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Basic Anesthesia Monitoring © D. J. McMahon rev cewood

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. 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

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.

‘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 ASA standards for monitoring are available on the ASA website: Standards of Care

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 CO 2. 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 (SpO 2 )

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. 2: Ventilation:

Respiratory Gas Monitors Gas from the patient circuit is drawn into an infrared measurement chamber. CO 2, N 2 O, 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} all assess the level of blockade. Tetany} 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.