Maintenance of Anesthesia

• Maintenance of Anesthesia overview

Learning Objectives

• Understand the difference between general anesthesia via inhalation anesthetics, total intravenous anesthesia (TIVA), and partial intravenous anesthesia (PIVA)
• Recognise physical/chemical properties of inhalant anesthetics and their effect
• List factors causing rapid alveolar anesthetic tension change (PA)
• Describe how inhalant anesthetics are metabolized
• Define minimum alveolar concentration (MAC), know isoflurane and sevoflurane values for main domestic species, and factors affecting MAC
• Describe the effect that inhalant anesthetic drugs have on the body

Currently Used Inhalants

• Isoflurane is purple
• Sevoflurane is yellow
• Desflurane is an inhalant anesthetic
• Nitrous Oxide (N2O) is also an inhalant anesthetic
• Halothane is not available in North America

Equipment

• Equipment needed for the maintenance of anesthesia:
  • Oxygen tanks
  • Anesthetic machine
  • Breathing circuits
  • Endotracheal tubes
  • Soda lime or other CO2 absorbent

Chemical & Physical Characteristics

• Desirable inhalants should be less reactive, more potent, and nonflammable through halogenation (adding Fl, Cl, or Br)

Properties Determining Method of Administration

• Boiling point
• Liquid density (specific gravity)
• Vapor pressure

Properties Determining Kinetics

• Solubility of gas:
  • Blood/gas partition coefficient
  • Oil/gas partition coefficient

Gas vs Vapor

• Gas exists in gaseous form at room temperature and sea level pressure (e.g., N2O)
• Vapor is the gaseous state of a substance that is a liquid at ambient temperature and pressure (e.g., isoflurane, sevoflurane, halothane, desflurane)
• Gas laws describe predictable gas behavior:
  • Boyle's law
  • Charles's law
  • Gay-Lussac's law
  • Dalton's law of partial pressure

Vaporization

• Vaporization: change in state from liquid to gas
• Equilibrium leads to no further molecule loss, and the gas phase saturates
• Vapor pressure indicates the ability to evaporate (enter the gas phase)
• Saturated vapor pressure indicates max molecule concentration in a vapor state
  • Temperature-dependent and unique for each agent

Saturated Vapor Pressure (SVP) vs Temp

• As liquid temperature increases, molecules escape/enter the gas phase
• Higher temperature means higher vapor pressure
• Lower temperature means lower vapor pressure

The Anesthetic Vaporizer

• Most anesthetic SVP does not allow for safe clinical use
• Vaporizers control the amount delivered to the patient
• Gas is diverted into two streams: bypass and vaporizing chamber
• Modern vaporizers are variable-bypass, concentration-calibrated, agent-specific, and temperature-compensated

Solubility of Gases

• This affects uptake and distribution in the body, thus induction and recovery speed
• Blood/gas partition coefficient:
  • A low number is more desirable for faster induction/recovery
  • Isoflurane > Sevoflurane which means faster induction of recovery
• Solubility in lipid (oil) correlates with anesthetic potency
• Oil/gas partition coefficient:
  • Isoflurane > Sevoflurane
• Less gas dissolves in solvent as temperature increases, and vice versa

Factors That Rapidly Change Alveolar PA

• Increased alveolar delivery and alveolar partial pressure controls anesthesia

Increased Alveolar Delivery

• Increased inspired anesthetic concentration
  • Increased agent vaporization
  • Increased vaporizer dial setting
  • Increased fresh gas flow
  • Decreased gas volume in the patient's breathing circuit

Increased Alveolar Ventilation

• Increased minute ventilation Tidal Volume x RR
• Decreased dead space ventilation

Decreased Removal From Alveoli

• Decreased blood solubility of anesthetic
• Decreased cardiac output
• Decreased alveolar-venous anesthetic gradient

Metabolism

• Illustrates the elimination routes of different volatile anesthetics
• Methoxyflurane has the highest percentage metabolized at 50-75%, while Nitrous oxide as the lowest at 0.004%
• in humans

Minimum Alveolar Concentration (MAC)

• MAC is the minimum alveolar concentration of inhaled anesthetic at 1 atmosphere that prevents movement in 50% of subjects exposed to supramaximal noxious stimulus
• Another way to say ED50 for inhalants
• Measuring potency and inverse relationship
• A highly potent inhalant such as halothane has a low MAC value.
• Corresponds to ED50
• MAC values are determined in the lab setting with healthy patients and the absence of other drugs or typical clinical circumstances
• The ED95 is 1.2 to 1.4 x MAC, meaning 95% of patients are anesthetized
• 1.5 x MAC = surgical anesthesia

MAC Values (%) For Various Species

• Lists the MAC values for Halothane, Isoflurane, Sevoflurane, Desflurane and Nitrous oxide for Dogs, Cats, Horses, Cows, Goats, Sheep, Pigs, Chickens, Mouse, Rabbit, Monkey, Goldfish and Humans

Factors That Effect MAC

• Lists the effects that different factors have on MAC

Increase MAC

• Hyperthermia increases MAC
• CNS Stimulation (e.g., ephedrine) increases MAC
• Increased metabolic rate and/or stress (e.g., hyperthyroidism) increases MAC

Decrease MAC

• Lower blood pressure decreases MAC
• Hypothermia decreases MAC
• CNS Depression (e.g., opioids, sedatives) decreases MAC
• Smaller body size/weight decreases MAC
• Age of animal (e.g., geriatric patient) decreases MAC
• Hyponatremia decreases MAC

No Change in MAC

• Higher blood pressure has no effect on MAC
• Anticholinergics have no effect
• Duration of anesthesia has no effect
• Gender has no effect
• Abnormal [K+]

Isoflurane

• Isoflurane (purple) stable in storage due to no preservative needed
• There is low blood gas solubility
• It more potent than sevoflurane
• There is fairly rapid induction and recovery, allowing for good muscle relaxation
• There is less cardiac depreesion that halothane, with a greater resp. depression
• Hypotension occurs due to vasodilation and decreased myocardial contractility
• Hypoventilation is common
• Less than 1% is metabolized, and is mostly eliminated by the lungs
• It has reasonable cost
• Mask induction happens due to odorous smell causing breath holding and bronchoconstriction
• Carbon monoxide is produced when exposed to desiccated CO2 absorbent

Sevoflurane

• Sevoflurane (yellow) is lower in blood-gas partition coefficient compared to isoflurane
• Has more rapid induction and recovery
• it is less potent than isoflurane and needs a higher vaporizer setting
• Similar in cardio-respiratory depression than isoflurane
• Possesses good muscle relaxation
• It can trigger malignant hyperthermia
• Mask induction occurs due to less odor, leading to smoother induction
• Only ~3% is metabolized by the body, with the rest of it being eliminated by the lungs
• CO2 absorbents degrade isoflurane to produce Compound A which has no clinical significance

Pharmacodynamics

• The mechanism of action on inhalant anesthetics is not fully understood
  • Multiple cell receptors and ion channels may be involved
  • Spinal cord and brain may be involved
• Reversible immobilization results in all species, which includes plants and protozoa
• CNS decreases cerebral metabolic rate (oxygen consumption); will cause either no change or an increase in cerebral blood flow; decreases cerebral perfusion pressure; increases ICP
• In Respiratory, the drug and species will specific depression of ventilation increasing CO2; bronchodilation occurs; and desflurane irritates airway
• In Cardiovascular can decrease cardiac output, and BP; arrhythmias associated with certain drugs may be exaggerated

TIVA and PIVA

• Anesthetic techniques that use IV infusions of drugs for anesthetic states
• TIVA should be limited to 1 hour, and prolonged recovery occurs
• Use supplemental O2
• An example of TIVA is triple drip infusion of ketamine, xylazine, and guaifenesin to produce general anesthesia on horses undergoing castration
• PIVA: "MLK" + reduced isoflurane concentration for TPLO

Guaifenesin (GG)

• Guaifenesin (GG) is also known as centrally acting muscle relaxant with sedative properties
• Co-administered with other anesthetic agents for the IV induction and/or maintenance of anesthesia
• Usually used in horses and Ruminants
• The mechanism is not clear, it affects the brain and spinal cord Have no analgesic properties
• It has wide therapeutic margins, but can cause cardio-resp depression if overdoes
• Elimination via hepatic metabolism and renal excretion, so rapidly eliminated in female ponies
• Rewarm before use to not let it precipitates out of solution when stored below room temp
• Should be prepared as 5 to 15% solution in 0.9% NaCl or 5% dextrose
• Has concentration dependent risk of hemolysis to 5% cattle and to 15% horses, and also thromobosis to 7%
• It there is Tissue necrosis if administered outside the vein, always use IV catheter to administer

Resources

• Chapter 16 (pages 297-322) in Lumb & Jones 5th ed
• Chapter 9 (pages 163-187) in the Handbook of Veterinary Anesthesia 5th ed