April 25, 2011
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%.
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)
Hypoxemia
Deficient oxygenation of blood.
Decreased oxygen tension in the
arterial blood (PaO2)
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)
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
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.
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.
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)
Pulse oximeter
 Patient movement (shivering, peripheral nerve
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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, Raynaud’s)
Venous pulsations (tricuspid regurg)
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
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
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.
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
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
 α angle – slope between II and III (increase in acute
bronchospasm)
Phase IV – Inspiratory downstroke
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)
Prolonged expiratory plauteau and
expiratory upstroke
 Mechanical obstruction to exhalation
 COPD
 Bronchospasm
Dips in expiratory plateau
 Spontaneous ventilation efforts
Cardiogenic oscillations
 Ventilator pressure relief valve pertubations
Elevated expiratory plateau
 Incorrect calibration
 Increased CO2 production / delivery
 Laparoscopic CO2 gas insufflation
 Decreased CO2 removal
 Hypoventilation
 Leak
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)
Prolonged inspiratory downstroke
and raised baseline
 Incompetent or missing inspiratory unidirectional
valve
 Inspiratory obstruction to gas flow (kinked tube)
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
Safety Features
 Pin index (cylinder) and diameter index (pipeline) safety
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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
Safety Features
 Gas flow proportioning – ensure minimum O2 of 25%
when N2O is used
 Vaporizer interlock system
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
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
 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
 Step 8 – Adjust / Check scavenging system (test pop
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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
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
Premedication
 Anxiolysis
 Minimization of gastric volume and acidity
 Antibiotic prophylaxis
 Antisialagogue effect
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 patient’s
oxygenation, ventilation, circulation, and temperature
shall be continually evaluated.
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)
Interventions
 Assess for airway obstruction, bilateral breath sounds,
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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
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
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 <0.4
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
Postop Hypoxemia
 Physiologically
 Low FiO2
 Hypoventilation
 VQ mismatch
 Shunt
 Pathologically
 Airway obstruction
 Atelectasis
 R Mainstem intubation
 Aspiration
 Pulmonary edema
 Pulmonary embolus
Shunt
 Perfused but not ventilated
 PaO2 will not rise with increased FiO2 once shunt
fraction approaches 30%
Dead Space
 Ventilated but not perfused
 Failure to maintain normal PaCO2 despite increased
MV (tv x rr)
Pulmonary Edema
Condition
Pathophysiology
 Congestive Heart Failure
  filling pressure,  CO
 Negative Pressure Pulmonary
 outside-inside pressure
Edema
 Acute Lung Injury and ARDS
gradient
 Permeability
ARDS Mechanical Ventilation
 TV < 6ml / kg
 PIP < 35 cm H2O
 Consider PEEP of 10 cm H2O