Anesthesia monitoring: Part I (Proceedings)


The word anesthesia means without sensation-–our goal is to provide unconsciousness, amnesia, analgesia and muscle relaxation for a variety of procedures both invasive and non-invasive.

• The word anesthesia means without sensation-–our goal is to provide unconsciousness, amnesia, analgesia and muscle relaxation for a variety of procedures both invasive and non-invasive. Our ability to carefully string our patients out along the line between life and death compromises homeostasis making close monitoring essential.

Why monitor?

• Anesthetic emergencies are difficult to predict

• Anesthetic emergencies happen quickly

• Anesthetic emergencies can be devastating

• It is better to be proactive rather than reactive

Our goal

• To be able to walk that line with confidence by maximizing the safety of the anesthetic experience

Morbidity and mortality (M&M)

• Morbidity refers to the prevalence of disease (related to the anesthetic event in this case)

• Mortality refers to the chances of death

• Certain problems are more likely to increase M&M

o Excessive bradycardia

o Cardiac depression

o Vasodilation

o Hypotension

o Arrhythmias

o Hypoventilation

o Hypoxemia

o Hypothermia

• Diligent monitoring allows us to recognize and treat potentially life threatening problems

Monitoring basics

• If you only had eyes, ears and hands...

o Heart rate

o Pulse quality and vasomotor tone

o Respiratory rate and character

o Reflexes and muscle tone

o Eye position

o Body temperature

• Monitoring multiple parameters gives you a more complete picture of the physiologic status of the patient

Heart rate

• Stay away from extremes...

• Bradycardia

o Heartrate is too slow when it is associated with decreased cardiac output, hypotension and/or poor perfusion

o Dog low 50's (with normal BP- also dependent on size, small dogs have higher heart rates...)

o Cat low 100's (with normal BP)

o It is also important to monitor blood pressure (BP) and end-tidal carbon dioxide (ETCO2)

• Tachycardia

o Decreases filling time of ventricle and increases myocardial oxygen consumption- a double whammy!

o Dogs 180-200 (size dependent)

o Cats 240-280 (size dependent)

• Some cause of extremes of heart rate (and potential ways to remedy them)

o Bradycardia

• Anesthetic overdose (lighten up)

• Opioid administration (give an anticholenergic)

• Alpha-2 agonist administration (reverse or no treatment)

• Hypothermia (rewarm)

• Hypoxia (oxygen therapy)

• 1st and 2nd degree A-V blockade (anticholenergics)

• High vagal tone (anticholenergics)

o Tachycardia

• Too light (deepen)

• Painful (give analgesics)

• Ketamine administration (no treatment)

• Anticholenergics (decrease dose next time)

• Inotropes (decrease infusion)

• Hypovolemia (restore volume)

• Hyperthermia (cool)

• Hypoxemia (oxygen therapy)

• Hypercarbia (ventilate or eliminate rebreathing of CO2)

• Anesthesia recovery (comfort or no treatment)

Pulse quality and vasomotor tone

• Palpation of a pulse is a subjective way of approximating blood pressure. It is done by evaluating the height and width of the pulse pressure wave form compared to normal

o Bounding pulse- vasodilation as seen in sepsis and hypovolemia, vessel is easily collapsible (complexes would appear tall and wide)

o Weak and thready pulse- vasoconstriction as seen with alpha-2 administration, poor cardiac function, tachycardia, small stroke volumes (complexes would appear small and narrow)

• Pulse quality is largely a reflection of stroke volume (the volume of blood pumped with each beat) and vessel size or vasomotor tone (vasodilation vs. vasoconstriction)

• Vasomotor tone

o Regulates both peripheral and visceral perfusion

• Vasodilation

- Improves peripheral perfusion

- Causes hypotension

• Vasoconstriction

- Impairs peripheral perfusion

- Improves blood pressure

o Assessing vasomotor tone

• Mucous membrane color and capillary refill time are good guides

- Pale = vasoconstriction, CRT less than 1 sec

- Pink = normal, CRT 2 sec

- Red = vasodilation, CRT greater than 2 sec

o Causes of vasodilation and vasoconstriction

• Vasodilation

- Systemic inflammation

- Sepsis

- Hypercapnia

- Hyperthermia

- Drugs (acepromazine, inhalants

• Vasoconstriction

- Hypovolemia

- Heart failure

- Hypothermia

- Drugs (alpha-2 agonists, sympathomimetics)

Pulse sites

• Femoral artery

o Large vessel but can be difficult to palpate in obese and heavily muscles animals

• Dorsal metatarsal artery (dorsal pedal)

o Smaller artery over dorsal aspect of metatarsals

o Very accessible, great for arterial catheter placement

o Can be difficult to palpate in vasoconstricted, hypotensive, or small patients

• Coccygeal artery

o Ventral tail (strongest at the base of the tail)

o can be stinky but is often useful for arterial catheters (must use aseptic technique)

• Radial artery

o Just proximal to the metacarpal pad

o Can be difficult to palpate in vasoconstricted, hypotensive, or small patients

• Lingual artery

o Ventral tongue near lingual frenulum

o Only useful in adequately anesthetized patients (great place to grab a pulse intra-op!)

o Arterial catheters can be placed here but hematomas are a problem

• Chest

o Good place to grab a heart rate in cats, small dogs and other small animals

Respiratory rate and character

• All anesthetic drugs provide some degree of respiratory depression

• A change in breathing is a good indication of a change in patient status

• Respiration is comprised of tidal volume (TV), respiratory rate (RR), and minute volume (MV)

o TV (Vt) = the volume of air in a single breath (10-20 mL/kg)

o RR (f) = the number of breaths per minute (8-15 br/min)

o MV (V) = total volume of air breathed per minute (150-250 mL/kg/min)

o TV X RR = MV

• Respiratory character

o Shallow breaths

• Decreased TV (may have an increased RR to normalize MV)

o Deep breaths

• Increased TV (may have a decreased RR to normalize MV)

o Apneustic

• Inspiratory hold (commonly seem with ketamine administration)

o Apnea

• Lack of spontaneous breathing

• Common after barbiturate induction (must assist breathing until drugs are metabolized/redistributed- apnea must be treated!)

• Too deep (lighten up)

• Hypothermic (warm)

• Over ventilated (back off)

o Bradypnea

• Slow RR

• Without ETCO2 monitor or a blood gas analyzer you don't know if this patient ventilating adequately (assist or sigh)

• Hypothermic (warm)

• Too deep (lighten up)

o Tachypnea

• Increased RR (many possible causes)

• Too light, too deep, hypoxia, hypercapnia, hyperthermia, hypotension, painful, septic, atelectic...

• Give some larger breaths, check body temp, check BP, assess pain, assess anesthetic level...

• Ventilation

o Hypoventilation and hyperventilation

• These can only be accurately assessed using an ETCO2 monitor or a blood gas analyzer

Reflexes and tone

• The presence of reflexes indicates a lighter level of anesthesia

o This is not always a bad thing!

o If it doesn't interfere with the procedure and the animal is stable, go with it

o The corneal reflex should always be present (unless paralyzed)

• Tone can be assessed as none, some or lots

o Jaw tone, anal tone, general muscle tone

o No tone at all may indicate that the patient is too deep or just right

o Some tone is ok and even good (as long as it doesn't interfere or cause harm)

o Lots of tone is not good (animal is awake and looking at you)

Eye position

• The eye is a little window into the CNS...

• Mydriasis is dilation, miosis is constriction

• If the eye is centrally facing

o Animal is either too deep or too light- look at pupil size

• Medium pupil- animal is probably light

• Dilated pupil- animal is too deep, immediate change is necessary

• Constricted pupil- animal is too deep, immediate change is necessary

• If the eye is ventral-medial

o Your animal is at a good anesthetic plane

• Drugs can affect eye signs and pupil size

o Opioids cause mydriasis

o Ketamine can maintain palpebral reflex


• Anesthesia depresses muscle activity, metabolism and thermoregulation

• Good range is 98-102 degrees Fahrenheit in dogs and cats perianesthesia

• Hypothermia can be an anesthetic- decrease your doses in hypothermic patients!

o Animals less than 98 are hypothermic

• 96-98 is mild hypothermia

• 94-96 is moderate

• 90-94 is severe (decrease anesthetic requirements, prolonged recovery)

• Less than 90 is moribund (CNS depression, death is imminent)

o Animals greater than 103.5 degrees Fahrenheit are hyperthermic (normal 100.5-102.5)

• Cell damage occurs at 108 degrees and above

• Malignant Hyperthermia (MH)

- Genetic predisposition seen in dogs, pigs, humans, horses and some cats

- Precipitated by inhalant anesthesia

- It is a rapid, relentless, progressive increase in body temp

- Can cause muscle rigidity and elevated potassium

- Treatment- aggressively cool, discontinue inhalant, administer Dantrolene, maintain on oxygen


• Your eyes, ears and hands can make excellent monitors if you know how to put them to work!

• Some words of wisdom to remember forever...

o Be aware

o Look at the whole picture (co-existing disease, drugs given and currents meds, procedure, species, breed...)

o Seek knowledge

• Knowledge builds confidence; the more you know the more confident you will be

o Enjoy yourself

• Anesthesia is fun


Dorsch JA, & Dorsch SE. Understanding Anesthesia Equipment. Baltimore: Williams & Wilkins, 1999

Haskins, SC. Monitoring Anesthetized Patients; Muir, WW. Cardiovascular System; McDonnell WN & Kerr, CL. Respiratory System; Bednarski, RM. Dogs and Cats; All In: Tranquilli WJ, Thurmon JC, Grimm KA, eds. Lumb & Jones Veterinary Anesthesia and Analgesia. Iowa: Blackwell, 2007

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