Level 1: Minimal sedation	Drug-induced state, during which patient responds normally to verbal commands. Cognitive function and coordination may be impaired. Ventilatory and cardiovascular function are unaffected
Level 2: Conscious sedation	Drug-induced depression of consciousness, during which patient responds purposefully to verbal commands, either alone or accompanied by light tactile stimulation. Patent airway is maintained without help. Spontaneous ventilation is adequate, and cardiovascular function is usually maintained
Level 3: Deep sedation	Drug-induced depression of consciousness, during which patient cannot be easily aroused but responds purposefully to repeated or painful stimulation. Patient may require assistance maintaining an airway. Spontaneous ventilation may be inadequate, and cardiovascular function is maintained
Level 4: General anesthesia	Patient is not able to be aroused even by painful stimuli. Patient often requires assistance in maintaining patent airway. Positive-pressure ventilation may be required owing to respiratory depression or neuromuscular blockade. Cardiovascular function may be impaired

From Bryson HM, Fulton BR, Faulds D: Propofol: an update of its use in anesthesia and conscious sedation. Drugs 50:513–559, 1995.

This chapter discusses all aspects of sedation and patient safety. Patient evaluation and risk assessment, presedation preparation, and issues of consent are discussed. The attributes of sedative drugs are discussed in the context of level of sedation and monitoring required.

Patient Evaluation and Preparation

Appropriate preprocedural evaluation of the patient’s history and physical findings reduces the risk of adverse outcomes (see Chapter 8).⁸ The clinicians responsible for sedation should familiarize themselves with specific and relevant aspects of the medical history, including abnormalities of major organ systems, previous adverse experience with sedation and analgesia, current medications and drug allergies, time of the last oral intake, and history of alcohol or recreational drug use. A thorough physical examination is required particularly to assess the heart and lungs in addition to the airway. It may be useful to consider the patient in terms of the American Society of Anesthesiologists (ASA) status classification (Table 7.2). There is usually no indication to perform laboratory tests, unless particular concern is raised during assessment.

Table 7.2 Definition of American Society of Anesthesiologists Status

Class 1	Patient has no organic, physiologic, biochemical, or psychiatric disturbance. Pathologic process for which operation is to be performed is localized and does not entail systemic disturbance
Class 2	Mild to moderate systemic disturbance caused either by the condition to be treated surgically or by other pathophysiologic processes
Class 3	Severe systemic disturbance or disease from whatever cause; it may be impossible to define degree of disability with finality
Class 4	Severe systemic disorders that are already life-threatening, not always correctable by operation
Class 5	Moribund patient who has little chance of survival but is submitted to operation in desperation

Although there are no data to support the provision of information about sedatives to patients, counseling may improve patient satisfaction. Patients undergoing sedation should be informed of the benefits, risks, and limitations associated with sedation and possible alternatives. This process should be part of the patient consent. Patients’ expectations regarding the discomfort they are likely to experience while sedated during the procedure should be realistic and as accurate as possible.

Procedural Monitoring

Patients undergoing endoscopic procedures should have continuous monitoring before, during, and after the administration of sedatives.⁹ Monitoring should be discontinued only when the patient is fully awake. The literature and medical opinion suggest that continuous recording of the patient’s level of consciousness, respiratory function, and hemodynamics reduces the risk of sedation-related adverse outcomes.⁸ Early detection of adverse events induced by sedatives, such as hypoxemia and cardiovascular compromise, allows intervention to prevent life-threatening complications. Each of the parameters monitored is addressed.

Level of Consciousness

Decreasing level of consciousness is associated with loss of reflexes that normally protect the airway and prevent hypoventilation. The response of patients to commands during sedation serves as a guide to their level of consciousness (see Table 7.1). Spoken responses also provide an indication that the patient is breathing. During procedures in which verbal responses are impossible, such as upper gastrointestinal (GI) endoscopy, other responses should be sought, including hand movements or a nod of the head. Lack of response to verbal or tactile stimuli suggests a greater level of sedation and should be treated accordingly.

Pulmonary Ventilation

Respiratory depression is probably the central event in sedation-related complications, and monitoring of ventilatory function reduces the risk of adverse outcomes. Ventilatory function can be monitored by observation of respiratory movement or by direct pulmonary auscultation. If ventilatory function cannot be observed, transcutaneous carbon dioxide (CO₂) and end-tidal CO₂ can be measured by capnography. Capnography measures CO₂ indirectly by light absorption in the infrared region of the electromagnetic spectrum. If a patient’s ventilation is compromised, CO₂ retention occurs, which is identified as an early event on capnography often before oxygen desaturation.¹⁰ Unless the breathing system is a closed circuit, the exact measurement of CO₂ is inaccurate. The presence of a CO₂ trace on capnography is reassuring that the patient is still breathing, however. Alternatively, respiratory activity can be continuously measured graphically using expired air CO₂ detectors.¹¹ This method can detect the early phases of respiratory depression. No studies have addressed whether capnography improves outcome in patients with conscious sedation.

Pulse Oximetry

Pulse oximetry is a noninvasive method of measuring oxygen saturation from a light signal transmitted through tissue, taking into account the pulsatile volume changes that occur. The probes differentiate the absorption of incidental light by the pulsatile arterial component from the static component—hence the term pulse oximeters. The pulse oximeter estimates the oxygen saturation by measuring the pulsatile signals across perfused tissue at two distinct wavelengths, which allows the differentiation of reduced hemoglobin and oxyhemoglobin. Oxyhemoglobin absorbs light in the infrared band, whereas reduced hemoglobin absorbs light in the red band. The functional oxygen saturation is defined as the ratio of oxyhemoglobin to all functional hemoglobins. The oxyhemoglobin dissociation curve is sigmoid, which limits the degree of desaturation that can be tolerated. Between 90% and 100% saturation, the partial pressure of arterial oxygen is maintained at a high level; however, at less than 90%, the curve becomes steeper, and small decreases in the oxygen saturation correspond to large decreases in partial pressure.

Various probes are available; however, the most commonly used probe is a reusable one for use on fingers or toes. Pulse oximetry in sedated patients has been shown to improve assessment of respiratory status.¹² The routine use of pulse oximetry in unsedated endoscopy is not always indicated, however.¹³ Although oximetry is not a substitute for monitoring ventilatory function, hypoxemia is more likely to be detected using oximetry in addition to clinical assessment. Pulse oximeter probes occasionally detect peripheral oxygen saturation poorly, however, resulting in a falsely low or absent oxygen saturation reading. Failure rates have been shown to be 7%,¹⁴ although the accepted error is 3% when compared with arterial blood gases.¹⁵ Failure is more common in patients who are ASA class 3 and higher, patients with hypertension or hypotension, patients with hypothermia, patients with pigmented skin, elderly patients, and patients with renal failure. Pulse oximeters require adequate pulsations to distinguish arterial blood light absorption from venous blood and tissue light absorption. The reading may be unreliable or absent if there is loss or diminution of the peripheral pulse; this may occur with blood pressure cuff inflation, improper positioning, hypothermia, hypotension, and peripheral vascular disease. Although the response time of the pulse oximeter is fast, there may be a significant delay in the alveolar oxygen tension and change in oximeter reading.¹⁶

Some shades of nail polish may cause significantly lower saturation readings. This problem can be overcome by placing the probe across the finger rather than from nail to finger dorsum. False alarms frequently occur during continuous monitoring and are usually due to motion artifact. Poor signal quality and sensor displacement are also common and can be avoided by changing position of the probe or warming a limb to improve circulation. Other maneuvers to reduce false alarms include delaying the time between detection of a low oxygen saturation and alarm activation, although this may lead to a delay in resuscitation in the event of a true alarm. Pulse oximetry is a valuable monitor because endoscopic procedures are performed with dim ambient light, which makes clinical assessment of a patient’s oxygenation difficult. Most patients undergoing an endoscopic procedure under sedation have supplemental oxygen administered. Their saturations remain at a normal level despite having profound respiratory depression, and this needs to be borne in mind.

Hemodynamic Measurements

Sedative agents and analgesics may have mild direct effects on hemodynamic control, such as hypotension and cardiovascular response to stress. These changes can be detected by regular measurement of pulse and blood pressure. Changes in blood pressure and pulse may represent responses to hypoxemia, oversedation, or possibly patient distress to procedure-induced pain. Although no evidence shows that blood pressure monitoring during endoscopy influences morbidity and mortality, it has been suggested that both regular blood pressure measurements and pulse be measured throughout procedures performed under sedation.¹⁷ There is wide geographic variation in this practice, however. Continuous electrocardiogram (ECG) monitoring should be considered in high-risk patients, such as patients with disturbances in cardiac rhythm or ischemic heart disease. The necessity for ECG monitoring has not been shown in clinical trials.

Supplemental Oxygen

Oxygen given via nasal cannulas or via a mask has been shown to reduce oxygen desaturation during endoscopy performed under sedation ^18,¹⁹ and should be given to all patients receiving sedation. However, because supplemental oxygen delays the onset of hypoxemia in sedated patients who are hypoventilating, it is important not to rely solely on oximetry to monitor ventilation but to employ additional techniques, including assessment of respiratory rate.