Time-constant adaptations in heart failure.

The circulatory compensatory mechanisms designed to cope quickly with physiological stress (e.g. sympathetic nervous system and the Frank-Starling mechanism) are less effective when there is chronic pathological stress, such as congestive heart failure (CHF). Other mechanisms come into play that operate over a longer time (e.g. activation of the renin-angiotensin-aldosterone system, myocardial hypertrophy and physiological deconditioning). Changes in blood vessels and skeletal muscle metabolism that result from inadequate delivery of oxygenated blood to working muscles belong to the group of mechanisms that develop slowly. When CHF therapy is successful, the abnormalities produced by this latter group of mechanisms will improve, but slowly. The concept that compensatory mechanisms have either short or long time constants for activation and reversal may explain why exercise tolerance improves much later than haemodynamics, which can be reversed acutely with vasodilator therapy.