Regional blood flow in congestive heart failure: concept of compensatory mechanisms with short and long time constants.

With physiologic stress to the cardiovascular system, some circulatory compensatory mechanisms are designed to restore homeostasis quickly (e.g., sympathetic nervous system activation and the Frank-Starling mechanism). These compensatory mechanisms are not nearly as effective when there is a chronic pathologic stress such as congestive heart failure (CHF). In this circumstance, other mechanisms that operate with longer time constants come into play (e.g., activation of the renin-angiotensin-aldosterone system, myocardial hypertrophy and deconditioning). The most successful chronic drug therapies of CHF are those that are designed to reverse the latter group of compensatory mechanisms, a process that is slow. It takes especially long to reverse those CHF-induced changes in blood vessels and skeletal muscle metabolism that are activated to cope with inadequate delivery of oxygenated blood to working muscles. The concept that compensatory mechanisms have either short or long time constants for activation, effectiveness and reversal may help explain why the improvement in exercise tolerance with effective heart failure therapy lags behind hemodynamic improvement.