Anesthesia Stages Quiz

Monitoring Anesthetic Depth

• The lecture objectives are to recognize signs associated with anesthetic planes, describe patient appearance in surgical planes, and list factors affecting patient depth monitoring.

What is General Anesthesia?

• General anesthesia involves losing consciousness and sensation, and includes muscle relaxation, analgesia, stress response suppression, and hypnosis.
• It's a reversible process.
• It can be induced using one drug or a combination.
• Drug effectiveness can vary depending on the species and procedure.

Classical Stages and Planes of Anesthesia

• The table details anesthetic levels, surgical stimulation reaction, palpebral reflex, eye and pupil reflex, jaw tone, heart rate, and respiratory rate.
• Stage I involves voluntary movement, some analgesia, and progressive ataxia leading to recumbency. It can be erratic.
• Stage II is characterized by struggling, vomiting, regurgitation, increased heart rate, and active reflexes, laryngospasm and dilated pupils
• Stage III, Plane I is minimal anesthesia, maintaining eye movements, suitable for minimally invasive procedures.
• Stage III, Plane II a medium plane where most surgeries are performed, having stabilized respiration and heart rate, strong corneal reflex and relaxed muscles.
• Stage III, Plane III is a deep plane described by decreased heart rate, slow, shallow abdominal breathing. Absence of jaw tone, no response to stimulation, and weak reflexes like palpebral and corneal.
• Stage III, Plane IV entails dilated pupils, hypotension possibly progressing to shock, slow irregular respirations and the lack of muscle tone and anal sphincter tone.
• Stage IV represents severe CNS depression, indicated by extreme pallor, prolonged CRT, respiratory arrest, absence of ocular reflex and flaccid muscle tone. Recovery in this stage requires urgent intervention.

Stage I

• Defined as the time from administering induction drugs until loss of consciousness.
• Characterized by voluntary movement, ability to turn or lift head, and some analgesia.
• Patients may exhibit signs like progressive ataxia, excitement, struggling, breath holding, and increased heart rate.

Stage II

• Marked by an exaggerated response to stimuli, vomiting or regurgitation, salivation, laryngospasm, and dilated pupils.
• This stage might feature increased heart rate, irregular breathing patterns, breath holding, and involuntary movement, potentially leading to delirium

Stage III

• Characterized by progressive loss of consciousness, reflexes, and muscle tone.
• Breathing patterns become slower and more regular. Vomiting and swallowing reflexes are abolished, and jaw tone is lost.

Stage III, Plane I

• Also known as a "light" plane, this stage continues until eyeball movement stops.
• Suitable for minimally invasive, or painful procedures.

Stage III, Plane II

• Usually a "medium" plane where most surgical procedures are performed.
• Characterized by progressive intercostal paralysis, and stabilized heart and respiratory rates.

Stage III, Plane III

• Considered a "deep" plane.
• Show a decrease in heart rate and slow, shallow abdominal breathing.
• The anesthetic level has minimized responses to surgical stimulation and caused profound muscle relaxation.

Stage III, Plane IV

• Marked by pupil dilation, and progressing towards hypotension, shock if not quickly reversed.
• Slow, irregular respirations along with the loss of anal sphincter tone.

Stage IV

• Represents profound CNS depression, possibly leading to death without immediate intervention.
• Key indicators include pale to white mucous membranes, prolonged CRT (>3 seconds), respirations cease and it is followed by cardiac arrest.

Summary of Monitoring Depth

• Check for palpebral reflex, eye position (ventral), lacrimation, nystagmus, corneal reflex, jaw tone, anal reflex, pedal reflex, and overall muscle tone. These can vary significantly between species.

What is the Goal for a Surgical Anesthesia Plane?

• Relaxed jaw tone easily opens with two fingers
• Mild to no palpebral reflex
• Ventral medial eye rotation
• Relaxed anal tone
• Smooth, regular respiratory pattern
• Normal heart rate
• Absence of swallowing, coughing, vomiting or laryngospasm.

Species Specific Signs

• Rodents: Observe feet, ears, and muzzle for pallor or cyanosis. Pedal reflex (pinching toes).
• Poultry: Examine comb and wattle for pallor; vent reflex.
• Rabbits: Use color of light reflected from the eye to assess circulation.
• Horses: Lacrimating, eye stays central, medial eye rotation, and corneal reflex persists.
• Cats: Potential for laryngospasm, retained cough, swallow/vomiting reflex.
• Cattle: Consistent and reliable eyeball rotation.

Factors Affecting Monitoring Depth

• Drugs, Ketamine, Propofol, Oxygenation adequacy,CO₂ retention and physical status are primary difficulty factors. Ketamine may exhibit less typical eye signs and still show blinking and swallowing reflexes, Propofol doesn't produce analgesia and can cause apnea.

Other Methods to Measure Anesthesia Depth

• Anesthetic gas analyzers can measure minimum alveolar concentration (MAC), although they are not always readily available.

Bispectral Index (BIS)

• BIS analyzes EEG values from the brain to assess the depth of anesthesia in animals or humans. A BIS of 60 or greater often indicates appropriate anesthetic level. BIS is frequently used in research.

Recovery

• Recovery progresses in reverse order of the anesthetic stages and species and patient responsiveness are different.
• Post-operative pain in horses can be severe due to delirium/dysphoria, especially when transitioning from inhalant anesthesia.
• Anesthetized patients require monitoring for a suitable time, depending on the species.

Resources

• Key readings include various veterinary anesthesia handbooks and manuals.