Week 13 - Temperature and Depth of Anesthesia Monitoring Flashcards
What is anesthesia awareness? What is its significance?
Unexpected and explicit recall by patients of events that occurred during anesthesia
-most patients don’t experience pain, but have vague auditory recall – sense of dreaming - generally not disturbing to patient
Significant because patients distressed by recall of intraop events may develop severe psychological sequelae, including PTSD
What are the goals of depth of anesthesia monitors?
- Ability to detect level depth of anesthesia and risk of awareness
- Determine if patient is unnecessarily too deep and at risk of prolonged recovery and increased morbidity and mortality
- No matter what the monitor is it should work similarly across all patient populations
- They should work the same regardless of meds and anesthetic technique employed
What are the basic principles of EEG analysis?
Information is collected by frontal electrodes and the signals are processed to yield an “index value” to determine the depth of anesthesia
Cells within the cerebral cortex provide synaptic activity and results in changes of voltage that can be detected by electrodes placed on the forehead and/or scalp
How does EEG activity relate to the depth of anesthesia?
There are various waveform patterns within certain frequency ranges, which correspond to various neurophysiological processes
These patterns are grouped into frequency bands in order of increasing frequency (Delta, Theta, Alpha, Beta)
Each band exhibits certain changes under the influence of anesthetic agents
What are the different EEG waves?
Delta: slowest frequency, seen in deep sleep/anesthesia, hypoxia, ischemia, poor metabolism
Theta: low frequency, seen under general anesthesia
Alpha: medium frequency, awake but eyes closed (relaxed or sedated state)
Beta: high frequency, awake state (alert)
What are the two EEG processing techniques?
Time Domain:
- voltage changes plotted against time
- burst suppression is identified in this domain
- complex signals can’t be fully analyzed by using time domain methods alone
Frequency Domain:
- advancement of microprocessing enabled fast calculations (Fourier transformation) = Analysis of frequency domain
- frequencies present in EEG compared with degree to which these frequencies are present
How does a BIS monitor work?
it converts the recordings from a frontal EEG to a single number (BIS index) and represents the level of consciousness by the patient
Ranges between 0 (isoelectric) to 100 (awake)
The changes in clinical state are represented by changes in phase coupling
*most widely used DoA monitor
What are the pros and cons of the BIS monitor?
Pros:
- Subjected to large randomized clinical trials to assess if it decreases incidence of awareness
- BIS algorithm was designed for use with propofol, versed and Iso
- Beneficial for TIVA
Cons:
- Some studies have been funded by Aspect Medical (the creator)
- BIS index may not accurately reflect the effects of all drugs
- Artifact from use of bovi, bair hugger, etc
How does N2O and Ketamine affect a BIS reading?
N2O: no change in BIS
Ketamine: BIS increases
*should decrease in both
What is the target range for the BIS monitor?
Generally accepted target range = 40-60
- very low probability of recall with BIS value less than 60
- the higher the BIS number –> the more awake
How does Entropy monitoring work?
The module takes the raw EEG and frontal EMG signals and applies an algorithm that quantifies the irregularity of the signal to provide a measure of depth of consciousness
Calculates and “entropy number” which ranges from 0 (minimum entropy – isoelectric EEG) to 1 (maximum entropy - white noise)
*deeper lever of anesthesia, which causes burst suppression and high regular patterns, would result in a lower entropy number
What are the two different entropy numbers the Entropy monitor calculates? What is the target range?
State Entropy (SE): calculated over the 0.8 Hz to 32 Hz band to reflect cortical processes *ranges from 0 to 91 (awake)
Response Entropy (RE): calculated over the 0,8 Hz to 47 Hz range, which includes the high frontal EMG-dominated frequencies *ranges from 0 to 100 (awake)
Target range for both values is 40-60
- if SE >60 – increase anesthetic dose
- a rise in RE of 5-10 points above SE even if SE <60 indicates inadequate analgesia
How does N2O and Ketamine affect a Entropy reading?
N2O: SE and RE are decreased (unlike with BIS)
Ketamine: SE and RE are increased
What are the benefits of warm patients?
- Reduced wound infections
- Reduced blood loss
- Reduced cardiac events
- Shortened hospital stays
*warming is recognized as standard of care by Medicare and SCIP as well as others
Where is the thermoregulatory site in humans?
Hypothalamus is the dominant thermoregulatory site
Thermal input from skin is secondary mechanism – also very important
*thermoregulation is based on many, redundant signals from nearly every type of tissue within the body
What are the three phases of processing thermoregulatory information?
Afferent Thermal Sensing – many cells in the body are temp sensors
Central Regulation – “set point” system vs thermoregulatory model
Efferent Responses – sweating, peripheral cutaneous vasoconstriction, and brown fat metabolism (more elaborate mechanisms: shivering, BP, and osmotic control adaptation)
What are the indications for temperature monitoring?
- Studies show significant reduction in temperature in anesthetized patients
- Large volumes of cold fluids administered
- Deliberate cooling or warming of a patient
- Pediatric surgery
- Patients with suspected or known temp regulatory problems – malignant hyperthermia
- Major surgical procedures
How does thermoregulation change during general anesthesia?
Pts under GA are unable to activate behavioral responses and must rely on autonomic defenses and external temperature management
All anesthetics markedly impair autonomic thermoregulatory control
- Propofol, Alfentanil, Precedex, Iso, and Des increase sweating threshold minimally, if at all (these agents help preserve warm defenses for some time)
- Propofol, Alfentanil, and Precedex produce significant decrease in vasoconstriction and shivering thresholds – Iso and Des decrease cold response threshold only slightly
How does thermoregulation change during neuraxial anesthesia?
Central temperature control is slightly impaired
- autonomic impairment is compounded by an impairment in awareness by the patient that they are becoming hypothermic
- misperception that an awake patient is able to regulate their temp
Shivering is common during neuraxial anesthesia, what are the four etiologies?
Shivering in response to core hypothermia***
Shivering in normothermic or hyperthermic pts who are developing fever
Direct stimulation of cold receptors in neuraxis by the injected local anesthetic (will usually see right away)
Non thermoregulatory muscular activity that resembles thermoregulatory shivering
What is the ASA standards for contact surface temperatures?
Maximum contact surface temp shall not exceed 48*C
Average contact surface temp shall not exceed 46*C during normal conditions
What are the different technologies that monitor temperature?
Thermistor: composed of metal oxide place into a wire
*advantages = small size, rapid response time, continuous readings, probes are interchangeable/disposable
Thermocouple: electrical circuit with two metals, one remains at constant temp, other is exposed to area where temp is being measured
*advantages = accuracy, small size, rapid response time, continuous readings, stability and probe interchangeability
Liquid Crystal: consists of flexible adhesive backing with plastic encased liquid crystals on black background
- advantages = safe, convenient, noninvasive, easy to apply, disposable, inexpensive
- disadvantages = rely on observer interpretation, not accurate
Infrared: electronic instrument, accurate
*tympanic thermometer – not useful in OR
What are the two different temperature compartments?
Core = Deep, vital internal organs
- uniform – varies between 35.7 and 37.8*C
- core temp should be monitored when significant changes in temp are expected
Periphery = normal thermoregulatory vasoconstriction maintains a temp gradient between the core and periphery of 2 to 4*C
-skin and axillary temperatures
What are the different temperature monitoring sites?
- PA: measured via PA cath, thought to be best method to measure temp although not reliable during thoracotomy or bypass
- Esophagus: temps can vary up to 4*C depending on probe location – should be in lower third or fourth of esophagus
- Nasopharynx: location close to hypothalamus, some studies have shown correlation with core temp, easily accessible during surgery – may cause epistaxis
- Forehead: liquid crystal strip provides easy noninvasive indication of core temp trends
- Tympanic: provides reliable reflection of core temp
- Axillary: easy, noninvasive site for surface temp
- Skin: easy, noninvasive surface temp
- Bladder: accurate measurement of core temp
- Rectal: good indication of core temp
What are the four major methods of heat loss?
- Radiation: loss of electromagnetic energy through infrared rays from warm body to colder objects in the room that don’t contact the body (60-70% of heat loss)
- Convection: transfer of heat to an air current
- Conduction: heat is lost through direct contact between the patient and colder objects
- Evaporation: occurs from the skin, respiratory tract, open surgical wounds, pneumoperitoneum, or wet towels/drapes that in contact with pts body
- Radiation > Convection > Evaporation > Conduction
- each 1*C decrease in temp = 7% decrease in metabolism
Who are the high risk populations for hypothermia?
Geriatrics
Pediatrics
Hypothyroid
Patients with hypothalamic lesions
*hypothermia = most common temp related disorder during anesthesia
How do forced air warming devices function?
Entrain ambient air through a microbial filter
Air is warmed using a thermostat controlled electric heater
Air is then blown through a hose that is attached to inflatable pt blanket
What are the advantages of forced air warmers?
- Safe, simple, effective
- Inexpensive (relatively speaking)
- Variety of blankets, both disposable and reusable
- Provide more calories to cost than other warming devices
- Fiberoptic laryngoscopes can be warmed using them
- OR table can be warmed preop
- Can also be used to cool
What are the disadvantages of forced air warmers?
- Electric power requirements make it unsuitable for field use
- Cumbersome to transfer or set up in CT
- May occasionally need to be removed to expose covered areas
- Most systems don’t permit the concurrent use of multiple blankets without two separate units
- Interference with BIS and DoA monitors
- ??increased risk of infection??
