Changes in respiratory pattern and arterial blood gases during sedation with propofol or midazolam in spinal anesthesia

Study Objectives: To investigate changes in respiratory pattern and arterial blood gases during sedation with propofol or midazolam in spinal anesthesia. Design: Randomized, placebo-controlled study. Setting: Operating room of a university-affiliated hospital. Patients: 40 ASA physical status I and II patients who required spinal anesthesia. Interventions: Spinal anesthesia with tetracaine and subsequent sedation with propofol (n = 15), midazolam (n = 15), or placebo (n = 10) was performed. Measurements: Respiratory pattern [rib cage contribution to the tidal volume (%RC) and phase shift between rib cage and abdominal movements (PSrc-ab)] with a respiratory inductive plethysmograph (Respigraph™) and arterial blood gas analysis (pH, pO 2, and pCO 2) were recorded. Main Results: Spinal anesthesia per se increased %RC by 35% without changing PSrc-ab values (1.00). Sedation with propofol and midazolam decreased %RC by 60% and by 40%, respectively. PSrc-ab increased in both groups following sedation, and the increase in this parameter was higher in the propofol group (1.12) than in the midazolam group (1.04). In the placebo group, %RC decreased by 20% without any change in PSrc-ab. The decrease in pO 2 was more significant in the propofol group (65.1 mmHg) than in the midazolam (74.2 mmHg) and placebo (83.1 mmHg) groups. Conclusion: Significant decreases in %RC and pO 2 during propofol sedation seem to depend on paradoxical respiration due, in part, to upper airway obstruction; therefore, attention should be directed to the respiratory pattern during sedation, especially with propofol.