Elevated Burst Suppression Ratio: The Possible Role of Hypoxemia

Abstract

To the Editor: We report a case characterized by the simultaneous occurrence of increased suppression ratio number (SR) and hypoxemia without a concomitant decrease in bispectral index (BIS). A 59-yr-old man was scheduled for tracheal granuloma removal by laser, using a rigid bronchoscopy. The patient’s history included two lobectomies for epidermoid carcinoma. Anesthesia induction was induced and maintained with infusions of propofol and remifentanil. A bolus of succinylcholine was injected to facilitate introduction of the bronchoscope. The lungs were mechanically ventilated through the lumen of the bronchoscope using a high frequency jet ventilator. Six minutes after the succinylcholine bolus, his Spo2 decreased dramatically below 80% despite ventilation with pure oxygen, and we also observed an increase of the SR. BIS was in the range of 40–60 and signal quality index was more than 75%. His mean arterial blood pressure was 80 mm Hg. The bronchoscope was withdrawn and replaced by an endotracheal tube; the lungs were mechanically ventilated using pure oxygen. High airway pressure and auscultation revealed bronchospasm. Spo2 improved after inhalation of a beta2-agonist and we observed a decrease of the SR (Fig. 1). The endotracheal tube was removed. The patient’s postoperative neurological examination was normal.Figure 1.: Acute decrease of Spo2 during rigid bronchoscopy. BIS, bispectral index; SR, burst suppression ratio; A, high frequency jet-ventilation; B, tracheal intubation; C to D, inhalation of beta2 agonist; E, recovery.In general, burst suppression patterns appear at BIS levels below 40. The effect of an increasing concentration of a volatile anesthetic on the electroencephalogram is a progressive slowing until burst suppression patterns occur. Our observation of an increased SR appearing while BIS was at an adequate anesthetic level could have been a coincidence, but we cannot exclude a relationship between SR and poor brain tolerance of hypoxemia. This hypothesis is supported by animal studies demonstrating that hypoxemia induces isoelectric electroencephalograms, oxygenation allowing the restoration of a baseline electroencephalogram (1), and a reduction in cerebral tissue damage (2). Ngai Liu, MD Thierry Chazot, MD Department of Anesthesiology Hôpital Foch Université Paris Ile-de-France Ouest Suresnes, France Catherine Mutter, MD Departments of Anesthesiology and Intensive Care Hôpital Hautepierre Strasbourg, France Marc Fischler, MD Department of Anesthesiology Hôpital Foch Université, Paris Ile-de-France Ouest Suresnes, France [email protected]