Oxygen is a highly reactive chemical species, providing many chemical and physical opportunities to detect its presence. Three main types of oxygen analyzers are seen in clinical practice: Paramagnetic oxygen analyzers, galvanic cell analyzers, and polarographic oxygen analyzers.

Paramagnetic gases are attracted to magnetic energy because of unpaired electrons in their outer shell orbits. Oxygen is a highly paramagnetic gas. Differential paramagnetic oximetry has been incorporated into a variety of operating room monitors. These instruments detect the change in sample line pressure resulting from the attraction of oxygen by switched magnetic fields. Signal changes during electromagnetic switching correlate with the oxygen concentration in the sample line.²

Galvanic cell analyzers meet the performance criteria necessary for operative monitoring. These analyzers measure the current produced when oxygen diffuses across a membrane and is reduced to molecular oxygen at the anode of an electrical circuit.³ The electron flow (current) is proportional to the partial pressure of oxygen in the fuel cell. Galvanic cell analyzers require regular replacement of the galvanic sensor capsule. In the sensor, the electric potential for the reduction of oxygen results from a chemical reaction. Over time, the reactants require replenishment.⁴

Polarographic oxygen analyzers are commonly used in anesthesia monitoring. In this electrochemical system, oxygen diffuses through an oxygen-permeable polymeric membrane and participates in the following reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻. The current change is proportional to the number of oxygen molecules surrounding the electrode. Polarographic oxygen sensors are versatile and are important components of gas machine oxygen analyzers, blood gas analyzers, and transcutaneous oxygen analyzers.⁵

The concentration of oxygen in the anesthetic circuit must be measured. Anesthesia machine manufacturers place oxygen sensors on the inspired limb of the anesthesia circuit to detect and alarm in the event that hypoxic gas mixtures are delivered to the patient. Carbon dioxide may reduce the usable lifetime of a galvanic oxygen sensor, so it is preferable to place the oxygen sensor on the inspired limb. Oxygen monitors require a fast response time (2 to 10 seconds), accuracy (±2% of the actual level), and stability when exposed to humidity and inhalation agents.

The removable, external oxygen sensors seen commonly on anesthesia machines such as the Narkomed and Drager Fabius are of the galvanic type. These devices should be calibrated against room air (FiO₂ = 0.21) daily, and also after 8 hours of use. These devices may also infrequently require calibration against FiO₂ = 1. As part of the preoperative checkout of the anesthesia machine, the clinician must confirm that the alarm limits of the inspired oxygen analyzer are set appropriately to alert to the presence of hypoxic mixtures. Inspired oxygen alarms cannot be relied upon to detect disconnection of the circuit.

According to the ASA Standards for Basic Anesthesia Monitoring,¹ Standard 2.2.1 states “During every administration of general
anesthesia using an anesthesia machine, the concentration of oxygen in the patient breathing system shall be measured by an oxygen analyzer with a low oxygen concentration limit alarm in use.”

The careful monitoring of the inspired oxygen concentration is of particular significance during low-flow anesthesia, in which the anesthesiologist attempts to minimize the fresh gas flow to the amount of oxygen necessary to replace the patient’s metabolic utilization. The gas mixture within the breathing circuit may become hypoxic if insufficient fresh gas flow is supplied, even if the fresh gas flow itself comprises pure oxygen.

This requirement to monitor inspired oxygen concentration may be waived by the responsible anesthesiologist under extenuating circumstances. There are no clinical contraindications to monitoring inspired oxygen concentration.

Adequate inspiratory oxygen concentration does not guarantee adequate arterial oxygen concentration.⁶ Consequently, ASA¹ Standard 2.2.2 mandates additional monitoring for blood oxygenation, including the provision of adequate lighting and exposure to assess the patient’s color by direct observation.

Pulse oximeters measure pulse rate and estimate the oxygen saturation of hemoglobin (Hb; SpO₂) on a noninvasive, continuous basis.⁷ The oxygen saturation of hemoglobin (as a percentage) is related to the oxygen tension (as a partial pressure, mm Hg) by the oxyhemoglobin dissociation curve. On the steep part of the curve, a predictable correlation exists between arterial oxyhemoglobin saturation (SaO₂) and partial pressures of oxygen (PaO₂). In this range, the SaO₂ is a good reflection of the extent of hypoxemia and the changing status of arterial oxygenation. For PaO₂ greater than 75 mm Hg, the SaO₂ reaches a plateau and no longer reflects changes in PaO₂. Coexisting medical conditions, such as hypercapnia, acidosis, and hyperthermia, cause the oxyhemoglobin dissociation curve to shift to the right and decrease the affinity of hemoglobin for oxygen (Fig. 25-1). This favors the unloading of oxygen from hemoglobin to peripheral tissues.

Pulse oximetry is based on the following premises:

Oxygen saturation is determined by spectrophotometry, which is based on the Beer–Lambert law. At a constant light intensity and hemoglobin concentration, the intensity of light transmitted through a tissue is a logarithmic function of the oxygen saturation of Hb. Two wavelengths of light are required to distinguish HbO₂ from Hb. Light-emitting diodes in the pulse sensor emit red (660 nm) and near infrared (940 nm) light. The percentage of HbO₂ is determined by measuring the ratio of infrared and red light sensed by a photodetector. Pulse oximeters perform a plethysmographic analysis to differentiate the pulsatile “arterial” signal from the nonpulsatile signal resulting from “venous” absorption and other tissues such as skin, muscle, and bone. The absence of a pulsatile waveform during extreme hypothermia or hypoperfusion can limit the ability of a pulse oximeter to calculate the SpO₂.

The SpO₂ measured by pulse oximetry is not the same as the arterial saturation (SaO₂) measured by a laboratory co-oximeter. Pulse oximetry measures the “functional” saturation, which is defined by the following equation:

Laboratory co-oximeters use multiple wavelengths to distinguish other types of Hb, such as carboxyhemoglobin (COHb) and methemoglobin (MetHb) by their characteristic absorption. Co-oximeters measure the “fractional” saturation, which is defined by the following equation:

In clinical circumstances where other Hb moieties are present, the SpO₂ measurement may not correlate with the actual SaO₂ reported by the blood gas laboratory. For example, MetHb absorbs red and infrared wavelengths of light in a 1:1 ratio corresponding to an SpO₂ of approximately 85%. Therefore, increases in MetHb produce an underestimation when SpO₂ >70% and an overestimation when SpO₂ <70%. Similarly, COHb also produces artificially high and misleading results; one study showed that at 70% COHb, the SpO₂ still measured 90%. In most patients, MetHb and COHb are present in low concentrations so that the functional saturation approximates the fractional value.⁹

The assessment of arterial oxygenation is an integral part of anesthesia practice. Early detection and prompt intervention may limit serious sequelae of hypoxemia. The clinical signs associated
with hypoxemia (e.g., tachycardia, altered mental status, cyanosis) are often masked or difficult to appreciate during anesthesia.

The appropriate use of pulse oximetry necessitates an appreciation of both physiologic and technical limitations. Despite the numerous clinical benefits of pulse oximetry, other factors affect its accuracy and reliability. Factors that may be present during anesthesia care and that affect the accuracy and reliability of pulse oximetry include dyshemoglobins, dyes (methylene blue, indocyanine green, and indigo carmine), nail polish, ambient light, light-emitting diode variability, motion artifact, and background noise. Electrocautery can interfere with pulse oximetry if the radiofrequency emissions are sensed by the photodetector. Surgical stereotactic positioning systems that make use of infrared position sensors may interfere with the infrared signals used by the pulse oximeter. Reports of burns or pressure necrosis exist but are infrequent. Inspecting the digits during monitoring can reduce these complications.

Recent developments in pulse oximetry technology reportedly may permit more accurate measurements of SpO₂ during patient movement, low-perfusion conditions, and in the presence of dyshemoglobins. Some of these instruments use complex signal processing of the two wavelengths of light to improve the signal-to-noise ratio and reject artifact. Studies in volunteers suggest that the performance of pulse oximeters incorporating this technology is superior to conventional oximetry during motion of the hand, hypoperfusion, and hypothermia.¹⁰,¹¹ Other pulse oximetry devices incorporate eight wavelengths of light to more accurately measure COHb and MetHb.⁹

Pulse oximetry has been used in all patient age groups to detect and prevent hypoxemia. The clinical benefits of pulse oximetry are enhanced by its simplicity. Modern pulse oximeters are noninvasive, continuous, and autocalibrating. They have quick response times and their battery backup provides monitoring during transport. The clinical accuracy is typically reported to be within ±2% to 3% at 70% to 100% saturation and ±3% at 50% to 70% saturation. Published data from numerous investigations support accuracy and precision reported by instrument manufacturers. Quantitative assessment of arterial oxygen saturation is mandated by the ASA monitoring standards,¹ and the convenience and safety of pulse oximetry has supplanted earlier techniques such as heated transcutaneous pO₂ electrodes.¹²

Pulse oximetry has wide applicability in many hospital and nonhospital settings. However, there are no definitive data demonstrating a reduction in morbidity or mortality associated with the advent of pulse oximetry. An older large randomized trial did not detect a significant difference in postoperative complications when routine pulse oximetry was used.¹³ However, a reduction of anesthesia mortality, as well as fewer malpractice claims from respiratory events, coincident with the introduction of pulse oximeters suggests that the routine use of these devices may have been a contributing factor.

There are no clinical contraindications to monitoring arterial oxygen saturation with pulse oximetry.

Arterial oxygen monitors do not ensure adequacy of oxygen delivery to, or utilization by peripheral tissues and should not be considered a replacement for arterial blood gas measurements or mixed central venous oxygen saturation when more definitive information regarding oxygen supply and utilization is required.

Unless the patient is in breathing room air, pulse oximetry is a poor indicator of adequate ventilation; patients who have been breathing 100% FiO₂ may be apneic for several minutes before desaturation is detected by the pulse oximeter. Once the P_AO₂ has fallen sufficiently to cause a detectable decrease in SpO₂, further desaturation may occur precipitously once the steep part of the HbO₂ dissociation curve is reached.

The patient’s expired gas is likely to be composed of a mixture of oxygen (O₂), nitrogen (N₂), carbon dioxide (CO₂), and anesthetic gases such as nitrous oxide (N₂O) and highly potent halogenated agents (sevoflurane, isoflurane, or desflurane). Anesthesiologists have long sought to measure the composition of expired gases noninvasively and in real time; these measurements can provide vital information regarding the patient’s respiratory condition and assist in the titration of volatile anesthetic agents.

In 1972, White and Wardley-Smith¹⁴ described the construction of a simple detector of anesthetic gases. Anesthetic gases will tend to partition into silicone rubber, changing the elastic properties of the rubber. A mechanical device can be constructed to maintain silicone rubber strips under light tension; this device is attached to the expiratory limb of the anesthesia circuit so that the strips are exposed to the patient’s expired gases. As the concentration of anesthetic gas rises, the length of the strips will change proportionally, and a mechanical linkage to a pointer on a gauge can display this change. Although the practical shortcomings of such a device are obvious from a contemporary viewpoint, this is only evidence of how much biomedical engineering sophistication has been applied to this important problem in the intervening 40 years. For instance, the response time of this simple device would be expected to be very slow. However, if piezoelectric crystals are coated with a layer of lipid, the concentration of anesthetic gases can be detected by changes in the resonant frequency of the crystal as anesthetic gases partition into the lipid coating. This idea makes use of the same physical behavior of anesthetic gases, but evolved in its implementation so that the response time is now sufficient to detect inspired, expired, and breath-to-breath changes in anesthetic gas concentrations.¹⁵ Nevertheless, this new device is still significantly limited by its inability to identify the gas it is detecting and by its inability to work with mixtures of different gases.

Mass spectrometry systems and Raman scattering devices addressed these shortcomings. Mass spectroscopy systems obtain a sample of expired gas, and bombard this gas mixture with electrons, creating ion fragments of a predictable mass and charge. These fragments are accelerated in a vacuum. A sample of this mixture enters a measurement chamber, where the fragment stream is subjected to a high magnetic field. The magnetic field separates the fragments by their mass and charge. The fragments are deflected onto a detector plate, and each gas has a specific landing site on the detector plate. The ion impacts are proportional to the concentration of the parent gas or vapor. The processor section of the mass spectrometer system calculates the concentration of the gases of interest. Mass spectrometers are large devices and were often multiplexed—that is, connected to multiple patients, taking expired gas samples from each in turn.¹⁶

Raman scattering results when photons generated by a high-intensity argon laser collide with gas molecules. The scattered photons are measured as peaks in a spectrum that determine the concentration and composition of respiratory gases and inhaled vapors. O₂, N₂, N₂O, CO₂, H₂O vapors and inhaled anesthetic agents are all simultaneously measurable using Raman scattering.¹⁷

Infrared absorption spectrophotometry (IRAS) gas analysis devices have now supplanted all of these earlier measurement techniques in clinical practice. Asymmetric, polyatomic molecules like CO₂ absorb infrared light at specific wavelengths. By transmitting light through a pure sample of a gas over the range of infrared frequencies, a unique infrared transmission spectrum (like a fingerprint) can be created for the gas (Fig. 25-2).

CO₂ strongly absorbs infrared light with a wavelength of 4.3 microns. At this wavelength there is minimal interference from other gases that may also be present, such as water vapor, oxygen, nitrous oxide, and inhaled anesthetic agents. Infrared light at this wavelength can be passed through a sample of gas to an infrared detector. As the concentration of carbon dioxide increases, the intensity of the light that reaches the detector decreases, in accordance with the Beer–Lambert law.

IRAS devices have five components: A multiple-wavelength infrared light source, a gas sampler, an optical path, a detection system, and a signal processor. Operating room IRAS devices can detect CO₂, N₂O, and the potent inhaled anesthetic agents as mixed together in a sample of the patient’s expired gas. The gas mixture is passed through the optical path of multiple infrared beams whose wavelengths are chosen to correspond to key features in the transmission spectra of the gases of interest. By analyzing the combination of absorption of infrared light at these wavelengths, the presence and concentrations of all of these gases can be determined simultaneously.¹⁹ The use of multiple wavelengths allows the gases to be identified automatically. Older IRAS devices used a hot electrical element to generate radiant infrared light over a broad range of wavelengths. An optical filter wheel was then used to cut out all but the desired wavelengths. Contemporary devices make use of small lasers and filters, designed such that they emit only at the desired wavelengths. This approach consumes much less electrical power, is physically less heavy and has led to the development of conveniently portable handheld gas analyzers.²⁰

Expired gas analysis allows the clinician to monitor inspired and expired concentrations of carbon dioxide and anesthetic gases simultaneously. These measurements require separate clinical interpretation. Critical events that can be detected by the analysis of respiratory gases and anesthetic vapors are listed in Table 25-1.

Monitoring of the partial pressure of expiratory CO₂ has evolved into an important physiologic and safety monitor. Capnography
is the standard of care for monitoring the adequacy of ventilation in patients receiving general anesthesia. It is also now mandated for use to monitor ventilation during procedures performed while the patient is under moderate or deep sedation.¹

There are no contraindications to the use of capnography, provided that the data obtained are evaluated in the context of the patient’s clinical circumstances. It is generally safe to use capnography for the monitoring of all patients.

Monitoring of expired anesthetic gases is only informative if detectable gases are used to maintain anesthesia. Infrared spectroscopy cannot detect Xenon, and is not informative if anesthesia is maintained using a total intravenous technique.

The sampling lines or water traps of expired gas analyzers may become occluded with condensed water vapor during prolonged use. Disconnecting the sampling line and flushing it with air from a syringe can sometimes clear it, but it may be necessary to replace these components. Elevating the sidestream sampling line above the ventilator circuit helps prevent the entry of condensed water. A humidity barrier is also useful, although this will increase the response time of the capnogram.

Although mass spectroscopy and Raman scattering are no longer seen in clinical practice, these technologies are able to detect the concentration of N₂ directly. Nitrogen monitoring provides quantification of washout during preoxygenation. A sudden rise in N₂ in the exhaled gas indicates either introduction of air from leaks in the anesthesia delivery system or venous air embolism. Infrared gas analyzers do not detect N₂ directly, and its concentration must be inferred as the amount remaining after other measurable gases are accounted for.

Although capnography provides a quantitative measurement of ETCO₂, it is not as accurate as blood gas analysis for the assessment of the partial pressure of arterial carbon dioxide. A gradient exists between the partial pressure of arterial carbon dioxide and ETCO₂; this gradient increases as the dead-space volume increases. In disease states characterized by increased dead space and ventilation–perfusion mismatch, such as emphysema or pulmonary embolism, or in iatrogenic single lung ventilation, an arterial blood gas analysis is necessary for an accurate determination of the partial pressure of arterial carbon dioxide.

Indwelling arterial cannulation permits the opportunity to monitor arterial blood pressure continuously and to have vascular access for arterial blood sampling. Intra-arterial blood pressure monitoring uses fluid-filled tubing to transmit the force of the pressure pulse wave to a pressure transducer that converts the displacement of a silicon crystal into voltage changes. These electrical signals are amplified, filtered, and displayed as the arterial pressure trace. Intra-arterial pressure transducing systems are subject to many potential errors based on the physical properties of fluid motion and the performance of the catheter–transducer–amplification system used to sense, process, and display the pressure pulse wave.

The behavior of transducers, fluid couplings, signal amplification, and display systems can be described by a complex second-order differential equation. Solving the equation predicts the output and characterizes the fidelity of the system’s ability to faithfully display and estimate the arterial pressure over time. The fidelity of fluid-coupled transducing systems is constrained by two properties: Damping (ζ) and natural frequency (f_n). Zeta (ζ) describes the tendency for fluid in the measuring system to extinguish motion, and f_n describes the frequency at which the measuring system is most prone to resonance. The fidelity of the transduced pressure depends on optimizing ζ and f_n so that the system can respond appropriately to the range of frequencies contained in the pressure pulse wave. Analysis of high-fidelity recordings of arterial blood pressure indicates that the pressure trace contains frequencies from 1 to 30 Hz. The “fast flush” test is a method used at the bedside to determine the natural frequency and damping characteristics of the transducing system. This test examines the characteristics of the resonant waves recorded after the release of a flush. Damping is estimated by the amplitude ratio of the first pair of resonant waves and the natural frequency is estimated by dividing the tracing speed by the interval cycle.²⁶

Multiple arteries can be used for direct measurement of blood pressure, including the radial, brachial, axillary, femoral, and dorsalis pedis arteries (Table 25-3). The radial artery remains the most popular site for cannulation because of its accessibility and the presence of a collateral blood supply. In the past, assessment of the patency of the ulnar circulation by performance of an Allen test has been recommended before cannulation. An Allen test is performed by compressing both radial and ulnar arteries while
the patient tightens his or her fist. Releasing pressure on each respective artery determines the dominant vessel supplying blood to the hand. The prognostic value of the Allen test in assessing the adequacy of the collateral circulation has not been confirmed.²⁷,²⁸

Three techniques for cannulation are common: Direct arterial puncture, guidewire-assisted cannulation (Seldinger technique), and the transfixion–withdrawal method.²⁹ A necessary condition for percutaneous placement is identification of the arterial pulse, which may be enhanced by a Doppler flow detection device in patients with poor peripheral pulses.³⁰ Ultrasound imaging with Doppler color flowmetry can provide valuable further assistance when the pulse is difficult to locate or the caliber of the vessel appears to be small.³¹

Arterial blood pressure transduction systems must be “zeroed” before use. The transducer is positioned at the same level as the right atrium, the stopcock is opened to the atmosphere so that pressure-sensing crystal senses only atmospheric pressure, and the “Zero Sensor” (or equivalent) option is selected on the monitoring equipment. This procedure establishes the calibration of the sensor and establishes the level of the right atrium as the datum reference point. For neurosurgical procedures in which the patient may be positioned in an upright or beach-chair position, it is a common practice to zero the transducer at the level of the Circle of Willis so that the arterial pressure tracing provides a reading that is adjusted for the height of the fluid column between the heart and the brain; it represents the arterial pressure at the base of the brain.

Direct arterial pressure monitoring requires constant vigilance. The data displayed must correlate with clinical conditions before therapeutic interventions are initiated. Sudden increases in the transduced blood pressure may represent a hydrostatic error because the position of the transducer was not adjusted after change in the operating room table’s height. Sudden decreases often result from kinking of the catheter or tubing. Before initiating therapy, the transducer system should be examined quickly and the patency of the arterial cannula verified. This ensures the accuracy of the measurement and avoids the initiation of a potentially dangerous medication error.

Traumatic cannulation has been associated with hematoma formation, thrombosis, and damage to adjacent nerves. Abnormal radial artery blood flow after catheter removal occurs frequently. Studies suggest that blood flow normalizes in 3 to 70 days. Radial artery thrombosis can be minimized by using small catheters, avoiding polypropylene-tapered catheters, and reducing the duration of arterial cannulation. Flexible guidewires may reduce the potential trauma associated with catheters negotiating tortuous vessels. After arterial cannulation has been performed, the tissues that are perfused by that artery should be examined intermittently for signs of thromboembolism or ischemia. During cannula removal, the potential for thromboembolism may be diminished by compressing the proximal and distal arterial segment while aspirating the cannula during withdrawal.

The standards for basic monitoring¹ stipulate that arterial blood pressure shall be determined and recorded at least every 5 minutes. This standard is usually met by intermittent, noninvasive blood pressure monitoring. However, continuous monitoring may be indicated by patient comorbidities or by the nature of the surgery to be performed.

Arterial catheters provide continuous monitoring of blood pressure and convenient vascular access to obtain blood samples for laboratory assays, including blood gas analysis to assess respiratory function. Placement of an arterial catheter can therefore be indicated by the need to meet any of these contingencies:

Arterial cannulation is regarded as an invasive procedure with documented morbidity. Ischemia after radial artery cannulation resulting from thrombosis, proximal emboli, or prolonged shock has been described.³³ Contributing factors include severe atherosclerosis, diabetes, low CO, and intense peripheral vasoconstriction. Ischemia, hemorrhage, thrombosis, embolism, cerebral air embolism (retrograde flow associated with flushing), aneurysm formation, arteriovenous fistula formation, skin necrosis, and infection have reportedly occurred as the direct result of arterial cannulation, arterial blood sampling, or high-pressure flushing. Patients with compromised collateral arterial supply, such as those with Reynaud’s phenomenon or thromboangiitis obliterans (Buerger’s disease), are at increased risk for ischemic complications.²⁹