Noninvasive Respiratory Support

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Chapter 28 Noninvasive Respiratory Support

In this chapter, supplemental oxygen therapy, humidification systems, adjunctive respiratory therapy, and noninvasive ventilation (NIV) are discussed. NIV encompasses various methods of positive pressure ventilation, including continuous positive airway pressure (CPAP), delivered without an endotracheal tube.1

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 (Sao2; 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 (Pao2) (see Chapter 1). Supplemental oxygen therapy is indicated to maintain the Pao2 above 8 κPa (60 mmHg) or the Sao2 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 (FIo2 >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 (Sao2 <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 FIo2. 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.

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, FIo2 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.

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.

Humidification Devices

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.