Anesthesia for patients with respiratory disease (Proceedings)
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.
Patient monitoringAssessment 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.