Sankaran Krishnan, MD, MPH
•Hypoxemia: Decreased partial pressure of oxygen in the blood
•Hypoxia: Decreased oxygen content in the tissues
•Normobaric oxygen therapy: Administration of oxygen under atmospheric pressure
•Hyperbaric oxygen: Delivery of oxygen under pressures that exceed atmospheric pressure
•High-flow oxygen: Delivery of oxygen at markedly higher flow rates than traditional flow rates used for oxygen therapies
Causes of Hypoxia and Hypoxemia
The causes of hypoxia and hypoxemia can be broadly grouped into 4 categories:
•Ventilation-perfusion mismatch: Resulting from differential ventilation and perfusion of the lungs, such as pneumonia, bronchiolitis, and atelectasis
—Central or neurogenic conditions, such as central hypoventilation syndromes, encephalopathies, or opioid or sedative overdose
—Neuromuscular conditions, such as spinal muscular atrophy, myopa-thies, myasthenia gravis
•Diffusion defect: Diseases or conditions that affect the alveolar-capillary barrier, such as interstitial lung disease
•Cytotoxic: Diseases or conditions that affect cellular respiration, such as cyanide toxicity
Determinants of Oxygen Delivery to Tissues
•The oxygen dissociation curve (Figure 111-1) illustrates the relationship between partial pressure of oxygen (PO2) and the percentage of hemoglobin saturation with oxygen (SpO2).
•The relationship between PO2 and SpO2 is not linear, but S-shaped. At higher PO2 levels, the curve flattens out, indicating that at higher PO2 levels, there is little incremental increase in SpO2.
•Fever, acidosis, and increased levels of diphosphoglycerate reduce the affinity for hemoglobin with oxygen, creating a “shift to the right” for the curve, which leads to unloading of O2 to the tissues.
•Oxygen delivery devices are compared in Table 111-1.
•Traditionally, maximum flows of 0.5–1.0 L/min for delivery of oxygen via a nasal cannula are used in newborns, and 2–4 L/min for older children and adults are used to prevent discomfort and drying of the nasal mucosa and other nasal mucosal complications.
•Oxygen is one of the most common interventions in respiratory disorders. However, if administered incorrectly, it can have clinically significant deleterious effects on the lung and other organs; therefore, like any prescribed medication, risks and benefits must always be carefully considered.
•Oxygen therapy should be monitored and dosage adjusted as per need, with the goal of timely discontinuation.
•Oxygen can be administered by using variable-flow or fixed-flow devices, at low or high flow rates. In variable flow systems, oxygen delivery is a function of entrainment of room air from the patient’s own inspiratory flow rate and volume, mixing with the delivered oxygen.
Adverse Effects of Oxygen Therapy
•Retinopathy of prematurity in neonates born preterm
•CO2 narcosis: Oxygen administration only corrects hypoxemia and not ventilatory failure with increased CO2. It is particularly important in those who have hypoventilation from neuromuscular weakness, where injudicious O2 therapy can mask impending respiratory failure or arrest.
•Possible increase of pulmonary blood flow, which can be deleterious in patients with certain cardiac pathologies
•Oxygen therapy is associated with free radical production and subsequent damage to lung tissue, even with relatively short durations of oxygen therapy.
•Persistent O2 therapy, especially at high inspired concentrations (fraction of inspired oxygen), can lead to pulmonary fibrosis and perhaps increase the risk for atelectasis.
Home Oxygen Use
•Oxygen for home use is typically delivered via nasal cannula for long-term use.
•Oxygen is also delivered to children with tracheostomies via “trach collar.”
•It can be used continuously or as pulsed (on-demand) therapy.
•There are three forms of domiciliary O2 delivery (Table 111-2).
—Compressed oxygen cylinders (large [“H/K” type], medium [“E” type], and portable types).
—O2 concentrators, stationary or portable. While there are exceptions, oxygen concentrators do not typically deliver oxygen at concentrations >40%.
Use of Oxygen While Flying
•Current U.S. Federal Aviation Administration rules do not permit travelers to carry their own oxygen tanks or liquid oxygen aboard commercial aircraft. Instead, patients may use a Department of Transportation– approved battery-powered portable oxygen concentrator.
•A prescription from a health care provider is necessary.
•These portable concentrators are available for rent from most home-care providers.
•Travelers are expected to bring enough 12-cell batteries for 1.5 times the anticipated duration of the flight.
•The clinician should be aware that commercial aircraft are typically pressurized up to an altitude of 8,000 feet only, beyond which there may be a drop in O2 saturations.
High-Flow Oxygen Therapy
•High-flow oxygen therapy is most often referred to as high-flow nasal cannula (HFNC) and is a relatively new noninvasive ventilation therapy delivered via a variety of devices.
•Current literature is limited, but it has been suggested that HFNC may be a relatively safe, well-tolerated, and feasible method for delivering oxygen to children with few adverse events reported.
•High flow is usually defined as a flow rate of ≥4 L/min but ranging from 4 to 20 L/min, with some studies reporting flows as high as 70 L/min.
•High-flow oxygen therapy may reduce the need for less tolerated and more invasive respiratory supports, such as continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP) and mechanical ventilation.
•In children, HFNC has been most frequently used for infants and young children who are hospitalized with bronchiolitis, particularly in patients who have respiratory distress despite receiving oxygen therapy who do not tolerate CPAP or BiPAP.
•There are reports of HFNC use in congenital heart disease, obstructive sleep apnea, and pulmonary edema.
•The use of HFNC in clinical practice continues to increase, particularly in inpatients. It is too soon to tell if home use will increase.
Possible Mechanisms of Action of HFNC
The mechanisms of action of HFNC are incompletely understood, because the flow rates delivered are greater than the normal minute ventilation of patients. Some possible effects of HFNC include
• Nasopharyngeal dead space washout
• Reduction of inspiratory resistance and work of breathing
• Providing positive end-expiratory pressure to the lungs
• Improvement of airway conductance and pulmonary compliance by providing adequate heating and humidification, thereby reducing the effects of dry air
Pressure Generated by HFNC
•The amount of pressure generated by HFNC is variable, depending on the flow rate, the diameter of the nasal cannula compared to the nares, the weight and size of the patient, and whether or not the mouth is closed.
•Recent studies have demonstrated limited pressure delivery as measured in the pharynx and esophagus, ranging from 2 to 4 cm H2O in both children and adults.
Patient Comfort with HFNC
•Studies in children outside the neonatal period and in adults have shown HFNC to be more comfortable and associated with less dyspnea and mouth dryness than oxygen delivered via face mask.
•Improved patient tolerance may be one of the reasons for the increasing use of high-flow oxygen therapy, despite the lack of conclusive evidence for clinical effectiveness.
Adverse Effects and Safety of HFNC
•Most studies demonstrate no or minimal adverse events.
•There are anecdotal reports of pneumothorax in younger infants, with flows exceeding 10 L/min, abdominal distention, and nasal bleeding.
•Unlike CPAP, there is no regulatory pressure relief valve; pressure effects may be seen if there is a minimal leak or no leak.
•Oxygen Therapy for Children (American Thoracic Society). www.thoracic.org/patients/patient-resources/resources/oxygen-therapy-children.pdf
•Oxygen Therapy for Children (World Health Organization). apps.who. int/iris/bitstream/10665/204584/1/9789241549554_eng.pdf
•When Baby Needs Oxygen at Home (American Academy of Pediatrics). www.healthychildren.org/English/ages-stages/baby/preemie/Pages/When-Baby-Needs-Oxygen-At-Home.aspx
•Home oxygen therapy can be helpful in the care of children with a wide variety of chronic respiratory diseases.
•Oxygen therapy is not benign and should only be prescribed when all risks and benefits are considered, with adequate supervision and monitoring.
•Pediatric pulmonologists should be involved in the care of all children prescribed home oxygen.
•HFNC is a relatively new approach, which is predominantly used in hospitals; recently, it appears to be finding its place in the homes of select patients. The safety of home high-flow oxygen therapy remains unclear.