Case Stem: A 70 year old man is to undergo cystoscopy and
transurethral resection of a bladder tumor under general anesthesia
through an LMA. He gave a history of mild asthma and used an
albuterol inhaler when necessary. Breathing room air (FiO2 = 0.21),
his pulse oximeter saturation reading (SpO2) was 94%.
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Hypoxia Reduction of oxygen supply to tissue below physiologic
levels. Decreased oxygen tension (PO2) inside the body at tissue
level or outside the body (hypoxic gas mixture)
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Hypoxemia Deficient oxygenation of blood. Decreased oxygen
tension in the arterial blood (PaO2)
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Yes. Age-dependent decrease in PaO2. Marshall and Whyche
equation Mean PaO2 (mmHg) = 102-0.33(age in years) Sorbini et al.
found PaO2 decreased from about 95 mmHg at 20 years of age to 73 at
75 years (about 4-5 mmHg per decade)
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No. Hypoxemia is considered to exist when the PaO2 is less than
60 mmHg which is equivalent to a hemoglobin O2 saturation of 90%
Using the Marshall Whyche equation 102-0.33(70) = 79 mmHg
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Pulse Oximeter Noninvasive device that provides an estimate
(SpO2) of the arterial hemoglobin saturation with oxygen. Uses
patient body part as in vivo cuvette through which 2 different
wavelengths of light are transmitted.
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Hemoximeter Used to analyze an arterial blood sample.
Laboratory cooximeter that uses six or more different wavelengths
of light to measure total hemoglobin, oxygenated hemoglobin,
deoxygenated hemoglobin, methemoglobin, carboxyhemoglobin and other
aberrations.
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Pulse oximeter Light-emitting diodes transmit red light at
wavelengths 660 nm and infrared light at 960 nm through the probe
site. Light is sensed by a single photodetector Ratio of
absorbances (660/990 nm) is related to hemoglobin O2 saturation
(Spectophotometry) Plethysmography detection of pulsatile flow (as
blood pulses, absorbance increases)
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Pulse oximeter Patient movement (shivering, peripheral nerve
stimulation, twitching) Presence of intense ambient light
Electrocautery use Administration of IV dyes with absorbance peaks
at 66o nm (methylene blue) Dyshemoglobinemias Nail polish Poor
pulsatile flow at probe site (hypotension, Raynauds) Venous
pulsations (tricuspid regurg)
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Methemoglobin Iron in heme moiety is oxidized (dapsone,
benzocaine, nitric oxide, prilocaine) to Fe3+ state rather than
Fe2+ state. Cannot carry O2 Shows similar absorbances at 660 and
940 nm (SpO2 tends toward 85%) Overestimates the fractional
saturation and underestimates the functional saturation
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Carboxyhemoglobin CO + Hb has similar absorbance to HbO2 at 660
nm, but very low absorbance at 94o nm. SpO2 overestimates
fractional saturation and underestimates functional saturation.
SpO2 will appear in the 90s Hemoximeter required to determine true
O2 sat
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Capnography Most use infrared spectroscopy to measure PCO2 A
built in barometer measures barometric pressure so that CO2 can be
displayed as a percentage. Gold standard for establishing presence
of ventilation.
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End-tidal CO2 Tension of CO2 in the exhaled gas at end of
exhalation. Represents the CO2 tension in the alveolar gas (PACO2)
Does not account for dead space ventilation Presence of CO2 depends
on Production of CO2 by the tissues CO and pulmonary blood flow to
carry CO2 Ventilation
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Capnogram Phase I Expiratory baseline Phase II Expiratory
upstroke Phase III Expiratory plateau Horizontal in healthy lungs
Upward Slope with obstructive airway disease Maximum expired CO2 is
considered the end-tidal angle slope between II and III (increase
in acute bronchospasm) Phase IV Inspiratory downstroke
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Elevated baseline CO2 Capnometer not properly calibrated to
zero Delivery of CO2 to breathing system through fresh gas inflow
Incompetent unidirectional valves Failure of CO2 absorber
(channeling, exhaustion, bypass)
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Prolonged expiratory plauteau and expiratory upstroke
Mechanical obstruction to exhalation COPD Bronchospasm
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Dips in expiratory plateau Spontaneous ventilation efforts
Elevated expiratory plateau Incorrect calibration Increased CO2
production / delivery Laparoscopic CO2 gas insufflation Decreased
CO2 removal Hypoventilation Leak
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Decreased expiratory plateau Incorrect calibration Air leak
into gas sampling system Hyperventilation Decreased CO2 production
(hypothermia) Increased arterial-alveolar CO2 gradient (VQ mismatch
/ pulmonary embolus)
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Prolonged inspiratory downstroke and raised baseline
Incompetent or missing inspiratory unidirectional valve Inspiratory
obstruction to gas flow (kinked tube)
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A-a gradient Measure of alveolar dead space ventilation 2.5cc /
Kg = volume of anatomic dead space (PaCO2 PETCO2) / PaCO2 = Ratio
of dead space to tidal volum Alveolar dead space increased by
ventilation in excess of perfusion or decrease in perfusion (shunt
has minimal effect) PaCO2-PACO2 nl 3-5 mmHg
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Safety Features Pin index (cylinder) and diameter index
(pipeline) safety systems Fail-safe valve pressure sensitive device
that interrupts flow of all hypoxic gases on the machine to their
flow control valves if the supply pressure of O2 in the high
pressure system falls below a threshold (between 12-20 psig) O2
supply failure alarm pressure below 30 psig O2 flow control knob
fluted and on the right Key-fill systems for vaporizers Pop-off
(pressure relief) valve
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Safety Features Gas flow proportioning ensure minimum O2 of 25%
when N2O is used Vaporizer interlock system
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Gas Leakage Breathing system Partially deflated tracheal tube
cuff Disconnection of sidestream gas analyzer Humidifiers Bag
Low-pressure machine components Cracked rotameter flow tubes
Incorrectly mounted vaporizers Vaporizer leak around agent filling
device Fracture in gas piping
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Machine Check for Leaks Drager Circle breathing system tubing
removed Insp and exp limb connected by tubing Resevoir bag removed
and replaced with test terminal with sphygmomanometer bulb
Pressurize with bulb to 50 cm H2O pressure should not decrease by
20 in 30 sec Test with vaporizers on Datex-Ohmeda One-way outlet
check valve at the common gas outlet Connect bulb and squeeze,
should not refill in 30 sec
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Step 1 Emergency Ventilation Equipment Step 2 Check O2 Cylinder
supply Step 3 Central pipeline supply Step 4 Low-pressure system
check (flow control valves and vaporizer status) Step 5 Leak check
of low-pressure system Step 6 Turn on machine master switch and
other electrical equipment Step 7 Test flowmeters
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Step 8 Adjust / Check scavenging system (test pop- off) Step 9
Calibrate O2 monitor Step 10 Check initial status of breathing
system (circuit, CO2 absorbent) Step 11 Leak check of breathing
system Step 12 Test ventilation system (connect resevoir bag to
Y-piece) Step 13 Check, calibrate, and set alarm limits Step 14
Check final status of machine
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Emergency Equipment Back-up ventilation equipment Emergency
airway equipment Cricothyroid kit / Difficult airway cart Working
flashlight Backup battery O2 tank and regulator Malignant
hyperthermia cart Code cart Fire extinguisher
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Premedication Anxiolysis Minimization of gastric volume and
acidity Antibiotic prophylaxis Antisialagogue effect
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Standard ASA Monitors Standard I Qualified anesthesia personnel
shall be present in the room throughout the conduct of all general,
regional, and monitored anesthetic care. Standard II During all
anesthetics, the patients oxygenation, ventilation, circulation,
and temperature shall be continually evaluated.
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Standard ASA Monitors Oxygen analyzer with low O2 alarm
Quantitative method of blood oxygenation (pulse ox) Ventilation
evaluation (chest rise, auscultation) Correct positioning of airway
devices End-tidal CO2 with airway devices Ventilator disconnection
alarm ECG, BP, HR (every 5 min for the latter 2) Body temperature
(if perturbations are expected)
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Interventions Assess for airway obstruction, bilateral breath
sounds, quality of breath sounds Check FiO2, ETCO2, HR, BP, SpO2
Bladder intake and output IV fluid intake Increase FiO2 to 100%
Consider assisted ventilation If no improvement, tracheal
intubation and PPV
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Common Leak Sites Incomplete tracheal cuff seal Elbow End-tidal
monitoring connections Inspiratory and expiratory hoses
Unidirectional valves, Pop-off valve, resevoir bag, bellows,
absorber, vaporizers, flowmeters, scavenging system
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Extubation Criteria Global criteria Return of consciousness
Demonstration of ability to protect airway Adequate reversal of NM
blockade Absence of hypothermia Presence of nl metabolic milieu
Respiratory criteria Vital capacity > 15ml per kg NIF < -20
cm H2O SpO2 > 90% on FiO2
Rapid Shallow Breathing Index Patient observed breathing a
T-bar or low Pressure support RSBI = RR (bpm) / tidal volume (L)
RSBI > 100 Patient will probably fail extubation If develops
diaphoresis, agitation, tachycardia, bradycardia, HTN, hypotension
failed trial
