Unit 6 - Misc. Monitors and Equipment Flashcards
how does train of four work
Delivers a series of 4 twitches at 2 Hz at 0.5-second intervals for 2 seconds (Hertz = cycles per second)
Height of the 4th twitch (T4) is compared to height of 1st twitch (T1)
what is the BEST method of assessing a deep neuromuscular block
deep block = no twitches present
post-tetanic count
what percent of Nm receptors must be blocked to lose T1 (no twitches)?
90%
fade occurs when T4/T1 ratio is less than:
1.0
what is the advantage of double burst stimulation
easier to detect fade than with TOF
describe double bust stimulation
2 short bursts of 50 Hz tetanus 0.75 seconds apart
visual/tactile fade with TOF difficult to detect when ratio:
> 0.4
clinical recovery from blockade achieved when TOF ratio:
> 0.9
reversal agent indicated when ratio < 0.9
% receptors blocked to lose T4 with TOF
80-85%
describe tetanus
rapid sequence of 50 Hz for 5 seconds
how long is TOF assessment affected after tetanus
Results of TOF inaccurate for up to 6 minutes after tetanus assessment
describe single-twitch
delivers stimulus ranging from 0.1-1 Hz
what is post-tetanic potentiation
twitch response is stronger after a tetanic stimulus than it was at baseline (d/t ACh mobilization in presynaptic nerve terminal)
Can assess depth of NM blockade when no twitches are present
Post-Tetanic Count
PTc that suggests T1 will return soon
6-10
PTc that suggest a deep block
3 PTc or less
how does cerebral oximetry work
Uses near infrared spectroscopy (NIRS) to measure cerebral oxygenation
Measures venous oxygen saturation
how does cerebral oximetry work
Uses near infrared spectroscopy (NIRS) to measure cerebral oxygenation
Measures venous oxygen saturation
where are cerebral oximetry stickers placed
Placed on patient’s scalp, generally over frontal lobe
when is post-tetanic count useful?
to assess for depth of blockade when no twitches are present (like with a deep block)
what makes up cerebral blood volume
1 part arterial to 3 parts venous
____% of blood in brain is venous
75
suggests reduction in cerebral oxygenation
25+% change in baseline
what does EEG measure
the differences between electrical potentials in multiple brain regions
EEG provides information about electrical activity of:
cerebral cortex
light sensors used by cerebral oximetry
3:
1. arterial hgb
2. venous hgb
3. rissue cytochromes
what type of brain waves
beta
brain waves increased with anesthesia induction
beta
brain waves associated with awake mental stimulation and “light” anesthesia
beta
brain waves from highest to lowest frequency
- beta
- alpha
- theta
- delta
characteristics of beta brain waves
high frequency, low voltage
brain waves assoc with awake but restful state with eyes closed
alpha
alpha brain waves
theta brain waves
delta brain waves
burst suppression
what are theta waves associated with
- general anesthesia
- children during normal sleep
what are delta brain waves assoc. with
- general anesthesia
- deep sleep
- brain ischemia/injury
brain waves predominant under GA
theta
delta
what is burst suppression assoc. with
- general anesthesia
- hypothermia
- CPB, cerebral ischemia (esp. unilateral)
MAC assoc. with isoelectricity on EEG
1.5-2
how does N2O affect brain waves
alone increases beta wave activity
how can sevo affect brain waves
can increase epileptiform EEG activity
how can etomidate affect brain waves
can cause myoclonus (not assoc. with epileptiform EEG activity)
how can ketamine affect brain waves
can cause ↑ high frequency cortical activity & pt may be deeper than EEG suggests
Provides a sensitive measure of brain tissue at risk of infarction
EEG
the brain can’t maintain electrical function without what 2 things
O2 & glucose
EEG changes that mays signify brain at risk for ischemia
Development of new delta waves during anesthesia
conditions that mimic cerebral ischemia
deep anesthesia, hypothermia, hypocarbia
how does BIS work
Uses a computer algorithm to translate raw EEG data into a number between 0-100
target BIS for general anesthesia
low probability of explicit recall
BIS waveforms as anesthesia gets deeper
- Lower frequency (slower)
- Higher amplitude (taller)
how can ketamine affect BIS
- increases high frequency activity
- can make BIS higher than the level of sedation/anesthesia
BIS value assoc. with burst suppression
0-40
BIS assoc. with light to moderate sedation
80
limitations of BIS monitoring
- 20-30 second lag
- inaccurate with w/ hypothermia, electromyographic interference (↑ muscle tone), and encephalopathy
- less accurate in children
target range for GA of patient safety index monitor (PSA)
25-50
how is patient safety index monitor (PSA) similar to BIS
- measures EEG
- runs data through an algorithm
- displays a number that indicates level of anesthetic depth
electromotive force (voltage) = current x impedance
law obeyed by electricity
Ohm’s Law
what must occur to cause electrical injury
Must be voltage difference for current to flow across an impedance
consequences of electrical injury
- arrythmias
- muscle contractions
- diaphragmatic paralysis
- thermal injury
threshold for touch perception of electrical shock
1 mA
max current for a harmless electrical shock
5 mA
“let go” current occurs before sustained contraction
10-20 mA
current assoc. with loss of consciousness
50 mA
current assoc. with V fib
100 mA
Greatest margin of safety against risk of macroshock
isolate ground from main power supply
what is a macroshock
comparatively larger amount of current applied to external body surface
what is a microshock
comparatively smaller amount of current applied directly to myocardium
High resistance of skin bypassed - takes significantly smaller amount of current to induce V-fib
maximum allowable current leak in the OR
10 microA
3 things that increase susceptibly to microshock
CVL, PAC, or pacing wires provide direct conductive pathway to heart
3 things that increase susceptibly to microshock
CVL, PAC, or pacing wires provide direct conductive pathway to heart
what is electrical grounding
OR systems designed to reduce risk of electric shock
OR power supply is not grounded; equipment is grounded
what is electrical grounding
OR systems designed to reduce risk of electric shock
OR power supply is not grounded; equipment is grounded
2 faults in the system for shock to occur in OR:
1) After 1st fault, OR power supply becomes grounded. There is no completed circuit and no shock
2) After the 2nd fault, the circuit is complete and electric shock occurs
required to supply ungrounded power to OR
Isolation transformer
purpose of line isolation monitor (LIM)
assesses integrity of ungrounded power system in OR
Tells you when OR becomes grounded and how much current could potentially flow through you or your pt if a 2nd fault occurs
Line isolation monitor (LIM)
does a line isolation monitor protect against micro or macro shock
nope, not by itself
Primary purpose of line isolation monitor
alert OR staff of 1st fault (means the OR has become grounded)
what should you do if line isolation monitor sounds
alert OR staff of 1st fault (means the OR has become grounded)
when will line isolation monitor sound
alarm will sound if sum of all currents > 2-5 mA but no risk of electric shock in this situation
current delivered by electrocautery
500,000 – 1 million Hz
serves as an exit point for monopolar ESU
Functional return electrode
provides large, low impedance surface area for current to exit the body
why doesn’t bipolar ESU need a return pad
- Tip of bipolar (forceps) contains active electrode and return electrode
- Electrical current not dispersed throughout patient’s body
factors that increase risk of burns with ESU
- fauly in return electrode
- electrode malfunction
factors that increase risk of burns with ESU
- fauly in return electrode
- electrode malfunction
why is the return pad for ESU so large
a large, low impedance surface area needed for electrical current to exit the body
the smaller the area that the electricity exits, the greater the intensity of the burn
how are burns at return pad site prevented
- entire surface of electrode should be in direct contact with patient’s skin
- return pad should not be placed over bony prominences or metal implants
- make sure gel isn’t dried out
how can metal jewelry cause a burn with ESU
by “re-concentrating” electrical current from ESU
what should be done if the patient can’t remove a piece of jewelry and cautery is going to be used
- site of surgery should be far away from location of jewelry
- jewelry should not be in direct path between ESU & return pad
- jewelry should be taped to skin to increase the surface area of contact between jewelry and skin
If capacitance-coupled return electrode used: any metal can form capacitive coupling & cause burns
2 drugs that are most likely to reduce reliability of BIS
- ketamine: increases high-frequency activity (can increase BIS)
- N2O: increases amplitude of high-frequency activity, reduces amplitude of low-frequency activity (doesn’t affect BIS)
