Haemodynamic Alterations Induced by Isoprenaline in Patients with Obstruction to Right Ventricular Outflow.

It is now well established that there are two fundamentally distinct types of obstruction to left ventricular outflow, the first resulting from a discrete, fixed narrowing at a valvular, supravalvular, or subvalvular level, and the second being produced by muscular hypertrophy in the left ventricular outflow tract. A number of designations, including obstructive cardiomyopathy (Goodwin et al., 1960) and idiopathic hypertrophic subaortic stenosis (IHSS) (Braunwald et al., 1960) have been applied to the latter condition. In patients with the discrete forms of obstruction to left ventricular outflow, the area of the stenotic outflow orifice remains constant during hemodynamic interventions which alter the systolic volume or the contractile state of the ventricle (Braunwald and Ebert, 1962), while with IHSS variations in the severity of obstruction may be induced by a variety ofphysiological and pharmacological stimuli. The dimensions of the obstructing orifice are not fixed in IHSS and the size of the left ventricular outflow tract varies, depending upon the contractile state of myocardium, the venous return, the systemic arterial pressure, and the systolic distending pressure (Whalen et al., 1963; Krasnow et al., 1963; Goodwin et al., 1964; Wigle, 1964). Although obstruction to right ventricular outflow is most commonly of the fixed type, in this chamber the obstruction may also result entirely from subvalvular muscular hypertrophy. The latter type of obstruction was first recognized in patients in whom the muscular hypertrophy developed as a result of severe valvular obstruction, and became unmasked by effective surgical treatment of the primary lesion (Kirklin et al., 1953; Brock, 1955). More recently, muscular obstruction to right ventricular outflow has also been found to occur in the absence of treated valvular pulmonary stenosis, accompanying IHSS in some instances, or occurring in patients with idiopathic right and left ventricular hypertrophy unaccompanied by obstruction to left ventricular outflow (Braunwalzl et al., 1964; Taylor, Bernstein, and Jose, 1964). The infusion of isoprenaline (isoproterenol), a drug that exerts a powerful stimulating action on the contractile state of the myocardium, either intensifies or provokes obstruction to left ventricular outflow in patients with IHSS, and measurement of the heemodynamic response to this drug provides a useful test for detecting dynamic obstruction to left ventricular outflow (Braunwald and Ebert, 1962; Whalen et al., 1963; Krasnow et al., 1963). This observation suggested that study of the response of the right ventricular outflow tract to isoprenaline might allow differentiation of the fixed from the dynamic forms of right ventricular outflow obstruction, and provided the basis for the present investigation.