Fig. 6. Effect of infusion of remifentanil on the mean blood concentration of propofol administered as an initial bolus dose of 2 mg/kg, followed by a constant infusion of 0.8 mg · kg⁻¹· min⁻¹. Open circles indicate blood levels of propofol (μg/ml; mean ± SD; n = 5). Thick black line indicates the duration of propofol administration,  i.e. , 105 min, starting at the arrow; the last blood levels were observed 30 min after its withdrawal. Thin black lines indicate 15-min infusion periods of remifentanil at 0.8, 1.6, and 3.2 μg · kg⁻¹· min⁻¹starting at the arrows. (In humans, propofol blood levels of 3–8 μg/ml are targeted and with infusion rates of 4–8 mg · kg⁻¹· h⁻¹. However, much higher levels are sometimes required.)