Findings recently published in Nature Scientific Reports by the University of Alabama at Birmingham’s Manimaran Ramani, M.D., indicate that in a rodent model prolonged oxygen exposure during the critical developmental period permanently impairs long-term hippocampal mitochondrial function.
Children and adolescents born preterm who require prolonged oxygen therapy in the neonatal intensive care unit often develop cognitive deficits, attention deficit disorder and autism spectrum disorder.
The exact mechanism by which children and adolescents born preterm develop cognitive and behavioral disabilities is not known. Previously, Ramani, an associate professor in the Division of Neonatology, and his team showed that young adult mice that are exposed to oxygen as newborns develop memory deficits and hyperactivity, findings similar to that of adolescent born preterm.
Although mitochondrial dysfunction and oxidative stress are associated with the pathogenesis of several neurodegenerative disorders such as Parkinson disease, the impact that early oxidative stress and mitochondrial dysfunction have on neurodevelopment is yet to be determined.
The hippocampus is the region of the brain that plays a key role in the formation and maintenance of long-term memory and is highly vulnerable to oxidative stress.
Neurons in the hippocampus are dependent on their mitochondrial function for the strengthening of synapses, a cellular response responsible for long-term memory.
This is the first preclinical study shows that early life oxygen exposure has a permanent negative impact on hippocampal mitochondria.
In their mouse model in which mice pups are exposed to oxygen, Ramani’s team expected a recovery in hippocampal mitochondrial function when assessed at young adult age.
On the contrary, the team was surprised by the fact that hippocampal mitochondrial dysfunction persists even after the initial oxidative stress is long gone.
“Premature infants require oxygen supplementation for their survival and maintaining lower oxygen saturation is known to increase mortality,” Ramani said.
“Hence, research going forward should focus on determining the role of other modalities of therapies such as antioxidants to counteract the toxicity effects of oxygen.”
Oxygen therapy is a life-sustaining intervention and remains the most commonly used drug in the care of sick newborns (Vento and Saugstad 2010).
In the 1940s and 1950s, flooding the newly designed incubators with 100% oxygen contributed to the survival of some premature infants. The first clinical reports, dating back to the early 1950s, linked unrestricted use of oxygen with an increased risk of retrolental fibroplasia (now known as retinopathy of prematurity) (Campbell 1951) and prompted a change in neonatal care practice to restrict oxygen use.
The resultant increase in mortality (Bolton and Cross 1974) was the first evidence of the central importance of adequate but not excessive oxygen delivery to the premature infant.
New evidence from the same period suggested that oxygen can be toxic at the cellular level by generating free radicals (Gerschman et al. 1954).
Over the ensuing decades, research has yielded an understanding that the immature antioxidant defense in newborns (particularly premature infants) predisposes this population to higher oxidative stress and cell damage (Vento et al. 2012; Torres-Cuevas et al. 2017).
Even brief exposure to excessive oxygen used in the resuscitation of newborns in the delivery room may lead to an increased risk of childhood leukemia (Naumburg et al. 2002) as well as adverse physiological adaptation at birth (Comroe 1939; Hutchison 1987; Saugstad et al. 2008).
While the deleterious effects of administering high concentrations of oxygen to newborns in respiratory distress became apparent over half a century ago, determining the optimal fraction of inspired oxygen (FIO2) that would prevent hypoxemia was challenging in the early decades of neonatology, as no reliable method to continuously measure oxygen concentrations in the blood was available.
In the 1980s, pulse oximetry measurement of oxyhemoglobin saturation provided a reliable and easier means to continuously monitor oxygenation compared with transcutaneous oxygen tension measurements (Durand and Ramanathan 1986).
Soon thereafter, it became the standard of care in neonatal intensive care units (NICU) (Van Meter et al. 2017).
Recognizing that a large number of neonates admitted to the NICU receive supplemental oxygen, there has been tremendous interest in determining the safest oxyhemoglobin saturation that would ensure adequate tissue oxygenation while preventing oxygen toxicity. With growing evidence that restricted use of oxygen decreases the incidence of retinopathy of prematurity and bronchopulmonary dysplasia, the American Academy of Pediatrics stated in 2007 that an oxyhemoglobin saturation between 85% and 95% provides a pragmatic range to guide oxygen therapy (American Academy of Pediatrics and the American College of Obstetricians and Gynecologists 2007).
In 2010, the International Liaison Committee on Resuscitation (ILCOR) revised their recommendations to initiate resuscitation of term newborns in room air (Perlman et al. 2010), whereas the optimal FIO2 in the resuscitation of preterm infants remains controversial (Rabi et al. 2015; Kapadia et al. 2017; Oei et al. 2017; Lui et al. 2018).
Through a collaborative effort by the Neonatal Oxygenation Prospective Meta-Analysis (NeOProM) study group (Askie et al. 2011), 5 randomized trials involving a total of ~5000 premature infants examined lower (85%–89%) and higher (91%–95%) saturation targets; unfortunately, the study yielded conflicting and controversial conclusions (Carlo et al. 2010; Schmidt et al. 2013, 2017; Stenson et al. 2013; Manja et al. 2015a; Tarnow-Mordi et al. 2016; Saugstad 2018). In addition, the optimal oxygen target in term neonates with hypoxic respiratory failure or newborns undergoing hypothermia therapy remains unknown and warrants further study. Table 1 summarizes the suggested oxygen therapy in newborns (Oei et al. 2018).
Suggested oxygen therapy in the delivery room and during neonatal intensive care unit (NICU) stay.
|Delivery room: initial FIO2||NICU stay: SpO2 target|
Note: FIO2, fraction of inspired oxygen; SpO2, oxyhemoglobin saturation as measured by pulse oximeter (Oei et al.2018).*Titrate FIO2 to Neonatal Resuscitation Program (NRP) target SpO2.
While the perfect oxygen saturation ranges for term and preterm infants remain uncertain, here we review data regarding the larger questions of the precision and accuracy of the current technology and the limitations of arterial oxygen saturation as a single marker of tissue oxygenation. Several common practices remain unjustified in the literature, including the tendency to more readily increase FIO2 during hypoxic events while taking a more lax approach to decreasing FIO2 when oxygen saturation is high; the continued use of the hyperoxia test in an era of readily available echocardiography; and preoxygenation (prior to intubation) in neonates to prevent oxygen debt (Sola 2008; Sola et al. 2014).
The fetus and basic concepts of oxygen physiology
Aerobic metabolism and maintenance of satisfactory cellular homeostasis are dependent on adequate oxygen delivery to and extraction by the tissues; only when this demand cannot be met does hypoxia ensue. Oxygen delivery, in turn, depends on oxygen content in the blood — influenced primarily by the concentration and percent saturation of hemoglobin — and cardiac output. Important considerations in the fetus include the higher affinity for oxygen in fetal hemoglobin (Hgb F), high Hgb concentrations in term fetuses, and higher cardiac output, factors that provide a substantial margin of safety for oxygenation (delivery is 3 times higher than demand). In addition, properties of Hgb F facilitate unbinding of oxygen in the tissues at lower partial pressures of arterial oxygen (PaO2) (Maurer et al. 1970). Therefore, Hgb F has the ability to more efficiently bind oxygen as the fetal blood circulates through the placenta and to more readily relinquish it within the tissues (Fig. 1).
Several physiological adaptations of the placenta and developing fetus ensure that the fetus is exposed to a relatively hypoxemic environment. Through an elaborate system of shunts (ductus venosus, ductus arteriosus, foramen ovale) and distribution of blood flow, the fetal circulation diverts the richest concentration of oxygenated blood to the most essential organs (the brain and heart) while minimizing blood flow to the lungs. The coronary and cerebral artery oxygen saturations in the fetus range between ≈58% and 65% (PaO2: 25–28 mm Hg) (Rudolph 1979, 2009; Prsa et al. 2014).
While PaO2 has a minimal impact on the oxygen content of blood, the fraction of oxygen dissolved in blood plays an essential role in the physiology of ventilation and cardiovascular regulation (Giussani et al. 2016). The oxygen tension in blood can exert its effect directly through a complex cascade of endogenous mediators that act directly on the vascular smooth muscle cells and by triggering chemoreflexes through the activation of chemoreceptors (Smith and Vane 1966; Ponte and Purves 1974; Giussani 2016). PaO2 has opposing effects in the systemic and pulmonary vasculature. Hypoxia results in increased cerebral blood flow (through cerebral vasodilation) but decreased pulmonary blood flow (through pulmonary vasoconstriction), while hyperoxia leads to cerebral artery vasoconstriction (Wolff 1936; Rudolph and Yuan 1966). Normoxia results in pulmonary vasodilation, but hyperoxia does not cause additional pulmonary vasodilation (Lakshminrusimha et al. 2006).
The amount of blood pumped into the pulmonary circulation is dynamic and changes during fetal development. In a clinical trial, providing 60% oxygen by face mask to pregnant women at 20–26 weeks gestation did not alter fetal pulmonary blood flow, whereas an increase in pulmonary blood flow was appreciated at 31–36 weeks gestation (Rasanen et al. 1998). Early in gestation, the cross-sectional pulmonary vasculature is low, maintaining a high pulmonary vascular resistance (PVR), and the lungs receive only ~13% of the cardiac output at 20 weeks gestation, which increases to 25%–30% at 30 weeks gestation owing to the proliferation of pulmonary vessels with a resultant drop in PVR. Cardiac output to the lungs then drops to 16%–21% near term gestation in response to active hypoxic pulmonary vasoconstriction secondary to the pulmonary vessels developing greater sensitivity to oxygen (Kinsella et al. 1994; Rudolph 2009; Rasanen et al. 1996; Prsa et al. 2014). Therefore, the effect of oxygen varies depending on the gestational age of the infant.
Oxyhemoglobin saturation, oxygen tension, and oxyhemoglobin dissociation equilibrium
Several important discoveries — including the discovery of oxygen in 1772 and hemoglobin in 1840, the demonstration in the 1850s that hemoglobin exists in 2 states (oxygenated and deoxygenated), and the development of analytical spectroscopy and the capacity to detect pulsatile (arterial) blood flow — all contributed to the development of pulse oximeters in the early 1980s (Van Meter et al. 2017). Soon thereafter, owing to its safety profile, its noninvasive nature, and the ability to continuously monitor oxygenation, the use of pulse oximeters became widespread in NICUs across the United States (Vijayakumar et al. 1997).
The differential light absorption of oxyhemoglobin (red spectrum of visible light) and deoxyhemoglobin (infrared spectrum of light) and the distinction between pulsatile and nonpulsatile flow constitute the principles of pulse oximetry. The sensor, which in newborns is applied to an extremity, contains 2 light-emitting diodes (LEDs), infrared and red light, that transmit light through the extremity and are received by a phototransistor on the opposite side of the LEDs. Pulsatile blood flow results in fluctuations in blood volume, thus changing the distance the light travels. The pulsatile component of the red to infrared light modulation ratio is calculated, and a microprocessor with built-in algorithms converts this ratio to pulse oxygen saturation (SpO2) based on a calibration curve (Chan et al. 2013; Tin and Lal 2015). Pulse oximeters are calibrated by data collected from healthy adults and correlated to arterial blood samples tested by co-oximetry (absorbance spectroscopy) that measures actual arterial oxyhemoglobin saturation (SaO2). Hgb F has an absorption comparable to that of predominant adult Hgb A, and therefore SpO2 measurement in newborns does not require a separate calibration (Harris et al. 1988; Rajadurai et al. 1992).
Hgb functions to carry blood to the tissues, and its configuration changes depending on whether it is in the deoxygenated (also known as tense) or oxygenated (relaxed) state. Once oxygen binds the protein, an alteration in its configuration facilitates further oxygen binding. Conversely, as oxygen is released from Hgb, a change in configuration towards the tense state enables unloading of oxygen (Bell 1999). This ability of Hgb to alter its structure promotes the binding and unloading of oxygen. In addition, the affinity of oxygen to bind Hgb depends on the partial pressure of oxygen in the blood. At higher PO2 (as in the pulmonary circulation), oxygen freely binds Hgb until it is fully saturated, whereas at the level of the tissues, oxygen is quickly released. This relationship can be better appreciated when plotting PaO2 against SaO2/SpO2 (or CaO2: arterial oxygen content), revealing the characteristic oxyhemoglobin dissociation curve (Fig. 1).
In a study to identify a better marker of oxygenation between oxyhemoglobin saturation (SaO2) and oxygen tension (PaO2) in hypoxemic patients, Ahmed et al. made strong arguments in favor of SaO2 (Ahmed et al. 2017). Over 70 000 arterial blood gases collected from pediatric patients with cyanotic congenital heart disease were analyzed, and PaO2 was plotted against SaO2. Owing to the nonlinear relationship between PaO2 and SaO2, a narrow range of PaO2 (30–35 mm Hg) in the hypoxemic range corresponds to a wide range of SaO2 values (28%–85%). In summary, since (i) SaO2 predominantly determines oxygen content and (ii) in hypoxemia the magnitude of SaO2 changes is greater than that of PaO2 changes, SaO2 serves as the better marker of oxygenation. However, as Hgb becomes fully saturated, any further rise in PaO2 does not alter SaO2 (representing the plateau in the oxyhemoglobin dissociation curve). Therefore, patients who receive supplemental oxygen and have SpO2 values in the high 90s may reach supraphysiological PaO2 values (>100 mm Hg), putting them at risk of oxygen toxicity.
An important consideration, particularly in extremely premature infants, is the effect blood transfusions have on the PaO2–SaO2 relationship (Fig. 1). Hgb F, the predominant hemoglobin in newborns, has a high oxygen affinity and, as a result, a lower P50 (partial pressure of oxygen at which 50% of Hgb is saturated by oxygen) of ≈18–19 mm Hg, whereas adult Hgb A has a P50 of ≈26–27 mm Hg (Emond et al. 1993; Maurer et al. 1970). Following transfusion of packed red blood cells (≈27 mL/kg) in extremely premature infants, Hgb F dropped from a mean baseline of 92% to 43%, while the value of P50 increased from 18.5 to 21 mm Hg (Fig. 1) (De Halleux et al. 2002). Consequently, if SpO2 were maintained at a constant value following adult blood transfusion, the newborn’s post-transfusion PaO2 would be higher. Therefore, in premature infants, SpO2 values in the upper 90s following several adult blood transfusions pose an even greater risk for oxygen toxicity, as PaO2 may approach values >100 mm Hg (Fig. 2) (Shiao 2005).
Goals of oxygen therapy
The role of oxygen therapy in hypoxemia is to (i) provide sufficient oxygen to the tissues, thus avoiding anaerobic metabolism and lactic acidosis; (ii) prevent hypoxic pulmonary vasoconstriction, which would further exacerbate hypoxemia and lead to hypoxia; and (iii) promote brain and somatic growth. The ideal oxygen therapy would meet these goals while minimizing formation of oxygen free radicals (Fig. 4).
In determining the lower oxygenation limit, 2 main factors need to be considered: (i) the oxygen level when hypoxic PVR becomes significant and (ii) the critical point when oxygen delivery (DO2) cannot sustain oxygen consumption (VO2). The alveolar partial pressure of oxygen (PAO2) has the greatest impact on pulmonary vascular reactivity (Moudgil et al. 2005). However, as PAO2 is not directly measured in clinical settings, PaO2 and SpO2 cutoffs have been evaluated in newborn animal models, which have shown a significant rise in PVR below a PaO2 of ≈45–50 mm Hg (Rudolph and Yuan 1966; Lakshminrusimha et al. 2009); in a lamb model of pulmonary hypertension induced by fetal ductus arteriosus ligation, maintaining SpO2 between 90% and 97% resulted in low PVR (Lakshminrusimha et al. 2009). The oxygenation target to identify the critical point below which VO2 decreases as DO2 decreases is more difficult to ascertain given that Hgb concentration has a greater impact on arterial oxygen content (CaO2) than oxyhemoglobin saturation, and because there are currently no reliable means to measure cardiac output continuously. Unpublished pooled data from newborn lamb experiments identified the CaO2 at ≈12 mL O2/mL when there is a notable reduction in the arteriovenous difference in oxygen. Once DO2 falls below the critical point, anaerobic metabolism and lactic acidosis ensue, consistent with tissue hypoxia. For example, a patient with a Hgb concentration of 10 g/dL who has an SpO2 of 85% (CaO2 ≈ 11.5 mL O2/mL) may fall below the critical point if cardiac output is low. An SpO2 of 93% under the same conditions would yield a CaO2 of ≈12.5 mL O2/mL, whereas an increase in Hgb to 13 g/dL (following a blood transfusion) would increase the CaO2 to ≈16 mL O2/mL. One needs to be mindful, however, that as Hgb drops, a compensatory increase in cardiac output can maintain oxygen delivery and thus prevent reaching of the critical point. An ongoing study funded by the NICHD (through the neonatal research network) and the NHLBI, the Transfusion of Prematures (TOP) trial (https://clinicaltrials.gov NCT01702805), aims to evaluate whether a liberal Hgb threshold (13 g/dL during the first week of postnatal life on respiratory support) or a restrictive strategy (Hgb of 11 g/dL) for blood transfusion improves neurological outcomes in preterm infants.
An interesting observation in the SUPPORT trial was that the increased mortality among infants randomized to the lower target SpO2 did not occur until after the first 2 postnatal weeks (Carlo et al. 2010). Vento et al. have speculated that the postnatal decrease in Hgb (especially in NICUs where there is low Hgb transfusion threshold) may result in low CaO2 values in preterm infants managed with low target SpO2 values (Fig. 5) (Vento 2014). Promoting placental transfusion at birth, minimizing iatrogenic blood loss, and using strategies to maintain higher Hgb levels should increase CaO2.
The upper limit of oxygenation should represent the level of oxygenation that results in toxicity, which in return leads to adverse effects (retinopathy of prematurity, bronchopulmonary dysplasia, and poor neurodevelopment). Experimental studies have shown that targeting PaO2 >80 mm Hg does not result in additional pulmonary vasodilation (Rudolph 1979; Lakshminrusimha et al. 2006, 2009). Furthermore, PaO2 >100 mm Hg following excessive use of oxygen in the resuscitation of babies with hypoxic–ischemic encephalopathy has been shown to be associated with poor neurodevelopmental outcomes (Kapadia et al. 2013).
SpO2 target limits
The final results from the NeOProM collaboration were recently reported (Askie et al. 2018). In this analysis, there was no significant difference between the lower SpO2 (85%–89%) and higher SpO2 (91%–95%) target range on the primary composite outcome of death or major disability at a corrected age of 18–24 months. However, patients who were randomized to the lower SpO2 target range were shown to have a higher risk of death and necrotizing enterocolitis, but a lower risk of retinopathy of prematurity (Askie et al. 2018). The results from this study suggest that an SpO2 target range of 91%–95% may be safer than a range of 85%–89% in extremely preterm infants (<28 weeks gestation) based primarily on an observed increase in the risk of death associated with the lower range (Bizzarro 2018).
Current evidence suggests that the safest SpO2 range in term infants with hypoxic respiratory failure and persistent hypertension of the newborn is 90%–95% (Vali and Lakshminrusimha 2017). A multi-center randomized study comparing SpO2 ≥94% to ≥90% in infants suffering from bronchiolitis has demonstrated that infants who were maintained at an SpO2 closer to 90% had less need for oxygen supplementation (56% versus 73%), a lower duration of oxygen use (5.7 h versus 27.6 h), and were discharged from the hospital sooner (40.9 h vs. 50.9 h) (Cunningham et al. 2015). Finally, in a large adult trial, patients who were expected to be admitted for >72 h in the intensive care unit were randomized to receive oxygen therapy to maintain PaO2 between 70 and 100 mm Hg (SpO2 between 94% and 98%; conservative group) or to allow PaO2 values up to 150 mm Hg (Spo2 values between 97% and 100%; conventional control group) (Girardis et al. 2016). Patients who were maintained at the lower oxygen range had lower risk of mortality, shock, liver failure, and bacteremia. Although children are not small adults and neonates (especially preterm) are not small children, from a physiological standpoint there may be some similarities in oxygen saturation targets across age groups while individuals receive supplemental oxygen (Fig. 6).
More information: Manimaran Ramani et al. Early Life Supraphysiological Levels of Oxygen Exposure Permanently Impairs Hippocampal Mitochondrial Function, Scientific Reports (2019). DOI: 10.1038/s41598-019-49532-z
Journal information: Scientific Reports
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