Morgan & Mikhail Chap 6 (Noncardiovascular Monitoring) Flashcards
Key Concept 1: Capnographs rapidly and reliably detect esophageal intubation
Capnographs rapidly and reliably detect esophageal intubation—a cause of anesthetic catastrophe—but do not reliably detect mainstem bronchial intubation.
Key Concept 2: Postoperative residual paralysis remains a problem in postanesthesia care…
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).
Respiratory monitoring before gas exchange monitors
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
Pulse oximeters are mandatory monitors for any anesthetic, including cases of moderate sedation. There are no contraindications.
Oximetry depends on the observation that oxygenated and reduced hemoglobin differ in their absorption of red and infrared light (Lambert–Beer law)
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 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 (Fio2) concentrations.
Carboxyhemoglobin (COHb) and HbO2
Because carboxyhemoglobin (COHb) and Hbo2 absorb light at 660 nm, 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%.
Other causes of Pulse Ox inaccuracy
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), a malpositioned sensor, and leakage of light from the light-emitting diode to the photodiode, bypassing the
arterial bed (optical shunting).
Capnography
Determination of end-tidal CO2 (ETco2) concentration to confirm adequate ventilation is mandatory during all anesthetic procedures. 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 dead space ventilation and a decrease in cardiac output may occur. Capnography is also used to gauge the success of ongoing resuscitation, where improvements in perfusion will be heralded by increases in end-tidal CO2. There are no contraindications.
Clinical Considerations of Capnography
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.
Capnographs display a waveform of CO2 concentration that allows recognition of a variety of conditions (Figure 6–3).
Anesthetic Gas Analysis
Analysis of anesthetic gases is essential during any procedure requiring inhalation anesthesia. There are no contraindications to analyzing these gases.
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
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.
Electroencephalography
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.
EEG Wave Types
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 often found in a resting adult with the 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, seizure disorders, 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.
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.
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 the 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
Evoked Potentials
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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 (in some cases) 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.
In general, intravenous anesthetic techniques
(with or without nitrous oxide) cause minimal changes, whereas volatile agents (sevoflurane, desflurane, and isoflurane) are best avoided or used at a constant low concentration.
Persistent obliteration of EPs is predictive of postoperative neurological deficit.
Cerebral Oximetry
Cerebral oximetry uses near-infrared spectroscopy (NIRS). Near-infrared light is emitted by a probe on the scalp (Figure 6–10). Receptors are positioned to detect the reflected light from both deep and superficial structures. As with pulse oximetry, oxygenated hemoglobin 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
Temperature
The temperature of patients undergoing anesthesia should be monitored during all but the shortest anesthetics.
Postoperative temperature is increasingly used as a
measurement of anesthesia quality.
Hypothermia is associated with delayed drug metabolism, hyperglycemia, vasoconstriction, impaired coagulation, postoperative shivering accompanied by tachycardia and hypertension, and increased risk of surgical site infections.
Hyperthermia can lead to tachycardia, vasodilation, and neurological injury.
Consequently, temperature must be measured and recorded perioperatively.
Urinary Output
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.
Peripheral Nerve Stimulation
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 must be monitored. In addition, peripheral nerve stimulation is helpful in detecting the onset of paralysis during anesthesia inductions or the adequacy of the block during continuous infusions with short-acting agents.
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 6–11).
Peripheral Nerve Stimulation Types of Stimulation
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.
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.
