Small Animal Anesthesia PDF

SECTION II S M ALL A NIM A L Anesthesia 11 CH A P TE...

SECTION II S M ALL A NIM A L Anesthesia 11 CH A P TE R A. Thomas Evans combined (hence the recommended ratio with ANESTHESIA MACHINES AND SYSTEMS nitrous of 2:1 nitrous to oxygen) I. Characteristics of a circle anesthesia machine F. 2 to 3 mL/lb/min (4 to 6 mL/kg/min)  metabolic A. Conservation of heat and moisture requirement for oxygen B. Absence of abrupt changes in anesthetic depth IV. N2O C. Partial pressure of carbon dioxide in arterial A. Compressed liquid, 750 psi when liquid present gas (PaCO2) depends on ventilation, not on fresh B. Newer machines have a device that shuts N2O off gas flow if oxygen is not flowing D. Disadvantages C. Analgesic 1. Composed of many parts D. Relaxant 2. Greater resistance E. More potent in humans 3. Difficult to clean F. Used in 2:1 ratio with oxygen 4. Not easily portable G. Second gas effect increases the concentration of 5. Inspired concentration not easily predicted at the “second gas” administered with nitrous low gas flows H. Diffusion hypoxia (outpouring of nitrous into the II. Oxygen cylinders alveoli) occurs when patient is removed from the A. Color coded: Green in the United States, white in anesthesia machine and immediately allowed to the rest of the world breathe room air (O2 only 20%) B. Full cylinder  1900 to 2200 psi (oxygen); be care- I. Gas regulators; pressure reduction valves ful of high pressure as follows: C. Pressure reduced via pressure reduction valve or 1. Reduces high pressure in cylinder to 50 psi pressure regulator 2. Central supply oxygen pressure already D. Attach to machine via a “yoke”; pin index safety reduced to 50 psi system is present V. Vaporizers E. “E” cylinder, 650 to 700 L when full; rented from A. Converts liquid anesthetic (isoflurane, sevoflu- company rane) to a vapor or gas state 1. Multiply pounds per square inch (psi) by 0.3 to B. Generally, 1.3 to 1.5 minimum alveolar concen- estimate liters in cylinder tration (MAC) will result in a moderate level of 2. 1 liter/min flow rate  11 hours (660 min) anesthesia; the actual vaporizer setting may have F. Oxygen supports combustion to be slightly higher than this. (MAC is in equilib- G. Open slowly; “righty tighty, lefty loosie” rium with the partial pressure of anesthetic in the H. Change when 100 to 200 psi remain. The anesthe- brain) sia machines are designed to run at a pressure C. Expensive, most are agent specific of 50 psi D. Should not be tipped over III. Oxygen flow meters E. Precision vaporizers: “Tecs” and “Matics” A. Measures gas flow in milliliters or liters per minute 1. Expensive B. Color coded (green) 2. Used with anesthetics with high vapor C. Oxygen goes through a graduated glass cylinder pressures with a floating ball or rotor 3. Compensated for temperature, gas flow rate, D. Oxygen flows should meet or exceed metabolic and back pressure requirement, which is suggested to be 10 to 15 mL/ 4. Dial on top indicates percentage being lb/min (20 to 30 mL/kg/min) for circle system administered E. If using nitrous oxide (N2O), oxygen flow should 5. Out-of-circuit location be at least 30% of total flow of oxygen and N2O 6. Need to be recalibrated 155 156 SECTION II SMALL ANIMAL F. Nonprecision b. Oxygen flow rate is what determines 1. Ohio no. 8 (ether 8) is historically used with classification methoxyflurane c. No pollution 2. This type of vaporizer is rarely used d. Can estimate anesthetic agent uptake and 3. Not temperature compensated oxygen consumption 4. Not flow compensated e. Heat and humidity conservation 5. Affected by back pressure f. Economy 6. Difficult to monitor the concentration of g. Closer monitoring and more knowledge anesthetic required 7. Technically difficult to use with non- h. Difficult to reanesthetize a patient that rebreathing systems wakes up 8. Halothane or isoflurane could be used if the i. Danger of hypercarbia wick is removed j. Buildup of trace toxic materials, such as 9. Inside-the-circle (VIC) location carbon monoxide, acetone, methane, 10. Factors that affect the output of vaporizers: hydrogen a. Temperature 2. Semiclosed b. Carrier gas flow rate; concentration of an- a. Oxygen flow rate, greater than esthetic should be increased when using 2-3 mL/lb/min low oxygen flow rates b. Some resistance c. Barometric pressure c. Some dead space d. Back pressure d. The rebreathing tubes are not part of the VI. Unidirectional valves, or “flutter” valves dead space A. Keep gas flow going in a circle e. Length of endotracheal tube (ETT) outside B. Prevents animal from rebreathing of mouth is considered mechanical dead C. Moisture may cause them to stick space D. May be dislodged during cleaning B. Nonrebreathing coaxial system (Bain system) VII. Rebreathing or reservoir bag 1. Use oxygen flow rate of 100 mL/lb/min; this A. Approximate size should be about 30 mL/lb recommended rate allows some “rebreathing”; (equivalent to 6 tidal volume; tidal volume to eliminate all rebreathing, one would need to is estimated to be 5 mL/lb) use a flow rate of 3 the minute ventilation B. Acts as a reservoir (minute ventilation equals breaths per minute C. Used to monitor breathing the tidal volume) D. Useful to check position of cuffed endotracheal 2. The Bain system is used with small patients tube (CETT) 3. Little or no dead space E. Allows “bagging” to prevent atelectasis, removes CO2 4. Little or no resistance to breathing F. Empty bag  too low O2 flow, too much scavenger, 5. Do not “flush” a Bain system; sudden large flow hole in bag may overpressurize and damage lungs VIII. CO2 absorption canister 6. Disconnection of the inner limb causes respira- A. Recommended volume  2 tidal volume tory acidosis B. Two formulations - soda lime and baralyme XI. Universal or Mera “F” system 1. Soda lime  94% calcium hydroxide, 5% sodium A. A coaxial circle system, with the inspiratory limb hydroxide, and 1% potassium hydroxide contained within the expiratory 2. Baralyme  80% calcium hydroxide and 20% B. Less bulky than traditional circle and may offer barium hydroxide more heat and humidification of the inspired C. Heat and water produced by the reaction gases D. Change absorbent after color changes 1⁄2 C. Occult disconnection or kinking of the inner limb E. Color change is time limited; check color change causes a huge increase in dead space and respira- after use change if granules are brittle tory acidosis F. “Channeling” may be a problem 1. Does not respond to increased minute IX. Pop-off valve ventilation A. Normally in the “open” position B. Allows exit of waste gases to the scavenger PREANESTHETIC AGENTS C. Prevents buildup of pressure within the anesthesia system I. Goals of premedication D. Can be either open or closed in a “closed” system A. Reduce stress E. Should be open in a “semiclosed” system B. Provide analgesia F. Should be either closed or partially closed if you C. Reduce vagal tone are breathing for the patient D. Reduce gastric volume X. Breathing systems E. Improve induction of anesthesia A. Rebreathing F. Improve recovery from anesthesia 1. Closed G. Decrease salivation a. Closed system  flow rate of 2 to H. Increase gastric pH 3 mL/lb/min (metabolic requirement) I. Amnesia  CHAPTER 11 Anesthesia 157 II. Anticholinergics C. Butyrophenone group of tranquilizers A. Glycopyrrolate, atropine 1. Droperidol (was a component of Innovar-Vet) B. Parasympatholytic (anticholinergic) 2. Azaperone (Stresnil) C. Side effects 3. Butyrophenone tranquilizers produce calming 1. Causes tachycardia and prevent fighting and cannibalism in pigs 2. Reduced intestinal motility (Stresnil) 3. Mydriasis, reduced tear production D. -2 Agonists: 4. Bronchial dilator 1. Xylazine (short-acting small animal [SA] and 5. Decreases salivation, which can become a long-acting large animal [LA] formulations), problem in animals that produce a large detomidine, dexmedetomidine, romifidine amount of saliva 2. These drugs act by reducing the level of the D. Used for intrathoracic cardiovascular cases neurotransmitter norepinephrine centrally E. Probably contraindicated in horses and and peripherally ruminants 3. Initial hypertension followed by hypotension III. Tranquilizers and sedatives 4. Sedation A. Acetylpromazine (acepromazine) (a phenothi- 5. Relaxation azine tranquilizer) 6. Analgesia 1. Actions 7. Adverse effects a. Sedation; sometimes for long periods a. Vomiting in small animals b. Antidysrhythmic b. Bradycardia, atrioventricular (AV) block c. Antiemetic; recommended to be used before c. Sensitization to epinephrine opioids d. Respiratory depression in some dogs; dogs d. Antihistamine effect may look cyanotic because of peripheral e. Decreases seizure threshold vasoconstriction f. Decreases MAC e. May cause bloat in dogs susceptible due to g. Little or no analgesic effect but will make gastrointestinal (GI) stasis opioids more effective f. Personality changes 2. Side effects g. Hyperglycemia; increased urine production a. Hypotension after large doses h. Abortion in cattle during last trimester (1) Small doses typically used in veterinary 8. Microdose of dexmedetomidine effective medicine (0.5 mcg/lb (1 mcg/kg), intravenous [IV] (2) treat overdose (hypotension) with administration in dogs) -agonists, fluids, and atropine (not 9. Reversed by yohimbine, tolazoline, atipa- epinephrine) mezole. Generally, for reversal of medetomi- b. Hypothermia dine, use the same volume of atipamezole c. May cause splenic enlargement 10. Can be absorbed through human skin abra- d. Some interference with clotting factors sions (wash off spills) e. May cause personality change E. Opioids (synthetic) and opiates (natural) f. Causes priapism in horses 1. These drugs work at various receptors; , , , B. Benzodiazepines: diazepam, midazolam,  in the spinal cord and brain (all opioids can zolazepam be combined with a low dose of acepromazine 1. Effect at -aminobutyric acid (GABA) receptor to increase sedation and analgesia) 2. Diazepam 2. Reverse opioids with naloxone or nalbuphine a. Intramuscular (IM) absorption not 3. Morphine dependable a. Commonly used in dogs, cats, and b. Passes blood brain barrier sometimes horses c. Anticonvulsant b. Dysphoria d. Muscle relaxant c. Excitement e. Does not mix with other agents d. Vomiting 3. Midazolam e. Histamine release a. More potent but shorter acting than f. Respiratory depression diazepam g. Bradycardia b. Water soluble (better for IM injections) h. Miosis in dogs and primates, mydriasis in c. Usually administered with opioids cats d. Little effect on circulatory or pulmonary i. Subcutaneous (SC), IM or slow IV adminis- systems tration (over 5 minutes), may cause hista- e. Can cause excitement when administered mine release and hypotension if adminis- alone tered too rapidly f. The antagonist for diazepam and midazolam 4. Meperidine is flumazenil a. 1⁄10 as potent as morphine 4. Zolazepam (component of Telazol) b. Vomiting rare a. Anticonvulsant c. Histamine release in dogs if given IV b. Muscle relaxant d. Short duration of action 158 SECTION II SMALL ANIMAL e. Used for premedication in pediatrics and F. Provides muscle relaxation geriatrics II. Barbiturates 5. Oxymorphone A. Onset and duration of action dependent of fat a. 5 to 10 as potent as morphine solubility, redistribution, and metabolism. Thio- b. Sedation pental is fat soluble; pentobarbital is not as fat c. Analgesia soluble d. Respiratory depression B. Thiopental is redistributed; pentobarbital de- e. Bradycardia pends on metabolism f. Vomiting rare C. Breathing is depressed; brief apnea is seen after g. Can be administered IV administration h. Duration: 1 to 3 hours D. Laryngospasm after thiopental is seen, especially 6. Hydromorphone in cats a. 6 as potent as morphine E. Thiopental causes reduced cardiac output and b. May cause vomiting blood pressure sensitivity to epinephrine is in- c. Sedation creased (ventricular premature contractions, d. Analgesia bigeminy) e. Duration is 1 to 3 hours F. Be careful in animals with cardiac disease 7. Butorphanol 1. Give more slowly a. Classified as an agonist-antagonist 2. Preoxygenate b. Controlled 3. Give thiopental with lidocaine c. Little respiratory depression G. Reduce dose of thiopental if animal is hypopro- d. Little cardiovascular depression teinemic ( 3.0 g/dL), acidotic, or extremely thin e. Sometimes combined with acetylpromazine H. Sighthounds f. Rare dysphoria (but does occur) 1. Tend to overdose at beginning g. Rare sedation (more common in geriatrics) 2. Leads to long recovery as redistribution sites h. 1 to 2 hours’ duration of action become filled 8. Buprenorphine 3. Hepatic metabolism is slower in sighthounds a. Another agonist-antagonist, buprenorphine I. Critically ill animals shunt blood to brain and has agonist properties 30 that of other internal organs. Thiopental shows greater morphine potency in these animals b. No vomiting J. Perivascular injection alkaline solution: Treat with c. Some respiratory depression saline, lidocaine, steroids, antibiotics d. Little cardiovascular depression K. Excitement during induction or recovery e. Used for longer postoperative analgesia 1. Injection too slow needs as duration of action appears clini- 2. Port on IV line distant from catheter cally to be 4 to 6 hours 3. Incorrect dose f. Buprenorphine is often combined with acet- III. Cyclohexamines: Ketamine, tiletamine ylpromazine to produce profound sedation A. Dissociative anesthesia or catalepsy and analgesia B. Animal appears awake but unaware of IV. Dissociative agents surroundings A. Ketamine, tiletamine C. Palpebral and laryngeal reflexes remain B. Low dose provides sedation; high dose provides D. Swallowing is present but ETT should anesthesia be used C. Metabolized by liver (dogs) and excreted through E. Sensitive to sound the kidney (cats) F. Rigid muscles (use sedative or tranquilizer to D. Side effects prevent) 1. Tachycardia G. Poor visceral analgesia, better somatic analgesia 2. Apneustic breathing H. Metabolized by the liver (dog) and excreted by 3. Increases intracranial pressure the kidney (cat) 4. May cause seizures I. Given IV or IM (oral in mean cats). Usually given 5. Analgesia (somatic) with diazepam, midazolam, acepromazine, zolaze- pam, medetomidine, or xylazine 1. Ketamine is usually mixed with diazepam for INJECTABLE ANESTHETIC AGENTS IV use. Diazepam is not absorbed well IM. I. Ideal injectable agent Midazolam can be substituted for diazepam A. Rapid onset, recovery 2. Ketamine-xylazine IM in cats can be used for B. Lack of tissue toxicity (what agent has tissue restraint and minor procedures toxicity?) a. Hypoventilation, hypertension, and C. Lack of adverse cardiovascular and respiratory decreased CO effects b. Possibility of aspiration reduced if xylazine D. Rapid metabolism (even in patients with liver or is given a few minutes before the ketamine kidney disease) 3. Tiletamine-zolazepam (Telazol) is used for re- E. Provides analgesia straint of mean dogs  CHAPTER 11 Anesthesia 159 a. Can be used SC, IM, and IV in cats; IM and D. Perivascular injection may cause a slough IV in dogs E. Used before ketamine-diazepam or thiopental b. Long recoveries c. Used as a restraint drug for wildlife INHALATION ANESTHETIC AGENTS d. Effects similar to ketamine J. Disadvantages I. Advantages of inhalation anesthesia 1. No reversal for ketamine A. Easy to change depth of anesthesia 2. Cardiac effects: Hypertension, tachycardia (not B. Elimination of inhalation agents occurs through recommended for hyperthyroid or hypertro- the lungs phic cardiomyopathy [HCM] cats) C. High concentration of oxygen can be delivered 3. Respiratory effects: Apneustic breathing D. Animals are usually intubated 4. Irritating to tissues II. Disadvantages 5. Increased salivation A. Cost of anesthetic machine 6. Increases in cerebrospinal fluid (CSF) and in- B. Slow induction traocular pressure C. Environmental pollution 7. Eyes remain open, pupil dilated, nystagmus III. N2O, ether (diethyl ether), halogenated compounds in cats (halothane, sevoflurane, isoflurane, methoxyflurane, 8. Cats often exhibit hallucination behavior in enflurane, desflurane) recovery A. Liquid at room temperature 9. Temporary blindness B. Stored inside the vaporizer and evaporated into IV. Propofol oxygen and then delivered to the breathing circuit A. A substituted phenol (cats are sensitive to phenol C. Methoxyflurane, halothane, enflurane, and desflu- compounds) rane are not routinely used in veterinary medicine B. Neutral pH at present C. Administer slowly IV IV. The goal of inhalation anesthesia is to maintain an opti- 1. Can be mixed with 2.5% thiopental equal mal and unchanging brain partial pressure as reflected amounts by the alveolar partial pressure. Alveolar partial pres- 2. Can give by intermittent injection or constant sure is important because “the partial pressure of the rate infusion (CRI) inhalation anesthetic in the brain is in equilibrium D. Disadvantages with the partial pressure in the blood which is in 1. Paddling or twitching during induction equilibrium with the partial pressure in the alveoli.” 2. Hypotension and tachycardia or bradycardia V. Induction of anesthesia occurs when an anesthetiz- 3. Apnea if it is given too rapidly ing partial pressure has been achieved in the brain. 4. Hemolysis and Heinz body formation in cats The brain can be considered the final site for a series (rapid injection and multiple injections) of concentration gradients. Delivered (vaporizer) E. Rapid metabolism and redistribution concentration  anesthesia circuit  inspired F. Premedication recommended  alveolar  arterial  brain G. Can be used in sighthounds VI. Physical properties of inhalation anesthetics H. Discard unused portion within 6 hours A. Vapor pressure V. Etomidate (Amidate) 1. The pressure a gas exerts on the walls of a A. Sedative and hypnotic closed container at standard temperature and B. Short acting, good muscle relaxation, but pressure (760 mm Hg at sea level) no analgesia 2. A measure of volatility, in other words, how C. IV in dogs and cats. IV injection may be painful easily a liquid evaporates D. Premedication recommended (diazepam, 3. Halothane and isoflurane have relatively high opioid) vapor pressures E. Metabolism rapid 4. Methoxyflurane (currently not manufactured in F. Advantages the United States) has a low vapor pressure. 1. Can administer repeated doses Sevoflurane is intermediate 2. Minimal effect on cardiovascular and hepatic 5. Vapor pressure at 20° C systems a. Halothane, 243 mm Hg 3. Works better in sick animals b. Isoflurane, 238 mm Hg G. Disadvantages c. Methoxyflurane, 23 mm Hg 1. Suppresses adrenal function d. Diethyl ether, 440 mm Hg 2. Nausea, vomiting, and excitement may occur e. Sevoflurane, 157 mm Hg during induction f. Desflurane, 669 mm Hg 3. Expensive B. Concentration (volumes %) can be converted to H. Open bottles should be discarded vapor pressure or partial pressure by multiplying VI. Guaifenesin (glyceryl guaiacolate [GG]) (volumes % atmospheric pressure). For exam- A. A muscle relaxant used in horses and cattle, small ple: 2% isoflurane is equal to 15.2 mm Hg (0.02 ruminants 760 mm Hg). Vapor pressure can also be con- B. Used in 5% and 10% solutions verted to volumes % (vapor pressure divided C. 10% solutions may cause hemolysis in cattle by atmospheric pressure) 160 SECTION II SMALL ANIMAL C. Partial pressure 3. If an inhalant has a long induction and recov- 1. A mixture of gases in a container will exert a ery time, the anesthetic is more than likely pressure on the walls of the container very soluble in blood. Conversely, if an inhal- 2. The proportion of total pressure from any one ant has a short induction and recovery time, gas in the mixture is called the partial pressure the anesthetic is more than likely relatively of that gas insoluble (Table 11-2) 3. Partial pressure equals the volumes % of a gas IX. Delivery of gases from breathing circuit to alveoli total pressure. In air, the oxygen concentra- depends on minute ventilation tion (20%) total pressure (atm)  the partial A. Increases in minute ventilation increase delivery pressure of oxygen. 0.20 760 mm Hg  152 mm B. Decreases in minute ventilation slow delivery Hg 152 mm Hg is the partial pressure of oxygen 1. Airway obstruction in air at sea level 2. Increased anatomic dead space 4. Even though the partial pressures of an anes- X. Transfer of gases from alveoli to blood depends on thetic in different tissues may be at equilib- the following: rium, the actual content of anesthetic in those A. Pulmonary blood flow tissues may be different. In other words, the B. Matching of ventilation to perfusion in the lung solubility of the anesthetic also plays a role C. Shunt 5. Concentration of a gas: A way of measuring the D. Concentration of anesthetic in blood amount of anesthetic expressed as volumes % XI. Transfer of anesthetics from the blood to tissues of partial pressure (2% isoflurane, 1% halo- depends on the following: thane, 2 1⁄2% methoxyflurane) A. Tissue blood flow VII. MAC B. Tissue/blood solubility coefficient A. The concentration (or partial pressure) of anes- C. Duration of exposure to the anesthetic thetic in the alveolus that will keep 50% of the XII. Factors that affect the concentration of anesthetics animals anesthetized during a painful stimulus in the lungs B. It is a method of comparing potency of inhalation A. An increase in ventilation will increase alveolar anesthetics concentration C. MAC reflects the brain partial pressure because B. A decrease in ventilation will lower alveolar con- the alveolar partial pressure is in equilibrium centration with the brain C. Animals with low cardiac output (shock) can also D. MAC relates very closely to the concentration of develop deep anesthesia the inhalant anesthetic required for maintenance D. Circulatory depression produces a positive anesthesia feedback E. 1.3 the MAC will keep 99% of the animals anes- XIII. The rate of induction of anesthesia is determined by thetized the rate of rise of the alveolar partial pressure. The F. Combinations of inhaled anesthetics have addi- following factors determine the rate of rise of alveo- tive effects of MAC (Table 11-1) lar concentration: VIII. Solubility A. Alveolar ventilation A. Solubility (partition coefficients) of anesthetics in B. The inspired concentration: If you use a higher blood and tissues determines the time necessary concentration of anesthetic, the alveolar concen- for equilibration between two phases to occur. tration will rise faster. You can decrease some of Phases refer to “blood phase,” “gas phase,” or the effects of tissue uptake by taking advantage of “brain phase” this “concentration effect” B. Solubility (of inhalation anesthetics) XIV. Factors that determine tissue uptake of anesthetic: 1. Blood-gas solubility coefficient determines up- A. Solubility take from the alveoli into the blood and thus B. Cardiac output the rate of induction of anesthesia C. Alveolar to venous gradient 2. Blood will hold more halothane (solubility XV. Recovery coefficient  2.5) than isoflurane (solubility A. Recovery from anesthesia reflects reversal of coefficient  1.5) the concentration gradients established during Table 11-1 Minimum Alveolar Anesthetic (MAC) Values for Some Common Species (%) Table 11-2 Solubility (Partition Coefficients) of Popular Anesthetics (37°C) Halothane Isoflurane Sevoflurane Anesthetic Blood/Gas Coefficient Cat 1.1 1.6 2.6 Dog 0.9 1.3 2.3 Halothane 2.5 Horse 0.9 1.3 2.3 Isoflurane 1.5 Pig 0.9 1.4 2.6 Sevoflurane 0.68  CHAPTER 11 Anesthesia 161 induction of anesthesia. The rate of recovery may 5. Check mucous membranes of the mouth, be influenced by the duration of prior administra- eyelids, footpads, ears, prepuce, vulva tion and the metabolism of the inhaled anesthetic B. Capillary refill time (CRT) B. Recovery is fastest following the short duration 1. Should be less than 2 seconds administration of inhaled anesthetics that are 2. Usually poor CRT after -2 agent poorly soluble in blood and tissues 3. Slow CRT may reflect low blood pressure XVI. N2O 4. Present after death A. Transfer of N2O to closed gas spaces: N2O will C. Heart rate and rhythm move into air filled spaces such as the gut, air si- 1. Acceptable rate for dogs is greater than 60 beats nuses, middle ear. Air introduced into the body per minute (beats/min). Some large-breed dogs, will also be affected. These air-filled spaces will however, are okay at a slower rate (i.e., in the increase in volume or pressure (pneumothorax) 50s) during anesthesia B. Diffusion hypoxia: After abruptly discontinuing 2. Acceptable rate for cats is 100 beats/min; 80 to the use of N2O, there will be an outpouring of N2O 90 beats/min might be acceptable before sur- into the lung from the blood, resulting a reduction gery starting if color, jaw tone, pulse are in available oxygen acceptable C. Second gas effect: The rapid uptake of N2O 3. High range for dogs is 150 to 160 beats/min; for increases effective minute ventilation and the cats, 180 to 200 beats/min rate of uptake of concomitantly inhaled gases 4. Normal heart rate for horses during anesthesia (isoflurane, oxygen) is 26 to 40 beats/min D. The uptake of nitrous also contracts the alveoli 5. Heart rate can be monitored by electrocardio- concentrating the remaining gases gram (ECG), using the esophageal stethoscope, counting pulse rate, pulse oximeter, or hand palpating the chest MONITORING THE ANESTHETIZED PATIENT D. Pulse strength and blood pressure (FROM THE LATIN MONERE, WHICH 1. A superficial pulse can be palpated in a num- MEANS “TO WARN”) ber of areas I. Qualitative assessment respiratory system. Some a. Common sites include radial, femoral, dor- terms concerning breathing patterns (pnoia  from sal pedal, lingual, metacarpal, recurrent the Greek “breath”) follow: metatarsal, palatine arteries A. Hyperventilation  a PaCO2 level in the arterial b. The lingual and the metacarpal are usually blood lower than 40 mm Hg accessible to the anesthetist from the front B. Hypoventilation  a PaCO2 level in the arterial of the surgery table blood higher than 40 mm Hg 2. The pulse strength is related to the pulse pres- C. Tachypnea  a fast breathing rate sure. The pulse pressure is the difference be- D. Bradypnea  a regular slow breathing rate tween the systolic pressure and the diastolic E. Hyperpnea  fast or deep respiration pressure F. Hypopnea  slow or shallow breathing a. Keep the mean pressure [MAP  diastolic G. Biot’s respiration  several breaths in a row fol- pressure  ( systolic pressure diastolic lowed by a period of apnea, repeating pressure)/3] above 60 mm Hg for dogs and H. Cheyne-Stokes respirations  breathing with in- cats and above 70 mm Hg for horses creasing rate and depth followed by decreasing b. A low blood pressure (hypotension) may be rate and depth followed by apnea, repeating due to deep anesthesia, vasodilation, blood pattern loss, dehydration, obstruction of blood flow, II. Respiratory rate and depth or a failing heart A. Normal rate during anesthesia in dogs is 8 to c. A high blood pressure (hypertension) may 20 breaths per minute. Toy breeds and obese be due to surgical stimulation, light anes- animals may breathe at a faster rate thesia, excessive fluid administration, B. Inhalation anesthetics tend to increase the thyroid tumor, adrenal tumor (pheochromo- breathing rate (but decrease the tidal volume) cytoma) or administration of a drug that C. Normal tidal volume  5 mL/lb (10 mL/kg) causes vasoconstriction (epinephrine, D. Breathing progresses from thoracic and abdomi- ephedrine, phenylephrine) nal to all abdominal to gasping as anesthesia E. Indirect blood pressure equipment: Doppler, deepens oscillometric III. Circulatory system 1. Use a correction factor of 14 mm Hg added to A. Mucous membrane color the value obtained by the Doppler in cats 1. Should be “pink” 2. Cuff width should be approximately 40% of the 2. May be bright red when carbon dioxide is high circumference of the leg. A cuff width less than (local tissue vasodilation) the circumference will give readings higher 3. Cyanosis indicates some problem with oxygen than the actual pressure delivery; this must be corrected immediately! 3. The oscillometric monitor does not work well 4. Pale mucous membranes indicate poor perfu- in cats because of the small size of the leg and sion of tissues arteries 162 SECTION II SMALL ANIMAL IV. Eye position, palpebral reflex b. The light is absorbed differently, depending A. The eye position tends to be centered when the on whether the hemoglobin is saturated patient is deep or light. Therefore, you would like with oxygen (oxyhemoglobin) or not (des- the eye to be rotated down and medially during oxyhemoglobin) maintenance anesthesia c. The percent of saturation of hemoglobin is B. Palpebral reflex is tested by tapping the medial or then displayed as a numeric value on the lateral canthus or by brushing the eye lashes. monitor Usually disappears about the time surgical anes- 3. Accuracy is affected by bright lights, abnormal thesia begins hemoglobins, movement of the patient, various V. Thermoregulation dyes, vasoconstriction, thickness of hair coat, A. Anesthetized animal body temperature falls color of hair coat during anesthesia tranquilizers and anesthetics 4. Keep the saturation 90% or greater prevent shivering a. Pulse oximetry will not warn of a develop- B. Hypothermia lowers MAC value, slows metabo- ing hypoxemia when the PaO2 is greater lism of drugs; use thermometer to document than 60 mm Hg C. Prevention b. 90% saturation of hemoglobin (arterial 1. Warm IV fluids blood) with oxygen equates to a PaO2 of 2. Low oxygen flow rates 60 mm Hg 3. Warm water heating blanket 5. Factors that affect how easily oxygen is re- 4. Bair hugger leased from the hemoglobin 5. Insulate operating table a. Low pH (acidosis), high temperature, high 6. Warm environment CO2 (respiratory acidosis), and increased 7. Dry the animal 2-3 diphosphoglycerate make it easier for VI. Catheter maintenance and fluid administration oxygen to be released from hemoglobin A. Keep catheter patent; sometimes this is difficult b. When the curve shifts to the right, oxygen owing to position under the drapes is released from the hemoglobin easier B. Monitor fluid administration (usual rate  5 ml/lb c. When the curve shifts to the left, oxygen is per hour or 10 mL/kg per hour). Use burette if not released as easy (held more tightly) by warranted hemoglobin VII. Blood loss 6. Venous blood normally has a hemoglobin satu- A. Sometimes difficult to gauge ration of 75% and a PvO2 of 40 mm Hg 1. Use suction devices to aspirate blood from the B. Capnography surgical site, but the canister may also contain 1. Capnography is a method of monitoring the saline, so it is an educated guess as to how amount of CO2 in the air that is breathed in much blood has been lost and out by the patient 2. Saturated sponges contain 5 or 6 (3 3) up to 2. CO2 is measured by collecting the gas and 10 (4 4) mL of blood transporting it via a tube to the machine (side B. A healthy animal can lose up to 15% of its blood stream) or measuring the CO2 via a sensor in volume without major problems. One way of esti- an airway adaptor (mainstream) mating blood volume is to take 8% of the animal’s 3. The highest part of the curve is the end weight in kilograms tidal CO2 C. Replacement 4. If the curve fails to go to the bottom (to the 1. Blood loss can be replaced with a balanced baseline), it is termed rebreathing electrolyte at 3 the volume of blood loss. 5. Low values ( 40 mm Hg) are due to overzeal- Three times the volume is used because only a ous ventilation, hypothermia, open thoracic portion of the balanced electrolyte remains in surgery, tube in the esophagus or cardiac the vascular compartment arrest (no CO2 would be measured during a 2. Blood loss can be replaced with whole blood cardiac arrest) at a 1:1 ratio because the blood stays in the 6. High values ( 40 mm Hg) are due to hypoven- vascular compartment tilation, hyperthermia, effects of premeds and 3. Other high-molecular-weight solutions such as anesthesia hetastarch can also be used, but a smaller vol- C. ECG ume is needed 1. Monitors electrical rhythm of the heart VIII. Quantitative assessment of anesthesia 2. During anesthesia the ECG is used primarily A. Pulse oximetry for detection of dysrhythmias and for monitor- 1. Measures the amount of oxygen bound to he- ing heart rate moglobin (percentage of total capacity) 3. There are many causes of artifacts in the ECG, 2. Sensor (usually a human earclip) is placed including patient movement, movement of the across a thin tissue (usually the tongue) leads, poor contact with the skin, cautery a. The sensor shines near infrared and infra- IX. Reflexes red light of different wave lengths through A. Swallowing reflex the tissue 1. Present during light anesthesia  CHAPTER 11 Anesthesia 163 2. Generally wait until the swallowing reflex re- C. Acute pain: Abrupt onset and short duration of pain turns before removing the ETT D. Chronic pain: Slow onset and long duration (can- B. Pedal reflex cer pain) 1. Obtained by pinching or squeezing the toe or E. Referred pain: Pain from a body part felt in a dif- footpad ferent part of the body 2. Not present during inhalation anesthesia F. Hyperesthesia: Increased sensitivity to a stimulus 3. Used in lab animals during pentobarbital G. Neuropathic pain: Pain from physical damage to a anesthesia nerve 4. Animals may have this reflex but not be aware H. Allodynia: Pain resulting from a stimulus that of pain does not normally provoke pain C. Ear-flick reflex I. Nociceptor: The afferent nerve ending that is sen- 1. Used primarily in cats sitive to a noxious stimulus 2. Present at all levels of anesthesia J. Nociception: Reception, conduction, and central 3. Not a good reflex to judge the depth of nervous processing of nerve signals generated by anesthesia the stimulation of nociceptors D. Corneal reflex K. Preemptive analgesia: Establishment of analgesia 1. Can be elicited by squirting eye washing before the onset of pain solution into the eye L. Somatic pain: Sharp, stabbing, well-localized pain 2. Present until deep levels of anesthesia are typically arising from skin, skeletal muscle, and attained the peritoneum E. Laryngeal reflex M. Visceral pain: Pain arising from visceral tissues 1. Present during light anesthesia that is not well located, radiates, and is often 2. Dogs generally do not have a strong laryngeal rhythmic reflex; however, cats have a strong one, espe- II. “Nociceptors” detect cially after thiopental, thus the use of lidocaine A. Heat, ischemia, distention, mechanical injury on the larynx before attempting to intubate B. Chemicals released: Prostaglandins, leucotrienes, X. Judging the depth of anesthesia bradykinin, proteolytic enzymes, histamine, A. In its simplest concept, the anesthetist needs to potassium ion, and serotonin keep the animal from moving or sensing pain dur- III. Why treat pain? ing the procedure A. What is painful for humans is probably painful for B. Learn to assess several parameters at the same animals time to get an overall picture of anesthetic depth B. Most animal owners expect their pets will be 1. Check jaw tone treated for pain 2. Look at the eye position C. Inhalants and tranquilizers do not have analgesic 3. Check the breathing pattern properties 4. Evaluate the blood pressure D. Poor recoveries from anesthesia if animal is 5. Count the heart rate having pain 6. Is there any response to surgery? E. Pain is not protective in the postoperative period 7. What does the pulse feel like? F. Pain can lead to the following: 8. What is the color of the mucous membranes? 1. Increased fear and anxiety Pink? Pale? 2. Poor appetite 9. What is the CRT? 3. Increased risk of infection 10. Check the vaporizer setting compared with 4. Lower survival rate with major illness the MAC value to see whether it makes sense IV. Physiology after evaluation of the animal’s anesthetic A. Two types of neurons that transmit pain impulses depth 1. A -fibers: Sharp pain 11. Sometimes, even though the patient seems 2. C fibers: Dull, throbbing pain that is difficult to anesthetized, movement occurs when the localize type or degree of stimulation changes B. The pain impulses are transmitted to the dorsal 12. Pinch toe to see whether there is any horn of the spinal cord, where they are either response suppressed or augmented C. Have “top-off dose” of injectable agent available C. The pain is then transmitted to the brain where and ready to inject the perception of pain occurs D. Do not allow the patient to remove the ETT V. Monitoring pain A. Intensity of pain depends on the surgical proce- dure; severe pain occurs after declawing, ortho- PAIN pedic procedures, amputations, ear ablations I. Definitions B. Amount of pain depends somewhat on the A. Pain: An unpleasant sensory or emotional individual animal experience 1. There appears to be a five-fold difference in B. Analgesia: The absence of pain achieved through the amount of analgesic required in people and drugs or other means that required in animals 164 SECTION II SMALL ANIMAL 2. Collies, malamutes, and huskies appear to d. Transdermal morphine fentanyl patch have little tolerance for pain (1) Several hours’ delay before blood level C. Pain appears to be strongest in the first 24 hours is achieved after surgery (2) Theoretically, butorphanol may reverse D. Animals hide pain from humans the fentanyl (do not use concurrently) VI. Signs of pain (3) Do not use under heat source A. Vocalization 5. NSAIDs B. Facial expression a. Aspirin, acetaminophen, ibuprofen, flunixin, C. Posture ketoprofen, ketorolac, carprofen, meloxicam D. Amount of activity b. Effective against somatic pain and, depend- E. Attitude ing on the drug, visceral pain F. Appetite c. Antiinflammatory, antipyretic G. Urinary and bowel habits d. They inhibit prostaglandin synthesis H. Lack of grooming e. Effective in 30 to 60 minutes I. Response to palpation f. Several species differences in their effects J. Physiologic changes such as heart rate, breathing (Tylenol toxicity in cats) rate, pupil size, pale mucous membranes g. Toxicity K. Laboratory findings of increased cortisol, (1) Stomach ulceration epinephrine (2) Renal toxicity VII. Methods of controlling pain (3) Prolonged bleeding times A. Some analgesia from endorphins, leu-enkephalin, (4) May antagonize cardiovascular drugs dynorphin, acupuncture such as enalapril B. Transcutaneous electric nerve stimulation (TENS) (5) Liver failure in some dogs has been C. Nonpharmacologic pain control reported 1. Good nursing care 6. Combinations 2. Opportunity for urination and defecation a. Tylenol-codeine: 10 mg/kg acetaminophen/ 3. Cold or heat packs 0.5 to 1 mg/kg codeine every 6 to 12 hours D. Pharmacologic pain control (do not give to cats) 1. More than one class of analgesic may be the b. Morphine at the time of surgery and then best protocol (balanced analgesia) ketoprofen at the end of surgery a. Opioid  nonsteroidal antiinflammatory c. Acepromazine  morphine or other opioid drug (NSAID) d. Local analgesics lidocaine, bupivacaine: Do b. The goal is to have the patient sleep and not exceed 2 mg/kg in dogs and 0.5 to 1 mg/kg move around comfortably in cats with bupivacaine 2. Methods of delivery injection (including e. Splash blocks, intraarticular, epidural, nerve epidural, intraarticular), oral, transdermal blocks, topical (EMLA [eutectic mixture of 3. Administer pain drugs before the pain occurs local anesthetics] cream) (preemptive analgesia) to prevent “windup,” or 7. -2 agonist: Medetomidine the buildup of chemical mediators within the 8. Dissociative: Ketamine spinal cord 9. Total intravenous anesthesia (IVA) 4. Opioids: Effective against “sore” or “achy” pain a. Fentanyl but not so effective for sharp pain b. Lidocaine a. Morphine, hydromorphone, oxymorphone, c. Morphine Demerol (meperidine), fentanyl, butorpha- d. Ketamine nol (mild to moderate pain, visceral pain), e. MLK (morphine, lidocaine, ketamine) buprenorphine (oral in cats) f. Propofol b. Intraarticular morphine: Instilled in the joint immediately before or after closure of the Supplemental Reading joint capsule c. Epidural morphine Greene SA. Veterinary Anesthesia and Pain Management (1) Contraindications include septicemia, Secrets. Philadelphia, 2002, Hanley & Belfus. skin infections, neurologic disorders, Muir WW, Hubbell JAE. Handbook of Veterinary bleeding disorders Anesthesia, 4th ed. St Louis, 2007, Mosby. (2) Complications include urinary Thurmon JC, Tranquilli WJ, Benson GJ. Essentials of Small retention, pruritus, respiratory Animal Anesthesia & Analgesia. Baltimore, 1999, depression Lippincott Williams & Wilkins.