AJP - Heart and Circulatory Physiology, Vol 253, Issue 5 1240-H1252, Copyright © 1987 by American Physiological Society
Interaction between cardiac chambers and thoracic pressure in intact circulation
R. Beyar, M. J. Hausknecht, H. R. Halperin, F. C. Yin and M. L. Weisfeldt
Department of Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
A comprehensive model that describes the interaction between the
cardiovascular system (CVS) and the intrathoracic pressure (ITP) based on a
lumped parameter vascular representation and a time-varying elastance
concept for the four cardiac chambers is presented. Special attention is
given to two possible mechanisms of interventricular interaction; the
constraining effects of the pericardium and direct interventricular
interaction that results from the fact that the two ventricles share a
common interventricular septum. The response of the CVS to positive and
negative perturbations in the ITP and to injection of fluid into the
pericardium was simulated and compared with experimental literature data.
The results show that 1) the total heart volume is relatively constant
throughout the cycle both for ITP of 0 and +15 mmHg, which is consistent
with experimental data in dogs, thus suggesting that intrinsic properties
of the cardiac chambers rather than a restricting pericardium is the
mechanism for that observation. 2) The pericardium has a major role in
modifying the transient and steady-state response to a step decrease in the
ITP with a transient decrease in left ventricle (LV) end-diastolic volume
followed by gradual increase afterwards. 3) The response to sudden
injection of fluid into the pericardial space is a larger transient
decrease in right ventricle than LV volume, which is consistent with
experimental data. 4) Transmission across the septum has a relatively minor
role in modifying the response of the CVS to negative pressure. Thus the
model reasonably predicts the effects of intrathoracic and pericardial
pressures on the circulation in a reflex-blocked animal and provides a
means for placing multiple potential mechanisms in proper hierarchial order
with regard to contributions to LV and overall CVS function.
