Monash Cardiovascular Research Centre, MonashHeart, Monash Health, Clayton, Victoria, Australia.
Department of Medicine, School of Clinical Sciences at Monash Medical Centre, Monash University, Clayton, Victoria, Australia.
Adv Physiol Educ. 2020 Sep 1;44(3):414-422. doi: 10.1152/advan.00043.2020.
Preload has been variously defined, but if there is to be a direct relationship with activity of the Frank-Starling mechanism in its action to increase the force and extent of contraction, preload must directly reflect myocardial stretch. The Frank-Starling mechanism is activated during any stretch of a cardiac chamber beyond its resting size, which is present immediately before contraction. Every left ventricle has an intrinsic and limited range of possible volumes at end diastole. There is a curvilinear relationship between left ventricular (LV) end-diastolic pressure (LVEDP) and LV end-diastolic volume (LVEDV), and, at maximal or near maximal LVEDV, there will be a high LVEDP. Within the possible range, the LVEDV will be determined by the extent of filling, any change in LVEDV will result in changed activity of the Frank-Starling mechanism, and change in LVEDV might, therefore, be considered to represent change in preload. On the other hand, it is the difference between the current and the maximal possible LVEDV (or the preload reserve) that may be of the most clinical relevance. There is a reciprocal relationship between preload and preload reserve, with minor or absent LV preload reserve indicating that there will be either minimal or no increase in stroke volume following intravenous fluid administration. As left atrial pressure can remain within the normal range when the LVEDP is elevated, it is LVEDP, and not left atrial pressure, that provides the most reliable guide to preload reserve in an individual at a specific period in time.
预负荷的定义多种多样，但如果要与心肌的 Frank-Starling 机制活动直接相关，从而增加收缩力和收缩幅度，那么预负荷就必须直接反映心肌的拉伸程度。Frank-Starling 机制在心脏腔室的伸展超过其静息大小（即在收缩前即刻存在）时被激活。每个左心室在舒张末期都有一个固有且有限的可能容积范围。左心室舒张末期压力（LVEDP）与左心室舒张末期容积（LVEDV）之间存在曲线关系，并且在最大或接近最大的 LVEDV 时，LVEDP 会很高。在可能的范围内，LVEDV 将由充盈程度决定，任何 LVEDV 的变化都将导致 Frank-Starling 机制的活性发生变化，因此，LVEDV 的变化可以被认为是前负荷的变化。另一方面，最具临床相关性的可能是当前和最大可能的 LVEDV（或前负荷储备）之间的差异。前负荷和前负荷储备之间存在相互关系，前负荷储备较小或不存在表明，在静脉输液后，心搏量的增加将非常有限或不存在。由于左心房压力在 LVEDP 升高时仍可能保持在正常范围内，因此 LVEDP 而不是左心房压力，是特定时间段内个体前负荷储备的最可靠指南。
