Although design plays an important role in selection of oxygen delivery devices, clinical assessment and performance ultimately determine which device should be selected.
By Kenneth Miller, MEd, RRT-ACCS, NPS, AE-C, FAARC
Oxygen administration is routinely utilized on the majority of patients admitted the emergency room or ICU with respiratory distress. Indications for oxygen administration include hypoxemia, increased working of breathing, and hemodynamic insufficiency. The overall goal of oxygen therapy administration is to maintain adequate tissue oxygenation while minimizing cardiopulmonary work. Signs of inadequate oxygenation include tachypnea, accessory muscle work, dyspnea, cyanosis, tachycardia and hypertension. Oxygen administration can also be utilized for chronic administration for patients with advance cardiopulmonary disease and can be administered during diagnostic assessment or assessment.
Currently, there is a wide array of oxygen delivery devices available to the respiratory therapist to utilize for administration. The choice of oxygen delivery devices depends on the patient’s oxygen requirement, efficacy of the device, reliability, ease of therapeutic application and patient acceptance. Although design plays an important role in selection of these devices, clinical assessment and performance ultimately determine how and which device should be selected.
Oxygen delivery devices range from very simple and inexpensive designs to more complex and costly. Oxygen percentage delivery can be inconsistent or precise depending on the type of administration device selected. Oxygen administration can be delivered via low-flow or high-flow systems, with humidity or not, and with a reservoir or not. Monitoring of oxygen delivery effectiveness includes arterial blood gas analyses, oxygen saturation monitoring, and clinical assessment. Oxygen can be considered toxic if percentages are delivered greater than 60%, and in the chronic carbon dioxide retention patient population it may diminish ventilator drive and produce life threatening hypercarbia. It can also cause absorption atelectasis by washing out nitrogen gas when delivered in high concentrations.2
Oxygen delivery devices have historically been categorized into three basic types based on their design: low-flow, reservoir, and high-flow. Regarding the inspiratory oxygen fraction (FiO2) range, oxygen systems can be divided into those indicated for low oxygen (<35%), moderate delivery (35%-60%) or high delivery (>60%). Some devices can deliver a wide range of oxygen percentages.3 When selecting an oxygen delivery device the respiratory therapist must address two key questions. First, how much oxygen can the device deliver? Second, is the FiO2 delivery consistent, or can it vary with changing respiratory patterns?
A review of the different oxygen delivery devices, clinical indications, and utilization will follow.
Typical low-flow oxygen systems provide supplemental oxygen often less than the patient’s total minute ventilation. Because the patient’s minute ventilation exceeds flow, the oxygen delivered by the device will be diluted with ambient air and thus the inspired oxygen delivery is less than anticipated. Low-flow oxygen delivery systems consist of nasal cannula, nasal catheters, and transtracheal catheters.
The standard nasal cannula delivers an FiO2 of 24-44% at supply flows ranging from 1-8 liters per minute (LPM). The formula is FiO2 = 20% + (4 × oxygen liter flow). The FiO2 is influenced by breath rate, tidal volume and pathophysiology.4 The slower the inspiratory flow, the higher the FiO2; the faster the inspiratory flow, the lower the FiO2. Since the delivered oxygen percentage is very inconsistent during respiratory distress, a nasal cannula is not recommended for acute severe hypoxemia or patients that breathe on a hypoxic drive where too high of an oxygen concretion may lead to respiratory depression. A nasal cannula utilizes no external reservoir of oxygen and relies on the patient’s upper airway as an oxygen reservoir. A humidification device is recommended for flows greater than 4 LPM to insure humidification of the dry inspired gas.5 Even with humidity, added flows 6-8 LPM can cause nasal dryness and bleeding. The best clinical indications for the nasal cannula are for patients who have a relatively stable respiratory pattern, who require low oxygen percentage, or who need supplemental oxygen during an operative or diagnostic procedure, or for chronic home care.
A nasal catheter is a soft paste tube with several holes at the tip. It is inserted into a nare, which needs to be changed every eight hours. This device has been replaced by the nasal cannula but it can be used for a patient that is undergoing an oral or nasal procedure.
Transtracheal catheters deliver oxygen directly into the trachea. There are washout and storage effects that promote gas exchange, as well as provide high-flow oxygen. High-flow transtracheal catheters may reduce the work of breathing and augment CO2 removal in the chronic oxygen user. Transtracheal oxygen therapy improves the efficiency of oxygen delivery by creating an oxygen reservoir in the trachea and larynx. Consequently, mean oxygen savings amount to 50% at rest and 30% during exercise. Transtracheal oxygen reduces dead space ventilation and inspired minute ventilation while increasing alveolar ventilation slightly, which may result in a reduction of the oxygen cost of breathing. As a result, patients using this device may experience improved exercise tolerance and reduced dyspnea.6 This delivery device is best used for home care and ambulatory patients who require long periods of mobility and do not feel comfortable wearing a nasal cannula.
Reservoir systems incorporate a mechanism for gathering and storing oxygen during inspiration and exhalation. Patients draw from the oxygen reservoir anytime their minute ventilation flow exceeds the device delivery flow. Types of reservoir devices include cannula and masks.
Reservoir cannulas improve the efficiency of oxygen delivery. These devices are designed to conserve oxygen. Hence, patients may be well oxygenated at lower flows. Liter flows up to 8 LPM have been reported to adequately oxygenate patients with a high-flow requirement. It has been concluded that the reservoir cannula provides effective oxygen delivery to patients at supply flows substantially less than the standard nasal cannula. The reservoir can be located under the nasal cannula or hang as a pendant around the patient’s neck. The device is aesthetically acceptable to patients and its widespread use in patients requiring chronic oxygen therapy could bring about significant financial savings.7 Similar to transtracheal oxygen, this device is best employed on chronic oxygen users who wish a greater degree of mobility than traditional oxygen systems provide.
To increase the oxygen concentration delivered, often a mask reservoir is utilized. The volume of the facemask is approximately 100-300 cm3 depending on size. It can deliver an FiO2 of 40-60% at 5-10 liters.8 The FiO2 is influenced by breath rate, tidal volume and pathology. A flow rate of greater than 5 LPM must be set to ensure the washout of exhaled gas and carbon dioxide retention. The mask is also indicated in patients with nasal irritation or epistaxis. It is also useful for patients who are strictly mouth breathers. However, the mask can be obtrusive, uncomfortable, and confining. It muffles communication, obstructs coughing, and impedes eating. It can also mask aspiration in the semi-conscious patient. A simple mask should be administered for only a few hours because of the low humidity delivered and the drying effects of the oxygen gas. This device is best used for short-term emergencies, operative procedures, or for those patients where a nasal cannula is not appropriate.
The non-rebreathing facemask is indicated when an FiO2 >40% is desired and for acute desaturation. It may deliver an FiO2 up to 90% at flow settings greater than 10 liters. Oxygen flows into the reservoir at 8-15 liters, washing the patient with a high concentration of oxygen. Its major drawback is that the mask must be tightly sealed on the face, which is uncomfortable and drying. There is also a risk of CO2 retention if the mask reservoir bag is allowed to collapse on inspiration. Humidification is difficult with this device, because of the high-flow required and the possibility of the humidifier popping off. This device is best utilized in acute cardiopulmonary emergencies where high FiO2 is necessary. Its duration should be less than four hours, secondary to inadequate humidity delivery and to variable FiO2 for patients who require a precise high oxygen percentage.9
High-flow oxygen delivery systems supply a given oxygen concentration at a flow equaling or exceeding the patient’s inspiratory flow demand. Often an air-entrainment or a blending system is used. As long as the delivered flow exceeds the patient’s total flow, an exact delivered FiO2 can be achieved.
A Venturi mask mixes oxygen with room air, creating high-flow enriched oxygen of a desired concentration. It provides an accurate and constant FiO2 despite varied respiratory rates and tidal volumes. FiO2 delivery settings are typically set at 24, 28, 31, 35 and 40% oxygen. The Venturi mask is often employed when the clinician has a concern about CO2 retention or when respiratory drive is inconsistent. The addition of humidification is not necessary with this device, secondary to the large amount of ambient entrainment that occurs to ensure the exact FiO2 is delivered.10 The Venturi mask is often utilized in the COPD patient population where the risk of knocking out the patient’s hypoxic drive is of concern.
An aerosol-generating device will deliver anywhere from 21 to 100% FiO2 depending on how it is set up. The flow is usually set at 10 LPM and the desired FiO2 is selected by adjusting an entrainment collar located on top of the aerosol container. The humidity device is connected to the flow meter, and wide bore tubing connects this to the patient’s mask. Wide bore tubing and the reservoir bag are placed in line to act as an oxygen reservoir to ensure an exact high FiO2 is delivered. This device adds water content to the patient and can assist in liquefying retained secretions. This oxygen delivery option is ideal for patients with tracheotomies because it allows for inspired air to be oxygenated, humidified, and even heated if necessary. They can be hooked up to a mask, tracheotomy mask, and even a T-piece. If the patient’s flow exceeds the total flow delivered (ambient entrainment and 10 LPM), the patient may retain CO2 and the FiO2 may be lower than desired.11 During inhalation, an aerosol mist should be seen coming from the mask or reservoir. To ensure accurate oxygen administration via this system, an oxygen analyzer should be used. This device can be used to ensure a precise oxygen delivery and also maintain humidification of artificial airways.
A relatively new oxygen delivery device is a high-flow nasal cannula (HFNC) system. Nasal oxygen has been administered at flows ranging from 10-60 liters. When this oxygen is warmed to body temperature and saturated to full humidity via molecular humidification, despite its high flows, it is deemed comfortable. High-flow oxygen (HFO) consists of a heated, humidified, high-flow nasal cannula that can deliver up to 100% heated and humidified oxygen at a maximum flow of 60 LPM via nasal prongs or cannula.
An air/oxygen blender can provide precise oxygen delivery independent of the patient’s inspiratory flow demands. Based on different bench and patient models, positive end-expiratory pressure may be generated.12 In these models, for approximately every 10 liters of flow delivered, about 1 cm/H2O of positive pressure is obtained.13 High-flow oxygen may help prevent escalation to more invasive respiratory interventions and can help facilitate ventilator liberation. It is best used to treat mild to moderate hypoxemia, to help aid with mucokinesis, and to provide an exact oxygen delivery percentage in patients with an inconsistent respiratory pattern. HFO delivery has been clinically utilized in a wide spectrum of patient care arenas. It has been administered to patient populations in critical care units, emergency departments, and end-of-life scenarios, and recently has migrated into the home care environment.14
In conclusion, oxygen administration is a common clinical intervention for patients with respiratory distress. Optimizing outcomes often depends on selecting the correct oxygen administration device. In selecting an oxygen delivery device, the respiratory therapist should include the following in their recommendation: the goal of oxygen delivery, the patient’s condition and etiology, and the performance of the device being selected. There are a plethora of oxygen delivery devices for the respiratory therapist to choose from to meet the desired clinical endpoint — selection depends on the clinical pathophysiology and the patient’s physiological response. Clinical assessment and monitoring are essential to ensure patient safety and to achieve desired clinical outcomes when administering oxygen.
Kenneth Miller, MEd, RRT-ACCS, NPS, AE-C, FAARC, is the educational coordinator and dean of wellness, respiratory care services, for Lehigh Valley Health Network in Allentown, Pa. For further information, contact [email protected]
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- Frat, JP, Thille, AW, Mercat A, Girault, C et al. High-Flow Oxygen through Nasal Cannula in Acute Hypoxemic Respiratory Failure. N Engl J Med 2015; 372:2185-2196.