Author + information
- Received October 2, 1993
- Revision received December 9, 1994
- Accepted December 15, 1994
- Published online May 1, 1995.
- Robert Johannes Menno Klautz, MDa,
- David Frank Teitel, MD*,1,
- Paul Steendijk, PhDa,
- Frank van Bel, MDa and
- Jan Baan, PhDa,*
- ↵*Address for correspondence: Dr. Jan Baan, Cardiac Physiology Laboratory, Department of Cardiology, Leiden University Hospital, P.O. Box 9600, 2300 RC Leiden. The Netherlands.
Objectives. We undertook the present study to determine whether afterload and contractility interact in the hearts of newborn lambs. We specifically investigated whether stepwise increases in afterload increase contractility.
Background. Several studies in the isolated and intact adult dog heart have shown that afterload and contractility are not independent determinants of cardiac performance; rather, they interact. Afterload and contractility are unlikely to interact in the newborn heart because the factors that may mediate the interaction in the adult are missing in the newborn.
Methods. We measured contractility at different steady state levels of afterload in seven newborn lambs under complete anesthesia. Contractility was measured by three different indexes: end-systolic pressure—volume relations (slope and volume position); preload-corrected first derivative of left ventricular pressure (dP/dtmax); and preload-corrected stroke work. Left ventricular pressure and volume were measured with a micromanometer and conductance catheter, respectively. Preload and afterload were manipulated by inflating or deflating a balloon catheter in the inferior vena cava and descending thoracic aorta, respectively. Data are expressed as mean value ± 1 SD.
Results. Stepwise increases in afterload increased contractility, independent of which of the three indexes was used. The slope of the end-systolic pressure—volume relation increased from a mean baseline value of 4.44 ± 2.43 to 6.69 ± 2.89 kPa/ml at the highest level of afterload. Concomitantly, volume at 14 kPa of the endsystolic pressure—volume relation decreased from 3.34 ±1.52 ml at baseline to 1.12 ± 0.83 ml at the highest afterload. The other two indexes showed qualitatively similar changes. Beats selected from unloading interventions on the basis of the same end-diastolic volume for each level of afterload showed no difference in stroke volume.
Conclusions. This study in newborn lambs demonstrates that stepwise increases in afterload increase contractility considerably and that this enables the heart to maintain stroke volume at different levels of afterload. This forms direct evidence for the existence of homeometric autoregulation in the intact newborn heart.
↵1 Dr. Teitel was the recipient of a Senior International Fellowship for this project from the National Institutes of Health, Bethesda, Maryland, and was Visiting Professor at the Cardiac Physiology Laboratory in Leiden during this study.
This study was presented in part at the 43rd Annual Scientific Session of the American College of Cardiology, March 1994, Atlanta, Georgia.
This study was supported in part by a grant from the Gisela Thier Foundation, Leiden, The Netherlands.
- Received October 2, 1993.
- Revision received December 9, 1994.
- Accepted December 15, 1994.
- American College of Cardiology