Week 13 - Temperature and Depth of Anesthesia Monitoring

Question 1

What is anesthesia awareness? What is its significance?

Answer 1

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

Question 2

What are the goals of depth of anesthesia monitors?

Answer 2

• 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

Question 3

What are the basic principles of EEG analysis?

Answer 3

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

Question 4

How does EEG activity relate to the depth of anesthesia?

Answer 4

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

Question 5

What are the different EEG waves?

Answer 5

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)

Question 6

What are the two EEG processing techniques?

Answer 6

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

Question 7

How does a BIS monitor work?

Answer 7

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

Question 8

What are the pros and cons of the BIS monitor?

Answer 8

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

Question 9

How does N2O and Ketamine affect a BIS reading?

Answer 9

N2O: no change in BIS

Ketamine: BIS increases

*should decrease in both

Question 10

What is the target range for the BIS monitor?

Answer 10

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

Question 11

How does Entropy monitoring work?

Answer 11

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

Question 12

What are the two different entropy numbers the Entropy monitor calculates? What is the target range?

Answer 12

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

Question 13

How does N2O and Ketamine affect a Entropy reading?

Answer 13

N2O: SE and RE are decreased (unlike with BIS)

Ketamine: SE and RE are increased

Question 14

What are the benefits of warm patients?

Answer 14

• 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

Question 15

Where is the thermoregulatory site in humans?

Answer 15

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

Question 16

What are the three phases of processing thermoregulatory information?

Answer 16

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)

Question 17

What are the indications for temperature monitoring?

Answer 17

• 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

Question 18

How does thermoregulation change during general anesthesia?

Answer 18

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

Question 19

How does thermoregulation change during neuraxial anesthesia?

Answer 19

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

Question 20

Shivering is common during neuraxial anesthesia, what are the four etiologies?

Answer 20

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

Question 21

What is the ASA standards for contact surface temperatures?

Answer 21

Maximum contact surface temp shall not exceed 48*C

Average contact surface temp shall not exceed 46*C during normal conditions

Question 22

What are the different technologies that monitor temperature?

Answer 22

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

Question 23

What are the two different temperature compartments?

Answer 23

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

Question 24

What are the different temperature monitoring sites?

Answer 24

• 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

Question 25

What are the four major methods of heat loss?

Answer 25

• 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

Question 26

Who are the high risk populations for hypothermia?

Answer 26

Geriatrics

Pediatrics

Hypothyroid

Patients with hypothalamic lesions

*hypothermia = most common temp related disorder during anesthesia

Question 27

How do forced air warming devices function?

Answer 27

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

Question 28

What are the advantages of forced air warmers?

Answer 28

• 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

Question 29

What are the disadvantages of forced air warmers?

Answer 29

• 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??