Airway maintenance in anesthesia (Proceedings)

The respiratory system is one of the most important physiologic systems that the anesthetist can affect during an anesthetic event. If respiration ceases, a patient can die within minutes. Most anesthesia involves the use of volatile inhalant anesthetics that must be delivered through the respiratory tract. This means that a patent and normally functioning airway is essential to ensure uptake of inhalant and maintenance of an adequate anesthetic place. The exchange of oxygen and carbon dioxide also relies on the normally functioning airway and respiratory system. Understanding how to maintain, monitor, and troubleshoot a patient's airway are essential jobs of the veterinary anesthetist.

The first step is to choose the method of providing inhalant anesthetic and oxygen to the patient. There are several options each with advantages and disadvantages. Providing inhalant by mask is technically the easiest strategy. It is important to keep the head and neck extended so that air flow is not compromised. A mask also offers the advantage of being useful for both induction and maintenance of anesthesia as well as providing oxygen to patients that are receiving intravenous anesthetics. Higher oxygen flow rates are necessary compared to the flow rates needed for intubated patients. The mask diaphragm must be tight fitting to the animals muzzle to prevent loss of gas and exposure of staff to waste gas pollution. The smallest mask possible minimizes dead space and re-breathing of carbon dioxide. The disadvantages of this type of anesthetic maintenance are that it is difficult to provide positive pressure ventilation to the patient and it is impossible to protect the airway from aspiration.

The endotracheal (ET) tube is the most efficient choice for administration of inhalant. When the tube is properly place and the cuff is inflated the anesthetist is able to provide inhalant as well has 100% oxygen to the patient. The ET tube also allows the anesthetist to maintain a patent airway, protect the airway from foreign material, allow application of efficient positive pressure ventilation, and to protect personnel from exposure to waste gases.

Anatomy of the ET tube

The three main parts are the ET tube, the cuff system (consisting of the pilot line and the cuff itself) and an adaptor that connects to the anesthetic circuit. The size of the ET tube is most often measured as internal diameter in millimeters. Some tubes are labeled in the French system. The French size is the external diameter in millimeters multiplies by 3. Many tubes also have a radio opaque line that allows the tube to been easily seen on radiographs. Many tubes have centimeter markings that allow the anesthetist to judge length in order to avoid endo-bronchial intubation.

Tube types

A Murphy tube is the most common type of ET tube. This type of tube has a "Murphy eye" at the end of the tube that helps prevent occlusion by foreign material. A Magill ET tube looks very similar to a Murphy tube but lacks the Murphy eye. Cole tubes are used primarily in birds and reptiles. These species have tracheas with complete tracheal rings and are predisposed to tracheal necrosis and rupture if cuffed ET tubes are used. A Cole tube is a type of un-cuffed ET tube that has a "shoulder" that sits in front of the larynx to help provide a seal without the use of a cuff.

Wire reinforced ET tubes are used primarily when the positioning of patient for a procedure or surgery could cause a normal ET tube to bend and kink preventing movement of gas. These tubes have a wire that is embedded in the wall of the tube and runs the length of the tube in a circular fashion that prevents it kinking when bent. Often, these tubes are too long for many small patients and do not lend themselves to being cut due to the wire that is embedded in the tube and connected to the adaptor. If the tube is long, it is important to make sure that the animal is not endo-bronchially intubated.

ET tube cuffs

High pressure/ low volume cuffs offer the advantage of being able to provide a tight, reliable seal of the airway, thereby preventing loss of gas and aspiration of pharyngeal fluid. At rest, the cuff has a small diameter therefore making it slightly easier to place. When the cuff is inflated, it will contact the tracheal wall and distend and deform the trachea to form a seal. The largest tube possible should be used so that the cuff will only be minimally inflated. The disadvantage of this type of tube is that it can cause tracheal necrosis if used for prolonged periods, especially if overinflated. Over-inflation can be prevented by using the smallest amount of air possible to prevent audible leaks at a peak inspiratory pressure of 20 cm H20.

Low pressure/ high volume cuffs have a large resting diameter. This can limit the size of tube that can be passed and can cause irritation of the upper airway. The cuff is also thin walled and floppy and can easily be torn. When inflated, this type of cuff will adapt itself to the tracheal wall and will not deform it as the high pressure/ low volume cuff will. Once inflated properly, is it possible to estimate the tracheal pressure by the pressure of the pilot balloon. The advantage of using the type of cuff is the reduction in cuff related complications due to tracheal ischemia. Compared to the high pressure cuff, these cuffs provide a less reliable seal of the airway and fluid and GI contents can wick past the folds in the cuff and enter the airway resulting in aspiration.

Preparing the ET tube for intubation

It is important to visually inspect the tube for any obstructions before use. The cuff of the ET tube should be inflated for at least one minute prior to use to make sure that there are no holes or leaks that could prevent a proper seal.

The diameter and length of the tube needed should be decided next. There are several ways to choose a tube diameter. One way is to palpate the animal's trachea and estimate the size. Tube size can also be estimated based on the size and breed of the patient. Another way to estimate ET tube size is to look at the area between the nares; this can often approximated the size of the trachea. Tube length can be determined by measuring from the thoracic inlet to the tip of the nose (or the incisors). Ideally, the tube should not extend beyond this point to prevent intubation of a single bronchus and ventilation of only one lung. The portion of tube that extends beyond the nose will contribute to dead space within the anesthetic circuit and can lead to re-breathing of CO2. Many tubes can be shortened by cutting them and replacing the adaptor.

Assessing correct placement of the ET tube

It is important to ensure the ET tube is placed within the lumen of the trachea. Endo-bronchial intubation and esophageal intubation must be avoided. Endo-bronchial intubation results in ventilation of only one lung and atelectasis of the other, non-ventilated lung. Oxygenation is generally decreased and the ventilated lung can be subject to barotrauma from hyperinflation. Signs of endo-bronchial intubation include coughing or bucking, absence of bilateral breath sounds, difficulty providing IPPV, panting, decrease in SpO2, and failure to stay anesthetized. The tube can be re-measured once in place by comparing the centimeter markings on the patient's tube to the markings on another ET tube and adjusting the length as necessary. A radiograph can also confirm endo-bronchial placement.

Esophageal intubation is another error that can occur, even with experienced anesthetists. Direct visualization of the arytenoids with the use of a laryngoscope is the gold standard for confirming tube placement. Lack of chest wall motion and decreased compliance of the reservoir bag with positive pressure ventilation can indicate esophageal intubation. However, this is not always a reliable assessment. Some other signs of esophageal intubation include lack of CO2 with capnography, decreased SpO2 readings, an increased amount of air needed to achieve a seal on the cuff, palpation of two firm tubes in the neck (the trachea AND the ET tube), vocalization (intubated patients will not be able to vocalize), and gastric distention from gas administration. If this occurs, the tube should be removed and a new, clean tube should be placed correctly.

Maintaining airway during anesthesia

Generally the cuff of the ET tube is sealed immediately following induction. It is important to recheck the seal about 5-10 minutes into anesthesia because once the muscles surrounding the trachea relax and anesthesia becomes deeper, a leak often develops. During the procedure it is also important to periodically recheck the cuff and re-inflate as needed since small leaks that were undetectable at first may appear during long procedures.

In some trauma patients, particularly patients with trauma or respiratory disease, blood or fluid may accumulate in the ET tube. These fluids can be seen inside the tube, they can be felt as the patient is "bagged", or the secretions can be heard with a stethoscope. If secretions are identified, the ET tube can be suctioned to remove them. This should be done using sterile technique to avoid introducing bacteria into the respiratory tract. The patient will be disconnected from gas supply and the sterile suction device is threaded into the lumen of the ET tube. Fluid is suctioned for a few seconds (about the length of a few breaths) and then reconnected to the oxygen and inhalant. It is helpful to give a few positive pressure breaths at this time to maximize oxygenation. This can be repeated as necessary. It is recommended that Sp02 be monitored during this process and that oxygen be provided if Sp02 decreases. If there is a large amount of thick mucus or if suctioning does not relieve the obstruction, the ET tube should be replaced.

Tracheal tube obstruction is a serious complication that must be recognized and remedied quickly. The tube can become kinked due to positioning, it can be obstructed with fluid or mucus, or the end of the tube may become lodged against the tracheal wall. Signs of ET tube obstruction include decreased compliance with ventilation, high inspiratory pressures as well as paradoxical chest wall movements. If a capnograph is used, changes are seen as a slope or notch on the expiratory phase which can progress to complete obstruction signaled by a decrease in CO2 to zero. The final result is circulatory collapse if not recognized and addressed. Once the problem is identified, the position of the tube can be changed, the airway suctioned as needed, or the tube can be completely replaced to alleviate the obstruction.

Regurgitation of gastric contents is fairly common during anesthesia. If not addressed, the patient is at risk of inhaling this fluid which can result in aspiration pneumonia. This is more likely to happen in a patient that has not been fasted, a patient that is not intubated, or a patient that has been intubated with an un-cuffed or improperly cuffed ET tube. If regurgitation does occur, the ET tube cuff should be checked to make sure that the seal is adequate. If possible, the patient's head should be lowered so that the fluid with easily drain out. The pharynx and esophagus should be suctioned and saline or water can be carefully infused into the mouth and esophagus to prevent caustic stomach acid from damaging the mucosa.

Certain procedures require special considerations for airway maintenance. Very short procedures may not require intubation and maintenance with injectable anesthetics or by mask by be adequate. Longer procedures require intubation to prevent waste gas exposure, to allow provision of IPPV, and to protect the airway.

Correction of laryngeal paralysis (arytenoid lateralization) leaves the patient at high risk of aspirating post-operatively. These patients should be kept intubated as long as possible and should be kept in sternal recumbence with the head and neck elevated to prevent passive regurgitation. The patient should be alert enough to sit up, swallow, cough and maintain their airway on their own before they are extubated.

Some surgeries of the maxilla and mandible may not be possible with an ET tube in place. A pharyngostomy tube can be placed in some cases. Usually the patient will be induced and intubated as usual and the surgeon will make an incision and pass a sterile wire-reinforced tube through the site. The tube will then be redirected down the trachea as the previous ET tube is removed. At the end of the procedure, the pharyngostomy tube can be removed and a normal ET tube can be placed as the patient is emerging form anesthesia.

Brachycephalic patients that undergo anesthesia can be challenging. Intubation can be difficult due to redundant tissue in the oral cavity, an elongated soft palate, and hypoplastic trachea. Most brachycephalic patients will require a smaller ET tube than a non-brachycephalic of similar size. It is important to be prepared for this possibility and to have ET tubes of various sizes available. Once the patient has received premedication and/or sedation they should be monitored closely as they may lose the ability to maintain their airway when sedated. Pre-oxygenation is recommended if not too stressful for the patient. An injectable induction protocol with a short acting agent is recommended so that the airway can be captured quickly. Mask induction is not recommended. Visualization of the arytenoid cartilages is often difficult and use of a laryngoscope is helpful.

Maintaining the airway during the recovery period

Ideally, recovering patients should be kept sternal with their head elevated and extended. When the patient is in sternal recumbency, it allows both lungs to expand equally. Keeping the head extended will keep the airway straight and open, and keeping the head and neck elevated will help to prevent passive regurgitation from occurring. Some animals may require supplemental oxygen in the recovery period. This is usually necessary in patients with respiratory disease and patients undergoing thoracotomy. Patients that have undergone procedures involving the head and neck are at risk for swelling that may lead to obstruction of the airway after extubation. This is also true for brachycephalic animals undergoing any procedure. It is important to maintain the ET tube as long as they will tolerate it. It is sometimes necessary to address swelling of the head, neck or airway with steroid administration. Dexamethasone sodium phosphate is a rapidly acting steroid that can be used for this purpose. Phenylephrine is a drug that causes vasoconstriction and can be instilled into the nares if congestion is preventing the movement of air. Ice can help as well if swelling is present. If the tongue or mucous membranes are edematous, topical application of glucose or dextrose can help to move the fluid out of the tissue and decrease swelling.

References

Tranquilli et al. Lumb and Jones Veterinary Anesthesia. Chapter 17

McCobb, "Airway Management and Intubation" Didactic Anesthesia Resident Training 1/22/07