Influence of lidocaine and bupivacaine on isolated guinea pig atria in the presence of acidosis and hypoxia.

In an isolated guinea pig atrial preparation, the bathing solution pH, PO2, and PCO2 were manipulated to mimic normal, acidotic, and hypoxic conditions. The effect of lidocaine and bupivacaine on spontaneous heart rate (HR) and contractile force (CF) was determined for 60 min under conditions of normal pH, PO2, and PCO2. Lidocaine (50 micrograms/ml) reduced HR by a maximum of 34.2 +/- 1.5% and CF by 38.9 +/- 8.1% (mean +/- SEM). Bupivacaine (5 micrograms/ml) reduced HR by a maximum of 30.1 +/- 1.9% and CF by 48.0 +/- 6.5%. Bupivacaine (10 micrograms/ml) caused a maximum HR reduction of 61.7 +/- 9.5% and CF reduction of 66.0 +/- 8.6%. Hypoxia or metabolic or respiratory acidosis did not further enhance the local anesthetic induced atrial depression. However, conditions of combined acidosis/hypoxia, while not significantly altering the HR and CF depression caused by lidocaine, did enhance bupivacaine-induced depression of HR and CF (93.6 +/- 6.3% and 95.2 +/- 4.8%, respectively). The effect of a protein-free bathing solution on the relative toxicities of lidocaine and bupivacaine is discussed.