Misc Equip

Mapleson Circuits

• Not commonly used in modern anesthesia workstations due to economic reasons for volatile anesthetic use
• Differ from Circle Systems in two ways:
  • Bidirectional gas flow
  • No CO2 absorber
• Depend on high fresh gas flow rates to eliminate CO2 and prevent rebreathing
• Still used in pediatrics, patient transport, procedural sedation, T-piece systems, and office-based airway management

Semi-Closed Systems

• Classification based on reservoir and rebreathing characteristics:
  • Open: No reservoir, no rebreathing
  • Semi-Open: Reservoir present, no rebreathing
  • Semi-Closed: Reservoir present, partial rebreathing
  • Closed: Reservoir present, complete rebreathing

Mapleson A (Magill Circuit)

• Functionally distinct from other circuits
• APL located near the patient
• Fresh gas flow enters from the distal end of the circuit
• Disadvantages:
  • Pollution
  • Can't be used with a ventilator
• Most efficient circuit for spontaneous ventilation if fresh gas flow equals minute ventilation
• Worse efficiency during manual ventilation due to APL location

Mapleson A (R3mix/Lack's Modification)

• Adaptation of Mapleson A to facilitate scavenging of gases
• Added coaxial expiratory limb and expiratory valve moved near the reservoir bag
• Disadvantages:
  • Increased work of breathing
  • Less efficient fresh gas flow than Magill

Mapleson B & C

• Rarely used
• Similar in assembly, with C having less dead space
• APL and FGF located proximal to the patient
• Extremely inefficient in scavenging and high gas flows
• High fire risk due to APL location

Mapleson D

• Like Mapleson A, but with APL and FGF flipped
• Most efficient during controlled ventilation since FGF pushes alveolar air toward the APL
• Effective at scavenging waste gases due to APL location away from the patient

Mapleson D (RE-mix/Bain Circuit)

• Modification of Mapleson D
• Coaxially placed FGF warms inspired gases
• Used commonly in pediatrics
• Advantages:
  • Exhaled gases scavenged
  • Recommend Pethick Test prior to use

Manual Resuscitators

• Nonrebreathing valve
• Components:
  • Self-inflating reservoir bag
  • Nonrebreathing valve (between vent bag and patient)
  • Intake valve (between vent and reservoir bag)
  • Reservoir valve (with 2 unidirectional valves)
• Disadvantages:
  • Lacks visual monitoring cue to patient respiratory effort
  • High PIP may be generated, risking barotrauma or gastric insufflation
  • Significant variability of tidal volume, PIP, and PEEP
  • Nonrebreathing valves generate resistance, increasing the work of breathing

Electroencephalogram (EEG)

• Direct assessment of nervous system function via extracellular electrical potentials
• Utility determined by:
  • Hemodynamic effects on electrophysiology
  • Pharmacologic effects on action potentials
• Anesthetics hyperpolarize neurons and affect neurotransmitter release/response at the synapse
• Presynaptic glutamate is inhibited, while postsynaptic GABA is potentiated

Nervous System Monitoring

• Historical practice: Physiologic responses to noxious stimuli are explained by the nociceptive-medullary-autonomic circuit
• Traditional physiologic monitors (HR, BP, SpO2) reflect supportive parameters of tissue maintenance
• Anesthetic state of the patient is inferred in this manner

Updated Theories

• CNS functions to exchange information via coordinated action potentials
• Cortical and subcortical structures are critically interconnected
• Growing evidence for top-down mechanism: Intracortical communication becomes interrupted
• Likely, a combination of top-down and bottom-up mechanisms is responsible for suppression of consciousness under anesthesia

EEG

• Pyramidal cells in the cerebral cortex produce large local field potentials via extracellular potentials
• Geometry of neurons creates a biophysical antenna whose potentials can be measured through the scalp
• Neurons exhibit oscillatory spiking and local field potentials that play a primary role in coordinating and modulating communication
• Typical mapping involves placement of 10-20 electrodes
• EEG activity reflects cerebral metabolism and blood flow
• EEG monitoring is useful when cerebral cortex tissue is vulnerable to ischemia or to guide pharmacologic therapy

EEG Waveforms

• Classified according to consistent patterns of amplitude and frequency:
  • Delta (0.5-4 Hz)
  • Theta (4-8 Hz)
  • Alpha (8-13 Hz)
  • Beta (13-30 Hz)
  • Gamma (30-80 Hz)
• "Activated" waveforms:
  • Beta (13-30 Hz)
  • Alpha (8-12 Hz)
• "Depressed" waveforms:
  • Theta (4-8 Hz)
  • Delta (1-4 Hz)

EEG-Based Indices

• Display a dimensionless variable to indicate the level of "wakefulness" created via proprietary algorithms
• GETA induces a change from high values to lower values that indicate states of sedation and unconsciousness
• BIS index: Range 0-100, with values of 60 suggesting reduced probability of consciousness
• GETA is commonly maintained between 45-60

EEG Index Disparities

• Uncertainties exist when using EEG-based indices to define brain states under general anesthesia
• Different anesthetics act at different molecular targets and neural circuits
• The different states of altered consciousness demonstrate different EEG signatures
• These various signatures created by different drugs are readily observed by EEG, but processed EEG indices can confuse or miss certain signals

Pharmacology

• Intravenous hypnotics and volatile anesthetics produce progressive slowing of EEG until burst suppression
• Activation of GABA receptors within the cortical and subcortical structures results in inhibition and disorganized communication
• Characterized by Alpha and Delta oscillations
• Volatiles: at higher doses, Theta bands are present
• N2O (NMDA and K+ sites of action) demonstrate Beta and Gamma oscillations, with synergism existing when combined with other inhalation agents

Miscellaneous

• Neuromuscular blockade: Overlapping frequencies between EEG and Electromyography (EMG) cause interference in values
• Vasoactive agents can affect EEG values
• Standard of care for patients with cardiac implantable electronic devices (CIEDs) undergoing magnet placement:
  • Have rescue equipment immediately available
  • Avoid activating electrosurgery in the area of the generator
  • Magnet placement: Position matters, with varying effects on pacemakers and ICDs

Anesthesia Circuits

• Occlude PT end: useful in MRI suites, allows for ease of connecting to ventilator, and O2 flush to fill circuit
• Disadvantages: disconnection and kinks often go unrecognized

Mapelson Circuits

• Mapelson E: modification of Ayre's T-piece, FGF proximal to patient, no reservoir bag, high fresh gas flows required
• Mapelson F/Jackson-Rees Circuit: modification of Mapelson E, added reservoir bag, commonly used for patient transport

Disadvantages of Mapelson Systems

• High fresh gas flows = high cost
• High pollution
• High FGF = less heat/humidification
• FGF rate difficult to ascertain in correlation to patient minute ventilation
• Inaccessible components depending on setup
• Not suitable for patients at risk of MH

Exceptions

• Mapelson A: only circuit with FGF near the reservoir bag
• Mapelson C: only circuit not corrugated
• Mapelson E: only circuit without a reservoir bag
• Bain & Lack: coaxial tubing

Manual Resuscitators

• Nonrebreathing valve: used consistently during patient transport, emergency resuscitation, or as a back-up supply

EEG

• Anteriorization: EEG pattern in deep anesthetic state, characterized by alpha and delta activity greater in anterior leads
• Burst Suppression: EEG pattern in deep anesthesia, characterized by high-frequency "burst" activity interspersed with flat "suppression" tracings
• Isoelectric: EEG pattern with zero amplitude, seen in deep states of GETA, coma, brain death, and cardiac arrest
• EEG-Based Indices: display a dimensionless variable to indicate level of "wakefulness", commonly used in BIS index

EEG Interpretation

• Accurately identifies consciousness/unconsciousness, stages of sleep, and coma
• Intraop, suggests anatomic or metabolic alterations of the brain
• Regional asymmetry seen with tumors, epilepsy, and cerebral ischemia

Awareness

• GETA requires: Unconsciousness, Amnesia, Analgesia, Immobility, Attenuation of autonomic response
• Awareness: ability to process, integrate, and store information in interaction with the environment
• Lack of awareness is inferred through EEG analysis

Processed EEG

• Manufactures devices that process reduced channel EEG signals
• Examines 4 components within the EEG associated with the anesthetic state
• Various devices: BIS sensor, PSArray, Narcotrend, Entropy Module, Cerebral State Monitor

Pharmacology

• Intravenous hypnotics & Volatile anesthetics produce progressive slowing of EEG until burst suppression
• Ketamine: creates false excitatory values, increases Theta and Beta/Gamma frequencies
• Dexmedetomidine: initially increases Alpha waves, at high doses, primarily Delta waves
• Opiates: decrease Beta, slow Alpha, and increase Delta, reduce wave variability

EMG

• Active or Evoked EMG: helps identify intact nerves, used to identify a nerve or anatomic variances in order to avoid cutting it
• Passive or Spontaneous EMG: commonly used in spine surgery to identify nerve irritation before injury

Cardiac Implantable Electronic Devices (CIEDs)

• Refers to any permanently implanted pacemaker, cardioverter-defibrillator, or cardiac resynchronization therapy devices
• Utilization is increasing due to growing geriatric populations and prevalence of cardiovascular disease

Pacemakers

• Pacemaker placement is indicated for symptomatic bradyarrhythmia and atrioventricular conduction defects likely to progress to complete heart block
• Have 1-2 leads placed atrial +/- ventricular and a pulse generator
• Generator transmits electrical pulse through the lead/s to depolarize local myocardium

Indications

• Expanded indications for pacemaker implantation include:
  • Sinus node dysfunction
  • Symptomatic bradycardia
  • AV block
  • Cardiomyopathy
  • Heart failure
  • Neurocardiogenic syncope
  • Post heart transplant
  • Terminating tachycardia
  • Congenital heart disease

Intraop Considerations

• Intraoperative complications related to CIEDs:
  • Inhibition of pacing
  • Inappropriate antitachycardia therapy
  • "Runaway" pacemaker
  • Reprogramming
  • Rate adaptive pacing interference
  • Myocardial burns
  • Complete device failure