Basic Monitoring

9faf5978d6e6d55b92bf8309a1d}/ID(AB1-M91)” href=”javascript:void(0)” title=”Hgb” onmouseover=”window.status=this.title; return true;” onmouseout=”window.status=”; return true;” onclick=”get_content(event,’AB1-M91′); return false;” target=”right”>Hgb molecule is capable of binding four oxygen molecules. Although oxygen dissolves in blood, binding oxygen to Hgb found in RBCs allows the blood to carry 70 times as much oxygen. In arterial blood, about 98.5% of the oxygen is bound to Hgb and only 1.5% is dissolved within the blood. V being specific to the monitoring techniques that should be used to assess a patient’s oxygenation, ventilation, circulation, and body temperature. These monitors are discussed in roughly the same order as the standards they fulfill.

▪ ASA STANDARD I

“Qualified anesthesia personnel shall be present in the room throughout the conduct of all general anesthetics, regional anesthetics and monitored anesthesia care.”

▪ PULSE OXIMETRY (ASA STANDARDS II AND IV)

Pulse oximetry is a noninvasive method used to determine oxygen levels in arterial blood. Similar to some gas analysis techniques, pulse oximetry is based on the principal that hemoglobin (Hgb) absorbs a specific wavelength of light differently whether it is saturated or desaturated with oxygen. Hgb is a protein contained in red blood cells (RBCs). Each Hgb found in RBCs allows the blood to carry 70 times as much oxygen. In arterial blood, about 98.5% of the oxygen is bound to Hgb and only 1.5% is dissolved within the blood. Hgb that has bound oxygen is termed oxyhemoglobin, and Hgb that does not have oxygen bound to it is termed deoxyhemoglobin.

A standard pulse oximeter contains a light source and a sensor. The light is passed through a portion of the patient’s body that is small enough that some of the light can pass through the body part to be detected by the sensor on the other side. A pulse oximeter uses two bandwidths of light: infrared (660 nm) to measure oxyhemoglobin and nearinfrared (940 nm) to measure deoxyhemoglobin. The amount of light that is absorbed and the amount of light that passes through the body part are dependent upon the amount of oxygen bound to the Hgb within the RBCs. Sensor readings of the light absorbed for both wavelengths are processed through an algorithm that compares the ratio of absorption for the two wavelengths. The pulse oximeter can then calculate and display a value that represents the percentage of arterial blood oxygenation. Normal values for humans range from 95% to 100%.

There are two types of pulse oximeters=”LK” xpath=”/CT{06b9ee1beed59419ac6a0214e6a9ecf4a0d34ac8c7cbab059779ea2bfe91b05f2934e9faf5978d6e6d55b92bf8309a1d}/ID(AB1-M91)” href=”javascript:void(0)” title=”Hgb” onmouseover=”window.status=this.title; return true;” onmouseout=”window.status=”; return true;” onclick=”get_content(event,’AB1-M91′); return false;” target=”right”>Hgb is. For example, if a patient’s Hgb is 16 g/dL and all of the available <A class=LK xpath="/CT{06b9ee1beed59419ac6a0214e6a9ecf4a0d34ac8c7cbab059779ea2bfe91b05f2934e9 foot and forearm for neonates. Less common sites include the cheek for adults and the penis for neonates.

Reflection probes have both a light source and a detector placed near one another. The amount of light reflected to the sensor is used for calculations. The forehead is the most common site for reflectance sensors.

Pulse oximeter probes can be either disposable or reusable (Fig. 33.1). Reusable probes are classified as “noncritical” items for disinfection purposes, and low-level disinfection between patients is suitable as long as the probe is placed over intact skin or does not become soiled with blood or body fluids. Any reusable probe that does become soiled should be treated with high-level disinfection or sterilization. Regardless of the treatment used, make sure the probe has dried completely before using it on the next patient.

▪ FIGURE 33.1 Reusable and disposable pulse oximetry probes.

▪ LIMITATIONS OF PULSE OXIMETRY

Although pulse oximetry information is extremely useful and has contributed to the safety of modern anesthesia, its limitations must be understood. A patient can have a reading in the normal range (95%-100%) and still have insufficient oxygen in the blood. What a pulse oximeter reading of 100% means is that of the Hgb that is available to transport oxygen in the arterial blood, 100% is occupied with oxygen. What it does not tell you is what percentage of Hgb is available to carry oxygen or what the total Hgb is. For example, if a patient’s Hgb is 16 g/dL and all of the available Hgb is carrying oxygen, the saturation reading would be 100%. If that same patient’s Hgb drops to 8 g/dL, the saturation reading can still be 100%, even though the total amount of oxygen carried in the arterial system has been reduced by half.

Another important factor to consider is the effect of other substances and the ability of Hgb to bind oxygen and their simultaneous effect on pulse oximetry readings. Carbon monoxide binds to Hgb 20 times more strongly than oxygen. The presence of carbon monoxide effectively lowers the oxygen-carrying capacity of blood (available amount of Hgb to carry oxygen). In addition, carbon monoxide bound to Hgb (carboxyhemoglobin) absorbs one of the wavelengths of light used by the pulse oximeter and interferes with the calculation of oxygen saturation. Therefore, patients with high carboxyhemoglobin levels may not have enough oxyhemoglobin, yet still have high pulse oximetry readings (e.g., patients with carboxyhemoglobin concentrations of 70%—more than two-thirds of the <A onmouseover="window.status=this.title; return true;" title=Hgb class=LK href="javascript:void(0)" xpath="/CT{06b9ee1beed59419ac6a0214e6a9ecf4a0d34ac8c7cbab059779ea2bfe91b05f2934e9faf5978d6e6d55b92bf8309a1d}/ID(AB1-M91)" div site.

Nail polish: Nail polish, particularly da2bfe91b05f2934e9faf5978d6e6d55b92bf8309a1d}/ID(AB1-M91)” href=”javascript:void(0)” title=”Hgb” onmouseover=”window.status=this.title; return true;” onmouseout=”window.status=”; return true;” onclick=”get_content(event,’AB1-M91′); return false;” target=”right”>Hgb including methemoglobin and sulfhemoglobin also interfere with both the ability of oxygen to bind to Hgb and pulse oximetry readings. Patients with methemoglobinemia typically have pulse oximetry readings of 85% despite severe reductions in oxygen-carrying capacity.

In most cases, a co-oximeter can be used to overcome the limitations of standard pulse oximeters mentioned above. Co-oximeters use multiple wavelengths of light (as many as eight) and can not only measure oxyhemoglobin but also
provide values for other states of Hgb, such as carbhemoglobin, carboxyhemoglobin, methemoglobin, and sulfhemoglobin, as well as the total amount of Hgb. Co-oximeters are commonly available with blood gas machines; however, pulse co-oximeters are available for bedside use as well.

Pulse oximeters also measure the volume of the sample body part and rely on the principle that during an arterial pulsation (increase in volume), the blood is fully oxygenated and this is when the reading should be taken. This is why most pulse oximeters provide a value for the heart rate and many display a waveform as they measure the rhythmic change in volume of the sample body. This property also leads to a limitation in pulse oximetry readings. When the pulse oximeter cannot detect regular, rhythmic changes in volume, it cannot give an oximetry reading. This can occur with peripheral vasoconstriction (peripheral vascular disease, vasospasm, cold body part, etc.), low blood flow (hypotension, reduced cardiac output), or arrhythmias that are irregular (atrial fibrillation, multifocal atrial tachycardia, etc.).

There are several sources of interference that can affect the accuracy and usability of a pulse oximeter. Although all pulse oximeters can experience problems, newer models include features and software to help minimize them. The most common of these are light, motion, nail polish (some blues, reds, and browns), hypoperfusion, and intravenous dyes. Problems and possible solutions are presented below:

Light: Ambient light normally affects both the saturation reading and the pulse rate. Shield the sensor with a surgical towel, drapes, etc. or change the sensor site.

Motion: Motion of the probe interferes with readings. This can occur when the probe is jostled (e.g., surgical personnel leaning agaisub>2 monitoring is to confirm the placement
of an endotracheal tube into the trachea. If the tube is placed in the trachea and the patient is ventilated, and there is sufficient cardiac output to deliver CO₂ to the lungs and there is sufficient gas exchange between the blood and the alveoli, the CO₂ monitor will register CO₂. If the endotracheal tube is placed in the esophagus, the CO₂ monitor will not register sustained CO₂ (Fig. 33.3). Although there may be some carbon dioxide in the esophagus and stomach, the concentration is normally low and will rapidly be depleted with ventilation of the stomach.

▪ FIGURE 33.2 A-E: Normal capnogram. A,B: Baseline represents continued inhalation (value should be zero) or lack of gas movement. B,C: Expiratory upstroke (sharp rise from baseline represents the beginning of exhalation and consists of a mixture of air and alveolar gas. C,D: Expiratory plateau (continued exhalation of alveolar gas, should be straight or nearly straight). D: End-tidal concentration (value at the end of exhalation); D,E: Inspiration begins (sharp downstroke as fresh gapter 32″ onmouseover=”window.status=this.title; return true;” onmouseout=”window.status=”; return true;” onclick=”get_content(event,’B01745940-DA4-C32′); return false;” target=”right”>Chapter 32. In this chapter, we focus on the practical application of that technology in anesthesia. The role of capnography within the ASA standards is to help ensure a patient is adequately ventilated during all anesthetics. Continuous readings of exhaled carbon dioxide (CO₂) can instantly communicate changes in the status of the patient and the anesthesia machine.

Although the numerical readings from a capnometer will satisfy the basic requirements, a capnograph with a continuous waveform is preferred. The shape of the waveform can yield additional diagnostic information about how a patient is being ventilated.

The “normal” capnogram is a waveform that represents the varying CO₂ level throughout the breath cycle over time (Fig. 33.2A-E).

Measured exhaled CO₂ is a function of CO₂ production by the body, delivery of the CO₂-containing blood to the lungs, gas exchange in the alveoli, and ventilation of the lungs to pick up CO₂ from the alveoli and eliminate it to the outside. One of the most important functions of CO

Machine problems leading to decreased ventilation (leaks, obstructions, depleted CO₂ absorber, etc.)

▪ FIGURE 33.4 Rising ETCO₂ levels as detected on a capnogram. (Adapted from Capnography Self-Study Guide. Rev. 1. Smiths Medical; 2008. Used by permission.)

Decrs.title; return true;” onmouseout=”window.status=”; return true;”>Fig. 33.3). Although there may be some carbon dioxide in the esophagus and stomach, the concentration is normally low and will rapidly be depleted with ventilation of the stomach.

Below is a discussion of the differential diagnosis of changes in measured CO₂ values or waveforms. Increased CO₂: An increase in the level of End-Tidal Carbon dioxide (ETCO₂) from previous levels can result from either an increase in the patient’s production of CO₂ or a decrease in ventilation (Fig. 33.4):

Increased metabolic rate (rising body temperature from blankets or external warming devices, thyrotoxicosis, malignant hyperthermia, etc.)
Increased cardiac output
Chemicals or metabolic products that have been administered to the patient that are converted to CO₂ (bicarbonate, lactate, CO₂ gas embolus, etc.)
Decreased respiratory rate or tidal volume
Decreased gas exchange with eventual rising CO₂ blood levels (pulmonary failure, chronic obstructive pulmonary disease [COPD], bronchospasm, etc.)
Machine problems leading to decreased ventilation (leaks, obstructions, depleted CO₂ absorber, etc.)

▪ FIGURE 33.4 Rising ETCO₂ levels as detected on a capnogram. (Adapted from Capnography Self-Study Guide. Rev. 1. Smiths Medical; 2008. Used by permission.)

Decreased CO₂: A decrease in the level of ETCO₂ from previous levels can result from a change in the body’s production of CO₂ (rare), the delivery of CO₂ to the lungs, or an increase in ventilation (Fig. 33.5):

Decreased metabolic rate or decreased core body temperature
Decreased cardiac output
Decreased pulmonary blood flow (pulmonary embolus)
Increased tidal volume or respiratory rate
Partial disconnect of CO₂-monitoring tubing

▪ FIGURE 33.3 ETCO₂ levels rapidly diminish or are not present with an esophageal intubation. (Adapted from Capnography Self-Study Guide. Rev. 1. Smiths Medical; 2008. Used by permission.)