Flashcards in UNIT 6 Monitors & Equipment Deck (80):
What components are present in the high pressure system of the anesthesia machine? What is the gas pressure in this region?
begins at the cylinder & ends at the cylinder regulators.
- hanger yoke
- yoke block w/ check valves
- cylinder pressure gauge
- cylinder pressure regulators
gas pressure = cylinder pressure
What components are present in the intermediate pressure system of the anesthesia machine? What is the gas pressure in this region?
begins at the pipeline & 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 = 50psi (if pipeline) or 45psi (if tank)
What components are present in the low pressure system of the anesthesia machine? What is the gas pressure in this region?
begins at the flowmeter tubes & ends at the common gas inlet.
- flowmeter tubes (Thorpe tubes)
- check valves
- common gas outlet
gas pressure = slightly above atmospheric pressure
What are the 5 tasks of oxygen in the anesthesia machine?
1. O2 pressure failure alarm
2. O2 pressure failure device (failsafe)
3. O2 flowmeter
4. O2 flush valve
5. ventilator drive gas (if pneumatic bellows)
Describe 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, but not impossible (>1 washer b/n the hanger yoke & stem of the tank may allow PISS to be bypassed)
air = 1,5
oxygen = 2,5
N2O = 3,5
Describe the diameter index safety system
DISS prevents inadvertant misconnections of gas hoses. Each gas hose and connector are sized and threaded for each individual gas
What are the maximum pressures and volumes for the cylinders that contain air, oxygen, and N2O?
air (yellow): 1900psi, 625L
oxygen (green): 1900psi, 660L
N2O (blue): 745psi, 1590L
weight full = 20.7lb
weight empty = 14.1lb
The bourdon pressure gauge on an O2 cylinder reads 500psi. If the flow rate is 4L/min, how long will this cylinder provide oxygen?
full = 660L/1900psi
660L/1900psi = X/500psi = 174L
174L/4lpm = 43.5mins
some books use 2000psi
Is it ever safe to use an oxygen cylinder in the MRI suite?
not unless it's made of nonmagnetic material such as 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
List 3 safety relief devices that prevent a cylinder from exploding when the ambient temperature increases.
gas cylinders should never be exposed to temp >130F (57C) --> fire/explosion
safety relief devices in the event of environmental fire (to prevent explosion)
- fusible plug made of Woods metal (melts at elevated temperature)
- frangible disk that ruptures under pressure
- valve that opens at elevated pressures
Give 1 example of how the oxygen pressure failure device (failsafe) might permit the delivery of a hypoxic mixture.
the failsafe device checks pressure (not flow)
if there is a pipeline crossover, then the pressure of the new gas will provide the pressure to defeat the failsafe device & the patient will be exposed to a hypoxic mixture
Give 4 examples of how the hypoxia prevention safety device (proportioning system) might permit the delivery of a hypoxic mixture.
1. oxygen pipeline crossover
2. leaks distal to the flowmeter valves
3. administration of a 3rd gas (helium)
4. defective mechanic or pneumatic components
What is the difference b/n the oxygen pressure failure device and the hypoxic prevention safety device?
oxygen pressure failure device (fail safe device)
- shuts off an/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 the O2 flow (i.e. N2O max = 75%)
Describe the structure and function of the flow tube.
internal diameter of flow tube is narrowest at the base & progressively widens along it's ascent
annular space = area b/n the indicator float & the side wall of the flow tube, also narrowest at the base & widest at the top.
This variable orifice architecture 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
flowmeters are made of glass = the most delicate part of the anesthesia machine. A leak will allow O2 to escape the low pressure system --> delivery of hypoxic mixture
if a leak develops in any of the other flowmeters, it won't reduce the FiO2 delivered to the patient
How do you calculate the FiO2 set at the flowmeter?
FiO2 = [ (21*air flow rate) + (100*oxygen flow rate) ] / total flow rate
An anesthesia machine uses fresh gas coupling. How do you determine the total Tv that will be delivered to the patient?
Vt total = Vt set on ventilator + FGF during inspiration - volume lost to compliance
When using a ventilator that couples FGF to Tv, what types of ventilator changes will impact Tv delivered to the patient?
making nearly any change will ultimately impact the Vt delivered to the patient:
Vt increases w/:
- decreased rr
- increased I:E ratio (1:2--> 1:1)
- increased FGF
- increased bellows height
Vt decreases w/
- increased rr
- decreased I:E ratio
- decreased FGF
- decreased bellows height
What is the vaporizer splitting ratio?
modern variable bypass vaporizers split fresh gas into two parts:
1. gas that enters the vaporzing chamber & becomes 100% saturated w/ IA
2. gas the bypasses the vaporizing chamber & doesn't pick up any IA
before leaving the vaporizer, these two fractions mix & this determines the final anesthetic concentration exiting the vaporizer
by setting the concentration on the dial, you determine the splitting ratio
What is the 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. This is generally d/t PPV or use of the O2 flush valve
compare and contrast the variable bypass vaporizer w/ the injector type vaporizer.
- Tec4, 5, 7; aladin; drager 19
- variable bypass
- flow over vaporization
- automatic temp compensation
- agent specific calibration
- elevation compensation
- tec6, drager D
- dual circuit (fresh gas isn't split)
- vaporized by heat, then injected into the fresh gas
- electronically heated to 39C
- agent specific calibration
- no compensation for elevation
What does the O2 analyzer measure and where is it located?
monitors O2 concentration (not pressure) and is the only device downstream of the flowmeters that can detect a hypoxic mixture.
What are 2 things you must do in the event of an oxygen supply line crossover?
1. turn on the O2 cylinder
2. disconnect the pipeline
Pressing the O2 flush valve exposes the breathing circuit to ___ O2 flow & ___ O2 pressure.
pressure 50psi (pipeline pressure
What are 2 risks of pressing the O2 flush valve?
barotrauma (if pressed during inspiration)
awareness (gas doesn't contain IA)
Describe the function of the ventilator spill valve in relation to using the O2 flush valve
if O2 flush valve is pressed during inspiration, the patient will be exposed to flows of 35-75L/min and a pressure of 50psi. If it is pressed during expiration, the excess flow will first fill the bellows then the rest is vented out the scavenger
compare and contrast VC & PC ventilation
VCV: delivers a preset Tv over a predetermined time. Since Tv is fixed, the inspiratory pressure will vary as the pt's compliance changes. Inspiratory flow is constant during inspiration
PCV: present inspiratory pressure over a predetermined time. Since pressure & time are fixed, Tv & inspiratory flow will vary depending on pt's lung mechanics. Tv goal may not be achieved.
A patient is receiving pressure controlled ventilation. What conditions can alter the Tv delivered to the patient?
- decreased compliance (pneumoperitoneum, trendelenburg)
- increased resistance (bronchospasm, kinked ETT)
- increased compliance (release of pneumoperitoneum, T-burg to supine)
- decreased resistance (bronchodilators, removing a/w secretions)
You notice that soda lime has become exhausted in the middle of a surgical procedure. What is the best action to take at this time?
You may be tempted to increased the MV. Although this removes a greater amount of CO2 from the body, it doesn't prevent the pt from rebreathing CO2 & may lead to hypercarbia
Instead, if you can't replace the CO2 absorbent, then you should increase FGF (convert circle system to semi-open system)
What is desiccation & how does it apply to soda lime?
water is required to facilitate the reaction of CO2 w/ the CO2 absorbent. The granules are hydrated to 13-20% by weight. When it is devoid of H2O, it is said to be desiccated.
- ethyl violet informs you about exhaustion but doesn't provide info about H2O content of the absorbent
in the presence of halogenated anesthetics, desiccated soda lime = increase CO production (des > iso >>> sevo) & compound A production (sevo)
List 7 ways to monitor for disconnection of the breathing circuit
4 ways to monitor for circuit disconnect: pressure, volume, EtCO2, vigilance
- precordial stethoscope
- visual inspection of chest rise
- respiratory volume monitors
- low expired volume alarm
- low peak pressure alarm
- failure of bellows to rise w/ an ascending bellows (not w/ descending or piston)
What are the OSHA recommendations regarding IA exposure for health care workers in the OR?
halogenated agents alone <2ppm
N2O alone <25ppm
together: <0.5ppm halogenated, <25ppm N2O
compare and contrast the 4 types of breathing circuits, and list examples of each.
- no rebreathing
- no reservoir
- insufflation, simple mask, NC, open drop
- no rebreathing
- mapleson, circle system if FGF > MV
- partial rebreathing
- circle system w/ FGF
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, the pt will rebreathe exhaled gas
- the 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
Which Mapleson circuit is most efficient for SV? Which is best for controlled veniltation?
- best = mapleson A (A > DFE > CB)
- worst = mapleson B
- best = mapleson D (DFE > BC > A)
- worst = mapleson A
Describe the mapleson breathing circuits
A: FGF, reservoir, APL, pt
B: reservoir, FGF, APL, pt
C: B w/ shorter circuit
D: reservoir, APL, FGF, pt
E: no reservoir, just FGF, pt (aka Ayre's T piece)
F: APL, reservoir, FGF, pt (aka Jackson-Rees)
What conditions decrease pulmonary compliance? How does this affect the peak pressure and plateau pressure?
Decreased pulmonary compliance is usually d/t a reduction in the static compliance (PIP & PP increase)
- endobronchial intubation
- pulmonary edema
- pleural effusion
- tension pneumo
- chest wall trauma
- abdominal insufflation
- Tburg position
- inadequate NMB
What conditions increase pulmonary resistance? How does this affect the peak pressure and plateau pressure?
Usually d/t a reduction in dynamic compliance (PIP increases, PP remains unchanged)
- kinked ETT
- endotracheal cuff herniation
- bronchial secretions
- compression of the airway
- foreign body aspiration
Describe 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 doesn't contain CO2
Discuss the significance of the alpha and beta angles on the capnograph.
increased alpha angle = expiratory airflow obstruction (i.e. COPD, bronchospasm, or kinked ETT)
beta angle is increased in some etiologies of rebreathing
in the case of CO2 absorbent exhaustion, the beta angle remains normal, but the baseline increases.
recall all of the abnormal CO2 waveforms you can (there are 9)
- airflow obstruction
- cardiac oscillation
- curare cleft
- low EtCO2
- high EtCO2
- inspired CO2
- bad unidirectional valve
- sample line leak w/ PPV
- pt w/ single lung transplant
thing of all the causes of increased & decreased EtCO2 that occur as a result of changes in CO2 production
increased EtCO2 d/t increased production:
- increased BMR (increased VO2)
- tourniquet or clamp removal
- NaHCO3 removal
- anxiety, pain
- increased muscle tone (post NMB reversal)
- medication side effect
decreased EtCO2 d/t decreased production:
- decreased BMR (decreased VO2)
- increased anesthetic depth
- decreased pulmonary blood flow
- decreased CO, hypotension
- pulmonary embolus
- V/Q mismatch
- medication side effect
Think of all the causes of increased & decreased EtCO2 that occur as a result of changes in alveolar ventilation or equipment malfunction.
decreased alveolar ventilation
- CNS depression
- residual NMB
- high spinal anesthesia
- NM disease
- metabolic alkalosis
- medication side effect
- CO2 absorbent exhaustion
- unidirectional valve malfunction
- leak in circuit
- increased apparatus dead space
increased alveolar ventilation
- inadequate anesthesia
- metabolic acidosis
- medication side effect
- ventilator disconnect
- esophageal intubation
- poor seal w/ ETT or LMA
- sample line leak
- airway obstruction
What wavelengths of light are emitted by the pulse oximeter? What law is used to make the SpO2 calculation?
2 wavelengths of light:
- red light (660nm); preferentially absorbed by deoxyHgb
- near infrared light (940nm); preferentially absorbed by oxyHgb
Beer-Lambert law is used; relates the intensity of light transmitted through a solution and the concentration of a solute within the solution
Which conditions impair the reliability of the pulse oximeter?
decreased perfusion (vasoconstriction, hypothermia, Raynaud's)
dysfunctional Hgb (carboxyHgb, MetHgb, but NOT HgbS or HgbF)
altered optical characteristics (methylene blue, indocyanine green, indigo carmine, NOT fluorescein)
nonpulsatile flow (CBP, LVAD)
motion artifact (shivering, movement)
other (electrocautery, venous pulsation, NOT jaundice or polycythemia)
What factors affect the accuracy of the NIBP measurement?
ideal bladder length = encircle 80% of the extremity
ideal bladder width = 40% of the circumference of the patient's arm
falsely increased BP:
- BP cuff too small
- BP cuff too loose
- BP measured on extremity below the level of the heart
falsely decreased BP
- BP cuff too large
- BP cuff deflated too quickly
- BP measured on extremity above the level of the heart
How does the site of measurement affect the BP reading?
as pulse moves from the aortic root toward the periphery, the SBP increased, DBP decreases, and pulse pressure widens. MAP remains constant
at the aortic root: SBP is the lowest, DBP is the highest, PP is the narrowest
at the dorsalis pedis: SBP is the highest, DBP is the lowest, PP is the iwdest
How does arm position affect the NIBP reading? How about when an arterial line is used?
blood in the circulation behaves like a column of fluid & follows the rules of hydrostatic pressure.
If BP cuff > heart, BP will be falsely decreased
If BP cuff < heart, BP will be falsely increased
For every 10cm change, the BP changes by 7.4mmHg
For every 1inch (2.5cm) change, the BP changes by 2mmHg
When an a-line is used, the level of the transducer is what's important (the height of the catheter doesn't matter).
What information can you learn from the arterial BP waveform?
systolic BP = peak
diastolic BP = trough
pulse pressure = peak-trough
contractility = upstroke
SV = area under the curve
closure of aortic valve = dicrotic notch
Discuss damping and the interpretation of the high pressure flush test.
optimal waveform morphology balances the amount of damping with the amount of distortion from the transducer system. The high pressure flush test helps us determine this when we flush the system and observe the oscillations that result
optimally damped: baseline is re-established after 1 oscillation
underdamped: baseline is re-established after several oscillations (SBP is overestimated, DBP is underestimated, MAP is accurate)
overdamped: baseline is re-established after no oscillations (SBP is underestimated, DBP is overestimated, MAP is accurate). Causes can 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 CVC or PA cath?
1. you must know the distance from the site of entry to the vena cava junction
2. you must know the distance from the VC junction to where the tip of the catheter should be placed
insertion site --> RA junction:
- SC = 10cm
- R IJ = 15cm
- L IJ = 20cm
- femoral = 40cm
- R median basilic = 40cm
- L median basilic = 50cm
RA junction --> catheter tip
- RA = 0-10cm
- RV = 10-15cm
- PA = 15-30cm
- PAOP position = 25-35cm
What are the 3 waves and 2 descents on the CVP waveform? What does each one signify?
a wave = RA contraction
c wave = tricuspid valve elevation into RA (RV contraction)
x descent = downward movement of contracting RV
v wave = RA passive filling
y descent = RA empties through open tricuspid valve
How do the waves and descents on the CVP waveform correlate w/ the electrical events in the heart?
A wave = RA contraction, just after P wave
C wave = RV contraction, just after QRS
X descent = RA relaxation, ST segment
V wave = passive filling of RA, just after T wave begins
Y descent = RA empties through tricuspid valve, after T wave ends
What factors increase or decrease the CVP?
- transducer below the phlebostatic axis
- RV failure
- tricuspid stenosis or regurg
- pulmonic stenosis
- constrictive pericarditis
- cardiac tamponade
- transducer above phlebostatic axis
What conditions cause loss of the a wave on the CVP waveform?
occurs when synchronized contraction of the RA is lost
- V pacing
What conditions cause an increased a wave on the CVP waveform?
large a wave is produced when the atria contracts and empties against high resistance
- tricuspid stenosis
- diastolic dysfunction
- MI, ischemia
- chronic lung dz --> RV hypertrophy
- AV dissociation
- junctional rhythm
- V pacing (async)
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 TV during RV systole --> increased volume & pressure in the RA & manifests as larger V waves
- tricuspid regurg
- acute increase in intravascular volume
- RV papillary m ischemia
How does the waveform change as the PA cath is guided into position? What are the normal pressures at each step?
1. RAP 1-10mmHg (CVP waveform)
2. RVP 15-30/0-8mmHg (larger, steeper waveform)
3. PAP 15-30/5-15mmHg ("step up" in the waveform + dicrotic notch)
4. PAOP 5-15mmHg (mirrors CVP waveform)
The tip of the PA cath should be positioned in West lung zone ___?
in this region, there is a continuous column of blood b/n the tip of the PAC & the LV. Since LVEDP reflects back through the pulmonary circulation, a tip positioned in zone III provides the most accurate LVEDP estimate.
What is the equation for mixed venous oxygen saturation?
SVO2 = SaO2- [VO2/(Q*1.34*Hgb*10)]
Q = CO
VO2 = O2 consumption
normal = 65-75%
What conditions are associated with a decreased SvO2? How about an increased SvO2?
1. increased consumption (stress, pain, thyroid storm, shivering, fever)
2. decreased delivery (decreased PaO2, decreased Hgb, decreased CO)
1. increased delivery (increased PaO2, increased Hgb, increased CO)
2. decreased consumption (hypothermia)
relate the phases of the cardiac action potential to the EKG
phase 0, depolarization (Na+ inward) = QRS
phase 1, initial repolarization (Cl- in, K+ out) = QRS
phase 2, plateau (Ca++ in, K+ out) = ST segment
phase 3, final repolarization (K+ out) = T wave
phase 4, resting phase (Na+ out) = end of T wave --> QRS
What region of the myocardium does each EKG lead monitor? What coronary arteries are monitored by each lead?
We commonly use 12 leads to look at the heart's electrical activity from a variety of different angles. We can divide these leads into 3 groups:
1. bipolar leads (3), I, II, III
2. limb leads (3), aVR, aVL, aVF
3. precordial leads (6), V1-V6
CxA = I, aVL, V5, V6 (lateral)
RCA = II, III, aVF (inferior)
LAD = V1-V4 (septum, anterior)
List the conditions that can cause L & R axis deviation
R axis deviation:
- acute bronchospasm
- cor pulmonale
L axis deviation
- chronic HTN
- aortic stenosis or regurg
- mitral regurg
recite the heart block poem
if "R is far from "P" then you have a first degree
longer, longer, longer, drop then you have a Wenckebach
if some "P"s don't get through then you have a Mobitz II
if "P"s and "Q"s don't agree then you have a third degree
What is the mechanism of action for each antiarrhythmic class (I-IV)? List examples of each.
I Na+ channel blockers
IA; mod phase 0 depression, prolongs phase 3 repol
- quinidine, procainamide, disopyramide
IB; weak phase 0 depression, shortened phase 3 repol
- lidocaine, phenytoin
IC; strong phase 0 depression
- flecainide, propafenone
II BB; slows phase 4 repol in SA node
- esmolol, metoprolol, atenolol, propanolol
III K+ channel blockers; prolong phase 3 repol (increased QT), increased effective refractory period
- amiodarone, bretylium
IV CCB; decreased conduction velocity via AV node
- verapamil, diltiazem
What EKG findings are consistent with Wolff Parkinson White syndrome?
- delta wave caused by ventricular preexcitation
- short PR interval
- wide QRS complex
- possible T wave inversion
What conditions increase the risk of torsades de pointes?
Other meds (methadone, droperidol, amio w/ hypokalemia)
No known cause
Type I antiarrhythmics
E-lyte disturbances (low K+, Ca++, or Mg++)
Syndromes (Romano Ward, Timothy)
What is the treatment for torsades de pointes?
reversing the underlying cause and/or shortening the QT interval
- cardiac pacing to increase HR will reduce the AP duration & QT interval
List 5 indications for cardiac pacemaker insertion
- symptomatic dz of impulse formation (SA node dz)
- symptomatic dz of impulse conduction (AV node dz)
- long QT syndrome
- dilated cardiomyopathy
What is the significance of the NBG pacemaker identification code?
position I = chamber paced
position II = chamber sensed
position III = response to sensed event
position IV = programmability
position V = pacemaker can pace multiple sites
How does atrial pacing affect the QRS complex? How about ventricular pacing?
a-paced = no QRS change
v-paced = widened QRS
What conditions increase the risk of failure to capture?
when the myocardium becomes more resistant to depolarization
- hypocapnia (affects K+ shift)
- fibrotic tissue buildup around pacing leads
- antiarrhythmic medications
How does the cerebral oximeter work? What value is considered a significant change from baseline?
utilizes near infrared spectroscopy to measure cerebral oxygenation (similar to SpO2)
- doesn't have the ability to detect pulsatile flow, thus it's primarily a measure of venous oxyHgb saturation & oxygen extraction
- decreased delivery --> increased extraction --> decreased venous Hgb sat
- a >25% change from baseline suggests a reduction in cerebral oxygenation
Describe the different types of EEG waveforms.
- high frequency, low voltage
- awake mental stimulation & "light" anesthesia
- 8-12 cycles/sec
- awake but restful w/ eyes closed
- 4-7 cycles/sec
- GA & children during normal sleep
- <4 cycles/sec
- GA, deep sleep, brain ischemia or injury
- GA, hypothermia, CPB, cerebral ischemia (esp if unilateral)
- very deep anesthesia, death
How do brain waves change during GA?
- induction = increased beta wave activity
- light anesthesia = increased beta wave activity
- theta & delta waves predominate during GA
- deep anesthesia produces burst suppression
- 1.5-2MAC, GA = isoelectricity
name 2 drugs that are likely to reduce the reliability of the BIS value
N2O (increases amplitude of high frequency activity & reduces amplitude of low frequency activity). This doesn't affect the BIS value
ketamine increases high frequency activity --> can produce a BIS that is higher than the level of sedation/anesthesia would otherwise suggest.
What is the difference b/n micro and macro shock?
macroshock: comparatively larger amount of current that is applied to the external surface of the body. Impedance of the skin offers a high resistance, so it takes a larger current to induce vfib
microshock: smaller amount of current applied directly to the myocardium. High skin resistance is bypassed, so takes smaller current to induce vfib
What are the key threshold values for macroshock and microshock?
1mA touch perception
5mA max for harmless shock
10-20mA "let go" current
50mA loss of consciousness
10mcA max allowable current leak in the OR