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Table of Contents
Year : 2021  |  Volume : 4  |  Issue : 1  |  Page : 46-49

Pulse oximeter with longer averaging time and missed chronic hypoxia in preterm infants

1 Department of Pediatrics, King Saud University Medical City, King Saud University, Riyadh, Saudi Arabia
2 Department of Pediatrics and Child Health, University of Manitoba, Winnipeg, Canada

Date of Submission03-Sep-2020
Date of Decision07-Oct-2020
Date of Acceptance21-Oct-2020
Date of Web Publication06-Jan-2021

Correspondence Address:
Nasser Saleh Alharbi
Department of Pediatrics, College of Medicine, King Saud University Medical City, King Saud University, Riyadh
Saudi Arabia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/JNSM.JNSM_105_20

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Background: Targeted oxygen saturation in preterm infants has been an area of debate for decades. Mild chronic hypoxia exposes some infants to significant comorbidities like pulmonary artery hypertension (PAH). The pulse oximeters vary in technical properties and setting; pulse oximeters with shorter SpO2 averaging time may provide a more accurate oxygen assessment. Aim: To evaluate the readiness of preterm infants for discharge based on the current unit's protocol which uses standard pulse oximetry with an averaging time of 20s, as opposed to a pulse oximeter with a shorter averaging time (2s). Methods: The study was a prospective observational pilot study included all infants <32 weeks' postmenstrual age (PMA) with no cardiovascular or respiratory pathology other than related to prematurity, such as bronchopulmonary dysplasia (BPD) and persistent ductus arteriosus. All infants underwent Echocardiography studies after the 2nd week of life and after 36 weeks to exclude PAH. All infants older than 36 weeks PMA who were off oxygen and ready to be discharged home as per unit's protocol underwent final oxygen assessment for a minimum of 6 h using motion resistant oximeter with a SpO2 short averaging time of 2s. Results: Thirty-five infants underwent the oxygen pulse oximetry testing. Of them, 42% were found to have chronic hypoxia (defined as 5% of recorded time with SpO2 ≤ 90%) and fulfilled the diagnostic criteria for BPD. Conclusions: A significant number of infants at 36 weeks' PMA with chronic hypoxia were missed using the current unit's oxygen assessment. With the prevalence being higher in infants diagnosed with BPD, a future study must be conducted to investigate the correlation between missed chronic hypoxia in infants with BPD and late-onset PAH.

Keywords: Bronchopulmonary dysplasia, hypoxia, preterm infant, pulmonary hypertension, sleep studies

How to cite this article:
Alharbi NS, Al-Katari AS, Al-Tirkawi K, Al-Faki W, Al-Ghamdi M, Iqbal SM. Pulse oximeter with longer averaging time and missed chronic hypoxia in preterm infants. J Nat Sci Med 2021;4:46-9

How to cite this URL:
Alharbi NS, Al-Katari AS, Al-Tirkawi K, Al-Faki W, Al-Ghamdi M, Iqbal SM. Pulse oximeter with longer averaging time and missed chronic hypoxia in preterm infants. J Nat Sci Med [serial online] 2021 [cited 2023 Mar 23];4:46-9. Available from: https://www.jnsmonline.org/text.asp?2021/4/1/46/306256

  Introduction Top

The oxygen saturation targets in preterm infants has been an area of controversy. Achieving the balance between adequate tissues oxygenation and avoiding oxygen toxicity in preterm infants (<37 weeks postmenstrual age [PMA]) is challenging. Previous evidence suggested that targeting a lower oxygen range of 85%–89% than a higher oxygen range of 91%–95% was associated with a higher risk of death and necrotizing enterocolitis, but a lower risk of retinopathy of prematurity treatment.[1],[2]

Targeting a lower oxygen saturation range (88%–92%) than a higher oxygen saturation range (92%–95%) in extremely premature infant (<29 weeks PMA) was associated with higher incidence of pulmonary artery hypertension (PAH) and elevated pulmonary vascular resistance.[3]

Infants with bronchopulmonary dysplasia (BPD) are at higher risk of developing late-onset PAH than non-BPD preterm infants and may have higher odds of mortality. The overall incidence of PAH among BPD patients is 17% and is higher in infants with severe BPD.[4],[5]

The factors that predispose preterm infants to PAH or BPD are similar: lung hypoplasia, growth restriction, infections, inflammation, and genetics.[6],[7],[8],[9],[10]

Intermittent hypoxia in children with sleep disordered breathing predisposes them to PAH;[11] however, it remains unclear whether it predisposes preterm infants with BPD to PAH. Animal studies suggested that in the high-risk group, even mild hypoxia can elicit striking PAH.[6],[7] In preterm infants with BPD and PAH, oxygen therapy can ameliorate the disease.[12]

The international practice guideline[13] recommends that the oxygen saturation should be maintained within the range 93%–95% and that chronic hypoxia should be avoided in children with BPD defined as either:

  1. Greater than or equal to 5% of recording time spent with an SpO2 ≤ 93% if measurements are obtained by continuous recording or
  2. At least three separate findings of SpO2 ≤93% if measurements are obtained intermittently.

The oxygen assessment tools used included pulse oximeters with variable technical properties. Most oximeters allow users to select SpO2 averaging time from 2 to 16s. The use of longer SpO2 averaging time can lead to less alarms but underestimate the short hypoxic events; hence, previous studies suggested that pulse oximeters with shorter SpO2 averaging time provide a more accurate oxygen assessment.[14],[15],[16]

In our standard unit's practice, preterm infants will be discharged on room air based on oxygen assessment using pulse oximeter with an SpO2 averaging time of 20s.

In this study, we hypothesized that the current standard of using a pulse oximeter with an SpO2 averaging time of 20s may fail to exclude chronic hypoxia at the time of discharge, a practice that may increase the risk of complications such as late PAH. Thus, we proposed a pulse oximeter with shorter averaging time to detect missed chronic hypoxia.

  Methods Top

The study was a prospective observational pilot study. All infants aged ≤32 weeks' PMA admitted to the neonatal intensive care unit at King Khalid University Hospital (KKUH) were eligible for enrollment in the study after obtaining parental informed consent. Patient recruitment was conducted between June 2013 and December 2014, after receiving approval from the local IRB at KKUH. Parents and the treating physicians were aware of the infant group assignments. Consent of participation was obtained from parents before enrollment.

Inclusion criteria

  1. Preterm babies aged <32 weeks' PMA
  2. Infants who are off oxygen supplement and ready for discharge as per standard unit protocol at 36 weeks' PMA
  3. Infants who passed the echocardiography study with no evidence of congenital heart defect, persistent ductus arteriosus (PDA), or PAH conducted at 2nd week of life and at 36 weeks' PMA.

Exclusion criteria

Infants with significant congenital anomalies especially those affecting the cardiac or respiratory systems and those with significant PAH suggested by echocardiography studies that were performed at the 1st week of life and at 36 weeks' PMA were excluded.

Study definitions

Bronchopulmonary dysplasia

It was defined based on the National Institute of Child Health and Human Development (NICHD) criteria. Thus, infants aged <32 weeks PMA and required oxygen supplementation for at least 28 days were classified as BPD patients; otherwise, they were classified as non-BPD infants based on the standard unit protocol for oxygen assessment. The standard unit protocol recommends the use of a pulse oximeter with an SpO2 averaging time of 20s and targets an SpO2 of >90%. The unit standard of oxygen weaning is by using a physiological reduction test.[17]

Chronic hypoxia

It was defined as 5% of recorded time with SpO2 of ≤90%, as per an oxygen determination study performed in infants at 36 weeks' PMA and beyond using a pulse oximetry for a minimum of 6 h, recorded with a motion resistant oximeter, with a short SpO2 averaging time of 2s by Masimo oximeter Radical-7.

Screening and Grading of PAH was by using echocardiography and the degree of PAH was classified into 4 groups: No pulmonary hypertension (HTN): when estimated right ventricular (RV) or mean pulmonary artery (PA) pressure < 30% systemic. Mild: when estimated RV or mean PA pressure 30%–50% systemic. Moderate: when estimated RV or mean PA pressure 50%–100% systemic. Severe: when estimated RV or mean PA pressure >100% systemic. We did direct assessment of RV systolic pressure from TR and simultaneous BP reading. We did direct assessment of PA pressure from PDA and simultaneous BP reading. Estimated mean PA pressure from PR jet with simultaneous BP readings. In infant with insufficient TR, PR and no PDA, the interventricular septal orientation during end-systole was used to indirectly estimate RV systolic pressure as follows: Round inter-ventricular septum at end-systole indicates RV systolic pressure < 50% systemic. Flat inter-ventricular septum at end-systole indicates RV systolic pressure 50%–100% systemic. Inter-ventricular septum bowing toward LV at end-systole indicate RV systole pressure >100% systemic.

Pulmonary artery hypertension

It was defined as an increase in pulmonary arterial pressure that develops later in infancy due to acquired causes; by contrast, persistent pulmonary hypertension was defined as a form of pulmonary HTN that occurs early in infancy as a normal physiological response to high PA pressure during the intrauterine period.

Congenital heart disease

It was defined as presence of any cyanotic or noncyanotic heart anomalies, vascular anomalies, and any heart structural changes that are associated with abnormal heart function except PDA.


All included infants underwent two echocardiography tests. The first one was done during the 2nd week of life and the second one at 36 weeks PMA. All echocardiographs were performed by a single experienced sonographer. A single experienced Pediatric cardiologist reviewed all studies to assess the cardiac anatomy, function, and PA.

A final pulse oximeter test was done for minimum of 6h recording (target: 12-24h recording), using a motion-resistant oximeter with a short SpO2 averaging time of 2s by Masimo oximeter Radical-7 for all infants who were ready to be discharged home off oxygen by the treating team.

Statistical analyses

Continuous variables were expressed as means and ranges, whereas, categorical variables were expressed as percentages. The analysis was performed using Excel program, Microsoft office 2016 software (Microsoft Corporation, Washington, USA). All tests of significance were set at P = 0.05.

  Results Top

A total of 48 preterm infants (40% of 120 preterm infants admitted to our unit for over a 15-month period) were enrolled in this study. Of them, 35 (73%) completed the study. Four of these infants were excluded as they were unable to fulfill the inclusion criteria, while eight were discharged on oxygen supplement. Thirty-five mothers (72.9%) had antenatal supervision visits. The infants' demographic data are shown in [Table 1]. Data regarding interventions and complications are shown in [Table 2]. Furthermore, none of the cohort infants has echocardiographic findings of PAH in any of the study visits; however, 11 premature infants (22.3%) had PDA after the 7th day of life, while 3 of them (6.3%) had a PDA at 36 weeks' PMA [Table 2].
Table 1: Infants' baseline data (n=35)

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Table 2: Interventions and complications (n=48)

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Nineteen infants were diagnosed with BPD, while the rest (16 infants) were classified as non-BPD preterm infants.

The oximeter recording in those who completed the study revealed that 16 infants (45.7%) spend >5% of recording time with SpO2 <90%, while 8 infants (22.9%) spend >10% of the recording time with SpO2 <90%, which is considered significant.

The subgroup analysis showed that the prevalence of having undetected hypoxia was much higher in children diagnosed with BPD [Table 3]. That is, 14 infants with BPD (73.7%) exhibited hypoxia, while only in 2 (14.2%) were classified as non-BPD (P < 0.05).
Table 3: Study results (n=35)

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  Discussion Top

All infants included in this cohort appeared suitable for discharge based on the standard pulse oximetry readings after 36 weeks, using the longer SpO2 averaging time. However, when a shorter SpO2 averaging time (2s) was used, results were quite different. Approximately 45.7% of preterm infants met the study definition of chronic hypoxia; incidence was higher in patients with BPD, and this may explain why some of the BPD patients in this cohort were in need of minimal flow of room air via nasal cannula (data not shown). This finding suggests the presence of a varying degree of hypoxia, which was overlooked using the standard pulse oximeter that utilizes a longer SpO2 averaging time. This finding was more prominent in the high-risk group (i.e., BPD patients) than in the non-BPD patients (89% vs. 12.5%). The inability to determine chronic hypoxia may contribute to the development of late-onset PAH in susceptible infants, such as BPD patients.

Patent ductus arteriosus is a common finding in preterm babies. Approximately 31.1% of the preterm infants included in the study had PDA. At time of discharge, 73% of infants showed resolution of the initial PDA after being treated conservatively, matching the results of a previous large cohort study from the same institute.[18]

Approximately 54.2% of the preterm infants included in the study met the NICHD criteria of BPD diagnosis. Mechanical ventilation was required in 89.4% of these BPD infants and in 7.1% of non-BPD infants, a finding that is consistent with previous reports.

Despite the reports of previous studies indicating the occurrence of PAH in 17% of BPD patients,[4] none of the cohort infants had PAH based on the results of echocardiography studies, which support the evidence that BPD/PAH relationship is progressive.

As a pilot study, several limitations were observed, which include the small sample size and low follow-up rate. The finding indicating that a significant proportion (73.7%) of infants with BPD was deemed fit for discharge and spent more than 5% of time with SpO2 <90% is troublesome. More importantly, the same finding was true for 12.5% of infants who were diagnosed with non-BPD as per the NICHD criteria.

As “physiological” O2 reduction test and NICHD criteria seem inadequate to diagnose hypoxia using a standard pulse oximetry, we suggest reviewing the safety of predischarge testing methods. Further studies with a larger number of patients are needed. Moreover, a follow-up study should be conducted to define the significant end point outcomes (e.g., neurodevelopmental impairments and increased burden of respiratory diseases) for this patient group.

  Conclusions Top

Preterm infants (especially those with BPD) should be assessed for signs of chronic hypoxia. Oxygen needs to be carefully assessed using a more efficient method, especially in those with BPD who still require room air minimal flow support. This group represents a possible risk for future PAH; hence, we suggest that a more accurate oxygen assessment should be taken into consideration.


This work was supported by the College of Medicine Research Center, Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia.

Financial support and sponsorship

College of Medicine Research Center, Deanship of Scientific Research, King Saud University, Riyadh, Saudi Arabia.

Conflicts of interest

There are no conflicts of interest.

  References Top

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Laliberté C, Hanna Y, Ben Fadel N, Lemyre B, Bijelic V, Barrowman N, et al. Target oxygen saturation and development of pulmonary hypertension and increased pulmonary vascular resistance in preterm infants. Pediatr Pulmonol 2019;54:73-81.  Back to cited text no. 3
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Trittmann JK, Gastier-Foster JM, Zmuda EJ, Frick J, Rogers LK, Vieland VJ, et al. A single nucleotide polymorphism in the dimethylarginine dimethylaminohydrolase gene is associated with lower risk of pulmonary hypertension in bronchopulmonary dysplasia. Acta Paediatr 2016;105:e170-5.  Back to cited text no. 6
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  [Table 1], [Table 2], [Table 3]


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