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BACKGROUND Transesophageal echocardiography (TEE) is now an established adjunct to routine echocardiography, its diagnostic impact making it an invaluable first-line diagnostic procedure in many cardiac conditions. However, there is no unanimity in the way the transesophageal procedure is carried out, especially with regard to the need for antibiotic prophylaxis, sedation, and the monitoring of oxygen saturation. HYPOTHESIS This study was prospectively undertaken (1) to determine the presence and magnitude of oxygen desaturation and (2) the changes in heart rate and blood pressure following sedation for routine TEE in an unselected and consecutive group of patients to identify those at high risk. METHODS Arterial oxygen saturation, heart rate, and systolic, diastolic, and mean blood pressure were monitored in 106 consecutive patients undergoing routine transesophageal echocardiography. Ninety-four (89%) patients received intravenous sedation with midazolam. RESULTS Three min after midazolam administration there was a drop in oxygen saturation from 97 +/- 2.5 to 95 +/- 2.9 (p < 0.001), in systolic blood pressure from 139 +/- 19.5 to 124.8 +/- 22.2 mmHg (p < 0.001), in diastolic blood pressure from 86.6 +/- 19.9 to 77.5 +/- 17.7 mmHg (p < 0.001), and in mean blood pressure from 108.3 +/- 18 to 95.6 +/- 28.8 mmHg (p < 0.001). After introduction of the transesophageal probe and during the examination, there was a further drop in oxygen saturation with a maximum drop at the 15th min of the examination (93.7 +/- 3.7 vs. 97 +/- 2.5, p < 0.001). The maximum blood pressure drop occurred at the 12th min into recovery: systolic blood pressure dropped from 139 +/- 19.5 to 118 +/- 20.8 mmHg (p < 0.001), diastolic blood pressure from 86.6 +/- 16.9 to 75.8 +/- 17.9 mmHg (p < 0.005), and mean blood pressure from 108.3 +/- 18 to 92.5 +/- 19.4 mmHg (p < 0.01). Patients with congestive heart failure had a greater drop in oxygen saturation compared with patients who were not in heart failure (p < 0.01). Twelve patients did not receive any sedation; however, they all showed a drop in oxygen saturation from 97.8 +/- 2.3 to 94.6 +/- 3.4 (p < 0.001), with a maximum drop at the 15th min during the transesophageal examination. CONCLUSION In patients with no chronic obstructive airway disease who are not in congestive heart failure, routine oxygen saturation monitoring is not deemed necessary during transesophageal examination. The cause of hypoxemia during the procedure is not only related to sedation but also to esophageal intubation.