The effect of cardiac autonomic blockade on ventricular response to changes in preload

Echocardiographic (E) assessment of left ventricular response to increases (1) and decreases (D) in preload of 9 volunteer male subjects (mean age 25 ± 2.0 years) was made during both control (C) and cardiac blocked (B) conditions. Cardiac autonomic blockade was produced by intravenous administration of atropine (0.04 mg/kg) and propranolol (0.2 mg/kg), while I was produced by 5° head-down tilt (T) for 90 min and D by lower body negative pressure to -40 Torr. Increases in resting heart rate of 44% and diastolic blood pressure of 13% occurred after B, (P < 0.05). During C, the alterations in preload produced mean changes in end-diastolic volume (EDV) ranging from 135 ± 10 cm³ for I to 96 ± 9.1 cm³ for D. Changes in stroke volume during condition C were significantly related to changes in EDV during preload alterations and conformed to a normal LV function curve and were described by SV - 0.6 EDV + 4.5 cm³ (r = 0.85; P < 0.001). Similarly during B, SV = 0.43 EDV + 5.8 cm³ (r = 0.75; P < 0.001). Comparison of the LV function curves produced during C and B showed that both slope of regression and r were significantly different (P < 0.01). These data indicate that cardiac autonomic blockade produces a downward shift in the LV function curve indicating a depression in myocardial contractility. In addition, the data indicate that changes observed in LV function during wide variations in preload are independent of autonomic nervous system influences and are independent of the intrinsic heart rate.