degree of oxygenation impairment. Our objective was to explore the relation of chest radiographic thoracic area (CRTA) with right-to-left shunt, right shift of the oxyhemoglobin dissociation curve and ventilation/perfusion ratio (VA/Q) in infants with BPD. Twenty-two infants born at median (IQR) gestation of 26 (24-28) weeks with BPD were prospectively studied at 39 (30-69) days. Inspired oxygen (FiO2) was varied to obtain transcutaneous oxygen saturation (SpO2) values between 85 and 96%. Shunt, shift and VA/Q were derived by plotting and analysing pairs of SpO2 and FiO2. CRTA was measured by free hand-tracing the perimeter of the thoracic area in anterio-posterior chest radiographs. Median (IQR) shunt was 8 (1-14)%, shift was 13 (11-19)kPa and VA/Q 0.42 (0.30-0.48). Median (IQR) CRTA/kg was 2495 (1962-2838)mm(2) and was significantly related to shift (r=0.674, p<0.001), VA/Q (r=-0.633, p<0.001), weight at study (r=-0.457, p=0.003) and day of life (r=-0.406, p=0.009), but not to shunt. CRTA in BPD is significantly related to oxygenation impairment as quantified by shift and VA/Q. CRTA can be used as a simple radiographic test to quantify BPD severity.

Using Measurements of Shunt and Ventilation-to-Perfusion Ratio to Quantify the Severity of Bronchopulmonary Dysplasia

BACKGROUND

Classifying the severity of bronchopulmonary dysplasia (BPD) by continuous numerical variables would facilitate follow-up of disease progression and quantified analysis of disease determinants.

OBJECTIVES

To non-invasively measure oxygenation impairment in BPD by the degree of right-to-left shunt, right shift of the oxyhaemoglobin dissociation curve and ventilation/perfusion (VA/Q) inequality and to explore their relation with clinical parameters.

METHODS

Prospective cohort study of 24 infants with a median (interquartile range, IQR) gestation of 25 weeks (24-27) and a birth weight of 0.70 kg (0.63-0.93), studied at 36 days (30-66), at a postmenstrual age (PMA) of 33 weeks (29-36). Inspired oxygen (FIO2) was varied to obtain three to five transcutaneous oxygen saturation (SpO2) values between 85 and 96%. Values of shunt, shift and VA/Q were obtained by plotting the paired data of SpO2 against FIO2 for each infant using a unique program. Right-to-left shunt, right shift of the oxyhaemoglobin dissociation curve and VA/Q were measured in infants born <32 weeks PMA receiving oxygen at 28 days.

RESULTS

The median (IQR) shunt was 8% (0.3-16.5), shift 14.5 kPa (10.9-19.4) and VA/Q 0.40 (0.30-0.48). Shunt, shift and VA/Q were significantly related to gestational age (GA) at birth, PMA at study, weight at study and weight gain per week.

CONCLUSIONS

Severity of pulmonary oxygenation impairment in BPD can be quantified at the cot-side by non-invasive measurement of shunt, shift and VA/Q. Low GA at birth, low weight at birth and at the time of study and impaired weight gain are significantly associated with the severity of oxygen-exchange impairment in infants with BPD.

Volumetric capnography in infants with bronchopulmonary dysplasia

OBJECTIVES

To assess the feasibility of using volumetric capnography in spontaneously breathing small infants and its ability to discriminate between infants with and without bronchopulmonary dysplasia (BPD).

STUDY DESIGN

Lung function variables for 231 infants (102 term, 52 healthy preterm, 77 BPD), matched for post-conceptional age of 44 weeks, were collected. BPD was defined as supplemental oxygen requirement at 36 weeks post-menstrual age. Tidal breath-by-breath volume capnograms were obtained by mainstream capnography. The capnographic slope of phase II (SII) and slope of phase III (SIII) were calculated and compared between study groups. The effect of BPD, tidal volume (VT), respiratory rate (RR), and prematurity on the magnitude of the slopes was assessed.

RESULTS

SII was steeper in infants with BPD (100 ± 28/L) compared with healthy preterm (88 ± 22/L; P = .007) and term infants (79 ± 18/L; P < .001), but this finding was attributed to differences in VT, RR, and gestational age. SIII was steeper in the BPD group (26.8 ± 14.1/L) compared with healthy preterm (16.2 ± 6.2/L; P < .001) and term controls (14.8 ± 5.4/L; P < .001). BPD was a significant predictor of SIII independently of VT, RR, and gestational age. The ability of SIII to discriminate between BPD and controls was significantly higher compared with lung clearance index (area under the curve 0.83 vs 0.56; P < .001).

CONCLUSIONS

Volumetric capnography may provide valuable information regarding functional lung alterations related to BPD and might be considered as an alternative to more involved lung function techniques for monitoring chronic lung disease during early infancy.

Bronchopulmonary dysplasia: clinical grading in relation to ventilation/perfusion mismatch measured by single photon emission computed tomography

Bronchopulmonary dysplasia (BPD) is a significant cause of morbidity in the preterm population. Clinical severity grading based on the need for supplemental oxygen and/or need for positive airway pressure at 36 weeks postmenstrual age does not yield reproducible predictive values for later pulmonary morbidity. Single photon emission computed tomography (SPECT) was used to measure the distribution of lung ventilation (V) and perfusion (Q) in 30 BPD preterm infants at a median age of 37 weeks postmenstrual age. The V and Q were traced with 5 MBq Technegas and Technetium-labeled albumin macro aggregates, respectively, and the V/Q match-mismatch was used to quantify the extent of lung function impairment. The latter was then compared with the clinical severity grading at 36 weeks, and time spent on mechanical ventilation, continuous positive airway pressure (CPAP) and supplemental oxygen. Of those with mild and moderate BPD 3/9 and 3/11 patients, respectively, showed significant V/Q mismatches. By contrast, 4/10 patients with severe BPD showed a satisfactory V/Q matching distribution. An unsatisfactory V/Q match was not correlated with time spent on supplemental oxygen or CPAP, but was significantly negatively correlated with time spent on mechanical ventilation. SPECT provides unique additional information about regional lung function. The results suggest that the current clinical severity grading can be improved and/or complemented with SPECT.

Postnatal maturation of breathing stability and loop gain: the role of carotid chemoreceptor development

Any general model of respiratory control must explain a puzzling array of breathing patterns that are observed during the course of a lifetime. Particular challenges are to understand why periodic breathing is rarely seen in the first few days after birth, reaches a peak at 2-4 weeks postnatal age, and disappears by 6 months, why it is prevalent in preterm infants, and why it reappears in adults at altitude or with heart failure. In this review we use the concept of loop gain to obtain quantitative insight into the genesis of unstable breathing patterns with a particular focus on how changes in carotid body function could underlie the age-related dependence of periodic breathing.

Air travel and the risks of hypoxia in children

In infants and children with chronic respiratory disease, hypoxia is a potential risk of aircraft travel. Although guidelines have been published to assist clinicians in assessing an individual's fitness to fly, they are not wholly evidence based. In addition, most evidence relates to adults with chronic obstructive pulmonary disease and thus cannot be extrapolated to children and infants. This review summarises the current literature as it applies to infants and children potentially at risk during air travel. Current evidence suggests that the gold standard for assessing fitness to fly, the hypoxia flight simulation test, may not be accurate in predicting in flight hypoxia in infants and children with respiratory disease. Further research is needed to determine the best methods of assessing safety of flight in infants and children.

Capnography in spontaneously breathing preterm infants with bronchopulmonary dysplasia

BACKGROUND

In adult patients with chronic obstructive pulmonary disease, there is a gradient between end-tidal carbon dioxide (EtCO(2)) and arterial carbon dioxide pressure (PaCO(2)), and the slope of the ascending phase of the capnogram is decreased due to obstruction. Corresponding data are lacking in infants with bronchopulmonary dysplasia (BPD).

OBJECTIVES

To compare PCO(2) -EtCO(2) gradient and capnogram shape in two groups of spontaneously breathing preterm subjects: infants with BPD and infants without respiratory disease (controls).

MATERIAL AND METHODS

Capnography was performed at 36 weeks postmenstrual age in 20 infants (12 BPD with oxygen dependency, 8 controls). Respiratory rate and the components of the respiratory cycle were measured. The PCO(2) -EtCO(2) gradient was calculated using EtCO(2) values and simultaneously sampled capillary values (PcCO(2)). Capnograms were compared between groups.

RESULTS

In BPD subjects, respiratory rate was increased (60 ± 16 bpm vs 43 ± 16 bpm, P = 0.009); a widened PcCO(2) -EtCO(2) gradient was observed (13 ± 4 mmHg vs 0 ± 7 mmHg, P = 0.0013); the ascending phase of the capnogram was not decreased, whereas the initial inspiratory phase was prolonged (0.32 ± 0.05 vs 0.24 ± 0.04, P = 0.001).

CONCLUSIONS

Compared with healthy infants, a higher PcCO(2) -EtCO(2) gradient was observed in infants with BPD, suggesting that ventilation-perfusion mismatch may be present in these infants. The capnogram did not exhibit the characteristic shape of airway obstruction.

The management of evolving bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is associated with increased mortality and significant long-term cardiorespiratory and neurodevelopmental sequelae. Treatment of evolving BPD in the neonatal intensive care unit (NICU) is challenging due to the complex interplay of contributing risk factors which include preterm birth per se, supplemental oxygen, positive pressure ventilation, patent ductus arterious, and pre- and postnatal infection. Management of evolving BPD requires a multimodal approach including adequate nutrition, careful fluid management, effective and safe pharmacotherapy, and respiratory support aiming at minimal lung injury. Among pharmacological interventions, caffeine has the best risk-benefit profile. Systemic postnatal corticosteroids should be reserved to ventilated infants at highest risk of BPD who cannot be weaned from the ventilator. Several ongoing randomised trials are evaluating optimal oxygen saturation targets in preterm infants. The most beneficial respiratory support strategy to minimise lung injury remains unclear and requires further investigation.

Lung function and respiratory symptoms at 11 years in children born extremely preterm: the EPICure study

RATIONALE

The long-term respiratory sequelae of infants born extremely preterm (EP) and now graduating from neonatal intensive care remains uncertain.

OBJECTIVES

To assess the degree of respiratory morbidity and functional impairment at 11 years in children born EP (i.e., at or less than 25 completed weeks of gestation) in relation to neonatal determinants and current clinical status.

METHODS

Pre- and postbronchodilator spirometry were undertaken at school in children born EP and classroom control subjects. Physical examination and respiratory health questionnaires were completed. Multivariable regression was used to estimate the predictive power of potential determinants of lung function.

MEASUREMENTS AND MAIN RESULTS

Spirometry was obtained in 182 of 219 children born EP (129 with prior bronchopulmonary dysplasia [BPD]) and 161 of 169 classmates, matched for age, sex, and ethnic group. Children born EP had significantly more chest deformities and respiratory symptoms than classmates, with twice as many (25 vs. 13%; P < 0.01) having a current diagnosis of asthma. Baseline spirometry was significantly reduced (P < 0.001) and bronchodilator responsiveness was increased in those born EP, the changes being most marked in those with prior BPD. EP birth, BPD, current symptoms, and treatment with beta-agonists are each associated independently with lung function z-scores (adjusted for age, sex, and height) at 11 years. Fifty-six percent of children born EP had abnormal baseline spirometry and 27% had a positive bronchodilator response, but less than half of those with impaired lung function were receiving any medication.

CONCLUSIONS

After extremely preterm birth, impaired lung function and increased respiratory morbidity persist into middle childhood, especially among those with BPD. Many of these children may not be receiving appropriate treatment.

A model investigation of the impact of ventilation-perfusion mismatch on oxygenation during apnea in preterm infants

Ventilation-perfusion (V/Q) mismatch is a prominent feature of preterm infants and adults with lung disease. V/Q mismatch is known to cause arterial hypoxemia under steady-state conditions, and has been proposed as the cause of rapid arterial oxygen desaturation during apnea. However, there is little evidence to support a role for V/Q mismatch in the dynamic changes in arterial oxygenation that occur during apnea. Using a mathematical model, we quantified the effect of V/Q mismatch on the rate of desaturation during apnea to ascertain whether it could lead to rates of up to 10%s(-1) as observed in preterm infants. We used a lung-body model for the preterm infant that incorporated 50 parallel alveolar-capillary units that were ventilated and perfused with the severity of V/Q mismatch (sigma) defined conventionally according to sigma=S.D. of the distribution of V/Q ratios. Average desaturation rate 10s from apnea onset was strongly elevated with worsening V/Q mismatch as a result of an earlier desaturation of low V/Q units compared with high V/Q units. However, V/Q mismatch had little impact after apnea onset, with peak desaturation rate only substantially increased if mismatching caused a lowered resting arterial O(2) saturation. In conclusion, V/Q mismatch causes a more immediate onset of desaturation during apnea, and therefore places preterm infants and adults with lung disease at risk of hypoxemic dips. However, V/Q mismatch does not accelerate desaturation rate beyond apnea onset and cannot, therefore, explain the rapid desaturation observed during recurrent apnea in preterm infants.