Effect of i.v. lignocaine on the breathing of patients anaesthetized with propofol.

Local anaesthetics are ventilatory depressants, but previous investigators have not commented on the effects on ventilatory timing. There is concern about the possible ventilatory depression caused by systemic absorption of local anaesthetics injected extradurally. We have studied ASA grade I patients anaesthetized with a propofol infusion and breathing spontaneously; they were given in random order lignocaine 1.5 mg kg-1 i.v. and an equivalent volume of 0.9% saline. Breathing was analysed using respiratory inductance plethysmography in 30-s periods for 4 min after injection, each period scaled to the 30-s period preceding injection. Lignocaine reduced minute ventilation. The greatest mean reduction in the 4 min was to 85%, occurring 2.5-3 min after injection; the greatest individual reduction was to 60-65%, which occurred by 30-60 s. Lignocaine decreased tidal volume and ventilatory rate by prolonging expiratory time. Lignocaine had no effect on or promoted bimodality of expiratory time. End-tidal carbon dioxide increased by a mean of 0.1%; the largest individual change was 0.3%. This suggests that lignocaine may have reduced the metabolic rate, affecting ventilation indirectly, but we conclude that lignocaine in a normal extradural dose should not be an important ventilatory depressant.