Download Anesthesia and Blood Gas Monitoring

Anesthesia and Blood Gas Monitoring
Cathy Ann Just, DVM
Overview
Veterinary anesthesiologists regularly use blood gas and acid-base analysis
for large animal cases, especially equine. In small animal practice,
anesthesiologists reserve blood gas and acid-base analysis for cases involving geriatric, sick or debilitated animals, emergencies, thoracotomies,
major abdominal procedures, and cases of cardiac or respiratory disease.
Small animal practices can benefit from blood gas information in procedures requiring some form of tranquilization, sedation, or general
anesthesia. This paper discusses the patient benefits of adding blood gas
and acid-base monitoring to many routine surgical procedures. Assessment of patient respiratory and metabolic function would be beneficial
in the following situations:
Condition
Geriatric, sick or debilitated animals
Emergencies
Thoracotomies
Major abdominal procedures
Pharmaceuticals
Anesthesia is a reversible
process intended to produce a
safe and effective means of
chemical restraint. A carefully
constructed and properly
executed anesthesia plan will
expedite clinical procedures
with minimal stress, pain, or
toxic side effects to the patient.
Numerous drugs, drug combinations, and routes of administration are available to the
practitioner to produce a
smooth, uneventful anesthetic
event.
All anesthetic drugs alter a
patient’s ability to maintain
homeostasis.
Respiratory
depression and hypoventilation
are among the most common
consequences of anesthesia,
and, if ignored or undetected,
will lead to apnea, hypoxemia,
and respiratory emergency.
Successful anesthesia manage-
Procedure
Extensive dental procedures,
diaphragmatic hernias
Pneumothorax, complex fracture repair
Lung lobectomy, tumor removal
GDV correction +/- splenectomy,
extensive gastrotomy/enterotomy
ment will maintain adequate oxygenation for the procedure through
recovery. Conversely, poorly managed anesthesia provides insufficient
tissue oxygen levels and can lead to cell death, organ failure, and
ultimately patient death. A thorough knowledge of available anesthetic
drugs and their potential side effects can help even experienced surgeons
improve anesthesia management.
Blood Gas Parameters
Respiratory homeostasis requires a balance between dissolved oxygen
(O2) and carbon dioxide (CO2) in blood. Blood gas analysis measures
the pressure exerted by these gases in arterial blood (PaO2 and PaCO2)
and is the industry standard for assessing respiratory function. Arterial
samples are most frequently acquired from the femoral, lingual, or
dorsal pedal arteries. Properly anesthetized animals should not experience discomfort from arterial sampling and the most frequently
encountered complication is bruising or hematoma formation. Understanding the effects and consequences of anesthesia on patient respiratory function will allow the veterinarian to intervene and avoid undesirable results.
Common Anesthetic Drugs and Their Effects on Respiratory Homeostasis
Drug Category
Example
Effect on Respiratory Function
Alpha2 Adrenergic Agonists
Xylazine (Rompun)
Medetomidine (Dormitor)
Severe respiratory depression and decreased
respiratory rate. Decreases respiratory center
sensitivity and raises threshold to increases
in CO2.
Barbituates
Phenobarbital, Thiopental
Dose-dependent respiratory depressant. Apnea.
Benzodiazepines
Diazepam (Valium)
Minimal direct respiratory effects. Can increase
respiratory effects of other drugs when used in
combination.
Dissociogenics
Ketamine, Telazol
Produces an apneustic breathing pattern which
can lead to increased CO2 and decreased
arterial pH.
Hypnotics
Propofol
Respiratory depression and apnea are common.
Dose-dependent depression of respiratory
system leading to decreased responsiveness to
CO2 and hypoxia.
Inhalants
Neuroleptanalgesia
Fentanyl + Droperidol
Respiratory depression. Can reverse respiratory
depression with Naloxone.
Opioids
Morphine, Oxymorphone,
Fentanyl
Dose dependent respiratory depression.
Increases threshold of respiratory center to
increases in CO2.
Tranquilizers
Acepromazine, Promazine
Decreased respiratory rate. Decrease respiratory
center sensitivity to increases in CO2.
(Continued)
Interpreting Respiratory Function Parameters from
Arterial Blood Samples
Parameter
Causes
system. Selected monitoring techniques should be simple,
specific, reliable, accurate, and complementary; never totally rely
on just one piece of monitoring equipment.
Consequences
Decreased PaO2
OXYGENATION
Hypoventilation
Inspiration of hypoxic gas mixtures
Impaired pulmonary gas exchange
A combination of any of the above.
TM
Hypoxemia
Compromised tissue oxygenation
Increased PaCO2
VENTILATION
Inadequate ventilation
- deep anesthesia
- inappropriate ventilator setting
CO2 administration/Equipment
malfunction
- exhausted absorbent
- one-way valve failure
Increased cellular production
- fever
Decreases pH causing respiratory
acidosis*. Respiratory acidosis can
result in cardiovascular failure
secondary to decreased myocardial
contractility and vasodilation.
Conclusion
Decreased PaCO2
Excessive ventilation
- inappropriate ventilator setting
- excessive manual ventilation
The VitalPath Blood Gas and Electrolyte Analyzer from Heska
Corporation is the latest addition to Heska’s advanced laboratory
analyzer platform. Among other key advantages, the VitalPath
analyzer offers fast, accurate results, a color LCD touchscreen,
simplified operation, and flexible sampling options. With as little
as 60 microliters of whole blood, serum, or plasma, the VitalPath
analyzer provides results in just 50 seconds for over 30 parameters. The flexible testing menu allows you to choose which tests
you would like to run for each sample. The analyzer
auto-calibrates after every sample, ensuring reliable results for
confident diagnosis and design of treatment plan.
The complex nature of anesthesiology and potential for a multitude of adverse events and responses implores the conscientious
clinician to include acid-base and blood gas analysis in their
anesthesia management program. The speed, accuracy, and
advanced capabilities of the VitalPath Analyzer readily
accommodate this need.
Prolonged return to spontaneous
respiration due to lack of CO2
stimulus.
*Do not treat respiratory acidosis with bicarbonate. Improve ventilation.
Tissue Perfusion
Oxygen is poorly soluble in plasma, therefore hemoglobin (Hgb), an
oxygen-carrying pigment, is required to ensure adequate quantities of
oxygen are delivered to the tissues. Oxygenated and deoxygenated Hgb
reflect different wavelengths of light; pulse oximetry takes advantage of
this difference. Using spectrophotometry, pulse oximetry monitors are
able to detect the patterns of reflected light and determine the percentage of oxygenated hemoglobin, the SpO2. Pulse oximetry is
non-invasive and simple to perform but it can be a problematic and
does not adequately reflect tissue oxygenation. The following factors
can contribute to inaccurate readings by a pulse oximeter: haired and or
pigmented skin, movement, hypotension, and peripheral vasoconstriction. Although mucous membrane color and SpO2 can predict tissue
perfusion, arterial blood gas is considered the industry standard for
measuring patient oxygenation and carbon dioxide elimination.
Remember an anesthetized animal with visibly pink mucous
membranes does not rule out the possibility of clinically significant
hypoxemia.
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monitor more than one body system and more than one parameter per
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