Ventilatory Response to Hypercapnia as Experimental Model to Study Effects of Oxycodone on Respiratory Depression

Opioid analgesics used to treat pain can cause respiratory depression. However, this effect has not been extensively studied, and life-threatening, opioid-induced respiratory depression remains difficult to predict. We tested the ventilatory response to hypercapnia for evaluating the pharmacodynamic effect of a drug on respiratory depression.

We conducted a randomized, placebo-controlled, double-blind, crossover study on 12 healthy adult males. Subjects received 2 treatments (placebo and immediate-release oxycodone 30 mg) separated by a 24-hour washout period. Subjects inhaled a mixture of 7% carbon dioxide, 21% oxygen, and 72% nitrogen for 5 minutes to assess respiratory depression. Minute ventilation, respiratory rate, tidal volume, flow rate, end-tidal CO2, and oxygen saturation were recorded continuously at pre-dose and 30, 60, 120, and 180 minutes post-dose. The primary endpoint was the effect on the ventilatory response to hypercapnia at 60 minutes post-dose, as assessed by the slope of the linear relationship between minute ventilation and end-tidal CO2.

At 60 minutes post-dose, subjects had a mean slope of 2.4 in the oxycodone crossover period, compared to 0.1 in the placebo period (mean difference, 2.3; 95% CI: 0.2 to 4.5; p = 0.035). Statistical significance was likewise achieved at the secondary time points (30, 120, and 180 minutes post-dose, p <0.05).

This model for testing ventilatory response to hypercapnia discriminated the effect of 30 mg of oxycodone vs. placebo for up to 3 hours after a single dose. It may serve as a method to predict the relative effect of a drug on respiratory depression.