Respiratory emergencies: respiratory support (Proceedings)

Support of respiratory function in critically ill patients is extremely important because inappropriate oxygenation and/or ventilation can quickly lead to an animal's demise. Also, respiratory infections, especially nosocomial infections, can be quite serious. Oxygen administration and mechanical ventilation are used to optimize oxygenation and ventilatory status. Humidification of inspired gases is performed to minimize respiratory tract injury. Aerosol therapy is performed to facilitate treatment of respiratory infections.

Oxygen Therapy

The primary indication for oxygen therapy is hypoxia. Hypoxia can result from (1) inadequate inspired partial pressure of oxygen, (2) impaired pulmonary function, (3) ineffective or inefficient oxygen transport in the blood, or (4) increased tissue oxygen consumption that is not matched by increased oxygen delivery. The need for oxygen therapy is determined by the clinical status of the patient. Clinical signs associated with hypoxia include cyanosis (as long as there is at least 5 g/dl of reduced hemoglobin in the blood), dyspnea, tachypnea, tachycardia, and anxiety. Oxygen is administered to patients with these clinical signs until it is determined that supplemental oxygen is unnecessary. Arterial blood gases (PaO2, SaO2) and pulse oximetery (SpO2) are helpful in determining the need for oxygen therapy. Although hypoxia is defined as PaO2 < 60 mmHg and SaO2 (SpO2) < 90%, oxygen should be administered to any critical patient with these parameters below normal values [normal PaO2 = 80 to 100 mmHg; normal SaO2 (SpO2) = 97% to 100%]. In the absence of blood gases and pulse oximetry, resolution of clinical signs during administration of oxygen would be evidence that the patient indeed needs oxygen therapy. Finally, it should be noted that supplemental oxygen is used to optimize oxygen delivery in patients with systemic inflammatory response syndrome or shock regardless of the clinical data.

Methods of supplemental oxygen administration include face masks (rigid or "baggie"), hoods, special cages or chambers, intranasal catheters, intratracheal catheters, endotracheal tubes, and tracheostomy tubes. Intranasal catheters are the most practical and efficient method for long-term oxygen administration. At relatively low flow rates (150 to 200 ml/kg/min) intranasal administration can achieve 40% to 50% inspired oxygen concentration. The actual flow rate used is determined by the state of pulmonary function. Monitoring during oxygen administration includes clinical signs, pulse oximetry, and arterial blood gases. An attempt is made to deliver the lowest possible flow rate that will abolish clinical signs and return SpO2 and PaO2 into the normal ranges. If supplemental oxygen fails to reverse hypoxia, ventilation therapy with positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP) is instituted until the patient can achieve normoxia without ventilatory support. Oxygen supplementation is discontinued when the state of hypoxia is reversed and the patient can remain normoxic on room air. Weaning from oxygen therapy is achieved by monitoring clinical signs, SpO2, and arterial blood gases while the oxygen flow rate is decreased and the oxygen is eventually turned off. Return of clinical signs or abnormal SpO2 (and/or PaO2) requires re-institution of the previous level of oxygen supplementation. During oxygen therapy an effort is made to decrease the level of supplementation such that the inspired oxygen concentration is 50% or less as soon as possible (within the first 8 to 12 hours) in order to prevent oxygen toxicity.

Positive Pressure Ventilation

The primary indication for positive pressure ventilation is ventilatory failure (inadequate gas transport in and out of the lungs). The level of ventilatory failure requiring positive pressure ventilation is defined as PaCO2 > 55 mmHg (normal PaCO2 = 35 to 45 mmHg) or minute ventilation < 100 ml/kg. Positive pressure ventilation is also used when oxygen therapy fails to reverse hypoxia and when intracranial hypertension is complicated by hypercarbia. Positive pressure ventilation is provided by manual or mechanical methods. Manual methods include the rebreathing bag of an anesthesia machine and the self-inflating ambu bag. Mechanical ventilators are used to control or assist the patient's respirations for extended periods. With controlled ventilation the ventilator totally controls the patient's respirations by determining the respiratory rate and tidal volume. Neuromuscular blockade may be necessary to achieve controlled ventilation. With assisted ventilation the patient is able to determine the respiratory rate and pattern, but the ventilator determines the tidal volume. Synchronized intermittent mandatory ventilation (SIMV) is a method whereby the machine delivers a set number of breaths per minute, but will not deliver an additional breath until the patient's respiratory rate drops below the rate set on the ventilator. Mechanical ventilators are either pressure-cycled or volume-cycled. With pressure-cycled ventilators the termination of inspiration is based on a preset pressure limit, whereas a preset tidal volume determines the termination of inspiration with a volume-cycled ventilator. Some volume-cycled ventilators have a safety mechanism whereby an upper pressure limit can be set to avoid the risk of overinflation. When a ventilator is used for improving oxygenation, PEEP or CPAP is applied (usually 5 to 10 cm H20) to overcome alveolar collapse and decrease the work of breathing. Sophisticated ventilators have built-in PEEP/CPAP settings. Disposable PEEP valves that attach to the exhalation limb of the breathing circuit can be used with ventilators that do not have PEEP/CPAP settings. A specialized type of ventilation therapy can be performed with a high-frequency ventilator. High-frequency ventilation uses a high respiratory rate and low tidal volume to allow transfer of gases via diffusion along a concentration gradient and, therefore, is useful for ventilation in the presence of tracheal disruption or whenever low mean airway pressures are desired.

Monitoring during ventilator therapy includes measurement of arterial blood gases (particularly PaCO2) and end-tidal carbon dioxide (ETCO2). The ETCO2 can be measured continuously to evaluate trends, but is typically 5 to 10 mmHg lower than PaCO2 which it estimates; therefore, periodic measurement of PaCO2 and correlation with ETCO2 is necessary for the truest picture of the patient's ventilatory status. Weaning from ventilatory support involves the gradual reduction of minute ventilation to allow the ETCO2 (and PaCO2) to increase enough to stimulate spontaneous respiration. Assisted ventilation or SIMV is continued until clinical signs, ETCO2 and blood gases normalize and remain normal during spontaneous breathing. Weaning from the ventilator is a slow process that may take several hours.

Humidification of Inspired Gases

Normally, inhaled air is heated and humidified by the nasopharynx and tracheobronchial tree such that alveolar air is 100% humidified at body temperature and contains 4 to 6 times the water vapor content of room air. Dry medical gases that bypass the upper airways increase humidification requirements and dessicate the respiratory mucosa resulting in viscous secretions, impaired mucociliary transport, inflammation, small airway closure, decreased functional residual capacity, reduced pulmonary compliance, and increased risk for infection. Compensatory increase in vaporization by respiratory mucosa results in cooling of the liquid surface and patient heat loss. Humidifiers are used in attempt to offset the detrimental effects of dry medical gases. Airways can be humidified with bubble humidifiers, heated bubble humidifiers, humidity exchange filters, and nebulizers. Bubble humidifiers work by bubbling the medical gas through water or saline on its way to the patient and result in a water vapor content that is only slightly greater than room air. Heated bubble humidifiers can supply inspired gases with 100% humidity, but require specialized equipment, and the temperature must be monitored closely to avoid thermal injury to the respiratory epithelium. Humidity exchange filters are disposable devices that interface between the endotracheal (or tracheostomy) tube and breathing circuit of a ventilator or anesthetic machine and trap exhaled moisture which then humidifies the inhaled gas. Nebulizers deliver water particles and, as such, will humidify the airways; however, prolonged use (more than 15 to 20 minutes) will result in overhydration and airway flooding.

Aerosol Therapy

An aerosol is a fine suspension of liquid (water, saline, or drug) droplets in a carrier gas (oxygen or compressed air) used to deliver substances directly to respiratory mucosal surfaces in order to prevent dessication (water), loosen secretions and stimulate coughing (saline), and/or treat respiratory disease (drugs). Aerosol deposition depends on the size of the particles and the rate and depth of breathing. The deepest (most peripheral) deposition of particles occurs with small particles (0.5 to 5.0 µm) delivered to a patient during slow, deep breathing. The instrument that creates and delivers an aerosol is referred to as a nebulizer, of which there are three general types: (1) jet nebulizer, (2) Babbington nebulizer, and (3) ultrasonic nebulizer. A jet nebulizer creates and delivers an aerosol with a narrow, high-velocity stream of gas directed over the end of a capillary tube that has the other end immersed in liquid. A Babbington nebulizer has a pressurized stream of gas that emerges from a slit in a hollow sphere over which fluid is dripped. An ultrasonic nebulizer uses a piezoelectric crystal close to a fluid reservoir which transforms electrical energy into high-frequency oscillations that eject aerosolized particles from the liquid surface. Ultrasonic nebulizers produce uniformly small (approximately 5 µm diameter) particles. Jet and Babbington nebulizers produce equivalent particle sizes averaging 3 to 5 µm in diameter.

Aerosols may be delivered to patients via face mask, enclosure in an oxygen cage or mist tent, or directly into a breathing circuit of a ventilator or anesthetic machine. Typically, aerosol administration is limited to 15 to 30 minutes every 4 to 8 hours because continuous administration may result in overhydration. Ancillary techniques include maintenance of normal hydration of the patient, thoracic physiotherapy (coupage), administration of bronchodilators (to facilitate peripheral delivery of particles), and use of a sterile delivery system (to prevent nosocomial infection). The most common drugs delivered via aerosol are antibiotics, particularly aminoglycosides for treating Gram-negative pneumonia. Because aminoglycosides are not absorbed into the bloodstream via the respiratory tract they can be used via aerosol without the risk of nephrotoxicity. Bronchodilators can be delivered via aerosol, but it is this author's preference to administer the bronchodilator by another route so that it is active prior to the delivery of the aerosol. Acetylcysteine was formerly given via aerosol to facilitate mucolysis, but it is irritating to respiratory mucosa and, therefore, no longer used for nebulization.