Monitoring the Anesthetized Patient Flashcards
T/F: heart rate speeds up when animals are light and decreases as animals get deeper
False
Monitoring
Is an imprecise art
- combo of subjective and objective signs
- balancing act between too much and too little
- low therapeutic index for most anesthetics
Depth of anesthesia
Theoretical construct to conceptualize anesthetic effects on CNS as discrete or continuous phases or states
- sedation and amnesia that prevents explicit recall (humans)
- no movement or hemodynamic responses to surgical stimuli
Insufficient unconsciousness
- purposeful, directed movement
- treat with a drug that produces unconsciousness (any GABA agonist)
Insufficient antinociception
- nonpurposeful movement associated with surgery
- treat with a drug that provides antinociception (opioids)
Components of anesthesia
- amnesia
- unconsciousness
- immobility
- analgesia
Analgesia is the absence of _____
Pain
- unconscious patients can not experience pain, so analgesia is unnecessary
- anesthetized patients can and do respond physiologically and autonomically to noxious stimuli
Inhalants
Minimum alveolar concentration
- medial ED50 response to supramaximal stimulation
- permits comparisons between agetns at equipotent levels
- MAC used to define endpoints
Consciousness is usually lost at _____
0.25-0.4 MAC
_____ is the endpoint often used in animal studies
Loss of righting reflex
- transition between Guedel stage 1 and 2
Unconsciousness occurs at anesthetic concentrations _____ those preventing movement
Below
- noxious stimulation can cause movement in unconscious animals
Movement does not equal _____
Consciousness
- anesthetized animals may move in response to noxious stimuli
- unconscious animals can produce complex movements at the level of the brainstem or spinal cord
- -> connection to cerebral cortex not required, movement during light anesthesia is generated in spinal cord rather than brain
Anesthetic-induced amnesia
Memory formation in humans blocked at MAC awake
- 0.25-0.4 MAC
- new memory formation in animals blocked at similar levels
Anesthetic-induced analgesia
Pain assessment can only occur at concentrations below MAC awake
- reflex responses to noxious stimuli reduced or lost at 1 MAC
- hyperalgesia occurs with some anesthetics at low levels (0.1-0.2 MAC, 0.1 - 0.3 ED50 for injectables)
Anesthetics cause ______
Dose-dependent cardiorespiratory depression
- reduce CO and bp
- reduce respiratory rate and volume
- reduce delivery of O2 to tissues
- anesthetic effects may exaggerate concurrent dz
What is required to monitor anesthetized patients?
- pre-existing and current physiologic status
- subjective and objective methods
- ability to assess multiple parameters and integrate them into a general assessment of patient’s current status
- means to document and track changes in real time
ACVAA monitoring recommendations
- assessment of circulation
- assessment of ventilation
- maintenance of an anesthetic record of events
- assignment of a responsible person to be aware of the patient’s status and prepared to intervene
Subjective
Open to individual interpretation
- simple, no special equipment
- educated guesses
Objective
Quantifiable
Subjective assessments
- reflex activity
- eye position
- muscle relaxation
- heart rate, pulse quality and strength
- respiratory rate, volume, character
- mucous membrane color and refill time
- response to noxious stimuli
Guedel’s stages of anesthesia
- 1: voluntary movement (awake or asleep, can be aroused)
- 2: involuntary excitement (loss of consciousness to onset of regular breathing)
- 3: surgical anesthesia (light, medium, deep)
- 4: medullary depression (extreme CNS/CV depression, death)
What is a major problem with subjective parameters?
Non-specific!
Differentials for tachycardia
- sympathetic stimulation or pain
- hypotension
- hypovolemia
- hypoxemia or anemia
- hypercarbia
- hyperthermia
- acute anaphylactoid response
- drugs (ketamine, thiobarbiturates, antimuscarinics)
- electrolyte disturbances
Differentials for bradycardia
- excessive sympathetic depression
- increased vagal tone
- hypothermia
- hyperkalemia
- elevated intracranial pressure
- hypoxemia
- drugs (opioids, alpha2 agonists)
Differentials for tachypnea
- sympathetic stimulation
- hypoxemia
- hypercarbia
- hyperthermia
- CSF acidosis
- drug effects (doxapram)
Differentials for bradypnea or apnea
- excessive respiratory depression
- recent hyperventilation
- hypothermia
- musculoskeletal weakness or paralysis
Lacrimation, pupil size, eye position
Unreliable
- varies with anesthetic type
- influenced by concurrently administered drugs
Common objective monitoring methods
- ECG
- bp
- hemoglobin saturation
- CO production/elimination
Less common objective monitoring methods
- inhaled anesthetic concentration
- neuromuscular responses
- EEG
- temporally processed data
- pleth volume index
ECG
Monitoring device that produces a continuous waveform representing the summation of electrical activity of the heart for a given axis or plane
All diagnostic ECGs in dogs are done in ______
Right lateral recumbency
ECG pros
Non-invasive
- provides HR and cardiac electrical rhythm in real time
- used to diagnose chamber enlargement and defects in cardiac impulse origin and conduction
ECG cons
- limited to assessment of changes in the x-axis
- does not provide info on cardiac pumping activity
- useful when combined with other methods that assess perfusion
Blood pressure
Q x R
- systolic: 100-160 mmHg
- diastolic: 60-90 mmHg
- mean arterial pressure: 60-80 mmHg
Direct blood pressure
Catheter inserted into an artery and connected to a pressure transducer
- changes in pressure converted to proportional changes in voltage and displayed on monitor
Direct arterial pressure pros
Provides systolic, diastolic, and mean arterial bp and hr
- gold standard of arterial pressure monitoring
- reliable under wide range of perfusion pressures
Direct arterial pressure cons
- invasive
- specialized equipment
- accuracy depends on calibration and selected artery
- does NOT measure flow or perfusion
- requires skill for placement
- potential for hematoma formation, air embolism, exsanguination, sepsis
Indirect blood pressure
Occlusive cuff is placed over accessible artery and inflated until artery is occluded
- pressure slowly released while noting the reappearance of perfusion in the artery
Indirect arterial pressure pro
- non-invasive
- doppler gives systolic bp and hr
- oscillometric provides systolic, diastolic, mean arterial pressures, hr
- easy
- assess trends
Indirect arterial pressure cons
- specialized equipment
- accuracy directly related to cuff fit
- doppler only gives systolic value (15-25 mmHg low in cats and small dogs)
- not reliable in presence of reduced peripheral perfusion, or patient movement
- does not quanify flow or perfusion
- not accurate or repeatable
Blood pressure
Useful for indirect assessment of adequacy of organ perfusion
- arterial bp = hr x sv x svr (**3 independent variables affecting bp!!)
- useful in monitoring anesthetic depth, assess IV fluid loading, inotrope efficacy
Pulse oximeter
Dual wavelength spectrophotometer
- arterial Hbg saturation by determining the difference in light absorption between oxyhemoglobin and total hemoglobin during pulsatile flow
Pulse oximeter pros
- non invasive
- simple to operate
- indicates presence of pulsatile flow and hemoglobin saturation
- normal SpO2 breathing air (21% O2) 92-95%
- increases to 98-100% when breathing 100% oxygen
Pulse oximeter cons
Slope of Hgb dissociation curve when SpO2 <90% makes pulse ox a brink of disaster monitor due to rapid fall in PaO2
- adversely affected by vasoconstriction, probe motion, and hypotension
- large number of false alarms
- limited probe placement locations
- misleading in face of severe anemia
Pleth variability index
Patient’s position of the Frank-Starling curve
- during mechanical ventilation, transpulmonary pressure increases during inspiration thus increasing preload on the heart
- not enough fluid positions your patient on the steep end of curve, making them more susceptible preload induced by the vent
- observed on pulse ox
PVI calculation
Automated measurement
- changes in plethysmographic waveform amplitude over the respiratory cycle
- percentage from 1 to 100% –> 1 = no pleth variablity, 100 = max pleth variability
Capnograph
Infrared spectrophotometer that determines CO2 levels in the last gas exiting the lungs during normal tidal expiration, where ETCO2 < PACO2 < PaCO2
Capnograph pros
- non invasive
- verifies ET placement
- provides rr
- verifies normal function of anesthetic circuit with lack of rebreathing
- verifies presence of circulation thru CO2 delivery to lungs
- accurate predictor of arterial PCO2 in ventilated patients
Capnograph cons
- expensive
- reduced accuracy in spontaneously breathing patients due to dead space ventilation (large difference between ETCO2 and PaCO2)
- false low readings with non-rebreathing circuits and high fresh gas flows due to sample dilution
