Chronotropic and inotropic supports are both required to increase cardiac output early after corrective operations for tetralogy of Fallot

To assess the respective roles of chonotropism, inotropism, and afterload reduction in increasing cardiac index early after corrective operations for tetralogy of Fallot, we measured vascular pressures and cardiac output and evaluated left ventricular dimension changes before and after a 35% rise in heart rate over baseline. This rise was induced by atrial pacing with intact atrioventricular conduction, isoproterenol, or atrial pacing together with dobutamine. With atrial pacing, left ventricular end-diastolic diameter decreased (38.7 +/- 4.3 to 34.2 +/- 5.6 mm, p less than 0.05), the shortening fraction (ratio of the difference between left ventricular end-diastolic and end-systolic diameters to left ventricular end-diastolic diameter) remained constant, and stroke volume index was reduced (28.8 +/- 4.5 to 19.7 +/- 4.6 ml/m2, p less than 0.05). As a result, cardiac index was left unchanged. When dobutamine was added as supplemental inotropic support, left ventricular end-diastolic diameter remained constant, shortening fraction increased (30% +/- 5.4% to 36% +/- 3.3%, p less than 0.05), and cardiac index rose significantly (3.04 +/- 0.61 to 4.18 +/- 0.85 L/min/m2, p less than 0.05). Heart rate acceleration with isoproterenol, combining chronotropism, positive inotropic support, and afterload reduction, slightly increased left ventricular end-diastolic diameter, significantly raised shortening fraction, and markedly enhanced cardiac index (3.03 +/- 0.55 to 4.9 +/- 1.09 L/min/m2). Atrial pacing with intact atrioventricular conduction, as an isolated chronotropic stimulus, is not suited to increase cardiac index early after operations for tetralogy of Fallot unless additional inotropic support is simultaneously provided.