As the number and complexity of endoscopic procedures increase, the role of sedation has been integral in patient and physician satisfaction. This article discusses the advances of computer-assisted and patient-controlled platforms. These computer-assisted and patient-controlled platforms use different anesthetics and analgesics, all with the intent of achieving improved consistency in the level of sedation, appropriate to the needs of patients, while also improving patient safety. These systems have been around for decades; however, few are approved for use in the United States, and several still require further study before broad clinical application.
Key points
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Ideal sedation requirements for endoscopy need to be matched to patient comfort, comorbidity risks, procedural discomfort, and the length of the procedure in order to minimize the unnecessary risk of deeper sedation or general anesthesia.
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Most patients require moderate sedation for relatively short procedures for which the ideal agent or administration model is still being sought.
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Patient- and computer-controlled sedation have the ability to impact the quality and safety of sedation for endoscopic procedures.
Introduction
Endoscopic procedures are vitally important in maintaining the health of a patient population; however, these procedures are invasive and cause patient discomfort and anxiety. Sedation has become a necessary part of the endoscopic procedure to ensure patient compliance through patient comfort. Optimally, sedation requirements for endoscopy need to be matched to patient comfort, comorbidity risks, the anticipated procedural discomfort, and the length of the procedure in order to minimize the unnecessary risk of deeper sedation or general anesthesia. Most patients require moderate sedation for relatively short procedures for which the ideal agent or administration model is still being sought.
Patient- and computer-controlled sedation have been long studied, more than 50 years; but these platforms are not yet accepted broadly as a means of sedation in the United States. As these technologies have advanced, so has the ability to impact the quality and safety of sedation for endoscopic procedures. A review of the current state of these technologic advances is discussed.
Introduction
Endoscopic procedures are vitally important in maintaining the health of a patient population; however, these procedures are invasive and cause patient discomfort and anxiety. Sedation has become a necessary part of the endoscopic procedure to ensure patient compliance through patient comfort. Optimally, sedation requirements for endoscopy need to be matched to patient comfort, comorbidity risks, the anticipated procedural discomfort, and the length of the procedure in order to minimize the unnecessary risk of deeper sedation or general anesthesia. Most patients require moderate sedation for relatively short procedures for which the ideal agent or administration model is still being sought.
Patient- and computer-controlled sedation have been long studied, more than 50 years; but these platforms are not yet accepted broadly as a means of sedation in the United States. As these technologies have advanced, so has the ability to impact the quality and safety of sedation for endoscopic procedures. A review of the current state of these technologic advances is discussed.
Sedation medications appropriate for computer and patient control
When selecting sedation agents for computer or patient control, the pharmacokinetic/pharmacodynamic (PK/PD) properties of the agent are paramount. Agents, such as midazolam, are less than ideal for infusions and computer or patient control, primarily because of their long onset and offset time. Fig. 1 shows theoretic plasma and effect site concentrations for a single bolus dose of midazolam. As can be seen, the effect site concentration is not at steady state until almost 10 minutes. This time frame prohibits the ability to titrate for endoscopy procedures, especially those that last less than 10 minutes, such as esophagogastroduodenoscopy (EGD).
In recent years propofol has become the preferred sedative because of its PK/PD properties, allowing for rapid onset/offset and rapid clear-headed recovery. Propofol, when titrated, allows for a rapid and steady state titration. Propofol has historically been considered a general anesthetic, as it has been used to induce a state of general anesthesia. However, when titrated, it can be used for sedation. Fig. 2 shows 3 plots of propofol infusion, all having a total of 187.5 mg delivered after 20 minutes. The top subplot shows a single bolus of propofol, with a maximum effect site concentration of approximately 6 μg/mL, a general anesthetic concentration. When that same amount of propofol is delivered in 4 boluses, the effect site concentration is significantly reduced to just more than 2 μg/mL, as shown in the middle subplot. The bottom subplot shows a steady state infusion, a preferred delivery method for sedation. In this case, a loading dose is delivered followed by a steady state delivery. Therefore, the peaks and valleys are managed, and the effect site concentration does not exceed approximately 1.5 μg/mL. The dose, and the method of delivery, defines the effect site concentration. Computer-controlled sedation allows for an increased degree of control of drug optimization, providing a potentially more stable sedation experience compared with traditional bolus dosing.
Using a drug such as propofol allows for procedural titration not possible with a drug such as midazolam, with a more significant onset/offset time. Fig. 3 (scale was changed to a maximum of 2 μg/mL compared with a maximum of 6 μg/mL in Fig. 2 ) shows an example of procedural titration for colonoscopy. A loading dose is delivered; the scope is inserted before reaching the maximum effect; the maximum effect is reached and maintained during the most noxious part of the procedure, looping during the variable anatomy of the sigmoid colon. Once the cecum has been reached, the rapid offset can be leveraged and the dose rate can be reduced or stopped, as in the example later. The effect site concentration rapidly decreases, so that when the scope is removed, patients are recovering, avoiding potential oversedation in the recovery room.
Propofol is not the only drug that can be titrated; there are others, such as dexmedetomidine, that have a PK profile allowing for titration/infusion. However, studies have shown its side effects, hemodynamic instability, and prolonged recovery limit its utility for procedural sedation. As in all cases, a medications pros and cons must be considered for both patients and the procedure.
Therefore, an ideal agent for computer- or patient-controlled sedation (PCS) would then be one whereby
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The medication titrated via a loading dose over several minutes can achieve the desired level of sedation.
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Increases can be done with incremental loading doses over a reasonable period of time.
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The infusion rate can be rapidly reduced, which will result in rapid patient recovery.
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There is a low side effect profile with hemodynamic stability.
Patient monitoring and oxygen delivery for computer- or patient-controlled sedation
Physiologic Monitoring
A computer-controlled system especially needs to assess patients’ physiology to some degree if it is going to also assess safety and not just effectiveness, which the bispectral index (BIS) and the automated responsiveness monitor provide.
The American Society of Anesthesiology (ASA) has provided guidance on what physiologic monitors should be used for sedation by a nonanesthesiologist as well as standards for basic anesthesia monitoring. In general, the requirements are blood oxygenation, respiration, blood pressure, heart rate, and level of sedation. If deep sedation is the targeted level of sedation, then exhaled carbon dioxide and electrocardiogram (ECG) should also be included.
When looking at the cascade of injury for sedation, it is important to monitor as far upstream in the cascade as possible. Fig. 4 shows the most common cascade of injury. An increase in drug delivery and/or a decrease in patient stimulus can result in oversedation. Oversedation can result in respiratory depression, when the results in hypoxemia, which if profound, can result in morbidity/mortality. In assessing the level of sedation, a transition to general anesthesia determined by nonresponsive to a trapezius squeeze is not necessarily a transition to a surgical plane of general anesthesia.
Clearly, per the ASA, monitoring of the level of sedation should be continually assessed. This element is the first in the cascade of injury, and the best monitor is the one that is located furthest from the injury.
As respiratory depression is next in the cascade, even for moderate sedation, it should be continually assessed, such as a precordial stethoscope or capnometer. After respiratory depression, hypoxemia should be monitored, typically via pulse oximetry.
If the drug is being titrated, the clinical team has the benefit to stop or reduce the medication, stopping the injury cascade of oversedation, apnea, hypoxemia, bradycardia/hypotension, and then morbidity/mortality.
Electroencephalogram Monitoring
Electroencephalogram (EEG) monitoring, predominately BIS being the focus of study, provides the ability to assess the effect of an anesthetic agent on the central nervous system. Controlled infusion of intravenous (IV) anesthetic agents based on the BIS provides the potential for a more controlled steady state infusion to a patient. BIS has been shown to be useful in predicting recovery time and the depth of sedation of patients; however, it has not been useful in ultimately predicting patient and endoscopist satisfaction. For this reason, it has not received widespread adoption for endoscopic sedation. In regard to use in a computer-controlled system, BIS presents unique problems. For example, at lighter levels of anesthesia (minimal/moderate sedation), even stimuli such as noise has an impact on the BIS, making the signal to noise ratio of the BIS value difficult to use in a closed loop system. However, at deep levels of sedation and anesthesia, the BIS value is not impacted by these mild external stimuli.
An Automated Responsiveness Monitor
A novel automated responsiveness monitor was developed for the SEDASYS System (Ethicon Endo-Surgery, Inc, Cincinnati OH) that uses audible prompts and tactile stimulus to assess if patients are in minimal to moderate sedation. This monitor is unique in that it is designed to assess sedation in a narrow range, balancing accuracy and the range of patient assessment. The monitor in the study by Doufas and colleagues shows the patient is responsive in all situations before transition to deep sedation. This system provides increased accuracy for minimally to moderately sedated patients but limited utility for patients targeted for deep sedation/general anesthesia.
Ocular Microtremor Monitor
Ocular microtremor (OMT) is a novel monitoring technology that looks at the microtremors in the eye and, through assessment of those tremors, is able to assess the degree of sedation/anesthesia. OMT has been shown to correlate well with BIS, indicating a potential that this may be another means of assessing the level of neuronal activity of patients. Its usefulness has yet to be proven in endoscopy and whether it will have the same shortcomings as BIS or will provide a more precise means of patient monitoring.
Oxygen Delivery
Oxygen delivery is critical in safely managing patients during sedation. By preoxygenating patients, you increase patients’ partial pressure of oxygen, allowing more time during respiratory depression to manage patients’ airway before they are desaturated. Fig. 5 shows an oxyhemoglobin dissociation curve with 3 slope areas highlighted. The slope when the Pa o 2 is greater than 10 kPa is at its most shallow. At this point on the curve, an apneic episode will result in little if any change in the patient’s oxygen saturation of arterial blood (SaO 2 ). If the apnea event were to continue, the patient becomes increasingly more susceptible to apnea episodes. As an apneic episode may not be total airway obstruction but shallow breathing, it is recommended to increase the level of oxygen delivery as patients decrease in saturation, again in a prophylactic manner. Ideally a computer-controlled sedation system should also provide a degree of automated oxygen delivery, increasing it in response to patients’ oxygen saturation.
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