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Critical Monitoring Issues for Non–Operating Room Anesthesia
The rise in the number of nonsurgical diagnostic and therapeutic interventions is driving the surge in volume of remote anesthesia and sedation services. These procedures have simultaneously become increasingly complex as a result of technological advances and sicker patient cohorts. Complicating matters further, although these patients remain at high risk for anesthesia-related complications, they may be scheduled for such procedures precisely because they are poor surgical candidates. Settings requiring non–operating room anesthesia (NORA) include the interventional radiology department, cardiac catheterization laboratory, endoscopy suite, emergency room, magnetic resonance imaging (MRI) suite, and hyperbaric oxygen chamber. The primary logistical challenges that abound in NORA include issues related to location, personnel, equipment, power supply, patient accessibility, and preprocedural screening.
One critical consequence of NORA expansion is a shortage of qualified anesthesiologist providers. This resource limitation has resulted in increased numbers of nonanesthesiologists providing sedation. The American Society of Anesthesiologists (ASA) categorizes sedation into four distinct categories: minimal sedation, moderate sedation, deep sedation, and general anesthesia ( Table 6-1 ). Monitored anesthesia care (MAC) is another term commonly encountered during sedation. MAC does not describe a particular level of sedation but rather the delivery of anesthesia care during a procedure. This may involve the administration of sedation, local anesthesia, or no anesthesia at all, but the provider remains responsible for the monitoring and medical care of the patient.
Table 6-1
Continuum of Depth of Sedation: Definition of General Anesthesia and Levels of Sedation/Analgesia
From American Society of Anesthesiologists. Continuum of depth of sedation: definition of general anesthesia and levels of sedation/analgesia—standards, guidelines and statements. Park Ridge, Ill: American Society of Anesthesiologists; 2009.
| Minimal Sedation (Anxiolysis) | Moderate Sedation/Analgesia (“Conscious Sedation”) | Deep Sedation/Analgesia | General Anesthesia | |
|---|---|---|---|---|
| Responsiveness | Normal response to verbal stimulation | Purposeful response to verbal or tactile stimulation | Purposeful response following repeated or painful stimulation | unarousable even with painful stimulation |
| Airway | Unaffected | No intervention required | Intervention may be required | Intervention often required |
| Spontaneous ventilation | Unaffected | Adequate | May be inadequate | Frequently inadequate |
| Cardiovascular function | Unaffected | Usually maintained | Usually maintained | May be impaired |
According to ASA mandate, general anesthesia is provided only by anesthesia professionals such as anesthesiologists, certified nurse anesthetists, and anesthesia assistants. However, the Centers for Medicare and Medicaid Services permits nonanesthesiologists to provide minimal, moderate, or deep sedation. This group of providers includes physicians, dentists, oral surgeons, and podiatrists. Great variation exists in these heterogeneous groups in their training, basic skills, and practice patterns, resulting in inconsistent care delivery.
In an effort to reduce anesthesia-related complications and improve outcomes, the ASA performed multiple closed claims analyses seeking to identify the root causes of adverse events during anesthesia. Initial analyses were focused on operating room–based general anesthetics. These sentinel studies revealed that the most adverse events were respiratory and frequently preventable with improved monitoring, specifically with the use of continuous pulse oximetry and capnography. Consequently, these monitors are now considered standard for oprerating room–based anesthetics and are nearly universally employed.
Subsequent analyses focusing on NORA have yielded similar results. The majority of complications in these studies were respiratory events. However, morbidity related to hypoventilation, hypoxemia, and hypothermia occurred at increased rates during NORA. Severity of injury and overall mortality also increased during remote location procedures. The endoscopy suite and cardiac catheterization laboratory were the most commonly identified locations. MAC was the most frequently documented anesthetic technique. Risk factors for complications during NORA included extremes of age, ASA III and IV physical health status ( Table 6-2 ), obesity, and emergency procedures. The quality of anesthesia care was more likely to be substandard in NORA claims in contrast to operating room–based claims, thus resulting in a call for improved monitoring and standardization of minimum monitoring requirements. The ASA now recommends adherence to the same basic monitoring standards for anesthesia used in both operating room and non–operating room procedures.
Table 6-2
American Society of Anesthesiologists Physical Status Classification System
Modified from American Society of Anesthesiologists. Physical Status Classification System. Park Ridge, Ill: American Society of Anesthesiologists.
| ASA 1 Healthy |
| ASA 2 Mild systemic disease |
| ASA 3 Severe systemic disease |
| ASA 4 Severe systemic disease that is a constant threat to life |
| ASA 5 Moribund and not expected to survive |
| ASA 6 Brain dead; organ donor |
ASA, American Society of Anesthesiologists.
Nonanesthesiologist investigations by emergency room physicians, radiologists, and gastroenterologists similarly underscore the importance of minimum standard monitoring. Improved monitoring of respiratory function, specifically pulse oximetry and capnography, facilitates the detection of respiratory insufficiency. For providers lacking formal anesthesia training, standard monitoring is essential to mitigate delays in the recognition of respiratory or cardiovascular insufficiency and ensure safe care delivery.
Despite the development of monitoring guidelines and overwhelming evidence of the benefits of basic monitors, uniform implementation of monitoring standards for NORA has not occurred. Surveys of sedation providers demonstrate inconsistent application of basic monitors. To effectively improve patient safety during NORA, it is imperative that all providers understand and consistently adhere to basic monitoring standards.
Basic Monitors
The ASA recommends that minimum standard monitors employed for anesthesia should enable an assessment of oxygenation, ventilation, and circulation. No distinction is made for type of anesthesia or location of anesthetic delivery. For healthy patients (ASA I to II physical health status) undergoing uncomplicated procedures, this should include continuous monitoring of oxygenation with pulse oximetry, respiratory rate and ventilation with capnography, and cardiac monitoring with electrocardiography. Noninvasive blood pressure measurements should be performed at least every 5 minutes. Measurement of patient body temperature is also recommended. Decisions regarding advanced invasive and noninvasive monitoring should be made on a case-to-case basis depending on patient health status and procedural complexity. Monitor alarm limits must be adjusted to age-appropriate vital sign thresholds and be clearly audible.
Pulse Oximetry
The introduction of continuous pulse oximetry during the 1980s has greatly improved provider recognition of periprocedural hypoxemia. The ASA now mandates continuous pulse oximetry during all anesthetics. Modern pulse oximeters calculate the arterial oxygen saturation based on the Beer-Lambert law. The measured arterial oxygen saturation is derived from the absorption of probe-emitted red and infrared light by hemoglobin within arterial (pulsatile) blood. The four main hemoglobin species within adult blood are oxyhemoglobin (HbO 2 ), deoxyhemoglobin (HbR), methemoglobin (metHb), and carboxyhemoglobin (COHb). Fetal hemoglobin (HbF) is present in neonatal blood. HbO 2 and HbR, the predominant species in normal individuals, absorb light at different wavelengths. HbO 2 absorbs near infrared light at a wavelength of 940 nm, and HbR absorbs red light at a wavelength of 660 nm. The pulse oximeter calculates arterial oxygen saturation according to the relative ratio of red and infrared light absorption.
Continuous pulse oximetry during anesthesia reduces the incidence and severity of periprocedural hypoxemia, although it is unclear whether this has resulted in reduced morbidity. Prospective studies have not shown improved outcomes resulting from pulse oximetry, but as Eichhorn pointed out in his 1993 editorial, the rate of hypoxia-related adverse events during anesthesia is relatively low, making it impractical if not impossible to perform a study powered to detect statistically improved outcomes related the use of pulse oximetry. Nonetheless, it is reasonable to conclude that enhanced detection of hypoxemia through routine continuous pulse oximetry has led to clinically significant improvements in care delivery.
Pulse oximetry has limitations. It does not provide information regarding ventilation. Also, because pulse oximeters rely on pulsatile blood flow to determine oxygen saturations, physiological states associated with decreased pulsatility, such as shock, severe vasoconstriction, and low cardiac output, may result in spurious pulse oximetry readings. The pulse oximeter probe is typically attached to fingertips. Nail polish on fingernails decreases tissue penetrance of probe-emitted light and obscures pulse oximetry readings. Finally, increased levels of abnormal hemoglobin variants such as methemoglobin and carboxyhemoglobin are associated with inaccurate pulse oximetry values and require alternative methods such as co-oximetry to measure arterial oxygen saturation.
Capnography
Capnography is the continuous measurement of the partial pressure of carbon dioxide over the respiratory cycle ( Figure 6-1 ). Capnography quantifies the amount of infrared radiation absorbed by molecules of carbon dioxide. The amount of infrared radiation absorbed has an exponential relationship to the partial pressure of carbon dioxide. The measured partial pressure of carbon dioxide at the end of exhalation is called the end-tidal carbon dioxide (ETCO 2 ). Classically, capnography has been used to assess ventilation in intubated patients. However, this technology may be used in nonintubated patients using nasal cannulas equipped with sidestream ports for sampling exhaled carbon dioxide. Before the introduction of capnography, visual inspection and impedance plethysmography were used to measure respiratory rate and to detect hypoventilation and apnea during sedation. These methods largely have been replaced by capnography, which also allows for the continuous quantification of respiratory rate.
Capnography provides essential quantitative and qualitative information regarding the adequacy of ventilation. During procedural sedation complicated by hypoventilation, capnographic waveforms typically exhibit two types of changes. Bradypneic hypoventilation, a common side effect of opioids, is hypoventilation caused by a decrease in respiratory rate. The capnographic tracing in bradypneic hypoventilation displays increased ETCO 2 and decreased rate. Conversely, hypopneic hypoventilation, as commonly occurs with hypnotics such as propofol, is associated with decreased tidal volume and respiratory rate. A greater proportion of each exhaled breath is dead space ventilation, and ETCO 2 is consequently reduced with a widened arterial carbon dioxide level to ETCO 2 gradient.
Studies evaluating capnography in nonintubated patients during NORA suggest that respiratory insufficiency may be detected by capnography well before hypoxemia is detected with pulse oximetry. In pediatric studies of remote procedural sedation in the emergency room and in MRI, capnography detected hypopneic hypoventilation 2 to 3 minutes before hypoxemia was detected with pulse oximetry. Similarly, in adult patients undergoing propofol sedation in the emergency room or in the endoscopy suite, capnography was shown not only to provide earlier detection of hypoventilation but to also decrease the incidence of hypoxemia. In patients undergoing procedural sedation receiving supplemental oxygen, capnography is crucial to the early detection of abnormal ventilation because hyperoxia delays the onset of hypoxemia. The utility of capnography is not limited to monitoring ventilation; it is also useful in assessing circulation, perfusion, and total body metabolism. Capnographic waveforms exhibit quantitative changes in many pathophysiological states. Causes of increased exhaled carbon dioxide include hypoventilation, rebreathing of exhaled carbon dioxide, and hypermetabolic states such as malignant hyperthermia and hyperthyroidism. Causes of decreased exhaled carbon dioxide include unplanned airway disconnection (i.e., circuit disconnection or extubation), hyperventilation, increased dead space ventilation, and pulmonary hypoperfusion (i.e., profound shock, low cardiac output, pulmonary embolism, or venous air embolism).
Electrocardiogram
Continuous electrocardiogram (ECG) monitoring is the simplest form of noninvasive cardiac monitoring. The ECG provides a graphic representation of the electrical activity of the heart, including its heart rate and rhythm. During anesthesia, continuous ECG monitoring is critical in the detection of myocardial ischemia, arrhythmias, or important electrolyte disturbances such as hyperkalemia. However, the ECG provides little information regarding myocardial contractility or function.
Both three-lead and five-lead ECGs are employed for standard monitoring purposes. The three-lead ECG incorporates the I, II, and III limb leads first described by Einthoven. This provides basic ECG information, and only one lead may be viewed at a time. For more complex cases in at-risk patients, five-lead monitoring is recommended. This requires the placement of an additional limb lead and a fifth (V) intercostal lead, allowing for a more comprehensive examination. During five-lead monitoring, valuable information may be gathered by viewing leads II and V simultaneously. Lead II is useful in the detection of arrhythmias and inferior wall ischemia, and lead V is helpful in the detection of anterior and lateral wall ischemia.
Noninvasive Blood Pressure
Noninvasive arterial blood pressure measurements should be performed at least every 5 minutes during NORA using either manual or automated methods. These measurements are classically performed by placing an appropriate-size blood pressure cuff around the upper arm. Cuff inflation above the systolic blood pressure results in brachial arterial compression interrupting blood flow. Blood flow resumes with cuff deflation, allowing for an estimation of arterial blood pressures. Manual blood pressure monitoring requires a sphygmomanometer and a stethoscope placed over the brachial artery. This method relies on auscultation of the Korotkoff sounds to determine the systolic and diastolic blood pressure. The mean arterial blood pressure (MAP) may be calculated based on these pressures.
Automated blood pressure monitoring uses the principles of oscillometry described by Von Recklinghausen. This method is based on the detection of oscillations transmitted as blood flow resumes in the previously compressed artery. Oscillometric blood pressure monitoring most accurately predicts the MAP, which occurs at the point of maximal oscillations. Systolic and diastolic blood pressure are calculated based on specific algorithms, but the systolic blood pressure normally corresponds to the beginning of oscillations and the diastolic blood pressure to when the oscillations cease.
Temperature
According to the ASA, “every patient receiving anesthesia shall have temperature monitored when clinically significant changes in body temperature are intended, anticipated or suspected.” Acceptable temperature monitoring sites include the axilla, nasopharynx, esophagus, rectum, and bladder. Malignant hyperthermia is the most urgent temperature-related complication of general anesthesia with volatile gases. However, hypothermia is far more common, because most anesthetics induce a state of poikilothermia.
Temperature monitoring is equally important during NORA. Hypothermia is prevalent in remote locations, because many areas are not equipped with effective heating systems. Furthermore, some locations require active cooling to prevent equipment from overheating. Hypothermia during anesthesia has been linked to multisystem morbidity, including myocardial ischemia, surgical site infections, and coagulation disorders. Ultimately, undetected and untreated hypothermia is associated with prolonged length of hospital stay and increased costs.
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