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Transient left atrial (LA) dysfunction (“stunning”) and the appearance of spontaneous echo contrast (“smoke”) have been reported in patients undergoing DC cardioversion of atrial fibrillation. It has been suggested that cardioversion-induced LA dysfunction may promote new thrombus formation leading to thromboembolic complications in AF patients even in the absence of demonstrable LA thrombus prior to cardioversion. In order to confirm previous observations of LA stunning end to investigate the determina nts of this response, we investigated LA function in a series of patients undergoing implantable defibrillator insertion and testing. Eight patients (67 ± 6 yrs, 5 M/3 F) with ischemic heart disease (n = 6) or cardiomyopathy (n = 2) and VTNF were studied. All patients were in NSR, had significant impairment of systolic LV function, and demonstrated inducible VT or VF in the EP lab. Intraoperative transesophegeal echocardiography with a biplane or multiplane probe was used to image the LA, mitral valve and LA appendage (LAA). Blood flow velocities were measured at the tips of the MV leaflets and LAA orifice by pulsed-wave Doppler before and 30 to 120 seconds after DC shocks of 15–20 joules with transvenous intracardiac electrodes lin SVC and RV apex, n = 7) or epicardial patches (n = 1) while in sinus rhythm. Measurements from three different cardiac cycles were averaged for each patient. Baseline LAA orifice flow velocity was 44 ± 16 cm/s and remained unchanged after DC shock, 46 ± 20 cm/s (p = ns). Similarly, MV peak A-wave velocity was 70 ± 34 cm/s at baseline and was not significantly different after DC shock, 69 ± 42 cm/s (p = ns). No evidence of LA “smoke” was seen in any patient before or after the shock. Conclusion As opposed to previous reports after cardioversion of atrial fibrillation, DC shocks alone do not produce LA dysfunction in patients in NSR even in the presence of left venticular dysfunction. LA stunning after DC shocks may require prior atrial fibrillation or specific atrial substrate.