Cardiodepressant effects of mexiletine in patients with severe left ventricular dysfunction

Mexiletine is thought to exert minimal negative inotropic actions, but its effects have not been evaluated in patients with severe congestive heart failure. The haemodynamic response to an oral loading dose of mexiletine (400 mg) was assessed in 20 patients with severe chronic heart failure. Mexiletine caused marked haemodynamic deterioration, with stroke work index decreasing in 18 of the patients. Two hours after mexiletine, mean cardiac and stroke work indexes decreased by 15% and 25%, respectively (both P less than 0.001), while heart rate and systemic vascular resistance increased by 10% and 20%, respectively (both P less than 0.05). Simultaneously, left ventricular filling pressure and right atrial pressure increased by 37% and 36%, respectively (both P less than 0.001), but mean arterial pressure did not change. Furthermore, clinical deterioration, with onset of dyspnoea at rest, developed in five patients at the time of peak haemodynamic effect. Plasma mexiletine concentrations were within the accepted therapeutic range of 0.5 to 2.0 micrograms.ml-1 in all but two of the patients. Nevertheless, the plasma concentration was an important determinant of haemodynamic effect. The stroke work index decreased by 38% in the patients with a mexiletine level above the median value of 1.3 micrograms.ml-1 (range 25 to 56%), but only 13% (range 15 to 43) in patients with lower plasma concentrations. In conclusion, although mexiletine may cause cardiodepressant effects in any patient with severe left ventricular dysfunction, dosing which results in a high (but still therapeutic) plasma level is more likely to cause haemodynamic deterioration.