CHAPTER 11. Anesthesia
A. Thomas Evans
ANESTHESIA MACHINES AND SYSTEMS
I. Characteristics of a circle anesthesia machine
A. Conservation of heat and moisture
B. Absence of abrupt changes in anesthetic depth
C. Partial pressure of carbon dioxide in arterial gas (Pa co2) depends on ventilation, not on fresh gas flow
D. Disadvantages
1. Composed of many parts
2. Greater resistance
3. Difficult to clean
4. Not easily portable
5. Inspired concentration not easily predicted at low gas flows
II. Oxygen cylinders
A. Color coded: Green in the United States, white in the rest of the world
B. Full cylinder = 1900 to 2200 psi (oxygen); be careful of high pressure
C. Pressure reduced via pressure reduction valve or pressure regulator
D. Attach to machine via a “yoke”; pin index safety system is present
E. “E” cylinder, 650 to 700 L when full; rented from company
1. Multiply pounds per square inch (psi) by 0.3 to estimate liters in cylinder
2. 1 liter/min flow rate = 11 hours (660 min)
F. Oxygen supports combustion
G. Open slowly; “righty tighty, lefty loosie”
H. Change when 100 to 200 psi remain. The anesthesia machines are designed to run at a pressure of 50 psi
III. Oxygen flow meters
A. Measures gas flow in milliliters or liters per minute
B. Color coded (green)
C. Oxygen goes through a graduated glass cylinder with a floating ball or rotor
D. Oxygen flows should meet or exceed metabolic requirement, which is suggested to be 10 to 15 mL/lb/min (20 to 30 mL/kg/min) for circle system
E. If using nitrous oxide (N 2O), oxygen flow should be at least 30% of total flow of oxygen and N 2O combined (hence the recommended ratio with nitrous of 2:1 nitrous to oxygen)
F. 2 to 3 mL/lb/min (4 to 6 mL/kg/min) = metabolic requirement for oxygen
IV. N 2O
A. Compressed liquid, 750 psi when liquid present
B. Newer machines have a device that shuts N 2O off if oxygen is not flowing
C. Analgesic
D. Relaxant
E. More potent in humans
F. Used in 2:1 ratio with oxygen
G. Second gas effect increases the concentration of the “second gas” administered with nitrous
H. Diffusion hypoxia (outpouring of nitrous into the alveoli) occurs when patient is removed from the anesthesia machine and immediately allowed to breathe room air (O 2 only 20%)
I. Gas regulators; pressure reduction valves as follows:
1. Reduces high pressure in cylinder to 50 psi
2. Central supply oxygen pressure already reduced to 50 psi
V. Vaporizers
A. Converts liquid anesthetic (isoflurane, sevoflurane) to a vapor or gas state
B. Generally, 1.3 to 1.5 × minimum alveolar concentration (MAC) will result in a moderate level of anesthesia; the actual vaporizer setting may have to be slightly higher than this. (MAC is in equilibrium with the partial pressure of anesthetic in the brain)
C. Expensive, most are agent specific
D. Should not be tipped over
E. Precision vaporizers: “Tecs” and “Matics”
1. Expensive
2. Used with anesthetics with high vapor pressures
3. Compensated for temperature, gas flow rate, and back pressure
4. Dial on top indicates percentage being administered
5. Out-of-circuit location
6. Need to be recalibrated
F. Nonprecision
1. Ohio no. 8 (ether 8) is historically used with methoxyflurane
2. This type of vaporizer is rarely used
3. Not temperature compensated
4. Not flow compensated
5. Affected by back pressure
6. Difficult to monitor the concentration of anesthetic
7. Technically difficult to use with non-rebreathing systems
8. Halothane or isoflurane could be used if the wick is removed
9. Inside-the-circle (VIC) location
10. Factors that affect the output of vaporizers:
a. Temperature
b. Carrier gas flow rate; concentration of anesthetic should be increased when using low oxygen flow rates
c. Barometric pressure
d. Back pressure
VI. Unidirectional valves, or “flutter” valves
A. Keep gas flow going in a circle
B. Prevents animal from rebreathing
C. Moisture may cause them to stick
D. May be dislodged during cleaning
VII. Rebreathing or reservoir bag
A. Approximate size should be about 30 mL/lb (equivalent to 6× tidal volume; tidal volume is estimated to be 5 mL/lb)
B. Acts as a reservoir
C. Used to monitor breathing
D. Useful to check position of cuffed endotracheal tube (CETT)
E. Allows “bagging” to prevent atelectasis, removes CO 2
F. Empty bag = too low O 2 flow, too much scavenger, hole in bag
VIII. CO 2 absorption canister
A. Recommended volume = 2× tidal volume
B. Two formulations – soda lime and baralyme
1. Soda lime = 94% calcium hydroxide, 5% sodium hydroxide, and 1% potassium hydroxide
2. Baralyme = 80% calcium hydroxide and 20% barium hydroxide
C. Heat and water produced by the reaction
D. Change absorbent after color changes ½
E. Color change is time limited; check color change after use change if granules are brittle
F. “Channeling” may be a problem
IX. Pop-off valve
A. Normally in the “open” position
B. Allows exit of waste gases to the scavenger
C. Prevents buildup of pressure within the anesthesia system
D. Can be either open or closed in a “closed” system
E. Should be open in a “semiclosed” system
F. Should be either closed or partially closed if you are breathing for the patient
X. Breathing systems
A. Rebreathing
1. Closed
a. Closed system = flow rate of 2 to 3 mL/lb/min (metabolic requirement)
b. Oxygen flow rate is what determines classification
c. No pollution
d. Can estimate anesthetic agent uptake and oxygen consumption
e. Heat and humidity conservation
f. Economy
g. Closer monitoring and more knowledge required
h. Difficult to reanesthetize a patient that wakes up
i. Danger of hypercarbia
j. Buildup of trace toxic materials, such as carbon monoxide, acetone, methane, hydrogen
2. Semiclosed
a. Oxygen flow rate, greater than 2-3 mL/lb/min
b. Some resistance
c. Some dead space
d. The rebreathing tubes are not part of the dead space
e. Length of endotracheal tube (ETT) outside of mouth is considered mechanical dead space
B. Nonrebreathing coaxial system (Bain system)
1. Use oxygen flow rate of 100 mL/lb/min; this recommended rate allows some “rebreathing”; to eliminate all rebreathing, one would need to use a flow rate of 3× the minute ventilation (minute ventilation equals breaths per minute × the tidal volume)
2. The Bain system is used with small patients
3. Little or no dead space
4. Little or no resistance to breathing
5. Do not “flush” a Bain system; sudden large flow may overpressurize and damage lungs
6. Disconnection of the inner limb causes respiratory acidosis
XI. Universal or Mera “F” system
A. A coaxial circle system, with the inspiratory limb contained within the expiratory
B. Less bulky than traditional circle and may offer more heat and humidification of the inspired gases
C. Occult disconnection or kinking of the inner limb causes a huge increase in dead space and respiratory acidosis
1. Does not respond to increased minute ventilation
PREANESTHETIC AGENTS
I. Goals of premedication
A. Reduce stress
B. Provide analgesia
C. Reduce vagal tone
D. Reduce gastric volume
E. Improve induction of anesthesia
F. Improve recovery from anesthesia
G. Decrease salivation
H. Increase gastric pH
I. Amnesia
II. Anticholinergics
A. Glycopyrrolate, atropine
B. Parasympatholytic (anticholinergic)
C. Side effects
1. Causes tachycardia
2. Reduced intestinal motility
3. Mydriasis, reduced tear production
4. Bronchial dilator
5. Decreases salivation, which can become a problem in animals that produce a large amount of saliva
D. Used for intrathoracic cardiovascular cases
E. Probably contraindicated in horses and ruminants
III. Tranquilizers and sedatives
A. Acetylpromazine (acepromazine) (a phenothiazine tranquilizer)
1. Actions
a. Sedation; sometimes for long periods
b. Antidysrhythmic
c. Antiemetic; recommended to be used before opioids
d. Antihistamine effect
e. Decreases seizure threshold
f. Decreases MAC
g. Little or no analgesic effect but will make opioids more effective
2. Side effects
a. Hypotension after large doses
(1) Small doses typically used in veterinary medicine
(2) treat overdose (hypotension) with α-agonists, fluids, and atropine (not epinephrine)
b. Hypothermia
c. May cause splenic enlargement
d. Some interference with clotting factors
e. May cause personality change
f. Causes priapism in horses
B. Benzodiazepines: diazepam, midazolam, zolazepam
1. Effect at γ-aminobutyric acid (GABA) receptor
2. Diazepam
a. Intramuscular (IM) absorption not dependable
b. Passes blood brain barrier
c. Anticonvulsant
d. Muscle relaxant
e. Does not mix with other agents
3. Midazolam
a. More potent but shorter acting than diazepam
b. Water soluble (better for IM injections)
c. Usually administered with opioids
d. Little effect on circulatory or pulmonary systems
e. Can cause excitement when administered alone
f. The antagonist for diazepam and midazolam is flumazenil
4. Zolazepam (component of Telazol)
a. Anticonvulsant
b. Muscle relaxant
C. Butyrophenone group of tranquilizers
1. Droperidol (was a component of Innovar-Vet)
2. Azaperone (Stresnil)
3. Butyrophenone tranquilizers produce calming and prevent fighting and cannibalism in pigs (Stresnil)
D. α-2 Agonists:
1. Xylazine (short-acting small animal [SA] and long-acting large animal [LA] formulations), detomidine, dexmedetomidine, romifidine
2. These drugs act by reducing the level of the neurotransmitter norepinephrine centrally and peripherally
3. Initial hypertension followed by hypotension
4. Sedation
5. Relaxation
6. Analgesia
7. Adverse effects
a. Vomiting in small animals
b. Bradycardia, atrioventricular (AV) block
c. Sensitization to epinephrine
d. Respiratory depression in some dogs; dogs may look cyanotic because of peripheral vasoconstriction
e. May cause bloat in dogs susceptible due to gastrointestinal (GI) stasis
f. Personality changes
g. Hyperglycemia; increased urine production
h. Abortion in cattle during last trimester
8. Microdose of dexmedetomidine effective (0.5 mcg/lb (1 mcg/kg), intravenous [IV] administration in dogs)
9. Reversed by yohimbine, tolazoline, atipamezole. Generally, for reversal of medetomidine, use the same volume of atipamezole
10. Can be absorbed through human skin abrasions (wash off spills)
E. Opioids (synthetic) and opiates (natural)
1. These drugs work at various receptors; μ, κ, σ, δ in the spinal cord and brain (all opioids can be combined with a low dose of acepromazine to increase sedation and analgesia)
2. Reverse opioids with naloxone or nalbuphine
3. Morphine
a. Commonly used in dogs, cats, and sometimes horses
b. Dysphoria
c. Excitement
d. Vomiting
e. Histamine release
f. Respiratory depression
g. Bradycardia
h. Miosis in dogs and primates, mydriasis in cats
i. Subcutaneous (SC), IM or slow IV administration (over 5 minutes), may cause histamine release and hypotension if administered too rapidly
4. Meperidine
a. 1/10 as potent as morphine
b. Vomiting rare
c. Histamine release in dogs if given IV
d. Short duration of action
5. Oxymorphone
a. 5 to 10× as potent as morphine
b. Sedation
c. Analgesia
d. Respiratory depression
e. Bradycardia
f. Vomiting rare
g. Can be administered IV
h. Duration: 1 to 3 hours
6. Hydromorphone
a. 6× as potent as morphine
b. May cause vomiting
c. Sedation
d. Analgesia
e. Duration is 1 to 3 hours
7. Butorphanol
a. Classified as an agonist-antagonist
b. Controlled
c. Little respiratory depression
d. Little cardiovascular depression
e. Sometimes combined with acetylpromazine
f. Rare dysphoria (but does occur)
g. Rare sedation (more common in geriatrics)
h. 1 to 2 hours’ duration of action
8. Buprenorphine
a. Another agonist-antagonist, buprenorphine has agonist properties 30× that of morphine
b. No vomiting
c. Some respiratory depression
d. Little cardiovascular depression
e. Used for longer postoperative analgesia needs as duration of action appears clinically to be 4 to 6 hours
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