Chapter 6: Noncardiovascular Monitoring

The previous chapter reviewed routine hemodynamic monitoring used by anesthesiologists. This chapter examines the vast array of techniques and devices used perioperatively to monitor neuromuscular transmission, neurological condition, respiratory gas exchange, and body temperature.

PRECORDIAL & ESOPHAGEAL STETHOSCOPES

Indications

Prior to the routine availability of gas exchange monitors, anesthesiologists used a precordial or esophageal stethoscope to ensure that the lungs were being ventilated, to monitor for circuit disconnections, and to auscultate heart tones to confirm a beating heart. Although largely supplanted by other modalities, the finger on the pulse and auscultation remain frontline monitors, especially when technology fails. Chest auscultation remains the primary method to confirm bilateral lung ventilation in the operating room, even though detection of an end-tidal CO₂ waveform is definitive to exclude esophageal intubation.

Contraindications

Esophageal stethoscopes and esophageal temperature probes should be avoided in patients with esophageal varices or strictures.

Techniques & Complications

A precordial stethoscope (Wenger chestpiece) is a heavy, bell-shaped piece of metal placed over the chest or suprasternal notch. Although its weight tends to maintain its position, double-sided adhesive disks provide an acoustic seal to the patient’s skin. Various chestpieces are available, but the child size works well for most patients. The bell is connected to the anesthesiologist’s ear by extension tubing.

The esophageal stethoscope is a soft plastic catheter (8–24FR) with balloon-covered distal openings (Figure 6–1). Although the quality of breath and heart sounds is much better than with a precordial stethoscope, its use is limited to intubated patients. Temperature probes, electrocardiogram (ECG) leads, ultrasound probes, and even atrial pacemaker electrodes have been incorporated into esophageal stethoscopes. Placement through the mouth or nose can occasionally cause mucosal irritation and bleeding. Rarely, the stethoscope slides into the trachea instead of the esophagus, resulting in a gas leak around the tracheal tube cuff.

Clinical Considerations

The information provided by a precordial or esophageal stethoscope includes confirmation of ventilation, quality of breath sounds (eg, stridor, wheezing), regularity of heart rate, and quality of heart tones (muffled tones are associated with decreased cardiac output). The confirmation of bilateral breath sounds after tracheal intubation, however, is best made with a binaural stethoscope.

PULSE OXIMETRY

Indications & Contraindications

Pulse oximeters are mandatory monitors for any anesthetic, including cases of moderate sedation. There are no contraindications.

Techniques & Complications

Pulse oximeters combine the principles of oximetry and plethysmography to noninvasively measure oxygen saturation in arterial blood. A sensor containing light sources (two or three light-emitting diodes) and a light detector (a photodiode) is placed across a finger, toe, earlobe, or any other perfused tissue that can be transilluminated. When the light source and detector are opposite one another across the perfused tissue, transmittance oximetry is used. When the light source and detector are placed on the same side of the patient (eg, the forehead), the backscatter (reflectance) of light is recorded by the detector.

Oximetry depends on the observation that oxygenated and reduced hemoglobin differ in their absorption of red and infrared light (Lambert–Beer law). Specifically, oxyhemoglobin (HbO₂) absorbs more infrared light (940 nm), whereas deoxyhemoglobin absorbs more red light (660 nm) and thus appears blue, or cyanotic, to the naked eye. The change in light absorption during arterial pulsations is the basis of oximetric determinations (Figure 6–2). The ratio of the absorptions at the red and infrared wavelengths is analyzed by a microprocessor to provide the oxygen saturation (SpO₂) of arterial blood based on established norms. The greater the ratio of red to infrared absorption, the lower is the arterial hemoglobin oxygen saturation. Arterial pulsations are identified by plethysmography, allowing corrections for light absorption by nonpulsating venous blood and tissue. Heat from the light source or sensor pressure may, rarely, result in tissue damage if the monitor is not periodically moved. No user calibration is required.

Clinical Considerations

In addition to SpO₂, pulse oximeters provide an indication of tissue perfusion (pulse amplitude) and measure heart rate. Depending on a particular patient’s oxygen–hemoglobin dissociation curve, a 90% saturation may indicate a PaO₂ of less than 65 mm Hg. Clinically detectable cyanosis requires 5 g of desaturated hemoglobin and usually corresponds to SpO₂ of less than 80%. Mainstem bronchial intubation will usually go undetected by pulse oximetry in the absence of lung disease or low fraction of inspired oxygen concentrations (FiO₂).

Because carboxyhemoglobin (COHb) and HbO₂ absorb light at 660 nm similarly, pulse oximeters that compare only two wavelengths of light will register a falsely high reading in patients with carbon monoxide poisoning. Methemoglobin has the same absorption coefficient at both red and infrared wavelengths. The resulting 1:1 absorption ratio corresponds to a saturation reading of 85%. Thus, methemoglobinemia causes a falsely low saturation reading when SaO₂ is actually greater than 85% and a falsely high reading if SaO₂ is actually less than 85%.

Most pulse oximeters are inaccurate at low SpO₂, and all demonstrate a delay between changes in SaO₂ and SpO₂. Other causes of pulse oximetry artifact include excessive ambient light, motion, methylene blue dye, venous pulsations in a dependent limb, low perfusion (eg, low cardiac output, profound anemia, hypothermia, increased systemic vascular resistance), malpositioned sensor, and leakage of light from the light-emitting diode to the photodiode, bypassing the arterial bed (optical shunting). Nevertheless, pulse oximetry can be an invaluable aid to the rapid diagnosis of hypoxia, which may occur in unrecognized esophageal intubation, and it furthers the goal of monitoring oxygen delivery to vital organs. In the recovery room, pulse oximetry helps identify postoperative pulmonary problems, such as hypoventilation, bronchospasm, and atelectasis. Advanced examination of the photoplethysmographic waveform can aid assessment of volume responsiveness and vascular impedence in mechanically ventilated patients.

Two extensions of pulse oximetry technology are mixed venous blood oxygen saturation (SvO₂) and noninvasive brain oximetry. The former requires the placement of a pulmonary artery catheter containing fiberoptic sensors that continuously determine SvO₂ in a manner analogous to pulse oximetry. Because SvO₂/ScvO₂ varies with changes in hemoglobin concentration, cardiac output, arterial oxygen saturation, and whole-body oxygen consumption, its interpretation is somewhat complex. Impaired oxygen delivery to the tissues (reduced arterial oxygen saturation, reduced cardiac output, increased tissue oxygen demand) results in a lower venous oxygen saturation. Higher venous oxygen concentrations occur when tissue oxygen delivery exceeds tissue demand or arterial venous shunting occurs. Measurement of central vein oxygen saturation (ScvO₂) is frequently used as a surrogate for SvO₂. SvO₂ measures the saturation of venous blood returning from the upper body, the cardiac circulation, and the lower body. Normally the SvO₂ is approximately 70%. ScvO₂ is usually a few percentage points higher as it does not reflect venous return from the cardiac circulation.

Noninvasive brain oximetry monitors regional oxygen saturation (rSO₂) of hemoglobin in the brain. A sensor placed on the forehead emits light of specific wavelengths and measures the light reflected back to the sensor (near-infrared optical spectroscopy). Unlike pulse oximetry, brain oximetry measures venous and capillary blood oxygen saturation in addition to arterial blood saturation. Thus, its oxygen saturation readings represent the average oxygen saturation of all regional microvascular hemoglobin (approximately 70%). Cardiac arrest, cerebral embolization, and severe hypoxia cause a dramatic decrease in rSO₂. Cerebral oximetry is routinely employed in the perioperative management of patients undergoing cardiopulmonary bypass. (See the section “Neurological System Monitors.”)

CAPNOGRAPHY

Indications & Contraindications

Determination of end-tidal CO₂ (ETCO₂) concentration to confirm adequate ventilation is mandatory during all anesthetic procedures, but particularly so for general anesthesia. Increases in alveolar dead space ventilation (eg, pulmonary thromboembolism, venous air embolism, decreased pulmonary perfusion) produce a decrease in ETCO₂ compared with arterial CO₂ concentration (PaCO₂). Generally, ETCO₂ and PaCO₂ increase or decrease depending upon the balance of CO₂ production and CO₂ elimination (ventilation). A rapid fall of ETCO₂ is a sensitive indicator of air embolism, in which both an increase in deadspace ventilation and a decrease in cardiac output may occur. There are no contraindications.

Techniques & Complications

Capnography is a valuable monitor of the pulmonary, cardiovascular, and anesthetic breathing systems. Capnographs in common use rely on the absorption of infrared light by CO₂. As with oximetry, absorption of infrared light by CO₂ is governed by the Beer–Lambert law.

A. Nondiverting (Flowthrough)

Nondiverting (mainstream) capnographs measure CO₂ passing through an adaptor placed in the breathing circuit (Figure 6–3). Infrared light transmission through the gas is measured and CO₂ concentration is determined by the monitor. Because of problems with drift, older flowthrough models self-zeroed during inspiration. Thus, they were incapable of detecting inspired CO₂, such as would occur with a breathing circuit malfunction (eg, absorbent exhaustion, sticking unidirectional valves). The weight of the sensor caused traction on the tracheal tube, and its generation of radiant heat can cause skin burns. Newer designs avoid these problems.

B. Diverting (Aspiration)

Diverting (sidestream) capnographs continuously suction gas from the breathing circuit into a sample cell within the bedside monitor. CO₂ concentration is determined by comparing infrared light absorption in the sample cell with a chamber free of CO₂. Continuous aspiration of anesthetic gas essentially represents a leak in the breathing circuit that will contaminate the operating room unless it is scavenged or returned to the breathing system. High aspiration rates (up to 250 mL/min) and low-dead-space sampling tubing usually increase sensitivity and decrease lag time. If tidal volumes (VT) are small (eg, pediatric patients), however, a high rate of aspiration may entrain fresh gas from the circuit and dilute ETCO₂ measurement. Low aspiration rates (<50 mL/min) can retard ETCO₂ measurement and underestimate it during rapid ventilation. Diverting units are prone to water precipitation in the aspiration tube and sampling cell that can cause obstruction of the sampling line and erroneous readings. Expiratory valve malfunction is detected by the presence of CO₂ in inspired gas. Although inspiratory valve failure also results in rebreathing CO₂, this is not as readily apparent because part of the inspiratory volume will still be free of CO₂, causing the monitor to read zero during part of the inspiratory phase.

Clinical Considerations

Capnographs rapidly and reliably detect esophageal intubation—a cause of anesthetic catastrophe—but do not reliably detect mainstem bronchial intubation. Although there may be some CO₂ in the stomach from swallowed expired air, this should be washed out within a few breaths. Sudden cessation of CO₂ during the expiratory phase may indicate a circuit disconnection. The increased metabolic rate caused by malignant hyperthermia causes a marked rise in ETCO₂.

The gradient between PaCO₂ and ETCO₂ (normally 2–5 mm Hg) reflects alveolar dead space (alveoli that are ventilated but not perfused). Any significant reduction in lung perfusion (eg, air embolism, decreased cardiac output, or decreased blood pressure) increases alveolar dead space, dilutes expired CO₂, and lessens ETCO₂. True capnographs (as opposed to capnometers) display a waveform of CO₂ concentration that allows recognition of a variety of conditions (Figure 6–4).

ANESTHETIC GAS ANALYSIS

Indications

Analysis of anesthetic gases is essential during any procedure requiring inhalation anesthesia. There are no contraindications to analyzing these gases.

Techniques

Techniques for analyzing multiple anesthetic gases include mass spectrometry, Raman spectroscopy, infrared spectrophotometry, or piezoelectric crystal (quartz) oscillation. Most of these methods are primarily of historical interest, as most anesthetic gases are now measured by infrared absorption analysis.

Infrared units use a variety of techniques similar to that described for capnography. These devices are all based on the Beer–Lambert law, which provides a formula for measuring an unknown gas within inspired gas because the absorption of infrared light passing through a solvent (inspired or expired gas) is proportional to the amount of the unknown gas. Oxygen and nitrogen do not absorb infrared light. There are a number of commercially available devices that use a single- or dual-beam infrared light source and positive or negative filtering. Because oxygen molecules do not absorb infrared light, their concentration cannot be measured with monitors that rely on infrared technology and, hence, it must be measured by other means (see below).

Clinical Considerations

A. Oxygen Analysis

To measure the FiO₂ of inhaled gas, manufacturers of anesthesia machines have relied on various technologies.

B. Galvanic Cell

Galvanic cell (fuel cell) contains a lead anode and gold cathode bathed in potassium chloride. At the gold terminal, hydroxyl ions are formed that react with the lead electrode (thereby gradually consuming it) to produce lead oxide, causing current, which is proportional to the amount of oxygen being measured, to flow. Because the lead electrode is consumed, monitor life can be prolonged by exposing it to room air when not in use. These are the oxygen monitors used on many anesthesia machines in the inspiratory limb.

C. Paramagnetic Analysis

Oxygen is a nonpolar gas, but it is paramagnetic, and when placed in a magnetic field, the gas will expand, contracting when the magnet is turned off. By switching the field on and off and comparing the resulting change in volume (or pressure or flow) to a known standard, the amount of oxygen can be measured.

D. Polarographic Electrode

A polarographic electrode has a gold (or platinum) cathode and a silver anode, both bathed in an electrolyte, separated from the gas to be measured by a semipermeable membrane. Unlike the galvanic cell, a polarographic electrode works only if a small voltage is applied to two electrodes. When voltage is applied to the cathode, electrons combine with oxygen to form hydroxide ions. The amount of current that flows between the anode and the cathode is proportional to the amount of oxygen present.

E. Spirometry and Pressure Measurements

Contemporary anesthesia machines measure airway pressures, volume, and flow to calculate resistance and compliance. Measurements of flow and volume are made by mechanical devices that are usually fairly lightweight and are often placed in the inspiratory limb of the anesthesia circuit.

The most fundamental detected abnormalities include low peak inspiratory pressure and high peak inspiratory pressure, which indicate either a ventilator or circuit disconnect, or an airway obstruction, respectively. By measuring VT and breathing frequency (f), exhaled minute ventilation (VE) can be calculated, providing some sense of security that ventilation requirements are being met.

Spirometric loops are usually displayed as flow versus volume and volume versus pressure (Figure 6–5). There are characteristic changes with obstruction, bronchial intubation, reactive airways disease, and so forth. If a normal loop is observed shortly after induction of anesthesia and a subsequent loop is different, the observant anesthesiologist is alerted to the fact that pulmonary or airway compliance, or both, may have changed. Mechanical ventilation and ventilators are discussed more completely in Chapter 57.

ELECTROENCEPHALOGRAPHY

Indications & Contraindications

The electroencephalogram (EEG) is occasionally used during cerebrovascular surgery to confirm the adequacy of cerebral oxygenation or during cardiovascular surgery to ensure that burst suppression or an isoelectric signal has been obtained before circulatory arrest. A full 16-lead, 8-channel EEG is not necessary for these tasks, and simpler systems are available. There are no contraindications.

Techniques & Complications

The EEG is a recording of electrical potentials generated by cells in the cerebral cortex. Although standard ECG electrodes can be used, silver disks containing a conductive gel are preferred. Platinum or stainless steel needle electrodes traumatize the scalp and have high impedance (resistance); however, they can be sterilized and placed in a surgical field. Electrode position (montage) is governed by the international 10–20 system (Figure 6–6). Electric potential differences between combinations of electrodes are filtered, amplified, and displayed by an oscilloscope or pen recorder. EEG activity occurs mostly at frequencies between 1 and 30 cycles/sec (Hz). Alpha waves have a frequency of 8 to 13 Hz and are found often in a resting adult with eyes closed. Beta waves at 8 to 13 Hz are found in concentrating individuals, and at times, in individuals under anesthesia. Delta waves have a frequency of 0.5 to 4 Hz and are found in brain injury, deep sleep, and anesthesia. Theta waves (4–7 Hz) are also found in sleeping individuals and during anesthesia. EEG waves are also characterized by their amplitude, which is related to their potential (high amplitude, >50 microV; medium amplitude, 20–50 microV; and low amplitude, <20 microV). Lastly, the EEG is examined for symmetry between the left and right hemispheres.

Examination of a multichannel EEG is at times performed intraoperatively to detect areas of cerebral ischemia, such as during carotid endarterectomy. Likewise, it can be used to detect EEG isoelectricity and maximal cerebral protection during hypothermic arrest. The strip chart EEG is cumbersome in the operating room, and often the EEG is processed using power spectral analysis. Frequency analysis divides the EEG into a series of sine waves at different frequencies and then plots the power of the signal at each frequency, allowing for a presentation of EEG activity in a more easily interpreted form than the raw EEG (Figure 6–7).

As inhalational anesthesia progressively deepens, initial beta activation is followed by slowing, burst suppression, and isoelectricity. Intravenous agents, depending on dose and drug used, can produce a variety of EEG patterns.

To reduce the incidence of awareness during general anesthesia, devices have been developed in recent years that process two-channel EEG signals and create a dimensionless variable to indicate level of wakefulness. The bispectral index (BIS) is most commonly used in this regard. BIS monitors examine four components within the EEG that are associated with the anesthetic state: (1) low frequency, as found during deep anesthesia; (2) high-frequency beta activation found during “light” anesthesia; (3) suppressed EEG waves; and (4) burst suppression.

Other devices attempt to include measures of spontaneous muscle activity, as influenced by the activity of subcortical structures not contributing to the EEG to further provide an assessment of anesthetic depth. Various devices, each with its own algorithm to process the EEG or incorporate other variables to ascertain patient wakefulness, may become available in the future (Table 6–1).

Controversy still exists as to the exact role of processed EEG devices in assessing anesthetic depth. Some studies have demonstrated a reduced incidence of awareness when these devices were used, whereas other studies have failed to reveal any advantage over the use of end-tidal inhalational gas measurements to ensure an adequate concentration of volatile anesthetic. Because individual EEG responsiveness to anesthetic agents and level of surgical stimulus are variable, EEG monitoring to assess anesthesia depth or titrate anesthetic delivery may not always ensure absence of wakefulness. Moreover, many monitors have a delay, which might only indicate a risk for the patient being aware after he or she had already become conscious (Table 6–2).

Clinical Considerations

To perform a bispectral analysis, data measured by EEG are taken through a number of steps (Figure 6–8) to calculate a single number that correlates with depth of anesthesia/hypnosis.

BIS values of 65 to 85 have been advocated as a measure of sedation, whereas values of 40 to 65 have been recommended for general anesthesia (Figure 6–9). Bispectral analysis may reduce patient awareness during anesthesia, an issue that is important to the public.

Many of the initial studies of BIS were not prospective, randomized, controlled trials, but were observational in nature. The monitor costs several thousand dollars and the single-use electrodes cost approximately $10 to $15 per anesthetic. Some patients with awareness have been identified as having a BIS of less than 65. However, in other patients with awareness, either there were problems with the recordings, or awareness could not be related to any specific time or BIS value.

Detection of awareness often can minimize its consequences. Use of the Brice questions during postoperative visits can identify a potential awareness event. Ask patients to recall the following:

• What do you remember before going to sleep?

• What do you remember right when awakening?

• Do you remember anything in between going to sleep and awakening?

• Did you have any dreams while asleep?

Close follow-up and involvement of mental health experts may avoid the traumatic stress that can be associated with awareness events. Increasingly, patients are managed with regional anesthesia and propofol sedation. Patients undergoing such anesthetics should be made aware that they are not having general anesthesia and might recall perioperative events. Clarification of the techniques used may prevent patients so managed from the belief that they “were awake” during anesthesia.

Can BIS predict outcomes? Some investigators have suggested that both hospital stay and mortality are increased in patients experiencing the so called “triple low” of low mean arterial blood pressure, low bispectral index score, and low minimum alveolar concentration of volatile anesthetics. Other investigations have failed to identify such an association.

EVOKED POTENTIALS

Indications

Indications for intraoperative monitoring of evoked potentials (EPs) include surgical procedures associated with possible neurological injury: spinal fusion with instrumentation, spine and spinal cord tumor resection, brachial plexus repair, thoracoabdominal aortic aneurysm repair, epilepsy surgery, and cerebral tumor resection. Ischemia in the spinal cord or cerebral cortex can be detected by EPs. Auditory EPs have also been used to assess the effects of general anesthesia on the brain. The middle latency auditory EP may be a more sensitive indicator than BIS in regard to anesthetic depth. The amplitude and latency of this signal following an auditory stimulus is influenced by anesthetics.

Contraindications

Although there are no specific contraindications for somatosensory evoked potentials (SEPs), this modality is severely limited by the availability of monitoring sites, equipment, and trained personnel. Sensitivity to anesthetic agents can also be a limiting factor, particularly in children. Motor evoked potentials (MEPs) are contraindicated in patients with retained intracranial metal, skull defects, or implantable devices, as well as after seizures and any major cerebral insult. Brain injury secondary to repetitive stimulation of the cortex and inducement of seizures is a concern with MEPs.

Techniques & Complications

EP monitoring noninvasively assesses neural function by measuring electrophysiological responses to sensory or motor pathway stimulation. Commonly monitored EPs are brainstem auditory evoked responses (BAERs), SEPs, and increasingly, MEPs (Figure 6–10).

For SEPs, a brief electrical current is delivered to a sensory or mixed peripheral nerve by a pair of electrodes. If the intervening pathway is intact, a nerve action potential will be transmitted to the contralateral sensory cortex to produce an EP. This potential can be measured by cortical surface electrodes, but is usually measured by scalp electrodes. To distinguish the cortical response to a specific stimulus, multiple responses are averaged and background noise is eliminated. EPs are represented by a plot of voltage versus time. The resulting waveforms are analyzed for their poststimulus latency (the time between stimulation and potential detection) and peak amplitude. These are compared with baseline tracings. Technical and physiological causes of a change in an EP must be distinguished from changes due to neural damage. Complications of EP monitoring are rare, but include skin irritation and pressure ischemia at the sites of electrode application.

Clinical Considerations

EPs are altered by many variables other than neural damage. The effect of anesthetics is complex and not easily summarized. In general, intravenous anesthetic techniques (with or without nitrous oxide) cause minimal changes, whereas volatile agents (halothane, sevoflurane, desflurane, and isoflurane) are best avoided or used at a constant low concentration. Early-occurring (specific) EPs are less affected by anesthetics than are late-occurring (nonspecific) responses. Changes in BAERs may provide a measure of the depth of anesthesia. Physiological (eg, blood pressure, temperature, and oxygen saturation) and pharmacological factors should be kept as constant as possible.

Persistent obliteration of EPs is predictive of postoperative neurological deficit. Although SEPs usually identify spinal cord damage, because of their different anatomic pathways, sensory (dorsal spinal cord) EP preservation does not guarantee normal motor (ventral spinal cord) function (false negative). Furthermore, SEPs elicited from posterior tibial nerve stimulation cannot distinguish between peripheral and central ischemia (false positive). Techniques that elicit MEPs by using transcranial magnetic or electrical stimulation of the cortex allow the detection of action potentials in the muscles if the neural pathway is intact. The advantage of using MEPs as opposed to SEPs for spinal cord monitoring is that MEPs monitor the ventral spinal cord, and if sensitive and specific enough, can be used to indicate which patients might develop a postoperative motor deficit. MEPs are more sensitive to spinal cord ischemia than are SEPs. The same considerations for SEPs are applicable to MEPs in that they are reduced in amplitude by volatile inhalational agents, high-dose benzodiazepines, and moderate hypothermia (temperatures <32°C). MEPs require monitoring of the level of neuromuscular blockade. Close communication with a neurophysiologist or monitoring technician is essential prior to the start of any case where these monitors are used to review the optimal anesthetic technique to ensure monitoring integrity.

CEREBRAL OXIMETRY AND OTHER MONITORS OF THE BRAIN

Cerebral oximetry uses near-infrared spectroscopy (NIRS). Using reflectance spectroscopy near-infrared light is emitted by a probe on the scalp (Figure 6–11). Receptors are likewise positioned to detect the reflected light from both deep and superficial structures. As with pulse oximetry, oxygenated and deoxygenated hemoglobin absorb light at different frequencies. Likewise, cytochrome absorbs infrared light in the mitochondria. The NIRS saturation largely reflects the absorption of venous hemoglobin, as it does not have the ability to identify the pulsatile arterial component. Regional saturations of less than 40% on NIRS measures, or changes of greater than 25% of baseline measures, may herald neurological events secondary to decreased cerebral oxygenation.

Reduced jugular venous bulb saturation can also provide an indication of increased cerebral tissue oxygen extraction or decreased cerebral oxygen delivery. Direct tissue oxygen monitoring of the brain is accomplished by placement of a probe to determine the oxygen tension in the brain tissue. In addition to maintaining a cerebral perfusion pressure that is greater than 60 mm Hg and an intracranial pressure that is less than 20 mm Hg, neuroanesthesiologists/intensivists attempt to preserve brain tissue oxygenation by intervening when oxygen tissue tension is less than 20 mm Hg. Such interventions center upon improving oxygen delivery by increasing FiO₂, augmenting hemoglobin, adjusting cardiac output, decreasing oxygen demand, or a combination of these methods.

TEMPERATURE

Indications

The temperature of patients undergoing anesthesia should be monitored during use of all but the shortest anesthetics. Postoperative temperature is increasingly used as a quality anesthesia indicator. Hypothermia is associated with delayed drug metabolism, increased blood glucose, vasoconstriction, impaired coagulation, postoperative shivering accompanied by tachycardia and hypertension, and increased risk of surgical site infections. Hyperthermia can likewise have deleterious effects perioperatively, leading to tachycardia, vasodilation, and neurological injury. Consequently, temperature must be measured and recorded perioperatively.

Contraindications

There are no contraindications, although a particular monitoring site may be unsuitable in certain patients.

Techniques & Complications

Intraoperatively, temperature is usually measured using a thermistor or thermocouple. Thermistors are semiconductors whose resistance decreases predictably with warming. A thermocouple is a circuit of two dissimilar metals joined so that a potential difference is generated when the metals are at different temperatures. Disposable thermocouple and thermistor probes are available for monitoring the temperature of the tympanic membrane, nasopharynx, esophagus, bladder, rectum, and skin. Infrared sensors estimate temperature from the infrared energy that is produced. Tympanic membrane temperatures reflect core body temperature; however, the devices used may not reliably measure the temperature at the tympanic membrane. Complications of temperature monitoring are usually related to trauma caused by the probe (eg, rectal or tympanic membrane perforation).

Clinical Considerations

Each monitoring site has advantages and disadvantages. The tympanic membrane theoretically reflects brain temperature because the auditory canal’s blood supply is the external carotid artery. Trauma during insertion and cerumen insulation detract from the routine use of tympanic probes. Rectal temperature probes have a slow response to changes in core temperature. Nasopharyngeal probes are prone to cause epistaxis, but accurately measure core temperature if placed adjacent to the nasopharyngeal mucosa. The thermistor in a pulmonary artery catheter also measures core temperature. There is a variable correlation between axillary temperature and core temperature, depending on skin perfusion. Liquid crystal adhesive strips placed on the skin are inadequate indicators of core body temperature during surgery. Esophageal temperature sensors, often incorporated into esophageal stethoscopes, provide the best combination of economy, performance, and safety. To avoid measuring the temperature of tracheal gases, the temperature sensor should be positioned behind the heart in the lower third of the esophagus. Conveniently, heart sounds are most prominent at this location. For more on the clinical considerations of temperature control, see Chapter 52.

URINARY OUTPUT

Indications

Urinary bladder catheterization is the most reliable method of monitoring urinary output. Catheterization is routine in some complex and prolonged surgical procedures such as cardiac surgery, aortic or renal vascular surgery, craniotomy, major abdominal surgery, or procedures in which large fluid shifts are expected. Lengthy surgeries and intraoperative diuretic administration are other possible indications. Occasionally, postoperative bladder catheterization is indicated in patients who have difficulty voiding in the recovery room after general or regional anesthesia.

Contraindications

Foley catheters should be removed as soon as feasible to avoid the risk of catheter-associated urinary tract infections.

Techniques & Complications

Bladder catheterization is usually performed by surgical or nursing personnel. The urologist may be needed to catheterize patients with strictures and other abnormal urethral anatomy. A soft rubber Foley catheter is inserted into the bladder transurethrally and connected to a disposable calibrated collection chamber. To avoid urine reflux and minimize the risk of infection, the collection chamber should remain at a level below the bladder. Complications of catheterization include urethral trauma and urinary tract infections. Rapid decompression of a distended bladder can cause hypotension. Suprapubic drainage of the bladder with tubing inserted through a large-bore needle is an uncommon alternative.

Clinical Considerations

An additional advantage of placing a Foley catheter is the ability to include a thermistor in the catheter tip so that bladder temperature can be monitored. As long as urinary output is high, bladder temperature accurately reflects core temperature. An added value with more widespread use of urometers is the ability to electronically monitor and record urinary output and temperature.

Urinary output is an imperfect reflection of kidney perfusion and function and of renal, cardiovascular, and fluid volume status. Noninvasive monitors of cardiac function and output (including echocardiography) provide more reliable assessments of the adequacy of intravascular volume. Inadequate urinary output (oliguria) is often arbitrarily defined as urinary output of less than 0.5 mL/kg/h, but actually is a function of the patient’s concentrating ability and osmotic load. Urine electrolyte composition, osmolality, and specific gravity aid in the differential diagnosis of oliguria.

PERIPHERAL NERVE STIMULATION

Indications

Because of the variation in patient sensitivity to neuromuscular blocking agents, the neuromuscular function of all patients receiving intermediate- or long-acting neuromuscular blocking agents should be monitored. In addition, peripheral nerve stimulation is helpful in detecting the onset of paralysis during anesthesia inductions or the adequacy of block during continuous infusions with short-acting agents.

Contraindications

There are no contraindications to neuromuscular monitoring, although certain sites may be precluded by the surgical procedure. Additionally, atrophied muscles in areas of hemiplegia or nerve damage may appear refractory to neuromuscular blockade secondary to the proliferation of receptors. Determining the degree of neuromuscular blockade using such an extremity could lead to potential overdosing of competitive neuromuscular blocking agents.

Techniques & Complications

A peripheral nerve stimulator delivers current (60–80 mA) to a pair of either ECG silver chloride pads or subcutaneous needles placed over a peripheral motor nerve. The evoked mechanical or electrical response of the innervated muscle is observed. Although electromyography provides a fast, accurate, and quantitative measure of neuromuscular transmission, visual or tactile observation of muscle contraction is usually relied upon in clinical practice. Ulnar nerve stimulation of the adductor pollicis muscle and facial nerve stimulation of the orbicularis oculi are most commonly monitored (Figure 6–12). Because it is the inhibition of the neuromuscular receptor that needs to be monitored, direct stimulation of muscle should be avoided by placing electrodes over the course of the nerve and not over the muscle itself. To deliver a supramaximal stimulation to the underlying nerve, peripheral nerve stimulators must be capable of generating at least a 50-mA current across a 1000-Ω load. This current is uncomfortable for a conscious patient. Complications of nerve stimulation are limited to skin irritation and abrasion at the site of electrode attachment.

Because of concerns of residual neuromuscular blockade, increased attention has been focused on providing quantitative measures of the degree of neuromuscular blockade perioperatively. Acceleromyography uses a piezoelectric transducer on the muscle to be stimulated. Movement of the muscle generates an electrical current that can be quantified and displayed. Indeed, acceleromyography can better predict residual paralysis, compared with routine tactile train-of-four monitoring used in most operating rooms, if calibrated from the beginning of the operative period to establish baselines prior to administration of neuromuscular blocking agents.

Clinical Considerations

The degree of neuromuscular blockade is monitored by applying various patterns of electrical stimulation (Figure 6–13). All stimuli are 200 μs in duration and of square-wave pattern and equal current intensity. A twitch is a single pulse that is delivered from every 1 to every 10 s (1–0.1 Hz). Increasing block results in decreased evoked response to stimulation.

Train-of-four stimulation denotes four successive 200-μs stimuli in 2 s (2 Hz). The twitches in a train-of-four pattern progressively fade as nondepolarizing muscle relaxant block increases. The ratio of the responses to the first and fourth twitches is a sensitive indicator of nondepolarizing muscle paralysis. Because it is difficult to estimate the train-of-four ratio, it is more convenient to visually observe the sequential disappearance of the twitches, as this also correlates with the extent of blockade. Disappearance of the fourth twitch represents a 75% block, the third twitch an 80% block, and the second twitch a 90% block. Clinical relaxation usually requires 75% to 95% neuromuscular blockade.

Tetany at 50 or 100 Hz is a sensitive test of neuromuscular function. Sustained contraction for 5 s indicates adequate, but not necessarily complete, reversal from neuromuscular blockade. Double-burst stimulation (DBS) represents two variations of tetany that are less painful to the patient. The DBS_3,3 pattern of nerve stimulation consists of three short (200-μs) high-frequency bursts separated by 20 ms intervals (50 Hz) followed 750 ms later by another three bursts. DBS_3,2 consists of three 200-μs impulses at 50 Hz followed 750 ms later by two such impulses. DBS is more sensitive than train-of-four stimulation for the clinical (ie, visual) evaluation of fade.

Because muscle groups differ in their sensitivity to neuromuscular blocking agents, use of the peripheral nerve stimulator cannot replace direct observation of the muscles (eg, the diaphragm) that need to be relaxed for a specific surgical procedure. Furthermore, recovery of adductor pollicis function does not exactly parallel recovery of muscles required to maintain an airway. The diaphragm, rectus abdominis, laryngeal adductors, and orbicularis oculi muscles recover from neuromuscular blockade sooner than the adductor pollicis. Other indicators of adequate recovery include sustained (≥5 s) head lift, the ability to generate an inspiratory pressure of at least –25 cm H₂O, and a forceful hand grip. Twitch tension is reduced by hypothermia of the monitored muscle group (6%/°C). Decisions regarding adequacy of reversal of neuromuscular blockade, as well as timing of extubation, should be made only by considering both the patient’s clinical presentation and assessments determined by peripheral nerve stimulation. Postoperative residual paralysis remains a problem in postanesthesia care, producing potentially injurious airway and respiratory function compromise and increasing length of stay and cost in the postanesthesia care unit (PACU). Reversal of neuromuscular blocking agents is warranted, as is the use of intermediate acting neuromuscular blocking agents instead of longer acting drugs. Quantitative monitors of neuromuscular blockade are recommended to reduce the incidence of patients admitted to the PACU with residual paralysis.

GUIDELINES