Interaction of extradural morphine and lignocaine on ventilatory response

We have evaluated the effects of lumbar extradural morphine and lignocaine on the ventilatory response to carbon dioxide. Twenty-four female patients were allocated randomly to receive extradural morphine 2 mg (group M), 2% lignocaine 10 ml (group L) or a combination of morphine 2 mg and 2% lignocaine 10 ml (group ML). On the day before surgery, resting ventilatory values including minute volume (VE) and tidal volume (VT), and ventilatory response to progressive hyperoxic hypercapnia (VE/PE'CO2) were measured. On the day of surgery, the same measurements were repeated 30 min after extradural injection. Ventilatory values at rest were not altered after extradural injection. Mean VE/PE'CO2 decreased significantly after extradural morphine (P = 0.002) and increased (P = 0.011) after extradural lignocaine. Mean VE 7.3 (VE at PE'CO2 7.3 kPa) decreased significantly after extradural morphine (P < 0.001) and increased after extradural lignocaine (P = 0.047). Extradural morphine and lignocaine did not significantly alter mean VE/PE'CO2 and mean VE 7.3: 14.6 (95% confidence intervals 12.1-17.1) to 15.3 (13.1-17.6) litre min-1 kPa-1 and 22.8 (18.1-27.5) to 22.8 (17.3-28.3) litre min-1, respectively. We conclude that extradural co-administration of morphine and lignocaine did not increase the risk of respiratory depression associated with morphine.