Dobutamine-inducible left ventricular obstruction: head-up tilt and autonomic balance

Dobutamine-inducible left ventricular obstruction lacks a sound clinical meaning. This phenomenon may be related to an abnormal response of the heart to the sympathetic stimulation, and head-up tilt would elicit intraventricular obstruction in patients known to develop it during dobutamine administration, through the synergistic effects of reduced preload, hypercontractility, and reflex increase in the cardiac sympathetic tone.

Twenty-one patients with normal coronaries and available dobutamine stress-echocardiography underwent 60 degrees -head-up tilt on echocardiographic and electrocardiographic monitoring in drug-free and nitroglycerin infusion (0.3 mcg/kg/min) protocols. Twelve patients had (group 1: 7 males, aged 61+/-7 years), and nine had not (group 2: 3 males, aged 60+/-9 years) developed left ventricular obstruction during dobutamine stress-echocardiography. Doppler echocardiographic measurements of left ventricular diameters, function and outflow tract flow velocity were performed; power heart rate variability was analysed on short electrocardiographic recordings (low/high frequency ratio); plasma catecholamines were determined at baseline and during the tilt manoeuvres.

Intraventricular obstruction did not develop in group 1. There were baseline differences in ventricular dimensions and mitral early-to-late velocity ratio between group 1 and 2, but hemodynamic behaviour during either tilt manoeuvre was similar. Low/high frequency ratio, that was similar at baseline and similarly increased on head-up tilt, increased significantly less on nitroglycerin-tilt in group 1. No difference in plasma catecholamines was found.

Reflex cardiovascular adaptive responses as those elicited by passive tilt are not involved in dynamic intraventricular obstruction in dobutamine-inducible obstruction patients, data indicating that left ventricular geometry and hypercontractility are not sufficient pathophysiological determinants.