Capnography monitoring in procedural sedation for bronchoscopy.

Abstract

Moderate sedation/analgesia, formerly known as “conscious sedation,” is commonly used during bronchoscopy for patients’ comfort and safety around the world. This is mostly because, during the procedure, patients may experience pain, excessive cough, and sensation of asphyxiation. These experiences may lead to anxiety, hypertension, and even arrhythmias. Moreover, they may leave behind an unpleasant memory of the procedure.1 Moderate sedation is defined by the American Society of Anesthesiologists (ASA) as “A drug-induced depression of consciousness during which patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation. No interventions are required to maintain a patent airway, and spontaneous ventilation is adequate. Cardiovascular function is usually maintained.”2 During bronchoscopy, patients may experience varying degrees of airway obstruction, due to bronchospasm, excessive cough, vocal cord edema, and bleeding, in addition to the presence of the bronchoscope in the airway. Moreover, strong cough reflex in some patients may necessitate additional doses of sedatives, compromising alertness. Collectively, these events may precipitate hypoventilation and/or apnea. Thus, in addition to the use of pulse oximetry to monitor oxygenation, the use of a ventilation monitor such as capnography during bronchoscopy might be beneficial. Pulse oximetry is thought to be an adequate monitor of ventilation. However, although it displays blood oxygenation relatively well, it actually makes monitoring ventilation challenging as it delays the detection of hypoventilation.3 Patients receiving supplemental oxygen during the procedure may not exhibit hypoxemia for a duration despite inadequate ventilation; and when detected, it might be too late to intervene and rescue. Physiologically, “hypoxemia secondary to respiratory depression occurs when an increase in alveolar CO2 (PACO2) produces a decrease in alveolar O2 (PAO2). Because pulse oximetry measures oxygen saturation (SpO2) instead of the partial pressure of oxygen in arterial blood (PaO2), the shape of the oxyhemoglobin dissociation curve dictates that PaO2 will be significantly below 100mm Hg before desaturation will be detected. Among patients breathing room air, this occurs with only modest increases in PaCO2. However, in the presence of supplemental O2, SpO2 may be maintained at greater than 90% despite truly spectacular increases in arterial carbon dioxide (PaCO2).” 4