Chapter 31
Anesthetic Considerations for Upper and Lower Respiratory Disease
Take a deep breath!
Rebecca A. Johnson
Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA
- Q. What are the pre-anesthetic considerations for patients with respiratory disease?
- A. The main purpose of pre-anesthetic assessment of patients with respiratory disease (or any other disease) is to identify those at risk for peri-anesthetic complications so that appropriate therapy can be instituted in a timely manner. As such, patient morbidity and even mortality will be reduced. Many general anesthetics and analgesics have profound effects on the respiratory system in normal patients and these effects will be amplified in one with respiratory disease. Therefore, a complete physical exam focusing on cardiothoracic auscultation is highly recommended. If respiratory disease is suspected then thoracic radiographs, pulse oximetry +/− capnometry, and arterial blood gas analyses are warranted.
Unless respiratory disease is severe, physical exam findings may be subtle and non-specific. Tachypnea may be present; however, that will not indicate that the animal is normoxic or normocapnic; respiratory frequency cannot be used to determine adequacy of ventilation since ventilatory deficiencies mainly manifest as impaired respiratory volumes (i.e., tidal volume, functional residual capacity, etc.). If disease is more severe, dyspnea may be present. Mucous membranes may be cyanotic (from hypoxia), bright red/flushed (from hypercapnia), or normal pink in color. Abnormal lung sounds may or may not be present, depending on the disease process, and may include crackles or wheezes. Severe cases can present with collapse and hyper- or hypothermia. Since these signs are variable, further diagnostics including radiographs will aid in the diagnosis of respiratory disease. Pulse oximetry should be viewed as a screening tool only for patient oxygenation (see Chapter 18). Capnometry is warranted if disorders concerning CO2 production or elimination (ventilation) are suspected. However, accurate readings require endotracheal intubation. Thus, arterial blood gas analyses are required to definitively identify and quantify the severity of hypoxia, hypo-, or hypercapnia and additional acid-base derangements associated with respiratory disease.
- Q. How can patients with respiratory disease be classified prior to general anesthesia?
- A. The American Society of Anesthesiologists (ASA) physical status classification system is a system for assessing patient condition prior to anesthesia and consists of five classes from healthy/normal to moribund. In patients with respiratory disease, this system can be adapted as follows:
- I. Normal, healthy patient.
- II. Patient showing dyspnea with moderate exertion.
- III. Patient showing dyspnea with mild exertion.
- IV. Patient with dyspnea at rest, constant threat to life.
- V. Terminal pulmonary disease, not expected to survive with or without intervention.
It is important to note that this classification system may not be appropriate for every patient with respiratory disease, nor is it a “risk assessment.” It simply allows the anesthetist to evaluate and classify the patient’s condition before anesthesia.
- I. Normal, healthy patient.
- Q. Why does the patient with respiratory disease present unique challenges with regards to general anesthesia?
- A. Patients with respiratory disease commonly have physiological abnormalities such as hypoventilation, ventilation-perfusion abnormalities, and ineffective gas exchange mechanisms. These impairments further compound the issues that patients with no respiratory disease encounter during general anesthesia since both CO2 elimination (via ventilation) and oxygenation are impaired in many normal anesthetized patients.
For example, current techniques in veterinary anesthesia commonly center on inhalant-based anesthetic procedures. If patients cannot exchange oxygen (O2) and/or remove carbon dioxide (CO2) efficiently, transfer of inhalants into the lungs might also be impaired. Inhalants are unique among pharmacologic agents as their uptake and elimination depend almost solely on normal respiratory function, with minimal metabolism in the body.
In addition, general anesthesia itself has profound effects on respiratory function. Patients routinely become hypoxemic if enriched oxygen mixtures are not used (inspired fraction of O2 at least 0.3–0.4). Moreover, the resting lung volume or functional residual capacity is reduced during anesthesia [1], lung compliance is reduced [2], and overall pulmonary system resistance increases considerably [2].
Significant atelectasis has long been associated with general anesthesia, contributing to anesthesia-associated hypoxemia [3]. The dead space to tidal volume ratio (VD/VT) is also increased during anesthesia (to ∼50% or more) [4]. Thus, ventilation-perfusion mismatching leading to hypoxemia is also extremely common. In addition, many drugs such as inhalant anesthetics and opioids depress minute ventilation in a dose-dependent manner, shifting the CO2-response curve to the right and reducing the slope (see Chapter 19). Altogether, these effects are further amplified in patients with ventilatory impairment.
- Q. What are some common ventilatory disorders in patients requiring anesthesia?
- A. Common upper and lower airway diseases/conditions that affect normal ventilation are listed in Table 31.1.
Table 31.1 Common causes of ventilatory dysfunction.
Upper airway
Lower airway
Other causes of respiratory
compromise
Laryngeal paralysis
Tracheal collapse
Brachycephalic syndrome
Feline upper respiratory disease complex
Canine infectious respiratory disease complex
Space-occupying mass
(hematoma, neoplasia,
foreign body)
Acute respiratory distress syndrome
Chronic obstructive pulmonary disease
Asthma
Severe pulmonary edema
Pulmonary thromboemboli
Pneumonia
Fibrosis
CNS depression (drugs, neurologic disease)
Neuromuscular disease (myasthenia gravis, tetanus, electrolyte abnormalities)
Restrictive extrapulmonary disorders
Marked obesity
- Q. What are the effects of sedatives on ventilation in dogs and cats?
- A. Sedatives and tranquilizers such as the phenothiazines (acepromazine) [5,6], alpha-2 agonists (dexmedetomidine) [7–9] and the benzodiazepines (diazepam, midazolam) [10] have at least some respiratory depressant effects in dogs and cats, although the clinical significance is minimal when each sedative is used alone. For example, medetomidine in dogs decreases respiratory frequency and increases PaCO2 but has no effect on PaO2
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