Oxygen comes packaged in three types of systems: compressed gas, liquid and oxygen concentrators. The trade-offs include size and weight of the device, storage capacity, cost and transfillability. The features of the systems are compared in table 1. Both liquid and compressed gas systems are becoming smaller and lighter.
Table 1. - Gas, liquid and concentrator oxygen systems
|Reliability||Good with regular service||Good but gauges may become inaccurate||Generally good but connector may freeze|
|Cost||Low but cost of electricity born by patient||Moderate||High|
|Power:wall current||Required||Not required||Not required|
|Transfilling||Good only on special units that allow transfilling||Limited||Excellent|
|Ambulatory use||Good with transfill systems to gas+conserver||Good with conserver||Good alone and with conserver|
|Stationary weight||35-50 lb||H cylinder 200 lb||Reservoir 120 lb|
|Use time at 2 L·min-1||Continuous||2.5 days||8.9 days, special system >30 days|
|Portable weight||Portable units are not presently available||E cylinder 22 lb with cart||6 lb with no conserver|
|Use time at 2 L·min-1||Unlimited||5 h||4 h|
|Portable weight with conserver||See gas transfill portable with conserver||M6 cylinder 4.5 lb||3.4 lb with conserver|
|Use time at 2 L·min-1||See gas transfill portable with conserver||12 h||10 h|
Conserver: oxygen conserving device. #: there is now a choice in oxygen delivery devices and systems that combine the benefits of conserving devices either gas or liquid systems. For example, the smallest liquid system weighs only 3.4 lb and provides 10 h of oxygen. The smallest gas system weighs 4.5 lb, refills from an oxygen concentrator and provides 12 h of oxygen. Availability of these systems varies by locality.
Liquid has been the standard transfill system. Now, portable compressed gas oxygen cylinders can be safely transfilled from a uniquely constructed oxygen concentrator. An additional advantage of these systems is that home oxygen deliveries are no longer required.
The continuous flow dual-prong nasal cannula is the standard means of oxygen delivery for the stable hypoxaemic patient. It is simple, reliable and generally well tolerated [31-34].
The nasal cannula delivers a low flow of pure oxygen entrained in a much larger volume of atmospheric air (20.9% oxygen). Each litre per minute of oxygen flow adds about 3-4% to the FI,O2. A rough approximation is that 1 L•min-1 increases the FI,O2 to 24%, 2 L•min-1 to 28%, and 3 L•min-1 to 32%. However, these small increases are usually sufficient to increase the arterial oxygen content to acceptable clinical levels.
The actual FI,O2 for any particular patient is variable, depending on the anatomy and patency of the nares and moment-to-moment variation in respiratory rate and pattern, as well as the underlying pathophysiological process. The FI,O2 is inversely related to the inspiratory rate, i.e. a more rapid inspiratory rate dilutes the oxygen flowing into the nares with more room air, thereby reducing the FI,O2.
Some studies indicate that mouth breathing impairs oxygen delivery, while others show no such reduction [31, 32]. Most mouth breathers have some nasal airflow as well. Since only a small nasal inspiratory flow is necessary, and some oxygen is stored in the nasal and sinus passages, nasal oxygen delivery is still beneficial to these patients.
Oxygen-conserving devices function by targeting oxygen delivery to early inhalation. These devices were developed in an effort to improve the portability of oxygen therapy by reducing the litre flow and thereby enabling patients to use a smaller and lighter ambulatory system, or a standard system for longer time periods . Other advantages include a reduction of overall costs of LTOT and the ability to treat refractory hypoxaemia more effectively. There are three distinct devices: reservoir cannulae, demand pulsing oxygen delivery devices and transtracheal oxygen. Their characteristics are summarised in table 2.
Table 2. - Comparison of oxygen-conserving devices
|Conserving device||Reservoir cannula||Demand pulse delivery||Transtracheal catheter|
|Conserving method||Store during exhalation||Early inspiration delivery||Store at end exhalation; bypass upper airway dead space|
|Efficiency gain (savings)||2:1-4:1||2:1-7:1||2:1-3:1|
|Reliability||Good||Mechanically complex||Mucus plug possible|
Effective with exercise
No nasal/ear irritation
Reduce minute ventilation
|Disadvantages||Bulky on face||Mechanically complex
Failure is possible
|Special care + training
Oxygen-conserving devices overall
The goal of prescribing oxygen-conserving devices is to improve portability, mobility and comfort and enable patients to be more active. They lessen the cost of home oxygen therapy by reducing number of home deliveries. This is in spite of the fact that oxygen-conserving systems are initially more expensive. As they are more efficient they are a prescribing option that can meet the needs of patients who require higher flow settings.
There is no evidence that humidification is necessary when oxygen is given by nasal cannula at flows <5 L•min-1, as evidenced by subjective complaints or severity of symptoms . There are no differences in subjective complaints or in severity of symptoms over time. These findings are explained by the low water vapour output of bubble humidifiers and small contribution of oxygen flow to the patient’s inspired minute ventilation, since most of the patient’s tidal volume consists of atmospheric gas. Moreover, oxygen flowing through the bubble humidifier is at room temperature; when it is raised to body temperature, the relative humidity falls.
This finding does not apply to patients receiving oxygen by tracheostomy or transtracheal oxygen (TTO), in whom the catheter has bypassed the upper airway. For these patients, humidification of inspired gas is essential even at low flow rates (1 L•min-1). TTO patients, who are at high risk for mucus ball formation, including those with high oxygen flows >5 L•min-1, may produce large amounts of mucus, or have a weak cough. These patients might benefit from heated and humidified oxygen.
Reservoir cannulas actually return some of the patient’s humidification at the temperature of exhaled gases. Hence, the patient receives his own humidification at higher than room temperature.
Reservoir cannulas operate by storing oxygen in a small chamber during exhalation for subsequent delivery during early phase inhalation. They are cycled by the patient’s nasal inspiratory and expiratory pressures. They are available in two configurations: Oxymizer and Pendant [34, 36]. The delivery efficacies of the two are roughly equivalent.
Compared with continuous flow oxygen, reservoir cannulas are two to four times as efficacious. They reduce oxygen usage by lowering the oxygen flow setting to 25-50% of that required for continuous flow oxygen to achieve equivalent Sa,CO2. They are also indicated for patients with high flow oxygen needs.
Demand pulsing oxygen delivery devices
Demand pulsing oxygen delivery devices deliver a small bolus of oxygen at the onset of inhalation . Connected between the nasal cannula and the pressurised oxygen source, they sense the start of inhalation through the nasal cannula, whereupon they immediately enable a short pulse of oxygen to flow to the patient. Because oxygen delivered at the beginning of inhalation reaches the ventilated alveoli, small oxygen pulses are very effective in oxygenating the patient.
These devices vary in their design features, including delivery strategy, missing breath alarms and battery life. Pulse demand devices have also been combined with a transtracheal oxygen catheter, which further improves the delivery efficacy of transtracheal oxygen delivery . The overall delivery efficacy of this combination is about equivalent to the most efficacious pulsed demand nasal delivery.
There have been recent concerns that pulsing devices may not maintain Sa,CO2 during exercise. Some newer pulsing devices have been specifically designed to maintain Sa,CO2 during exertion .
TTO is delivered directly by the insertion of a catheter percutaneously between the second and third tracheal rings . Conservation occurs because the anatomic reservoir is increased to include the airways above the catheter insertion site. TTO can reduce the flow rate of oxygen by ~50% at rest and 30% during exercise compared with nasal cannula delivery .
Although TTO is considered in the category of oxygen conserving devices, it is considerably different from the approaches discussed earlier. TTO reduces inspired minute ventilation , which may lessen the work of breathing, conserve energy expenditure while lessening dyspnoea. High flow via a transtracheal catheter reduces total dead space volumes in an amount proportional to the increase in flow rate. The pleural pressure-time index and tension-time index for the diaphragm decreases, which may account for the decrease in dyspnoea and increase in exercise tolerance seen in these patients.
TTO requires a trained team of clinicians to evaluate, educate and monitor patients . The ideal candidate for TTO has a strong desire to remain active, is willing to follow the care protocol, is not experiencing frequent exacerbations, has a care-giver who is willing to assist with problem solving and details of care, and lives within 2 h of the institution or has equivalent follow-up in the home community . TTO is of particular benefit to patients who are unwilling to accept the visible nasal cannula and thereby reject oxygen therapy.
Relative contraindications include high-dose steroids (e.g. prednisone >30 mg?day-1) and conditions that predispose to delayed healing, e.g. diabetes mellitus, connective tissue disease or severe obesity. Absolute contraindications include subglottic stenosis or vocal cord paralysis, herniation of the pleura into the insertion site, severe coagulopathy, uncompensated respiratory acidosis and inability to practice self-care.
Complications of TTO are infrequent but can be serious. They include catheter displacement, bacterial cellulitis, subcutaneous emphysema, haemoptysis, severed catheter and mucus balls. Mucus balls can develop on the catheter due to the drying effect of the oxygen, increased sputum production and poor adherence to cleaning schedules; they may cause coughing, catheter blockage and tracheal obstruction, with serious consequences. Daily cleaning prevents mucus ball formation in most patients.