ASA Standard Monitors

There are 2 Basic Standards put out by the American Society of Anesthesiologists (ASA) that guide monitoring during any procedure or surgery require anesthesia. The first standard is “a qualified anesthesia personnel shall be present in the room throughout the conduct of all general anesthetics, regional anesthetics, and monitored anesthesia care”. The second standard is “during all anesthetics, the patient’s oxygenation, ventilation, circulation, and temperature shall be continually evaluated”. Continually is defined as “repeated regularly and frequent checks, no more than 5 minutes apart”. Let’s break down the 4 components further.

Starting with oxygenation, there are two major components to this. The first involves the anesthesia machine/ventilator. If you are in an anesthetic case that utilizes the ventilator, you will need a gas analyzer that can detect the concentration of inspired gases, including an FiO2 analyzer. Your ventilator should also be equipped to alarm if a hypoxic mixture of gas is being delivered to the patient. Hypoxic mixtures can be accomplished when using nitrous oxide or if there is a oxygen source failure from either the central gas supply or via gas cylinders. These two components are typically built into every ventilator. Most modern ventilators also have the capacity to analyze expired gases as well. The second component to oxygenation is pulse oximetry which analyzes the patient’s hemoglobin saturation with oxygen. A pulse oximeter uses the Lambert-Beer Law, which is a physics property that expresses how much light is absorbed by matter. There are two types of pulse oximetry: fractional oximetery (oxyhemoglobin/[oxyhemoglobin + deoxyhemoglobin + methemoglobin + carboxyhemoglobin]) and functional oximetry (oxyhemoglobin/[oxyhemoglobin + deoxyhemoglobin]).

In the operating room, we typically use functional oximetry (SpO2). How does a pulse ox work? It emits two wavelengths of light; red (660 nm) and infrared (940 nm). Deoxyhemoglobin absorbs more light in the red band and oxyhemoglobin absorbs more light in the infrared band. A photoplethysmography is used to identify pulsatile blood flow (alternating current, AC) and non-pulsatile flow (direct current, DC). The pulse ox then creates a ratio of (AC/DC)660/(AC/DC)940 and a built in algorithm converts this to an SpO2 percentage. The pulse ox is most accurate between 70-100%. The calibration of the pulse ox was done on healthy volunteers with whom they did not test SpO2’s less than 70%. Low perfusion states also decrease the accuracy and the signal of the pulse ox. These include hypovolemia, hypothermia, cardiac arrest, arrhythmias, cardiac bypass, vasoconstriction, tourniquet, and BP cuff inflation. There are two dyshemoglobinemias that commonly come up in practice and on test questions: carboxyhemoglobin and methemoglobin. Carboxyhemoglobin usually shows a pulse ox reading about 90%, overestimating the SpO2. You should suspect carboxyhemoglobinemia for patients who were exposed to smoke or fires. Methemoglobin is formed when iron goes from +2 ferrous to +3 ferric state. The ferric state creates a left shift on the oxygen dissociation curse and holds onto oxygen tighter than normal. Methemoglobin absorbs equal amounts of red and infrared light which leads to an SpO2 reading around 85%. Causes of methemoglobinemia include local anesthetics (ie. Benzocaine), chlorate’s, antimalarials, antineoplastics, sulfonamides, dapsone, and metoclopramide. Other causes for falsely decreased SpO2 readings include IV dyes (methylene blue > indocyanine green > indigo carmine), dark nail polish, shivering or surgical motion, ambient light, or a malpositioned sensor. If for some reason monitors need to be disconnected for a short period of time such as flipping a patient prone for surgery, physical signs of oxygenation function include color of skin and mucous membranes.

Ventilation is typically monitored visually (for MAC cases with chest rise and condensation on a face mask), tidal volumes (if doing general anesthesia and connected to a ventilator), and with capnography (used for both MAC and GA cases). Capnography provides both qualitative (capnography waveform tracing) and quantitative (unit is mmHg). Inspired CO2 is also useful for indicating when a gas absorbent should be replaced (most have a visual color change as well when exhausted). In most operating room, the technique for measuring end-tidal CO2 is via infrared spectrography. Each molecule (CO2 and anesthetic gases) have a specific absorbance of infrared light that can be quantified. Occasionally when doing a intubation outside of the operating room, you may see a colorimetric CO2 analyzer which connects between the endotracheal tube and the ventilator circuit. This is a pH sensitive test paper that changes color when exposed to CO2.

When looking at a capnogram, there are 4 phases to the waveform. Phase 1, the beginning of the rise in waveform, is the initiation of expiration. Phase 2, the sharp increase in waveform, is the expiration of dead space and alveolar gas. Phase 3, the top of the waveform, is the alveolar plateau. Phase 4, the decline in the waveform, the the initiation of inspiration. Clinical uses of capnography include confirmation of endotracheal intubation, monitoring of adequacy of ventilation in controlled or spontaneously ventilated patients, and noninvasive estimate of PaCO2. There is around a 2-5 mmHg difference between end-tidal CO2 and PaCO2 (PaCo2 > EtCO2). This gradient increases with increased dead space, age, pulmonary disease, low cardiac output, pulmonary embolism, and hypovolemia. Causes of increased CO2 production (hypermetabolic state) include fever, sepsis, malignant hyperthermia, hyperthyroidism, and shivering. Causes of decreased EtCO2 include disconnection of ETT from circuit, decreased cardiac output, hypovolemia, pulmonary embolism, hypothermia and hyperventilation. Qualitative assessment of a capnogram can pick up on ventilation changes such as rebreathing of CO2, obstruction, bronchospasm, return of spontaneous respiratory efforts, cardiac oscillations, and incompetent inspiratory valve. Most modern day ventilators will also have an audible apnea alarm that will go off if the patient’s airway is disconnected from ventilator or if it doesn’t sense end-tidal CO2 for a prolonged period of time.

Circulation is assessed by continual blood pressure and electrocardiogram monitoring. Blood pressure monitoring can be invasive (arterial blood pressures) or noninvasive (BP cuff). Arterial BP monitoring allows for heart beat to heart beat blood pressure readings. It also gives a provider access to sample Arterial blood pressure waveforms also provide qualitative information. As you move further away from the aorta (central arterial pressure), the systolic amplification increases, however the MAP is unchanged, dicrotic notch is delayed, and the pulse pressure widens. Arterial blood pressures also can provide information about the patient’s volume based on pulse pressure variation (PPV). In ventilated patients who are in sinus rhythm, a PPV of greater than 13% in indicative of a positive response to fluid resuscitation to correct hypotension. Noninvasive blood pressure monitoring utilizes oscillometry which is able to detect the MAP or the strong pulse felt by the BP cuff as the tourniquet pressure is released. The SBP and DBP are derived from an algorithm (MAP = 1/3 SBP + 2/3 DBP). In terms of the cuff, it should wrap comfortable around a patient’s extremity (generally the upper arm, bicep). If the cuff is too small, you will get falsely elevated blood pressures. If the cuff is too big, you will get falsely decreased blood pressures.

The last point to hit home about blood pressure is that it varies by position and height. For example, if a patient is in the sitting position (“beach chair”) for a shoulder surgery, the blood pressure at their arm is different than the blood pressure in their brain because of the change in height. The conversion factor for this difference in height is a change of 10cm = a change in 7.4 mmHg in the blood pressure (conversely a change in 15 cm of height = a change of 10 mmHg in blood pressure). The relationship is inversely related, meaning a positive change in height will result in a negative change in blood pressure (the blood pressure in the brain is lower than the blood at the arm in a beach chair position). The second component of monitoring circulation is a continuous electrocardiogram. For most adults, we use a 5 lead system. In pediatrics, typically we use 3 leads. The most commonly monitored leads are Lead II and Lead V. Lead II is best for visualizing the p-wave, letting you know if your patient is in sinus rhythm. Lead V is the best lead to observed for ST depression or elevation related to ischemic changes to the anterior or lateral wall of the heart.

The last component of monitoring a patient under general anesthesia is temperature. Monitoring temperature is required only if clinically significant changes in body temperature are anticipated. General anesthesia falls under this category as it causes profound vasodilation and creates a major heat shift from a patient’s core into their periphery and leads to hypothermia if not addressed (redistribution). The most common places to measure a core temperature in the operating room include nasopharyngeal, oropharynx, esophagus, and pulmonary artery. Other areas that used but less accurate include bladder (impacted by urine flow rate), axillary (varies by skin perfusion), and skin (varies by site and exposure to air). Hyperthermia (greater than 37.2 degrees celsius) can be seen in sepsis, malignant hyperthermia, or iatrogenic causes (forced-air warmer, surgical drapes, operating room lights on the patient). However, hypothermia (less than 36 degrees Celsius) is much more common and has detrimental consequences such as increased risk for surgical site infection, coagulopathies, slowed emergence, and potentially cardiac collapse.