Basic Anesthesia Monitoring
rev 2016-01-18
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Basic Anesthesia Monitoring
© D. J. McMahon 150114
rev cewood 2015-01-18
Basic Anesthetic Monitoring
Key Points:
-
Know the four parameters of the ASA monitoring standard
Know the best and worst locations for temperature monitoring
Know the reason for peripheral nerve stimulators
What are the four modes of nerve stimulators
Know about BiS monitors
The primary goal of anesthesia is to keep the patient as safe as
possible in the perioperative period.
Continuous monitoring of the patient during and after surgery allows
the clinician to identify problems early, when they can still be
corrected.
Disturbances that occur during surgery include,
but are not limited to:
- Airway obstruction, Respiratory depression, Apnea.
- Cardiac depression, Arrhythmias, Bradycardia, Tachycardia.
- Hypertension, Hypotension.
- Hypervolemia, Hypovolemia, Fluid Shifts.
- Hypothermia, Hyperthermia
Basic monitoring includes ongoing evaluation
of the major body systems.
Standards of Care
‘Guidelines’ specify what is usually expected, while
‘Standards’ specify what is always expected.
The current standards of anesthesia monitoring are published by
the American Society of Anesthesiologists (ASA). Monitoring
standards are not law except in several states, but for all practical
purposes they might as well be.
Failure to follow nationally published standards puts the provider
at risk for credentialing problems and lawsuits.
The ASA standards were most recently updated in 2015.
ASA standards for monitoring are available on the ASA website:
https://www.asahq.org/quality-and-practice-management/standards-and-guidelines
The ASA Standards for
Basic Anesthetic Monitoring
Standard I states that an 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.
Oxygenation: Inspired oxygen.
Hemoglobin saturation with a pulse oximeter
and observation of skin color.
Ventilation:
Capnography. Tracheal intubation must be verified clinically and
by detection of exhaled CO2.
Mechanical ventilation must be monitored with an audible
disconnect monitor.
Circulation:
ECG monitoring, blood pressure measurement at least every
five minutes, and continuous monitoring of peripheral circulation
by palpation, auscultation, plethysmography, or arterial pressure.
Temperature: Thermometry if changes are anticipated, intended, or suspected.
1: Oxygenation:
Oxygen Saturation (SpO2)
2: Ventilation:
Anesthesia machines have ventilator disconnect alarms
and built-in flow meters (spirometers). These include
high and low limit alarm settings.
Continuous measurement of exhaled tidal volume can
detect circuit leaks and hypoventilation.
Excessive airway pressure can result in patient injury, so
anesthesia machines also include overpressure relief
valves ("pop-off" valves), with overpressure alarms.
Respiratory Gas Monitors
Gas from the patient circuit is drawn into an infrared measurement
chamber.
CO2, N2O, and inhaled anesthetic agents all absorb infrared light,
at slightly different frequencies. “Sidestream” infrared (IR)
sampling.
These monitors provide breath-by-breath gas analysis, and display
the respiratory rate.
Limitation of IR analyzers: moisture can cause blockage of the gas
path.
Oxygen is detected within the same monitor, by a fuel cell or a
paramagnetic sensor.
3: Circulatory Monitoring:
ECG and Blood Pressure
4: Temperature Monitoring
Lower esophageal temperature is normally a good
reflection of core or blood temperature.
Upper esophageal and nasopharyngeal temperature are
affected by airway temperature, so are less accurate.
Tympanic membrane temperature is also a good
indication of core temperature but it is not practical in
the surgical setting.
Monitoring of skin temperature is nearly useless.
Esophageal Stethoscope (with Temp Probe)
Integration of monitors for
patient and anesthesia machine:
Peripheral Nerve Stimulators
Peripheral nerve stimulation monitoring is not required by the ASA
standards. However, it is an important safety monitor in patients
receiving neuromuscular blocking drugs.
Clinical monitoring of neuromuscular blockade during an anesthetic
is difficult without a nerve block monitor.
Train-of-four monitoring
Twitch
Tetany
}
} all assess the level of blockade.
}
Double-burst stimulation assesses return of muscle strength at the
end of the case.
(Assessment of strength is important at the end of an anesthetic
before a decision is made to extubate the patient.)
Comparison of four modes of nerve block monitor:
Common placement of nerve block monitor electrodes:
Alternative placements of PNS electrodes:
Neurological monitoring is not mentioned in the ASA standard, but
it is frequently done.
EEG is not practical for the anesthesia provider, but the bispectral
index (BiS monitor) is now increasingly accepted.