Basic Anesthetic Monitoring
ASA Standards for Basic
Anesthesia Monitoring
from VM Dept of Anesthesiology, 2007
rev 1/11
dmcmahon
Basic Anesthetic Monitoring
The primary goal of anesthesia is to keep the
patient as safe as possible in the
perioperative period. Anesthesia and surgery
are serious invasions on the physiologic stability
of the human body. Careful monitoring of the
patient during and after surgery allows the
anesthesiologist to identify problems early, when
they can still be corrected. Proper monitoring of
the patient can reduce the risks involved in
anesthesia and surgery.
The primary goal of anesthesia
• Some of the physiologic disturbances that occur
in the perioperative period include, but are not
limited to apnea, respiratory depression, airway
obstruction, cardiac depression, hypertension,
hypotension, hypervolemia, hypovolemia,
arrhythmias, blood loss, fluid shifts, weakness,
bradycardia, tachycardia, hyperthermia, and
hypothermia. Basic monitoring must include
ongoing evaluation of the major body
systems.
Standards of Care
• Proper monitoring standards are well-defined. Federal and state
governments, as well as national and local professional groups all
have tried to set guidelines or standards. (Guidelines specify what is
usually expected, and standards specify what is always expected.)
The most widely accepted current anesthesia monitoring standards
are those that have been published by the by the American Society
of Anesthesiologists (ASA). For the most part, monitoring standards
are not law (exceptions include New Jersey and New York), but for
all practical purposes they might as well be. Failure to follow
nationally published standards sets the practitioner up for
credentialing problems, lawsuits, and the like. The ASA standards
were initially published in 1986, and were most recently updated in
1993. Copies of the ASA standards for monitoring are available from
the ASA.
The ASA Standards for Basic
Anesthetic Monitoring
• Standard I states that a qualified anesthesia provider will be present
with the patient throughout the anesthetic. Standard II states that the
patient's oxygenation, ventilation, circulation, and temperature will
be continually monitored. Assessment of oxygenation involves two
parts: measurement of inspired gas with an oxygen analyzer and
assessment of hemoglobin saturation with a pulse oximeter and
observation of skin color. Assessment of ventilation is by clinical
assessment and preferably capnography. Tracheal intubation must
be verified clinically and by detection of exhaled CO2. Mechanical
ventilation must be monitored with an audible disconnect monitor.
Assessment of circulation involves continuous ECG monitoring,
blood pressure measurement at least every five minutes, and
continuous monitoring of peripheral circulation by such means as
palpation, ausculation, plethysmography, or arterial pressure
monitoring. The patient's temperature must be measured if
changes are anticipated, intended, or suspected.
Monitoring Devices
• Oxygen Analyzers Oxygen analyzers are an integral
part of the newer anesthesia machines and have been
added onto most of the older machines. The purpose of
the oxygen analyzer is to confirm that oxygen is being
delivered to the patient and that concentration of oxygen
in the gas mixture is adequate. Isolated incidents have
occurred where the gas in the hospital "green line" was
argon or something else other than oxygen. The oxygen
analyzer provides one last check before the gas mixture
is delivered to the patient. For the analyzer to be useful,
it must be calibrated and the low-limit alarm must be
working.
Oxygen analyzers
• The two main types of oxygen analyzers are
galvanic (fuel cell), and the polarographic.
The systems we use are the polarographic type.
This involves a semipermeable membrane, an
electrolyte solution, and a battery source. The
battery polarizes the electrodes and the current
that is generated varies with the amount of
oxygen present. The electrode/electrolyte
cartridge requires periodic replacement. The
main disadvantage of this system is the slow
response time.
Pulse Oximeters
• The pulse oximeter provides continuous monitoring of
hemoglobin saturation using a two-wavelength light
absorption technique. The monitor filters out the effects
of ambient light, tissue, skin pigment, tissue, and venous
blood. It focuses on the pulsatile absorption which due to
pulsatile arterial blood. Pulse oximeters were developed
in the early 1980's and rapidly proved their value in
anesthesia. Pulse oximetry allows rapid, beat-by-beat,
noninvasive monitoring of blood oxygenation. Pulse
oximetry have been a major advance in improving the
safety of anesthesia.
R=(AC red / DC red) / (AC infrared / DC
infrared)
• The value of R is correlated to saturation
percentage in a "look-up" table. The data in the
"look-up" table was derived from human testing.
For example, an R of 1 corresponds to a
saturation of 85% in the average patient. The
accuracy is 1 to 2 percent.
• Disadvantages of pulse oximetry are that it is
motion sensitive, and that substances like
carbon monoxide, methemoglobin, and some
dyes affect the readings.
Capnography
• The most common method of exhaled CO2 measurement is
sidestream infrared (IR) capnography. Gas from the circuit is drawn
into an infrared measurement chamber. CO2, N2O, H20, and
inhaled anesthetic agents all absorb infrared light, but at slightly
different frequencies. Newer monitors have precise light sources
and filters that specifically measure the individual gases. These
monitors provide breath-by-breath gas analysis. Problems with IR
capnographs are that moisture can cause blockage of the gas path,
and that they can't measure oxygen or nitrogen.
• Other methods of measuring exhaled gases include RAMAN
scattering and mass spectrometry. These systems measure oxygen
and nitrogen directly, as well as carbon dioxide. They are, however,
more expensive and more complicated devices
Automatic blood pressure
monitors
• Current automatic noninvasive blood pressure monitors
work on the oscillometric technique. The cuff inflates well
above the systolic pressure and then deflates slowly.
The monitor first senses oscillations as the cuff drops to
systolic pressure. The point at which the oscillations are
the strongest is read as the mean pressure. Most of
these devices calculate the diastolic pressure after they
measure the systolic and mean pressures.
• The system is normally very reliable and accurate, but
motion (especially shivering) on the part of the patient or
the surgeon leaning against the cuff will cause false
readings or failure to get a reading. Patient injury is
possible if the tubing becomes kinked. Values may be in
error if the cuff is not the proper size.
ECG Monitor
• The ECG monitor can provide a lot of information to the
anesthesiologist. Arrhythmia detection and identification of
tachycardia and bradycardia are important uses. The ECG monitor
may also provide the first indication of myocardial ischemia.
However the absence of ST depression does not guarantee that
ischemia is not present. Lead placement is important in ischemia
detection. The most sensitive lead is lead V5, detecting about 75%
of ischemic episodes. Lead II plus lead V5 raise the detection rate to
80%, whereas leads II, V4, and V5 together detect 98% of ischemic
events.
• Current top-of-the line monitors do automated ST analysis which is
more reliable than individual practitioner assessment as long as the
measurement points are correct.
Ventilation Monitors
• Current anesthesia machines have ventilator
disconnect alarms and built-in spirometers. The
spirometers have high and low limit alarm
settings. Continuous measurement of exhaled
tidal volume can detect circuit leaks and
hypoventilation. The spirometers on the
anesthesia machines may give false readings if
moisture blocks the innerworkings.
• Current anesthesia machines also have
overpressure alarms and overpressure "pop-off"
valves. Patient injury can occur before these
high pressure alarms are triggered
Temperature Monitors
• Monitoring of skin temperature is nearly
useless. Upper esophageal and
nasopharyngeal temperature are affected
by airway temperature. Lower esophageal
temperature is normally a good reflection
of core or blood temperature. Tympanic
membrane temperature is also a good
indication of core temperature but it is not
practical in the operating room
environment.
Peripheral Nerve Stimulators
• Peripheral nerve stimulation (PNS) monitoring is not
required by the ASA standards. However, it is an
important safety monitor in patients who a receiving
neuromuscular blocking drugs. Train-of-four monitoring
assesses the level of nondepolarizer blockade and
double-burst stimulation assesses return of strength at
the end of the case. Clinical monitoring of neuromuscular blockade during an anesthetic is difficult
without a PNS monitor. Clinical assessment of strength
is important, however, at the conclusion of an anesthetic
before a final decision is made to extubate the patient