Oxygen supplementation and airway management (Proceedings)
Oxygen delivery to the tissues must be prioritized in any critical patient.
Oxygen delivery to the tissues must be prioritized in any critical patient. It can be maximized by careful consideration of pulmonary gas exchange, hemoglobin concentration for oxygen transport, and tissue perfusion for delivery of oxygen to the cells. If respiratory distress is present, it is often very obvious that oxygen supplementation is required. It is important to recognize, however, that oxygen supplementation may be very beneficial in situations when the presence of hypoxemia might not be intuitively obvious by observation. Tachypnea may be wrongly attributed to pain when it really is caused by hypoxemia. Dogs that are flat out because of shock or neurologic involvement may be unable to manifest the typical clinical signs of respiratory distress. The observation of pink mucous membranes does not rule out the possibility of clinically significant hypoxemia, since membranes will remain pink until the PaO2 has dropped below 60 mmHg (normal 85-100 mmHg). Similarly, we have great difficulty detecting cyanosis in the animal with very pale mucous membranes, because insufficient perfusion of the peripheral tissues by blood may preclude the observation of deoxyhemoglobin. In such situations, we must use other tools to infer a requirement for oxygen supplementation: for example the presence of refractory tachycardia or hypotension, ventricular arrhythmias, severe mental depression, or tachypnea. The provision of supplemental oxygen is indicated in every emergency trauma situation, and indeed in every shocky patient until it has been established that the animal is stable without it. High concentrations of oxygen can be easily achieved by use of several methods.
Methods of oxygen administration
Several possible methods of oxygen administration are commonly used. Each method has specific advantages and disadvantages, and different methods often work best in different patients.
Masks, bags, hoods
In each case, oxygen is pumped into a contained area over the head or muzzle of the animal. Most oxygen masks are made of transparent plastic, through which the animal can be observed. Several methods have been advocated by which increased concentrations of oxygen can be achieved, including "Flowby" jet-stream canopy O2, placement of a plastic bag over the head into which oxygen is pumped, and the use of an Elizabethan collar with plastic wrap covering the front.
o Easy to use
o Quickly placed in position in emergency situations
o Depending on flow rates and tightness of fit, very high oxygen concentrations can be achieved
o Because only the head is covered, the clinician can still work with the animal for diagnostic tests or therapeutics
o May not be well tolerated by dyspneic animals
o Not effective in animals that are moving around
o Animals can overheat extremely quickly, especially if they are large and rapidly breathing. The clinician must observe carefully for evidence of excessive panting and increases in body temperature, which could be very detrimental in the dyspneic animal.
o Carbon dioxide may build up to high concentrations, especially if there is no avenue for outflow from the hood or mask. Hypercarbia can lead to significant respiratory acidosis.
For administration of oxygen by nasal tube, a rubber urinary catheter is commonly used. Catheters may vary in size from 5 to 10 French, depending on the size of the animal. The catheter is measured from the nares to the medial canthus of the eye, and marked with a small piece of tape. A few drops of lidocaine are instilled in one nostril, and the animal's nose is elevated to prevent the lidocaine from dripping out. In a few minutes, local anesthesia of the nostril should have been achieved, and catheter insertion can be begun. The catheter is inserted gently into the nostril in a ventromedial direction, and advanced to the marker. Once the catheter is in place, it is bent around under the alar fold, and sutured or glued in place on the side of the face. For the most secure placement, a suture should be placed as close to the nasal-cutaneous junction as possible. The nasal catheter is attached to an oxygen delivery system, with flow rates of 100-200 ml/kg/min. In very dyspneic animals, bilateral nasal oxygen lines can be used. Some animals can be best managed using human bilateral nasal "prongs" that only penetrate 1 cm or less into the nasal cavity. Inspired oxygen concentrations of 30-50% can easily be achieved using this type of system. If the nasal catheter is guided further into the nasopharynx under sedation, oxygen concentrations of up to 80% may be achieved in some animals.
o Catheter is easy to place
o Well tolerated by the majority of dogs
o The clinician is free to work with the animal, evaluate mucous membrane color, complete a physical examination, and pursue diagnostic and therapeutic measures as indicated
o The animal is free to move around within the confines of a cage
o Some animals will not tolerate the nasal line, and seem to undergo a significant amount of discomfort, with pawing at the face or sneezing
o Inspired oxygen concentrations may not be high enough for very dyspneic animals, particularly if they are mouth breathing
o Not useful in animals with nasal or pharyngeal disease, injury or pain
o This technique is more difficult to apply in cats and brachycephalic dogs, when the nasal catheter may not stay in place
For administration of transtracheal oxygen, a catheter is placed transcutaneously into the trachea, and oxygen is insufflated directly into the airway. For placement of a tracheal catheter, a small patch of skin on the ventral midline of the neck is clipped and scrubbed. Lidocaine is used to provide local anesthesia. A through-the-needle catheter is placed into the airway using the same method as might be used for a transtracheal wash. The catheter is secured in place with glue, sutures, or tape, and oxygen is administered via a delivery system at rates of 50-100 ml/kg/min. Higher oxygen concentrations can be achieved in the airway using this technique, because there tends to be less mixing with inhaled room air.
o Bypasses the nasal cavity and pharynx, especially useful for animals with injury or obstruction of these areas
o Well tolerated
o Consistent delivery of oxygen in spite of movement of the animal
o Dislodgement of the catheter can lead to subcutaneous insufflation of oxygen
o Kinking of the catheter can occur at the entry site in the neck
o Excessive desiccation of the mucous membranes may occur, since the oxygen is being insufflated directly into the trachea, by-passing the nasal turbinates.
o Oxygen must be humidified if it going directly into the trachea.
Oxygen cages are now supplied by a number of manufacturers. As well as providing a higher concentration of inspired oxygen, a good oxygen cage should also allow control of internal cage temperature and humidity. A good oxygen cage should be capable of reaching oxygen concentrations in excess of 80%, for use with severely dyspneic animals.
o Non-invasive and very well tolerated, especially by cats
o High oxygen concentrations can be achieved
o The animal is hidden behind glass walls, and cannot be manipulated, examined, or treated by the clinician
o Large dogs may become overheated
o Oxygen cages are expensive to obtain
o Oxygen cages can be wasteful of oxygen, since each time the door is opened the oxygen inside is lost and must be replaced
The respiratory tract has a variety of sophisticated defense mechanisms to ensure that it is protected from the inhalation of dry, cold, contaminated air. In the normal animal, air traveling through the turbinates is warmed and saturated with water vapor before it reaches the pharynx. Due to turbulent air flow, particles larger than 10 microns collide with the mucus surfaces and can be removed. Clinical techniques that bypass the nasal turbinates, such as tracheostomy, nasopharyngeal or tracheal administration of oxygen, and positive pressure ventilation all result in considerable damage to the respiratory mucosa. In the normal animal, once past the turbinates, smaller particles can penetrate deep into the trachea and bronchioles, but collide with the mucus layer of the ciliated epithelium, and are carried up to the pharynx by the delicate cilia of the mucociliary escalator.
Efforts to preserve airway function are directed at support of two of the most important defense mechanisms of the respiratory tract. First, the function of the mucociliary escalator must be optimized by ensuring that respiratory mucus is fluid and easily moved in an orad direction by the fragile cilia of the respiratory epithelium. Secondly, the cough mechanism must be encouraged, as it is one of the most important means by which material can be eliminated from the airway.
Since 90% of airway mucus consists of water, the most important way to optimally maintain airway clearance and normal airway hygiene is to maintain normal hydration. Systemic dehydration results in drying of the mucus and mucociliary layer, and should be carefully avoided. The clinician must walk a fine line between over hydration and resulting pulmonary edema, and the extent of hydration required to optimize airway clearance. In the presence of inflammatory conditions such as bronchopneumonia, maintenance of adequate hydration is vital. Intravenous fluid therapy is usually beneficial, and diuretics should be avoided unless the patient is in severe distress. In contrast, noninflammatory conditions such as pulmonary edema or hemorrhage may be associated with relatively little additional mucus production, and in this case the lung and airway should be maintained in a much dryer state.
The second option for increasing the moisture content of airway mucus is by humidification of inspired air. The term humidification refers to the saturation of air with water vapor. The amount of water vapor is determined by the temperature of the inspired gas - the warmer the air, the more water vapor it contains. Normally, air passing through the turbinates is humidified and warmed to body temperature as it contacts the mucosal surface. If the turbinates are by-passed, for example by nasopharyngeal or tracheal oxygen administration, or by tracheostomy, the airway mucosa is damaged by cold, dry air. Even in an airway that was otherwise normal, such irritation leads to increased mucus production, damage to the respiratory epithelial cells, and edema. This injury may be even more profound in the previously injured airway.
Humidification of inspired air may be achieved in several ways. Most commonly, supplemental oxygen is humidified by bubbling through water prior to administration to the patient. Oxygen is bubbled through a flow meter to which a jet humidifier is attached. By bubbling through the water at room temperature, the oxygen is passively humidified, and can therefore assist in preserving the integrity of the respiratory mucosa. This type of humidification should be used whenever supplemental oxygen is being administered to the awake patient.
A number of different options are available for the anesthetized and intubated patient. Specially designed heated humidifiers can be inserted into the breathing circuit. By heating the inspired air to body temperature, the content of water vapor is increased, closely approximating the action of the turbinates. An alternative, and less expensive option for short-term anesthesia, is the use of an "artificial nose" in the breathing circuit. These are small, disposable plastic, heat and moisture exchangers which are attached to the end of the endotracheal tube.
While humidification saturates the air with water vapor, nebulization loads the inhaled air with small spherical droplets of water or saline. The droplets are then deposited at various levels of the respiratory tract, depending on their size. The smaller the droplet, the deeper it penetrates, impacting the airway mucosa due to brownian motion, gravity, and when there are changes in direction of air flow. Droplets greater than 10 microns impact the mucosa of the upper airway and trachea. In contrast, droplets less than 0.5 microns are small enough to reach the alveoli, and are exhaled rather than showering out on the mucosal surface. Most nebulizers in clinical use create droplets in the 2-5 micron range, with some variability in size of the droplets that are produced. In this size range, the droplets penetrate and impact the mucosa at the level of the small airways.
The most commonly used nebulizers create droplets by use of ultrasound. Alternatively, disposable nebulizers are available that are driven by gas pressure from an oxygen source. Disposable nebulizers offer the potential advantages of minimal potential for transmission of infection between patients, and the option to provide oxygen supplementation simultaneously with nebulization.
Various solutions can be administered by nebulization. Bland solutions such as sterile water or saline are most commonly used, with 0.9% saline solutions thought to be most effective for moistening secretions. The animal should inhale the nebulized solution for 15-20 minutes, 4-6 times daily, and coupage should be performed at the same time to stimulate coughing. Although the benefits of nebulization have not been documented in veterinary patients, clinical experience suggests that it is very beneficial for patients with viscous respiratory tract secretions, particularly those with bronchopneumonia.
Coupage and physical therapy
It is impossible to over-emphasize the importance of the cough mechanism for clearance of respiratory tract secretions, particularly in the presence of bronchopneumonia. Coughing promotes movement of material into the pharynx, where it can be swallowed or expectorated. Once the secretions have been moistened, the simplest way to stimulate coughing is by inducing an increased tidal volume during respiration using mild exercise such as walking, if the patient is stable.
Coupage is performed by repeatedly and firmly striking the chest wall with a cupped hand, which stimulates the cough reflex and helps to "break up" secretions in the airways, and is usually well tolerated. Patients with bronchopneumonia should be coupaged for 5-10 minutes several times daily, especially in if they are unable to stand and move around. As an alternative that may save time in the busy veterinary clinic, vibrating massagers designed for human use are readily available and inexpensive, and when wrapped around the chest, seem to be effective in mobilization of secretions. The use of postural drainage may also be helpful in some patients. In order to maximize gravitational drainage from the bronchi, coupage can be performed with the patient in lateral recumbency with the worst lung uppermost. Since this manipulation may worsen hypoxia, it may not be well tolerated by the most unstable patients.