April 25, 2011 Case Stem: A 70 year old man is to undergo cystoscopy and transurethral resection of a bladder tumor.
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Transcript April 25, 2011 Case Stem: A 70 year old man is to undergo cystoscopy and transurethral resection of a bladder tumor.
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
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
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
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,
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