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The effect of cardiac autonomic blockade on ventricular response to changes in preload
Insights
Cardiac autonomic blockade depresses left ventricular (LV) contractility, shifting the LV function curve downward. However, LV response to preload changes remains independent of autonomic nervous system influences.
Area of Science:
- Cardiovascular Physiology
- Autonomic Nervous System
- Cardiac Function
Background:
- The autonomic nervous system significantly influences cardiac function.
- Understanding left ventricular (LV) response to preload variations is crucial for assessing cardiac health.
- Previous studies have explored factors affecting LV performance, but the specific impact of autonomic blockade on LV function curves requires further elucidation.
Purpose of the Study:
- To assess the echocardiographic (E) left ventricular (LV) response to changes in preload under both control and cardiac-blocked conditions.
- To determine the effect of autonomic blockade on LV function curves.
- To investigate the independence of LV preload recruitable stroke volume from autonomic influences.
Main Methods:
- Echocardiography was used to assess LV response in 9 male subjects.
- Cardiac autonomic blockade was induced using atropine and propranolol.
- Preload alterations were achieved through head-down tilt (increases) and lower body negative pressure (decreases).
Main Results:
- Cardiac autonomic blockade resulted in a significant downward shift of the LV function curve, indicating depressed myocardial contractility.
- Stroke volume changes were significantly related to end-diastolic volume changes under both control and blocked conditions.
- The relationship between stroke volume and end-diastolic volume, described by linear regression, showed significant differences in slope and correlation during autonomic blockade.
Conclusions:
- Cardiac autonomic blockade significantly depresses left ventricular contractility.
- The observed changes in left ventricular function during preload variations are independent of autonomic nervous system influences.
- These findings highlight the direct impact of autonomic blockade on intrinsic myocardial contractility.
Abstract:
Echocardiographic (E) assessment of left ventricular response to increases (I) 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 degrees 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 less than 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 less than 0.001). Similarly during B, SV = 0.43 EDV + 5.8 cm3 (r = 0.75; P less than 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 less than 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.