OXYGEN THERAPY

Oxygen delivery to the tissues depends on arterial oxygen content, cardiac output, and individual organ perfusion. Arterial oxygen content depends on the hemoglobin concentration and arterial oxygen saturation (S_ao₂; see Equation 1-13). The latter is, in turn, dependent on the oxyhemoglobin dissociation curve (see Fig. 1-14) and the arterial partial pressure of oxygen (P_ao₂) (see Chapter 1). Supplemental oxygen therapy is indicated to maintain the P_ao₂ above 8 κPa (60 mmHg) or the S_ao₂ above 90%. Only low concentrations of supplemental oxygen are required to treat hypoxemia due to hypoventilation. In contrast, high concentrations of oxygen may be required to treat hypoxemia due to ventilation/perfusion mismatch. Oxygen is ineffective if hypoxemia is due to pure shunt. High-concentration oxygen therapy in the setting of hypoventilation may mask the development of severe hypercarbia and respiratory acidosis (see Chapter 1).

Impaired gas exchange is almost universally present following cardiac and thoracic surgery, so oxygen is indicated for the duration of a patient’s stay in the intensive care unit (ICU). It should also be continued following discharge from the ICU if a patient is receiving opioid analgesia because that predisposes the patient to nocturnal desaturation.² Supplemental oxygen therapy during the perioperative period has also been shown to reduce the incidence of surgical wound infections.³

Excessive oxygen therapy is not without risk. High-concentration oxygen promotes absorption atelectasis and, in patients with severe chronic obstructive pulmonary disease (COPD), it may exacerbate hypercarbia (see Chapter 27). Administration of high oxygen concentrations (F_Io₂ >0.6) for longer than several days may potentially cause direct pulmonary toxicity and should be avoided if possible.⁴ However, supplemental oxygen should not be withheld in the presence of clinically significant hypoxemia (S_ao₂ <85% to 90%).

Oxygen Delivery Systems

Noninvasive methods of oxygen delivery can be divided into variable-performance and fixed-performance systems. In order to provide a fixed concentration of oxygen, the system must deliver gas at a flow rate that matches the patient’s peak inspiratory flow rate. This is approximately 25 to 35 l/min at rest but can rise to over 60 l/min with respiratory distress. If delivered flow is less than peak inspiratory flow, entrainment of room air dilutes the inspired gas and results in a reduced F_Io₂. Piped and bottled oxygen and air sources are dry and must be humidified if administered at high flow rates (see later material).

Variable-Performance Systems

Variable-performance systems have a flow rate or a reservoir size that is not sufficient to prevent entrainment of air at high-peak inspiratory flow rates.

Nasal Cannulas.

Nasal cannulas provide a low level of supplemental oxygen. They are well tolerated and allow the patient to eat and expectorate without having to remove the cannulas. The oxygen flow fills the nasopharyngeal and oropharyngeal reservoirs at end-expiration and is then inspired into the lungs, even when the mouth is open. Flow is usually set at 2 to 4 l/min, which is equivalent to an F_Io₂ of less than 0.3. Higher flow rates require humidification and may be uncomfortable.

Semirigid Face Masks.

Semirigid face masks increase the effective size of the airway reservoir and thus provide a higher oxygen concentration than do nasal cannulas, but they are less practical because they must be removed when eating. The simple face mask has an oxygen inlet at the base and holes at the sides for expiration and air entrainment. The F_Io₂ depends on the oxygen inflow rate and the patient’s tidal volume, respiratory rate, and respiratory pattern. A minimum flow rate of 5 l/min is required to prevent the rebreathing of expired gas. The partial rebreather mask and nonrebreather mask allow delivery of higher F_Io₂ levels but are designed for prehospital use and are not recommended in the ICU, where high F_Io₂ therapy is best given through a humidified fixed-performance system.

Manual Resuscitators.

The components of manual resuscitators (such as the Laerdal and the Ambu) are illustrated in Figure 28-1. As with other variableperformance systems, F_Io₂ depends on the oxygen flow, tidal volume, and respiratory rate, but it can be increased by attaching an oxygen reservoir bag to the air entrainment inlet. The resuscitation bag is designed for short-term emergency use when augmentation of a patient’s respiratory effort by manual ventilation is required. Care should be taken with a patient who has respiratory distress but good respiratory drive because spontaneous breathing through a manual resuscitator increases the work of breathing, potentially worsening the respiratory distress.

Figure 28.1 Manual resuscitator. A, During inspiration, when the self-inflating bag is compressed, flow from the oxygen supply is directed into the reservoir. B, If the reservoir becomes full, which may occur if minute ventilation is less than the oxygen flow, a pressure-release valve prevents a dangerous buildup of pressure within the device. C, During expiration, the self-inflating bag refills from both the oxygen supply and the reservoir bag. If the oxygen supply and the reservoir bag provide insufficient gas to refill the device, room air is entrained through the entrainment valve. The “duck-billed valve” at the patient end of the device ensures that minimal rebreathing of respiratory gas occurs and also allows the incorporation of a spring-loaded PEEP valve that is connected to the exhalation port of the breathing circuit if it is required. Modern valves (e.g., the Ambu Mark III) have very low inspiratory and expiratory resistance. PEEP, positive end-expiratory pressure.

Fixed-Performance Systems

Fixed-performance systems provide a constant F_Io₂ that is independent of a patient’s peak inspiratory flow. This is achieved by having either a high flow rate or a large-capacity reservoir.

Venturi Mask.

With the Venturi mask system, oxygen inflow is connected to a specific color-coded entrainment device at the base of the mask that provides a set F_Io₂ at a set oxygen inflow rate. Various entrainment devices can provide an F_Io₂ of 0.24 to 0.5, with an oxygen inflow of 4 to 15 l/min and a total flow delivered to the patient (including entrained air) of 35 to 45 l/min. If the patient’s peak inspiratory flow exceeds this total flow, a lower F_Io₂ is inspired. The Venturi mask is ideal for a patient with COPD who has a low to moderate oxygen requirement but is at risk for hypercarbia with uncontrolled oxygen therapy.

High-Flow Nonrebreather Mask

High-flow nonrebreather systems can provide an F_Io₂ up to 1.0. They consist of high-flow oxygen and air sources, a blender that mixes the oxygen and air to the desired concentration, a flow meter capable of delivering flows that can match the patient’s peak inspiratory flow rate, a humidifier, wide-bore tubing, and a close-fitting face mask. Patients who have high F_Io₂ requirements and are using this system must be closely monitored because any deterioration in clinical status may mandate the rapid institution of invasive ventilation.

HUMIDIFICATION

Humidity is a measure of the amount of water vapor in a gas. It may be expressed as a partial pressure (kPa), an absolute value (g/m³), or a relative value (%). The last is the absolute mass of water divided by the maximum mass of water that could exist in vapor form in the gas at a certain temperature.

During quiet nose breathing, inspired gas is warmed to body temperature and fully saturated with water vapor before it reaches the carina (absolute humidity 43.4 g/m³, relative humidity 100%, partial pressure 6.3 κPa at 37°C at sea level). Humidification occurs primarily in the nose, where inspired gases are exposed to a large surface area of highly vascular mucosa. Mouth breathing, particularly at high peak inspiratory flow rates, results in less humidification by the upper airway.

Normal mucociliary function of the large airways is dependent on adequate humidification of the inspired gas. Abnormal mucociliary function results in cytologic damage and leads to the accumulation of tenacious secretions and microatelectasis. At a relative humidity of 75% (32.5 g/m³ at 37°C), mucociliary function is impaired; at 50%, it fails entirely. Changes in mucociliary function occur within minutes to a few hours if low-humidity gases are inspired. This is the rationale for providing humidification of inspiratory gas of at least 32.5 g/m³ to all patients who are receiving high flows of inspiratory gases and to those whose upper airways have been bypassed using an endotracheal tube.

ADJUNCTIVE RESPIRATORY THERAPIES

Patients unable to generate high expiratory flow rates because of respiratory muscle weakness or obstructive airway disease may be unable to cough effectively. This leads to the accumulation of excessive secretions and entry of foreign material into the lungs, with the development of atelectasis, impaired gas exchange, and pneumonia. When cough is poor, lung-expansion techniques and maintenance of normal mucociliary clearance become important.