The presence of disease has been shown to be positively associated with increased anesthesia-related mortality. Indeed, the possibility of rapid decompensation when sedative or anesthetic drugs are administered in the presence of respiratory disease makes anesthesia in these patients particularly challenging.
The presence of disease has been shown to be positively associated with increased anesthesia-related mortality. Indeed, the possibility of rapid decompensation when sedative or anesthetic drugs are administered in the presence of respiratory disease makes anesthesia in these patients particularly challenging. In many of these patients, anesthesia and sedation is being administered to perform diagnostic testing to characterize their respiratory disease.
An understanding of the pathophysiology of respiratory disease, awareness of the risks associated with respiratory diagnostic procedures, and advanced planning for complications are important components of sound anesthesia management in patients with respiratory disease. The purpose of this presentation will be to briefly review monitoring of patients with respiratory disease, and to discuss anesthetic management for respiratory diagnostic techniques.
Assessment of respiratory function is particularly important to prevent morbidity and mortality associated with respiratory diagnostic procedures. Blood gas analysis is considered the 'gold standard' in veterinary respiratory monitoring during anesthesia, but its use is limited in most cases. Placement of an arterial catheter requires skill, time, and is costly. Moreover, blood gas analysis using blood from an arterial catheter does not provide instantaneous results unless an indwelling optode system is used. However, an understanding of how pulmonary pathology will affect blood gases is essential to anesthesia management of the patient with respiratory disease. The following is a list of thumb rules that apply to respiratory physiology and monitoring:
• A value of 60 mm Hg is generally considered to be the lower safety limit for transient, acute arterial oxygen tension. This value corresponds to 90% saturation of hemoglobin and is also the value at which a significant increase in chemoreceptor activity is stimulated. Anemia, cardiovascular function, duration of hypoxemia, and other factors may diminish the 'safety margin' for hypoxemia.
• The level of hypercapnia (increased arterial carbon dioxide in the blood) that is dangerous depends on many factors:
o Oxygen concentration of inspired gas and metabolic acid-base status in the animal. An elevation in CO2 will result in decreased blood pH and can cause hypoxemia in animals breathing room air (21% oxygen concentration). For example, an increase in arterial carbon dioxide tension from 40 to 60 mm Hg would result in a decrease in pH from 7.4 to 7.3 in the normal animal. This is not dangerous in healthy animals. Concurrently, oxygen tension in the arterial blood would drop to 60-65 mm Hg from a normal value of 85-100 mm Hg. A pre-existing acidosis, anemia, or hypoxemia may accentuate the abnormalities described for the healthy animal.
o Hypoventilation-induced hypoxemia can be prevented by supplementing oxygen. An inspired oxygen concentration of 50% or higher will prevent physiologically significant hypoxemia due to hypoventilation.
• Pulse Oximetry is the most commonly useful monitoring modality in anesthetized patients with respiratory disease. Pulse oximetry can be unreliable when hypotension, movement, or vasoconstriction is present. In addition, lingual probe placement is difficult to maintain during procedures where the oral cavity and/or tongue may be manipulated (i.e., bronchoscopy, laryngoscopy). In spite of these negative characteristics, the importance of the information provided by the instrument makes it and essential tool in patients undergoing anesthesia for respiratory diagnostic procedures.
• Capnography is used to noninvasively measure adequacy of ventilation. Capnography is extremely useful in anesthetized and intubated patients. However, many diagnostic procedures do not allow for endotracheal intubation.
General principles of management
• Preoxygenation is always a good idea in animals with pulmonary parenchymal disease or in those likely to have upper airway obstruction or tracheal collapse.
• The veterinarian should have the supplies and equipment necessary to induce general anesthesia rapidly, perform intubation (or tracheostomy), and provide artificial ventilation when necessary. Animals with respiratory disease can rapidly decompensate and the clinician must be ready to provide rapid therapy.
• The practitioner should be prepared to provide increased monitoring and support in the post-anesthesia period. In particular, assessment of patient oxygenation, breathing pattern, and adequacy of ventilation are monitored more aggressively after anesthesia.
• Oxygen supplementation by chamber, tent, or nasal catheter may also be required in the post anesthesia period.
Evaluation of the nasal cavity using a rigid or flexible endoscope is an extremely stimulating procedure and general anesthesia is a requirement. Conventional anesthesia protocols are often used for these procedures because endotracheal intubation does not usually interfere with the diagnostic procedure. Response to pharyngeal /nasal manipulations can be decreased through pretreatment with topical local anesthetics. When nasal biopsy is a part of the procedure, blood loss can be significant. It is essential that the pharyngeal region be cleared of blood and blood clots prior to terminating anesthesia. In addition, the use of a cuffed endotracheal tube will prevent aspiration of blood and obstruction of the trachea. Nasal packing, with or without epinephrine, phenylephrine, or oxymetazoline, may be used to control mucosal bleeding prior to termination of anesthesia.
Removal of the endotracheal tube should only be done after the pharynx has been cleared of blood and bleeding controlled. The tube may be removed with the cuff partially inflated if the clinician is concerned about possible aspiration of blood or fluid. Postoperative monitoring should be extended until the animal is clearly able to maintain sternal recumbence, swallow, and manage its airway.
Laryngoscopy is often performed to assess laryngeal function. Thiopental, propofol, diazepam/ketamine, and isoflurane have all been used for assessing laryngeal function. In one investigation, thiopental provided the best exposure to the larynx of the three injectable agents. Regardless of the protocol used, a light plane of anesthesia is required to avoid obscuring subtle changes in laryngeal function. Unfortunately, a light plane of anesthesia is often associated with significant patient resistance to examination and inadequate laryngeal exposure. To alleviate this problem, doxapram HCl, a respiratory stimulant, can be given to increase respiratory drive, velocity of air flow, and magnify changes in airway pressure during inspiration and exhalation. Thus, paradoxical laryngeal movement will be accentuated in animals with laryngeal paralysis. It is important to have an observer note the timing of inspiration and exhalation so that the individual examining the larynx can determine if abduction occurs during inspiration, as it should, or if it is paradoxic.
Transtracheal aspiration/tracheal wash
A tracheal wash may be performed without invading the oral cavity by passing a through-the-needle catheter through the cricothyroid ligament or between tracheal rings and into the trachea (i.e., transtracheal wash). Often, this procedure can be done with local anesthetic. In some animals, mild to moderate sedation is also beneficial. When sampling of airways is to be done using a transoral approach, hypnosis is necessary to allow a sterile endotracheal tube to be passed through the larynx and into the trachea (i.e., endotracheal wash). After mild to moderate premedication, thiopental, valium-ketamine, or propofol may be used to induce anesthesia. Due to its noncumulative nature, propofol is used frequently for this procedure. It should be given slowly and to effect to avoid excessive anesthetic depth, hypotension, and apnea. Because propofol can cause hypoxemia and/or apnea, patient monitoring, oxygen supplementation, and the provision for artificial ventilation must be part of planning for the procedure. In general, a dose of 1-8 mg/kg may be used for induction of anesthesia. A continuous rate of infusion of 0.1-0.4 mg/kg/min may be used during a procedure. Alternately, repeated boluses of 0.5-1 mg/kg can be given as needed to maintain hypnosis and relaxation.
Bronchoscopic examination and airway lavage of patients with respiratory disease can yield valuable diagnostic information in patients with respiratory disease, and can be particularly challenging from an anesthetic managment standpoint. Low-dose premedication of the patient (i.e., butorphanol) is useful to minimize anxiety associated with anesthetic preparation and preoxygenation. Placement of an intravenous catheter is essential for those patients to allow for rapid management of anesthetic depth. Propofol is used frequently for bronchoscopy as repeated administration is not associated with prolonged recoveries (see transtracheal aspiration/tracheal wash above). Bronchoscopy can be performed through a pre-placed endotracheal tube in medium to large dogs and anesthesia and oxygen can be administered via an anesthesia machine. In most cases, bronchoscopy is done without endotracheal intubation. Oxygen may be supplied to the animal through a port in the bronchoscope or by inserting a soft catheter into the trachea and insufflating oxygen into the trachea next to the bronchoscope. When oxygen is insufflated, care must be taken to avoid overinflation of the lungs and barotrauma if the bronchoscope becomes wedged in the trachea or bronchus. Movement of the endoscope may cause the oxygen insufflation catheter to become dislodged or to be advanced too far into the airways. Pulse oximetry is an important monitoring tool during and after bronchoscopy. Significant desaturation (<90%) should be treated by withdrawing the bronchoscope and administering supplemental oxygen until saturation values return to normal. After bronchoscopy, and especially after bronchoscopy with bronchoalveolar lavage, patients should receive supplemental oxygen and be monitored until they are able to maintain normal hemoglobin saturation while breathing room air spontaneously. As with the other respiratory diagnostic procedures, the clinician should be prepared to intubate and ventilate the patient in the event of emergency.
Transthoracic fine needle aspiration of pulmonary tissue/thoracic masses
The level of anesthesia required for fine-needle aspiration is dependent upon patient mentation and location of the target mass. When general anesthesia is required, it is important to avoid positive pressure ventilation during and after aspiration. It is especially important to monitor oxygenation, ventilation, and respiratory pattern after transthoracic fine needle aspiration so that early diagnosis of and intervention for hemorrhage or pneumothorax can be initiated should it occur.
Diagnostic thoracoscopy is less invasive than thoracotomy. Although the procedure may be performed without general anesthesia in human beings, this is not the case for veterinary patients. There is no single, ideal choice of drugs used for this procedure. Indeed drug choices depend most on the condition of the animal. In some cases, neuromuscular blockade is used in conjunction with general anesthesia to abolish movement associated with spontaneous ventilation. Thoracoscopy may be performed with or without collapsing the lung in the operative thorax. Collapse of the lung in the operative thorax increases the working space available to the thoracoscopist. One-lung ventilation with selective intubation using a double-lumen endotracheal tube is the most common way to accomplish deflation of the lung in human medicine. Unfortunately, these specialized tubes are made for human beings and may not be appropriate for the sizes of canine or feline patients. In veterinary patients, a bronchial blocker or Fogarty catheter is frequently used to isolate the lung to be collapsed. These devices can be placed prior to intubation with an endotracheal tube with the catheter and endotracheal tube lying next to one another in the trachea. Alternatively, the catheter can be passed through the lumen of the endotracheal tube in larger dogs. The lung may then be collapsed by application of positive pressure to the pleural space, or negative pressure to the lumen of the catheter/blocker. It is not unusual for arterial oxygenation to be decreased below normal and carbon dioxide to be increased above normal during anesthesia and one-lung ventilation. Because one-lung ventilation is somewhat difficult to establish (especially so in small patients) and may carry risks associated with bronchial blocker placement, two-lung ventilation with sustained pneumothorax has been investigated as an alternative to one-lung ventilation. While the technique does not cause significant cardiovascular impairment in healthy dogs, the effects of sustained pneumothorax might be clinically significant in dogs with decreased vascular volume, pulmonary disease, or cardiovascular disease. As with other intrathoracic procedures, removal of intrapleural gas, oxygen supplementation, and postoperative patient assessment are important principles of postanesthesia management.
Gross et al. Anesthetic Considerations for Endoscopy. In: Macarthy TC: Veterinary Endoscopy. 2005, 21-27.
Johnson LR. Clinical Canine and Feline Respiratory Medicine 2010