inhalant anesthetics

• Control of airway
• High FiO₂ (fraction of inspired oxygen)
• Rapid changes in depth
• Rapid recovery - ventilation
• Compatible with renal or hepatic disease
• Cost effective

• 1. Breathed in
• 2. Absorbed - alveoli into blood
• 3. Absorbed - blood to brain

To acheive an adequate partial pressure in the brain to cause the desired level of CNS depression.

• What is added
• -Inspired Concentration
• -Alveolar Ventilation
• What is taken away
• -Solubility of anesthetic in blood
• -Cardiac output

• Directly proportional
• Higher % concentration - faster to desired partial pressure

• Function of vaporizer setting and fresh gas flow into the circle system
• % concentration at induction is higher than at maintenance
• -The higher the inspired partial pressure, the more rapidly the alveolar partial pressure approaches the inspired partial pressure

• Better ventilation = faster increase in alveolar %
• Depends on tidal volume and RR
• Rapid shallow breathing = slow rise of alveolar pressure (dead space rebreathing)
• Spontaneous vs. Mechanical (much more efficient)

• Time required for flow through a container to equal the capacity of the container
• Lung = FRC/V_alveolar
• Anesthetic circuit = circuit capacity/FGF
• (FRC = functional residual capcity, FGF = fresh gas flow)

• Anesthetic Solubility
• Cardiac Output

• Capacity of blood to take up anesthetic
• High solubility = more drug moves into blood from alveoli
• Drug dissolved in blood = not available as a diffusible gas
• High Sol = more molecules required to saturate blood before PP can increase
• Speed of onset is closely related to anesthetics solubility

• Index of solubility of anesthetic agent in the blood
• Measured at equilibrium
• -PP are equal
• -Concentrations are not equal
• High number = high solubility
• Methoxyflurane>Halothane>Isoflurane>Sevoflurane

• Blood flow through lungs removes inhalant from alveoli, lowers P_A, delays onset
• High CO = large distribution throughout body
• = less than 8% total to brain
• Low CO = Heart-Lung-Brain perfusion preserved
• = very high % of total CO goes to brain
• = large amount of anesthetic to brain

• Rapid in
• -sick/debilitated animals
• -CV compromise
• Slow in
• -stressed/excited animals

• Good alveolar ventilation
• Low CO
• Low anesthetic solubility

• P_A high enough to guarantee sufficient P_Br
• MAC = minimum alveolar concentration
• _{% at which 50% of animals will not move in response to surgical stimulus}
• more than 1 MAC to guarantee surgical anesthesia in 100% of patients
• 1.2-1.3 x MAC required for most animals

• Gas phase - partial pressure = % conc x atmospheric pressure
• Use alveolar concentration in MAC determination for convenience
• -vaporizer setting delivers a percent concentration
• -do not need to know barometric pressure

halothane<isoflurane<sevoflurane

comparison of potency (halothane has lower MAC and is more potent)

• hypothermia
• pregnancy
• old age
• hypothyroidism
• hypoxemia (<40mmHg)
• hypercarbia (>95mmHg)
• concomitant anesthetic drugs (inj, inhal, sed, analgesics)

• hyperthermia
• hyperthyroidism
• hypernatremia
• CNS stimulants
• -increase catecholamines
• -ephedrine, amphetamines

• Movement of drug from brain back into blood
• Movement from blood to alveolus
• Removal from alveolus into breathing system and out scavenger

(same factors influence anesthetic uptake)

• Isoflurane, Sevoflurane, Desflurane - potent!
• Low margin of safety (TI 2-4)
• OD
• -severe ventilatory depression
• -severe cardiovascular depression
• -severe CNS depression
• -Death

• unknown!
• collection of many end points that are site specific

• EEG wave changes
• -awake: low amp, high freq
• -inhalation anesthesia: high amp, low freq
• -ISO, Sevo, Des 2 MAC or > = isoelectric pattern
• All agents depress seizure activity
• No analgesia

• VASODILATION
• increased CBF
• decreased CMR (metabolic rate)
• increased ICP
• hyperventilation (PaCO₂ < 30mmHg) reduces CBF with iso, sevo, des

• Dose dependent
• Contractility
• -not myocardial depressants around MAC in health
• -may decrease contractility in pt w/heart dz
• -may decrease contractility at conc > 1xMAC

• Dose dependent vasodilation (decreased resistance)
• Decreased SV
• Dose dependent increase in HR (suppressed w/opioids)
• usually remains CONSTANT
• Arterial BP decreases (less dramatic w/Des)

• Sensitization of myocardium to catecholamine induced arrhythmias
• -stress/excitement can increase endogenous catecholamine levels
• Iso, Sevo, Des - least effect

• 1. Mode of Ventilation and PaCO₂
• _{2. Noxious Stimulation}
• 3. Duration of Anesthesia
• 4. Concurrent drug administration

• Spontaneous vs. Mechanical
• Mechanical or IPPV (Intermittent positive-pressure ventilation)
• -increased intrathoracic pressure
• -decreased venous return
• PaCO₂ if increased
• -direct depressant on heart and peripheral vessels - dilation
• -indirect via SNS increasing arterial BP

• Sympathetic stimulation
• Increase arterial BP
• Increase HR and CO
• Decreased or prevented by inhaled anesthetics
• 1.5 to 2 x MAC

• After several hours: CO and HR may increase
• Cause unknown - dose? body position?
• More important in research than clinically

• Sedatives and Injectables
• -decreases anesthetic requirements (MAC)
• -decrease effects from inhalant
• -may accentuate CV depression
• Positive Inotropes or Vasopressors
• -do not affect anesthetic requirements
• -counteract unwanted CV depression

• Decrease minute ventilation (dose-dep) (Apneic index)
• Decrease sensitivity to PaCO₂
• -depress resp. control center in medulla and peripheral chemoreceptors
• Severely depress hypoxic drive
• Depress hypoxic pulmonary vasocontstriction

• Decreased lung volume (FRC)
• -atelectasis is common
• Bronchodilation
• -increased dead space ventilation
• Airway irritation d/t pungency
• -Des>Iso
• -prevents mask induction w/Des

• Depression of hepatic function and hepatocellular damage
• -mild and transient or permanent
• Reduce intrinsic hepatic clearance of drugs
• Reduced blood flow
• Iso, Sevo, Des - maintain adequate O2 supply so liver injury less likely

• Dose-Depending decrease
• -renal blood flow
• -glomerular filtration
• -urine output
• Primarily pre-renal corresponding to decreased BP
• Reversed after anesthesia

• Breakdown of Sevo causes increased serum fluoride concentration
• -potentially nephrotoxic
• -horses? no effect
• Sevo degradation in CO2 absorbents produces Compound A (nephrotoxic to rats)
• -no evidence in dogs, cats, horses
• -avoid low O2 flow rates w/prolonged anesthesia

• Varying degrees of metabolism in liver
• relative % metabolized by drug
• Des<Iso<Sevo
• Recovery mainly dependent on elimination through lung

• Malignant Hyperthermia
• -potentially life threatening genetic myopathy
• -swine and humans
• -other species too
• -any agent can cause this

• Rapid rise in body temp (treat quick, may cause death)
• Increased RR, dyspnea, apnea
• Tachycardia
• Metabolic acidosis
• Muscle rigidity
• Increase in end tidal CO2

• Dantrolene IV (muscle relaxant, antipyretic)
• Stop inhalant anesthesia
• 100% O2
• Body cooling

• Slow rise in alveolar partial pressure
• Slow to reach MAC
• Slow to achieve adequate brain concentration
• Slow to overdose
• Slow to recover
• More metabolism of drug
• More hangover

• Rapid rise in alveolar partial pressure
• Fast to reach MAC
• Fast to achieve adequate brain concentration
• Fast to overdose
• Fast to recover
• Less drug metabolism
• Less hangover

halothane<isoflurane<sevoflurane<desflurane