4.1 Oxygen Therapy & Non-Invasive Respiratory Support

Oxygen Therapy & Non-Invasive Respiratory Support

Physiological Principles of Neonatal Oxygenation

Oxygen delivery (DO2) to neonatal tissues depends on cardiac output, hemoglobin concentration, and oxygen saturation. While term infants possess mature respiratory control and antioxidant defenses, preterm infants are born in a state of developmental vulnerability. In utero, the fetus exists in a relatively hypoxic environment (fetal PaO2 is approximately 25 to 30 mmHg), which is essential for normal organ development, particularly vascular development. At birth, the transition to extrauterine life exposes the neonate to room air (21% oxygen), causing a rapid increase in arterial oxygenation. When supplemental oxygen is introduced during resuscitation or ongoing care, it must be managed with extreme precision to avoid the cellular injuries associated with oxygen toxicity.

Oxygen Toxicity: Pathophysiologic Pathways and Free Radicals

Oxygen toxicity is mediated by the generation of reactive oxygen species (ROS) or free radicals, such as superoxide anions, hydrogen peroxide, and hydroxyl radicals. Preterm infants have significantly lower levels of protective antioxidant enzymes (e.g., superoxide dismutase, catalase, and glutathione peroxidase), which typically double in concentration during the last weeks of gestation. Excess oxygen exposure leads to oxidative stress, initiating cellular necrosis, apoptosis, lipid membrane peroxidation, and inflammatory cascades.

Retinopathy of Prematurity (ROP)

Retinopathy of Prematurity is a biphasic vascular disease of the retina directly linked to hyperoxia and fluctuating oxygen levels:

• Phase 1 (Vaso-obliteration): Exposure of the immature retina to hyperoxia causes vasoconstriction and ceases the normal outward migration of blood vessels from the optic nerve toward the periphery. This hyperoxic exposure downregulates vascular endothelial growth factor (VEGF).
• Phase 2 (Vasoproliferation): As the infant matures and metabolic demands of the peripheral retina increase, the unvascularized retina becomes severely hypoxic. This tissue hypoxia triggers a massive upregulation of VEGF and erythropoietin, driving abnormal, disorganized neovascularization (new vessel growth). These fragile vessels can leak, bleed, cause vitreous hemorrhage, and lead to fibrotic scarring. In severe cases, this leads to tractional retinal detachment and permanent blindness.

Bronchopulmonary Dysplasia (BPD)

In the developing lung, hyperoxia and free radicals disrupt the alveolarization process. Exposure to high concentrations of oxygen damages the alveolar-capillary membrane, leading to increased vascular permeability, pulmonary edema, and inflammatory cell infiltration. This alveolar damage inhibits the normal division of primitive saccules into mature alveoli, resulting in fewer, larger alveoli with a significantly reduced surface area for gas exchange. Fibrotic changes and muscularization of pulmonary arterioles also occur, predisposing the infant to chronic lung disease and pulmonary hypertension.

Monitoring and Oxygen Saturation Targets

Managing neonatal oxygenation requires strict adherence to evidence-based oxygen saturation (SpO2) targets. Clinical trials (such as SUPPORT, BOOST-II, and COT) comparing lower targets (85%–89%) to higher targets (91%–95%) demonstrated that while lower targets reduced ROP, they significantly increased mortality and necrotizing enterocolitis (NEC). Conversely, higher targets increased ROP but decreased mortality. Consequently, international consensus guidelines recommend targeting an ongoing SpO2 of 90% to 95% for infants born at less than 32 weeks gestation.

To ensure compliance with these targets, oximeter alarm limits must be set tightly and adjusted dynamically:

• Low Alarm Limit: Set at 89% to prevent hypoxemia.
• High Alarm Limit: Set at 95% (or 96% depending on institutional policy) to prevent hyperoxia when the infant is receiving supplemental oxygen. If the infant is in room air (21% FiO2), a high alarm is not clinically necessary.

Delivery Room Transition Targets

During the immediate post-natal transition, SpO2 levels rise slowly over the first 10 minutes. Resuscitation guidelines (NRP) dictate targeting pre-ductal saturations (measured on the right hand or wrist) that mimic this normal physiological curve:

Resuscitation should begin with room air (21% FiO2) for infants >= 35 weeks and 21% to 30% FiO2 for infants < 35 weeks, adjusting the blender up or down to remain within these transition ranges.

Non-Invasive Respiratory Support Modalities

Low-Flow Nasal Cannula (LFNC)

Low-flow nasal cannula is used for infants with mild respiratory distress or those weaning from higher support.

• Flow Rates: Typically 0.01 to 2 L/min.
• Blender Necessity: Supplemental oxygen delivered via cannula must always be blended with medical air. Utilizing 100% wall oxygen is an exam trap; an air-oxygen blender must be used to dial in the precise FiO2 (21%–100%).
• Pressure Delivery: LFNC does not provide reliable or measurable positive end-expiratory pressure (PEEP).

Heated Humidified High-Flow Nasal Cannula (HFNC)

HFNC delivers heated and humidified gas at higher flow rates, providing a modest and variable amount of distending pressure.

• Flow Rates: 2 to 8 L/min (typically initiated at 3 to 5 L/min).
• Conditioning of Gas: Gas must be heated to 37°C and humidified to 100% relative humidity (44 mg H2O/L). This prevents mucosal drying, thick secretions, ciliary paralysis, and airway cooling, which can trigger bronchospasm.
• Cannula Sizing: The outer diameter of the nasal prongs must not exceed 50% to 60% of the internal diameter of the infant's nares, with an absolute upper limit of 80%. A tight seal prevents gas escape, converting the open high-flow system into a closed pressurized system, which can generate excessively high pressures and cause air leak syndromes (pneumothorax, pulmonary interstitial emphysema).

Continuous Positive Airway Pressure (CPAP)

CPAP delivers a constant positive pressure to the airway during spontaneous breathing, maintaining lung volume and improving gas exchange.

• Mechanism of Action: CPAP recruits atelectatic alveoli, increases functional residual capacity (FRC), splints the upper airway and chest wall, and reduces the work of breathing.
• Modes of Delivery:
  • Bubble CPAP: Gas flows through the circuit and vents underwater. The depth of the tube in the water determines the pressure (e.g., 6 cm depth = 6 cmH2O PEEP). The bubbles create high-frequency pressure oscillations (chatter) that are transmitted to the infant's lungs, which may improve CO2 clearance and alveolar recruitment.
  • Ventilator-Derived CPAP: Delivered by a mechanical ventilator using a variable or constant flow system.
• Typical Settings: Pressures are started at 5 to 6 cmH2O and titrated up to 8 to 10 cmH2O based on work of breathing, grunting, retracting, and FiO2 requirements.

Nursing Management and Clinical Complications

• Nasal Septum Integrity: Nasal interfaces (prongs or masks) put infants at high risk for tissue necrosis, septal deviation, and columella loss. Nurses must inspect the nose every 2–4 hours, ensure correct prong sizing (avoiding tight fit), maintain a 1–2 mm space between the prong bridge and the septum, and rotate between prongs and masks. Protective barriers (e.g., pectin-based barriers) must be applied.
• Gastric Decompression ("CPAP Belly"): Constant pressure causes gas to enter the stomach, leading to abdominal distention, diaphragmatic splinting, and feeding intolerance. An orogastric tube (OGT) must be placed and left uncapped (vented) to continuously decompress the stomach.

Clinical Scenarios & Exam Traps

• Ductal-Dependent Congenital Heart Defects: In ductal-dependent cardiac lesions (e.g., Hypoplastic Left Heart Syndrome, transposition of the great arteries), administering high levels of oxygen is contraindicated. Oxygen is a potent pulmonary vasodilator. Dropping pulmonary vascular resistance (PVR) leads to massive pulmonary overcirculation ("pulmonary steal"), depleting systemic perfusion and resulting in hypotension, lactic acidosis, and shock. Target SpO2 in these infants is restricted to 75% to 85%.
• PPHN vs. Ductal Defects: In contrast, in Persistent Pulmonary Hypertension of the Newborn (PPHN), high oxygen levels (target SpO2 > 95%) are used therapeutically to decrease PVR and resolve right-to-left shunting.