Influence of pulmonary factors on pulse oximeter saturation in preterm infants

Permissive hypercapnia and oxygenation impairment in premature ventilated infants

AIM

In permissive hypercapnia high levels of carbon dioxide (CO2) are tolerated in ventilated preterm infants to minimise lung injury, but hypercapnia could directly impair oxygenation. We aimed to quantify the association of elevated CO2 with oxygenation impairment in preterm infants by measuring the right-to-left shunt and the ventilation/perfusion (VA/Q) ratio.

METHODS

Pre-existing datasets from preterm infants during the acute phase of respiratory distress syndrome or with evolving or established bronchopulmonary dysplasia were analysed. Non-invasive paired measurements of the fraction of inspired oxygen (FIO2) and transcutaneous oxygen saturation (SpO2) were used to calculate the degree of right-to-left shunt, right shift of the FIO2 versus SpO2 curve and the VA/Q.

RESULTS

A total of 75 infants (43 male) with a median (IQR) gestational age of 26.4 (24.7-27.7) weeks were studied at 7 (2-31) days. Thirty-six infants (48 %) had an arterial partial pressure of CO2 (PaCO2) above 6 kPa. The PaCO2 was independently associated with the right shift of the curve [adjusted p < 0.001, unstandardised coefficient; 2.26, 95 % CI: 1.51-2.95] and the right-to-left shunt [adjusted p = 0.016, unstandardised coefficient; 1.86, 95 % CI: 0.36-3.36] after adjusting for confounders. An increase of the PaCO2 from 5 to 8 kPa, corresponded to a right shift of the curve of 20.2 kPa or a decrease in the VA/Q from 0.66 to 0.24.

CONCLUSIONS

Increased carbon dioxide levels were significantly associated with impaired oxygenation in preterm infants with respiratory distress syndrome or bronchopulmonary dysplasia.

Functional morphometry: non-invasive estimation of the alveolar surface area in extremely preterm infants

BACKGROUND

The main pathophysiologic characteristic of chronic respiratory disease following extremely premature birth is arrested alveolar growth, which translates to a smaller alveolar surface area (SA). We aimed to use non-invasive measurements to estimate the SA in extremely preterm infants.

METHODS

Paired measurements of the fraction of inspired oxygen and transcutaneous oxygen saturation were used to calculate the ventilation/perfusion ratio, which was translated to SA using Fick's law of diffusion. The SA was then adjusted using volumetric capnography.

RESULTS

Thirty infants with a median (range) gestational age of 26.3 (22.9-27.9) weeks were studied. The median (range) adjusted SA was 647.9 (316.4-902.7) cm2. The adjusted SA was lower in the infants who required home oxygen [637.7 (323.5-837.5) cm2] compared to those who did not [799.1 (444.2-902.7) cm2, p = 0.016]. In predicting the need for supplemental home oxygen, the adjusted SA had an area under the receiver operator characteristic curve of 0.815 (p = 0.017). An adjusted SA ≥688.6 cm2 had 86% sensitivity and 77% specificity in predicting the need for supplemental home oxygen.

CONCLUSIONS

The alveolar surface area can be estimated non-invasively in extremely preterm infants. The adjusted alveolar surface area has the potential to predict the subsequent need for discharge home on supplemental oxygen.

IMPACT

We describe a novel biomarker of respiratory disease following extremely preterm birth. The adjusted alveolar surface area index was derived by non-invasive measurements of the ventilation/perfusion ratio and adjusted by concurrent measurements of volumetric capnography. The adjusted alveolar surface area was markedly reduced in extremely preterm infants studied at 7 days of life and could predict the need for discharge home on supplemental oxygen. This method could be used at the bedside to estimate the alveolar surface area and provide an index of the severity of lung disease, and assist in monitoring, clinical management and prognosis.

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.

Automated oxygen control in preterm infants, how does it work and what to expect: a narrative review

BACKGROUND

Automated oxygen control systems are finding their way into contemporary ventilators for preterm infants, each with its own algorithm, strategy and effect.

OBJECTIVE

To provide guidance to clinicians seeking to comprehend automated oxygen control and possibly introduce this technology in their practice.

METHOD

A narrative review of the commercially available devices using different algorithms incorporating rule-based, proportional-integral-derivative and adaptive concepts are described and explained. An overview of how they work and, if available, the clinical effect is given.

RESULTS

All algorithms have shown a beneficial effect on the proportion of time that oxygen saturation is within target range, and a decrease in hyperoxia and severe hypoxia. Automated oxygen control may also reduce the workload for bedside staff. There is concern that such devices could mask clinical deterioration, however this has not been reported to date.

CONCLUSIONS

So far, trials involving different algorithms are heterogenous in design and no head-to-head comparisons have been made, making it difficult to differentiate which algorithm is most effective and what clinicians can expect from algorithms under certain conditions.

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.

Pulmonary Gas Exchange Improves over the First Year in Preterm Infants with and without Bronchopulmonary Dysplasia

BACKGROUND

Right shift of the peripheral oxyhaemoglobin saturation (SpO2) versus inspired oxygen pressure (PIO2) curve is a sensitive marker of pulmonary gas exchange.

OBJECTIVES

The aim of this study was to assess the impact of prematurity and bronchopulmonary dysplasia (BPD) on gas exchange and right-to-left shunt in the neonatal period, and its evolution over the first year of life.

METHOD

We assessed shift and shunt in extremely preterm (EP) and very preterm (VP) infants at 36 and 44 weeks' postmenstrual age (PMA), and at 1-year corrected postnatal age (cPNA). PIO2 was decreased stepwise to achieve SpO2 between 85 and 98%. Shift and shunt were calculated from paired SpO2/PIO2 measurements using customized software. Results were examined cross-sectionally at each time point, and longitudinally using generalized linear regression. Term infants were assessed at 44 wk PMA as a comparative reference.

RESULTS

Longitudinal modelling showed continuous decline in shift in EP and VP infants during the first year of life. There was no difference in shift compared to term infants at 44 wk PMA (p = 0.094). EP infants with BPD had higher shift than infants without BPD at 36 wk PMA (p < 0.001) and 44 wk PMA (p = 0.005) but not at 1-year cPNA.

CONCLUSIONS

In the absence of lung disease, prematurity per se did not result in reduced gas exchange at 1-year cPNA. We report ongoing, significant improvements in pulmonary gas exchange in all preterm infants during the first year of life, despite evidence of early deficits in gas exchange in EP infants with BPD.

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.

Achieved Oxygenation Saturations and Outcome in Extremely Preterm Infants

Infants in the Neonatal Oxygenation Prospective Meta-analysis trials were randomized to SpO₂ targets of 85% to 89% or 91% to 95%. Group allocation was masked. Different outcomes are likely partially attributable to differences in achieved SpO₂. Infants randomized to the lower range had higher than intended readings. SpO₂ distributions of infants in the low-range group of the Benefits of Oxygen Saturation Targeting II UK trial who died or developed necrotizing enterocolitis were centered around 90% to 92%. These achieved SpO₂ distributions caution against using lower SpO₂ target ranges early or throughout the clinical course in extremely preterm infants.

Hypoxemic and hyperoxemic likelihood in pulse oximetry ranges: NICU observational study

OBJECTIVE

Describe the likelihood of hypoxemia and hyperoxemia across ranges of oxygen saturation (SpO₂), during mechanical ventilation with supplemental oxygenation.

DESIGN

Retrospective observational study.

SETTING

University affiliated tertiary care neonatal intensive care unit.

PATIENTS

Two groups of neonates based on postmenstrual age (PMA): <32 weeks (n=104) and >36 weeks (n=709).

MAIN MEASURES

Hypoxemia was defined as a PaO₂ <40 mm Hg, hyperoxemia as a PaO₂ of >99 mm Hg and normoxemia as a PaO₂ of 50-80 mm Hg. Twenty-five per cent was defined as marked likelihood of hypoxemia or hyperoxemia.

RESULTS

From these infants, 18 034 SpO₂-PaO₂ pairs were evaluated of which 10% were preterm. The PMA (median and IQR) of the two groups were: 28 weeks (27-30) and 40 weeks (38-41). With SpO₂ levels between 90% and 95%, the likelihoods of hypoxemia and hyperoxemia were low and balanced. With increasing levels of SpO₂, the likelihood of hyperoxemia increased. It became marked in the preterm group when SpO₂ was 99%-100% (95% CI 29% to 41%) and in the term group with SpO₂ levels of 96%-98% (95% CI 29% to 32%). The likelihood of hypoxemia increased as SpO₂ decreased. It became marked in both with SpO₂ levels of 80%-85% (95% CI 20% to 31%, 24% to 28%, respectively).

CONCLUSIONS

The likelihood of a PaO₂ <40 mm Hg is marked with SpO₂ below 86%. The likelihood of a PaO₂ >99 mm Hg is marked in term infants with SpO₂ above 95% and above 98% in preterm infants. SpO₂ levels between 90% and 95% are appropriate targets for term and preterm infants.

Effect of a smaller target range on the compliance in targeting and distribution of oxygen saturation in preterm infants

BACKGROUND

Following recent recommendations, the oxygen saturation (SpO₂) target range for preterm infants in our nursery was narrowed towards the higher end from 85%-95% to 90%-95%. We determined the effect of narrowing the SpO₂ target range on the compliance in target range and distribution of SpO₂ in preterm infants.

METHODS

Before and after changing the target range from 85%-95% to 90%-95%, infants <30 weeks of gestation receiving oxygen were compared during their admission on the neonatal intensive care unit. For each infant, distribution of SpO₂ was noted by collecting SpO₂ samples each minute, and the percentage of time spent with SpO₂ within 90%-95% was calculated. Oxygen was manually adjusted. Hypoxaemic events (SpO₂ <80%) where oxygen was titrated were analysed.

RESULTS

Data were analysed for 104 infants (57 before and 47 after the range was narrowed). The narrower range was associated with an increase in the median (IQR) SpO₂ (93% (91%-96%) vs 94% (92%-97%), p=0.01), but no increase in median time SpO₂ within 90%-95% (49.2% (39.6%-59.7%) vs (46.9% (27.1%-57.9%), p=0.72). The distribution of SpO₂ shifted to the right with a significant decrease in SpO₂ <90%, but not <80%. The count of minute values for Sp0₂ <80% decreased, while the frequency and duration of hypoxaemic events and oxygen titration were not different.

CONCLUSION

Narrowing the target range from 85%-95% to 90%-95% in preterm infants was associated with an increase in median SpO₂ and a rightward shift in the distribution, but no change in time spent between 90% and 95%.

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.

Improving manual oxygen titration in preterm infants by training and guideline implementation

To study oxygen saturation (SpO₂) targeting before and after training and guideline implementation of manual oxygen titration, two cohorts of preterm infants <30 weeks of gestation needing respiratory support and oxygen therapy were compared. The percentage of the time spent with SpO₂ within the target range (85-95%) was calculated (%SpO₂-wtr). SpO₂ was collected every minute when oxygen is >21%. ABCs where oxygen therapy was given were identified and analyzed. After training and guideline implementation the %SpO₂-wtr increased (median interquartile range (IQR)) 48.0 (19.6-63.9) % vs 61.9 (48.5-72.3) %; p < 0.005, with a decrease in the %SpO₂ > 95% (44.0 (27.8-66.2) % vs 30.8 (22.6-44.5) %; p < 0.05). There was no effect on the %SpO₂ < 85% (5.9 (2.8-7.9) % vs 6.2 (2.5-8) %; ns) and %SpO₂ < 80% (1.9 (1.0-3.0) % vs 1.7 (0.8-2.6) %; ns). In total, 186 ABCs with oxygen therapy before and 168 ABCs after training and guideline implementation occurred. The duration of SpO₂ < 80% reduced (2 (1-2) vs 1 (1-2) minutes; p < 0.05), the occurrence of SpO₂ > 95% did not decrease (73% vs 64%; ns) but lasted shorter (2 (0-7) vs 1 (1-3) minute; p < 0.004).

CONCLUSION

Training and guideline implementation in manual oxygen titration improved SpO₂ targeting in preterm infants with more time spent within the target range and less frequent hyperoxaemia. The durations of hypoxaemia and hyperoxaemia during ABCs were shorter. What is Known: • Oxygen saturation targeting in preterm infants can be challenging and the compliance is low when oxygen is titrated manually. • Hyperoxaemia often occurs after oxygen therapy for oxygen desaturation during apnoeas. What is New: • Training and implementing guidelines improved oxygen saturation targeting and reduced hyperoxaemia. • Training and implementing guidelines improved manual oxygen titration during ABC.

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.