Unit 6: Equipment & Monitors Flashcards
What components are present in the high pressure system of the anesthesia machine? What is the gas pressure in this region?
-Hanger Yoke
-Yoke block with check valves
-Cylinder pressure gauge
-Cylinder pressure regulators
Gas Pressure = Cylinder Pressure
What components are in the intermediate pressure system of the anesthesia machine?
Begins at the pipeline and ends at the flowmeter valve
-pipeline inlets
-pressure gauges
-ventilator power inlet
-oxygen pressure failure system
-oxygen second stage regulator
-oxygen flush valve
-flowmeter valve
Gas Pressure = 50 psi (if using pipeline) and 45 psi (if using tank)
What components are present in the low pressure system of the anesthesia machine?
Begins at the flowmeter tubes and ends at the common gas inlet
-flowmeter tubes (Thorpe Tubes)
-vaporizers
-check valves (if present)
-common gas outlet
Gas Pressure = slightly above atmospheric pressure
What are the 5 tasks of oxygen in the anesthesia machine?
- O2 pressure failure alarm
- O2 pressure failure device (failsafe)
- O2 flowmeter
- O2 flush valve
- Ventilator drive gas (if pneumatic bellows)
What is the pin index safety system?
PISS prevents inadvertent misconnections of gas cylinders
-pin configuration on each hanger yoke assembly is different for each gas, making unintended connections of the wrong gas unlikely
**presence of more than one washer between the hanger yoke assembly and the stem of the tank may allow bypassing of the PISS
What is the diameter index safety system?
DISS prevents inadvertent misconnections of gas hoses
-each hose and connector are sized and threaded for each individual gas
What are the max pressures and volumes for cylinders that contain air, oxygen, and nitrous oxide?
Air: 625 L – 1900 psi
Oxygen: 660 L – 1900 psi
Nitrous Oxide: 1590 L – 745 psi
The bourdon pressure gauge on an O2 cylinder reads 500psi. If the flow rate is 4 L/min, how long will this cylinder provide oxygen to the pt?
What is the equation?
Step 1: How much O2 is in the cylinder?
[tank capacity (L) / full tank pressure (psi)] x [contents remaining (L) / gauge pressure (psi)]
Step 2: How long will it last?
contents remaining (L) / FGF rate (L/min)
Example:
660L / 1900psi = X/500 psi = 174 L
174L / 4L per min = 43.5
Is it ever safe to use an O2 cylinder in the MRI suite?
Never take a cylinder into the MRI scanner unless it is made of a non-magnetic material (i.e. aluminum)
An MRI safe cylinder will have two colors: most of the tank is silver, and only the top is the color that signifies the gas it contains
What are the 3 safety relief devices that prevent a cylinder from exploding when the ambient temp increases?
- A fusible plug made of Wood’s metal (melts at elevated temps)
- A frangible disk that ruptures pressure
- A valve that opens at elevated pressures
never expose a cylinder to temps higher than 130F (57*C)
Give an example of how the O2 pressure failure device (failsafe) might permit the delivery of a hypoxic mixture
The failsafe device responds to pressure (not flow) –> if there is a pipeline crossover, the pressure of the second gas will produce pressure to defeat the failsafe device
Give 4 examples of how the hypoxia prevention safety device (proportioning device) might allow the delivery of a hypoxic mixture
- Oxygen pipeline crossover
- Leaks distal to the flowmeter valves
- Administration of a third gas (helium)
- Defective mechanic or pneumatic components
What is the difference between the oxygen pressure failure device and the hypoxia prevention safety device?
Oxygen Pressure Failure Device = Fail-safe Device
-shuts off and/or proportionately reduces N2O flow if O2 pressure drops below 20psi
Hypoxia Prevention Safety Device = Proportioning Device
-prevents you from setting a hypoxic mixture with the flow control valves
-limits N2O flow to 3x O2 flow (N2O max = 75%)
Describe the structure and function of the flowmeters
Annular space = area between the indicator float and the side wall of the flow tube – narrowest at the base and widest at the top
-this “variable orifice” provides a constant gas pressure throughout a wide range of flow rates
-Laminar flow is dependent on gas viscosity (Poiseuille)
-Turbulent flow is dependent on gas density (Graham)
What is the safest flowmeter configuration on the anesthesia machine?
O2 flowmeter should always be furthest to the RIGHT
-a leak will allow O2 to escape the low-pressure system which could result in the delivery of a hypoxic mixture
-O2 should be closest to the manifold outlet (right side) because if a leak occurs in any other flowmeter, it won’t reduce the FiO2
How do you calculate the FiO2 set at the flowmeter?
FiO2 = (Air flow rate x 21) + (Oxygen flow rate x 100) / Total flow rate
Ex: air at 1 L/min and O2 at 3 L/min
(1 x 21) + (3 x 100) / 4 = 80.25 (80%)
How do you determine the total tidal volume that will be delivered to the patient?
Vt total = Vt set on ventilator + FGF during inspiration
Step 1: Convert FGF from L/min to mL/min
- 4 L/min = 4000 mL/min
Step 2: Multiply FGF by I:E ratio
- 4000 mL/min x (1/3) = 1300 mL/min
Step 3: Calculate tidal volume per breath
- 1300 mL / 10 breaths per min = 133 mL
Step 4: Add volume set on ventilator to FGF during inspiration
- 500 + 133 = 633 mL tidal volume
- (4000 x 1/3) / 10 = 133 mL + 500 mL = 633 mL
When using a ventilator that couples fresh gas flow to tidal volume, what increases and decreases the tidal volumes delivered?
Tidal Volume Increases with:
- decreased RR
- increased I:E ratio (1:2 to 1:1)
- increased FGF
- increased bellows height
Tidal Volume Decreases with:
- increased RR
- decreased I:E ratio
- decreased FGF
- decreased bellows height
**when using a ventilator that couples FGF to tidal volume – making nearly any change on the vent settings will ultimately impact the Vt delivered to the pt
What is the vaporizer splitting ratio?
Modern variable bypass vaporizers split fresh gas into 2 parts:
- Some fresh gas enters the vaporizing chamber and becomes 100% saturated w/ a volatile agent
- The rest of the gas bypasses the vaporizing chamber and doesn’t pick up any volatile agent
-before leaving the vaporizer, these two fractions mix, and this determines the final anesthetic concentration exiting the vaporizer
What is the pumping effect?
Pumping effect can increase vaporizer output
-anything that causes gas that has already left the vaporizer to re-enter the vaporizing chamber can cause the pumping effect
-generally due to positive pressure ventilation or the use of the O2 flush valve
*modern anesthesia machine design mitigates this risk
Describe the variable bypass vaporizer. What are examples?
-Splitting Ratio = variable bypass (vaporizer splits fresh gas)
-Method of Vaporization = flow over
-Temp Compensation = automatic
-Calibration = agent specific
-Position = out of circuit
-Elevation Compensation = yes
Ex: Datex-Ohmeda Tec 4, 5, 7AU – Aladin – Drager Vapor 19, 2000
Describe the injector-type vaporizer. What are examples?
Desflurane
-Splitting Ratio = dual circuit (fresh gas is not split)
-Method of Vaporization = gas/vapor blender (heat creates vapor that is injected into the fresh gas)
-Temp Compensation = electronically heated to 39*C
-Calibration = agent specific
-Position = out of circuit
-Elevation Compensation = no
Ex: Datex-Ohmeda Tec 6 – Drager D-Vapor
What does the oxygen analyzer measure, and where is it located?
Monitors oxygen concentration (not pressure)
-the only device downstream of the flowmeters that can detect a hypoxic mixture
What 2 things must you do in the event of an oxygen supply line crossover?
- Turn ON the oxygen cylinder
- Disconnect the pipeline oxygen supply **KEY STEP
Pressing the oxygen flush valve exposes the breathing circuit to ___ O2 flow and ___ O2 pressure
Oxygen Flow = 25-75 L/min
Oxygen Pressure = 50 psi (pipeline pressure)
What are the two risks of pressing the O2 flush valve?
Barotrauma: pressing during inspiration
Awareness: gas from flush valve doesn’t pass through the vaporizers, excessive use adds gas to the breathing circuit that doesn’t contain a volatile diluting the partial pressure of the agent
Describe the function of the ventilator spill valve in relation to using the O2 flush valve
Inspiration:
1. drive gas compresses bellows
2. drive gas closes spill valve
3. fresh gas from the ventilator goes to the pt
Expiration:
1. expired gas refills the bellows
2. bellows fill completely
3. when circuit pressure >2-4 cmH2O expired gas is directed through the spill valve to the scavenger
What is volume controlled ventilation?
Delivers a preset tidal volume over a predetermined time
-tidal volume is fixed so the inspiratory pressure will vary as the patient’s compliance changes
-inspiratory flow is held constant during inspiration
Fixed: tidal volume, inspiratory flow rate, inspiratory time
Variable: peak inspiratory pressure
What is pressure control ventilation?
Delivers a preset inspiratory pressure over a redetermined time
-pressure and time are fixed so tidal volume and inspiratory flow will be variable and dependent of the pt’s lung mechanics
-if airway resistance rises or lung compliance decreases, then tidal volume will suffer
Fixed: peak inspiratory pressure and inspiratory time
Variable: tidal volume and inspiratory flow
What conditions can alter the tidal volume delivered to the patient during pressure control ventilation?
Tidal Volume Decreases With:
-decreased compliance (pneumoperitoneum or Trendelenburg position)
-increased resistance (bronchospasm or kinked ETT)
Tidal Volume Increases With:
-increased compliance (release of pneumoperitoneum or going from trendelenburg to supine)
-decreased resistance (bronchodilator therapy or removing airway secretions
What is the best action to take when the soda lime becomes exhausted in the middle of a surgical procedure?
If you can’t replace the CO2 absorbent –> increase the fresh gas flow to convert the circle system into a semi-open configuration
What is desiccation and how does it apply to soda lime?
-Water is required to facilitate the reaction of CO2 with Co2 absorbent – the granules are hydrated to 13-20% by weight
-Desiccation = when the absorbent is devoid of water
In the presence of halogenated anesthetics, desiccated soda lime increases the production of carbon monoxide (Des > Iso»»Sevo) and compound A in presence of Sevo
*carbon monoxide can cause carboxyhemoglobinemia
*compound A may cause renal dysfunction
What are the 7 ways to monitor for disconnection of the breathing circuit?
Via pressure, volume, ETCO2, and your own vigilance
- Precordial stethoscope
- Visual inspection of chest rise
- Capnography
- Respiratory volume monitors
- Low expired volume alarm
- Low peak pressure alarm
- Failure of bellows to rise with an ascending bellows
What are the OSHA recommendations regarding inhalation anesthetic exposure for health care workers in the OR?
Halogenated agents alone <2 ppm
Nitrous Oxide alone <25 ppm
Halogenated agents + Nitrous Oxide <0.5 ppm and 25 ppm respectively
What are the four types of breathing circuits? List examples of each
Open: non-rebreathing, no reservoir
-insufflation, simple face mask, nasal canula, open drop
Semi-Open: non-rebreathing, has a reservoir
-mapleson circuit (FGF dependent on design) and circle system (FGF > minute ventilation)
Semi-Closed: partial rebreathing, has a reservoir
-circle system (FGF < minute ventilation)
Closed: complete rebreathing, has a reservoir
-circle system (very low FGF and APL closed)
What is the purpose of the unidirectional valves in the breathing circuit?
To ensure that gas moves in one direction
-if a valve becomes incompetent – pt will rebreathe exhaled gas
-definitive fix is to correct the valve
-if this cannot be done, then a closed or semi-closed system should be converted to a semi-open system by increasing FGF
Which Mapleson circuit is most efficient for spontaneous ventilation? Which is best for controlled ventilation?
Spontaneous Ventilation: A > DFE > CB
-best = Mapleson A
-worst = Mapleson B
*Always Do Continous Breathing
Controlled Ventilation: DFE > BC > A
-best =Mapleson D
-worst = Mapleson A
*Dead Bodies Can’t Assist
What conditions decrease pulmonary compliance? How does this affect the peak pressure and plateau pressure?
Decreased pulmonary compliance is usually due to a reduction in static compliance (PIP and PP Increase)
- endobronchial intubation
- pulmonary edema
- pleural effusion
- tension pneumothorax
- atelectasis
- chest wall trauma
- abdominal insufflation
- ascites
- trendelenburg position
- inadequate muscle relaxation
What conditions increase pulmonary resistance? How does this affect the peak pressure and plateau pressure?
Increased pulmonary resistance is usually due to a reduction in dynamic compliance (PIP increases and PP is unchanged)
-kinked ETT
-ETT cuff herniation
-bronchospasm
-bronchial secretions
-compression of the airway
-foreign body aspiration
What are the 4 phases of the normal capnograph?
Phase I (A-B): exhalation of anatomic dead space
Phase II (B-C): exhalation of anatomic dead space + alveolar gas
Phase III (C-D): exhalation of alveolar gas
Phase IV (D-E): inspiration of fresh gas that does not contain CO2
What is the significance of the alpha and beta angles on the capnograph?
Alpha Angle: increase in the angle signifies an expiratory airflow obstruction (i.e. COPD, bronchospasm, or kinked ETT)
Beta Angle: increased in some (but not all) etiologies of rebreathing – specific to rebreathing caused by a faulty unidirectional valve (will appear normal in exhausted CO2 absorbent)
What is the cause of the following abnormal CO2 waveforms?
What are the causes of increased end-tidal CO2 as a result of changes in CO2 production?
Increased CO2 Production & Delivery to the Lungs:
-increased BMR (increased VO2)
-malignant hyperthermia
-thyrotoxicosis
-fever
-sepsis
-seizures
-laparoscopy
-tourniquet or vascular clamp removal
-sodium bicarb administration
-anxiety
-pain
-shivering
-increased muscle tone (after NMB reversal)
-medication side effect
What are causes of decreased end-tidal CO2 as a result of changes in CO2 production?
Decreased CO2 Production & Delivery to the Lungs:
- decreased BMR (decreased VO2)
- increased anesthetic depth
- hypothermic
- decreased pulmonary blood flow
- decreased CO
- hypotension
- pulmonary embolus
- V/Q mismatch
- medication side effect
What are causes of Increased end-tidal CO2 as a result of changes in alveolar ventilation or equipment malfunction?
Decreased Alveolar Ventilation:
- hypoventilation
- CNS depression
- residual neuromuscular blockade
- COPD
- high spinal anesthesia
- neuromuscular disease
- metabolic alkalosis (if spont breathing)
- medication side effect
Equipment Malfunction:
- rebreathing
- CO2 absorbent exhaustion
- unidirectional valve malfunction
- leak in breathing circuit
- increased apparatus dead space
What are causes of decreased end-tidal CO2 as a result of changes in alveolar ventilation (4) or equipment malfunction (6)?
Increased Alveolar Ventilation
-hyperventilation
-inadequate anesthesia
-metabolic acidosis (if spont breathing)
-medication side effect
Equipment Malfunction
-ventilator disconnect
-esophageal intubation
-poor seal with ETT or LMA
-sample line leak
-airway obstruction
-apnea
What wavelengths of light are emitted by the pulse ox? What law is used to make the SpO2 calculation?
based on Beer-Lambert law (relates the intensity of light transmitted through a solution and the concentration of the solute within the solution)
Two Wavelengths:
-red light (660 nm) is preferentially absorbed by deoxyhemoglobin (higher in venous blood)
-near-infrared light (940 nm) is preferentially absorbed by oxyhemoglobin (higher in arterial blood)
What conditions impair the reliability of the pulse ox?
-Decreased Perfusion: vasoconstriction, hypothermia, reynaud’s syndrome
-Dysfunctional Hgb: carboxyhemoglobin, methemoglobin, NOT HgbS or HgBF
-Altered Optical Characteristics: methylene blue, indocyanine green, indigo carmine, NOT fluorescein
-Non-Pulsatile Flow: CBP, LVAD
-Montion Artifact: shivering/movement
-Other: electrocautery, dark skin, venous pulsation, NOT jaundice or polycythemia
What factors affect the accuracy of the NIBP cuff measurement? What causes falsely increased and decreased BP?
Ideal bladder LENGTH is long enough to wrap around 80% of the extremity
Ideal bladder WIDTH is 40% the circumference of the arm
Falsely Increased = too small cuff, too loose cuff, BP is measured on extremity below the level of the heart
Falsely Decreased = too large cuff, cuff is deflated too rapidly, BP is measured on extremity above the level of the heart
How does the site of measurement affect the blood pressure reading?
As the pulse moves from the aortic root towards the periphery, the SBP increases, DBP decreases, and pulse pressure widens – MAP remains constant through arterial tree
-At the aortic root – SBP is the lowest, DBP is the highest, and PP is narrowest
-At the dorsalis pedis – SBP is highest, DBP is lowest, and PP is widest
How does the arm position affect the NIBP reading? How about when an arterial line is used?
-If BP cuff location is above the heart – reading will be falsely decreased (less hydrostatic pressure)
-If BP cuff location is below the heart – reading will be falsely increased (more hydrostatic pressure)
*For every 10cm change, BP changes by 7.4mmHg
*For every inch change, BP changes by 2 mmHg
A-line: transducer level is what’s important
What information can you learn from the arterial BP waveform?
-SBP = Peak of waveform
-DBP = Trough of waveform
-Pulse Pressure = Peak - Trough
-Contractility = Upstroke
-Stroke Volume = Area under the curve
-Closure of Aortic Valve = Dicrotic notch
Discuss damping and the interpretation of the high pressure flush test on an arterial line
High pressure flush test helps determine the balance between the amount of damping with the amount of distortion from the transducer
-Optimally Damped System = baseline is re-established after 1 oscillation
-Under Damped System = baseline is re-established after several oscillations (SBP is overestimated, DBP is underestimated, and MAP is accurate)
-Over Damped System = baseline is re-established with no oscillations (SBP is underestimated, DBP is overestimated, and MAP is accurate) – causes include an air bubble or clot in the pressure tubing or low flush bag pressure
How do you determine the appropriate distance to thread a central line or PA catheter?
- You must know the distance from the site of entry to the vena cava junction
- You must know the distance from the vena cava junction to where the catheter’s tip should be placed
- Add these two numbers to determine this distance from the insertion site to the tip of the catheter
What are the 3 waves and 2 descents on the CVP waveform? What does each signify?
A wave = right atrial contraction
C wave = tricuspid valve elevation into right atrium
X descent = downward movement of contracting right ventricle
V wave = right atrium passive filling
Y descent = right atrium empties through open tricuspid valve
How do the waves and descents on the CVP waveform correlate with the electrical events in the heart?
A wave = right atrial contraction – just after P wave
C wave = right ventricular contraction – just after QRS
X descent = right atrial relaxation – ST segment
V wave = passive filling of right atrium – just after T wave begins
Y descent = right atrium empties through open tricuspid valve – after T wave ends
What factors increase CVP? (9)
-Transducer below the phlebostatic axis
-Hypervolemia
-RV failure
-Tricuspid stenosis or regurgitation
-Pulmonic stenosis
-PEEP
-VSD
-Constrictive pericarditis
-Cardiac tamponade
What factors decrease CVP?
Transducer above the phlebostatic axis
Hypovolemia
What conditions cause loss of the A wave on the CVP waveform?
Occurs when synchronized contraction of right atrium is lost
-A-Fib
-V-pacing if underlying rhythm is asystole
What conditions cause an increased A wave on the CVP waveform?
Large A wave is produced when the atria contracts and empties against a high resistance
- tricuspid stenosis
- diastolic dysfunction
- myocardial ischemia
- chronic lung disease leading to RV hypertrophy
- AV dissociation
- junctional rhythm
- V-pacing (asynchronous)
- PVCs
What conditions cause a large V wave on the CVP waveform?
Tricuspid regurg allows a portion of the RV volume to pass through the closed but incompetent tricuspid valve during RV systole – this increases the volume and pressure in the RA and manifests a large v waves
-tricuspid regurg
-acute increase in intravascular volume
-RV papillary muscle ischemia
How does the waveform change as the PA catheter is guided into position? What are the normal pressures at each step?
What West lung zone should the tip of the pulmonary artery catheter should be positioned in?
Zone 3
-there is a continuous column of blood between the tip of the PAC and the LV
-since LVEDP reflects back through the pulmonary circulation a tip positioned in zone 3 provides the most accurate estimation of LVEDP
What is the equation for mixed venous oxygen saturation?
SvO2 = SaO2 - (VO2 / Q x 1.34 x Hgb x 10)
Q: cardiac output (L/min)
VO2: oxygen consumption (mL O2/min)
Hgb: amount of hemoglobin (g/dL)
SaO2: loading of hgb in arterial blood (%)
**Normal = 65-75%
What conditions are associated with a decreased SvO2?
Increased O2 Consumption: stress, pain, thyroid storm, shivering, fever
Decreased O2 Delivery: decreased PaO2, decreased Hgb, decreased CO
What conditions are associated with a increased SvO2?
Decreased O2 Consumption: hypothermia, cyanide toxicity
Increased O2 Delivery: increased PaO2, increased Hgb, increased CO
What are the phases of the cardiac action potential and how does each phase relate to the EKG?
Phase 0: Depolarization – Na in – QRS
Phase 1: Initial Repolarization – Cl in, K out – QRS
Phase 2: Plateau – Ca in, K out – ST segment
Phase 3: Rinal Repolarization – K out – T wave
Phase 4: Resting Phase – K leak – End of T wave to QRS
What region of the myocardial does each EKG lead monitor? What coronary arteries are monitored by each lead?
Lateral Region: I, aVL, V5, V6 –> Circumflex Artery
Inferior Region: II, III, aVF –> Right Coronary Artery
Septum Region: V1, V2 –> LAD
Anterior Region: V3, V4 –> LAD
What conditions cause a right axis deviation?
-COPD
-Acute bronchospasm
-Cor pulmonale
-Pulmonary HTN
-Pulmonary embolus
What conditions cause a left axis deviation?
-Chronic HTN
-Left bundle branch block
-Aortic stenosis
-Aortic insufficiency
-Mitral regurgitation
Recite the heart block poem
If “R” if far from “P”, than you have a First Degree
Longer, longer, longer, drop than you have a Wenckebach
If some “P”s don’t get through than you have a Mobitz II
If “P”s and “Q”s don’t agree than you have a Third Degree
What is the mechanism of action for class I antiarrhythmics? List examples
Na Channel Blockers
Ia: moderate depression of phase 0 – prolongs phase 3 repolarization (K channel block –> increases QT)
-ex) Quinidine, Procainamide, Disopyramide
Ib: weak depression of phase 0 – shortened phase 3 repolarization
-ex) Lidocaine, Phenytoin
Ic: strong depression of phase 0 – little effect on phase 3 repolarization
-ex) Flecainide, Propafenone
What is the mechanism of action for class II antiarrhythmics? List examples
Beta-Blockers
-slows phase 4 depolarization in SA node
-ex) Esmolol, Metoprolol, Atenolol, Propranolol
What is the mechanism of action for class III antiarrhythmics? List examples
K Channel Blockers
-prolongs phase 3 repolarization (increases QT)
-increases effective refractory period
-ex) Amiodarone, Bretyium
What is the mechanism of action for class IV antiarrhythmics? List examples
Ca Channel Blockers
-decreases conduction velocity through AV node
-ex) Verapamil, Diltiazem
What EKG findings are consistent with Wolff-Parkinson-White syndrome?
- Delta wave caused by ventricular preexcitation
- Short PR interval (<0.12 seconds)
- Wide QRS complex
- Possible T wave inversion
What conditions increase the risk of torsades de pointes?
POINTES
- Phenothiazine
- Other meds (methadone, droperidol, amiodarone w/ hypokalemia)
- Intracranial bleed
- No known cause
- Type I antiarrhythmics
- Electrolyte disturbances (low K, Ca, or Mg)
- Syndromes (Romano-Ward, Timothy)
What is the treatment for torsades de pointes?
Acute treatment includes reversing the underlying cause and/or shorten the QT interval:
-magnesium sulfate
-cardiac pacing to increase the HR will reduce action potential duration and the QT interval
What are the 5 indications for cardiac pacemaker insertion?
- Symptomatic diseases of impulse formation (SA node disease)
- Symptomatic diseases of impulse conduction (AV node disease)
- Long QT syndrome
- Dilated cardiomyopathy
- Hypertrophic obstructive cardiomyopathy
What does each position of the NBG pacemaker identification code represent?
Position 1 = Chamber Paced
Position 2 = Chamber Sensed
Position 3 = Response to Sensed Event
Position 4 = Programmability
Position 5 = Pacemaker can pace multiple sites
How does atrial pacing affect the QRS complex? How about ventricular pacing?
Atrial Pacing – electrical signal travels through the AV node and QRS maintains it normal, narrow appearance
Ventricular Pacing – electrical signal is delivered beyond the AV node and QRS takes on a wide appearance
What conditions increase the risk of failure to capture?
-Hyper- and hypokalemia
-Hypocapnia (intracellular K shift)
-Hypothermia
-Myocardial infarction
-Fibrotic tissue buildup around the pacing leads
-Antiarrhythmic medications
How does the cerebral oximeter work? What value is considered a significant change from baseline?
Cerebral oximetry utilizes near-infrared spectroscopy (NIRS) to measure cerebral oxygenation
- relies on the fact that cerebral blood volume is 1 part arterial to 3 parts venous (75% of blood in the brain is on the venous side of circulation)
- since NIRS can’t detect pulsatile blood flow, it is primarily a measure of venous oxyhemoglobin saturation and oxygen extraction
- decreased cerebral oxygen delivery –> increased cerebral oxygen extraction –> decreased venous hemoglobin saturation
*A >25% change from baseline suggests a reduction in cerebral oxygenation
What are the different types of EEG waveforms?
How do brain waves change during general anesthesia?
-Induction of GA –> increased beta wave activity
-Light anesthesia –> increased beta wave activity
-General anesthesia –> theta and delta waves predominate
-Deep anesthesia –> produces burst suppression
At 1.5-2.0 MAC – general anesthetics cause suppression or isoelectricity
What are two drugs that are most likely to reduce the reliability of the BIS value?
Nitrous Oxide – increases amplitude of high frequency activity and reduces the amplitude of low frequency activity
Ketamine – increases high frequency activity (can produce a higher BIS valve than the level of sedation/anesthesia would otherwise suggest)
What is the difference between macro- and microshock?
Macroshock = comparatively larger amount of current that is applied to the external surface of the body – skin’s impedance offers a high resistance, so it takes larger current to induce v-fib
Microshock = comparatively smaller amount of current that is applied directly to the myocardium – high resistance of the skin is bypassed, so it takes significantly smaller amount of current to induce v-fib
*central line, PA catheter, or pacing wires provide a direct conductive pathway to the heart – increase pt’s susceptibility to microshock
What are the key threshold values for macroshock and microshock?
Macroshock:
1 mA = threshold for touch perception of shock
5 mA = max current for a harmless shock
10-20 mA = “let go” current occurs before sustained contraction
50 mA = loss of consciousness
100 mA = v-fib
Microshock:
10 uA = max allowable current leak in the OR
100 uA = v-fib
What is the role of the line isolation monitor? What should you do if it alarms?
It assesses the integrity of the ungrounded power system in the OR – tells you how much current could potentially flow through you or a patient if a second fault occurs
-primary purpose is to alert to a first fault
-does NOT (by itself) protect you or the patient from macro or microshock
-will alarm when 2-5 mA of leak current is detected
-if alarm sounds, the last piece of equipment that was plugged in should be unplugged
