Oxygen monitoring in the NICU

Oxygen monitoring and treatment is a vital part of neonatal care and has been used for treatment of cyanosis in preterm infants for over 120 years [1]. From 1900 to 1950, preterm infants were treated with increased levels of inspired oxygen, as it was found to decrease cyanosis and episodes of apnea [1]. During the 1950s, it was found that increasing levels of oxygen led to an increased number of infants with retrolental fibroplasia, now called retinopathy of prematurity (ROP).

This led to restrictions in how much oxygen preterm infants received, which again led to an increase in mortality and rates of cerebral palsy [1]. Since then, there has been debate on the optimal supplementation of oxygen and how to monitor oxygen levels in the preterm infant, and even though many studies have examined the relationship between oxygen levels and neonatal mortality and morbidity, it is still debated how to optimally monitor oxygenation and the optimal oxygen levels [2].

Oxygen monitoring in the preterm infant

Oxygen monitoring can be performed using either invasive or non-invasive methods. Invasive methods include blood gas analysis, while non-invasive methods include pulse oximetry, transcutaneous monitoring and near-infrared spectroscopy/NIRS among others. For the last decades, blood gas analysis has been the gold standard for determining oxygenation levels in the NICU [3]. The arterial blood gas is precise and gives a direct measure of the oxygen levels in the blood but is invasive (either through an arterial puncture or most commonly from an indwelling catheter) and can lead to significant blood loss [3]. One study showed that neonatal blood sampling, with blood gases being the most common blood sample, led to a blood loss of nearly 60% of the endogenous blood volume in the first 2 weeks of life and that increasing rates of blood loss was associated with development of bronchopulmonary dysplasia (BPD) [4].

The capillary blood gas gives good approximations of the arterial blood gas but cannot be used to estimate the partial pressure of oxygen in the blood [5]. Research has shown that preterm infants can be subject to as many as 50 painful procedures over the first 4 weeks of life, with blood gas analyses being one of the most common [6]. As neonatal pain has been associated with adverse neurological outcome, painful procedures, such as capillary sampling, should be limited as much as possible [7].

Pulse oximetry first gained use in the NICU in the 1980s and is now considered standard-of-care in neonatal care to monitor oxygenation [2]. It is instantaneous and non-invasive, and the current generation of sensors have reduced motion errors significantly and increased the clinical reliability [8]. Due to the dissociation curve of hemoglobin, pulse oximeters are most precise in SpO₂ ranges of 70-95% [9]. This means that only relying on pulse oximetry for monitoring neonatal oxygenation carries a risk of overlooking hypoxia and/or hyperoxia, both of which are deleterious to the neonate.

A large study by Wackernagel and colleagues showed the discrepancy between SpO₂ readings and arterial oxygen saturation and oxygen tension in neonates [9]. Among over 27,000 SpO₂/SaO₂ pairs in 1908 patients, 57% of cases showed a PaO₂ < 6 kPa (hypoxia), while the SpO₂ reading was > 90%, and 19% of cases showed a PaO₂ > 11 kPa (hyperoxia) while SpO₂ was < 95%. This means that relying only on SpO₂ for monitoring neonatal oxygenation carries a non-negligible risk of overlooking both hyperoxia and hypoxia. This makes the authors conclude that “pulse oximetry readings did not fulfill the performance requirements for titrating oxygen supplementation in neonatal patients”. Furthermore, pulse oximetry results are dependent on the patient’s skin color, due to the skin’s absorption of near-infrared light. This has led to concerns about racial discrepancy in pulse oximeters [10]. Vesoulis and colleagues showed that there is a “modest but consistent difference in SpO₂ error between black and white infants, with increased incidence of occult hypoxemia in black infants” [11].

Transcutaneous monitoring is also non-invasive and can be used to estimate arterial oxygen and carbon dioxide levels. Transcutaneous monitoring has traditionally been done by placing a heated sensor on the skin that increases the capillary blood flow and amount of oxygen diffusing to the sensor. Due to different diffusion rates, monitoring tcpCO₂ can typically be achieved using lower temperatures of 38-42°C, which is not feasible for tcpO₂, where temperature has to be kept at 43-44°C to achieve precise results [12]. This has fuelled fear about the risk of skin burns on the sensitive neonatal skin, though the reports of burns in recent decades are scarce. Newer generation transcutaneous sensors using optical technology have been developed, though they still need to operate at 42-43°C [13]. Several studies have shown that high transcutaneous oxygen levels and oxygen variability is associated with a higher risk of ROP, and that transcutaneous monitoring of oxygen leads to less oxygen variability than SpO₂ monitoring [14, 15].

NIRS (near-infrared spectroscopy) is non-invasive and uses near-infrared light to estimate regional tissue saturation [16]. It has been used in neonatology primarily to monitor regional cerebral oxygenation, but also to assess splanchnic tissue perfusion and its correlation to the course of necrotising enterocolitis [17]. Although NIRS has been shown to be able to reduce the burden of hypoxia and hyperoxia in preterm infants, it has not yet been proved to reduce neonatal morbidity, though a large multi-center study is undergoing to help answer this [18, 19].

Conclusions:

Titrating the correct amount of oxygen to a neonate is a difficult balance, where many questions about levels of oxygenation and methods to monitor oxygenation are still unanswered. The different monitoring methods contain advantages and disadvantages, which make some authors argue that oxygen monitoring in neonates should ideally consist of a combination of the different methods [20].

References

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