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 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 CO 2. 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 CO 2 measurement is sidestream infrared (IR) capnography. Gas from the circuit is drawn into an infrared measurement chamber. CO 2, N 2 O, H 20, 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 V 5, detecting about 75% of ischemic episodes. Lead II plus lead V 5 raise the detection rate to 80%, whereas leads II, V 4, and V 5 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 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
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