Arterial mechanical impedance is a sensitive stress response monitor during general anesthesia

Arterial mechanical impedance is a characteristic of arterial dynamics that is determined by photoplethysmographic amplitude and direct arterial pressure, while mechanical impedance is indicated by stiffness, viscosity, and inertia. We compared the parameters of mechanical impedance and photoplethysmographic amplitude to estimate the magnitude of stress response in patients undergoing general anesthesia by measuring direct arterial pressure. After sedating each patient with propofol, photoplethysmographic amplitude and mechanical impedance were measured as baseline control values, then 3 min after fentanyl administration (2 microg kg(-1)), the parameters were measured again as post-fentanyl values. Thereafter, a direct laryngoscopy procedure was performed and post-laryngoscopy values for the parameters were determined. The magnitudes of response to each event were compared by using the ratio of the measurements from the preceding event. Then, comparisons of the magnitudes were performed after coordinating each response in the same direction. Our results showed that both stiffness and viscosity of mechanical impedance had greater variations than photoplethysmographic amplitude. In conclusion, we propose stiffness and viscosity derived from arterial mechanical impedance as sensitive parameters to monitor stress responses during general anesthesia.