Cardiac performance with increasing age
cardiac performance with increasing age have been reported in dogs. Older (8-12 years
old) beagles were found to have a reduced maximum heart rate in response to isoproterenol
administration than younger (1-4 years old) beagles (Yin et al. 1979). Also, increased
impedance to left ventricular ejection has been demonstrated at exercise in 10-14-year-old
beagles compared with younger individuals (Yin et al. 1981) demonstrating that ventricular
afterload increases in older dogs.
These changes are thought to be caused by reduced cardiovascular response to b-adrenergic
stimulation due to uncoupling of an intracellular pathway possibly related to cyclic
AMP or protein kinase phosphorylation. Receptor sensitivity is reduced in the presence
of catecholamines, and increased catecholamine concentrations have been reported
in old people, who develop similar reductions in function. Chronically increased catecholamine
concentrations also occur as one of the physiological mechanisms in cardiac compensation,
such as in response to reduced cardiac output in endocardiosis.
Reduced response to b-adrenergic stimulation in the peripheral vasculature decreases vasodilation
which was demonstrated in the study of beagles at exercise (Yin et al. 1981)
as the lower impedance in the young dogs was eliminated by administration of beta-blocker.
Myocyte contraction and relaxation times are prolonged with increasing age, a phenomenon
thought to be due to reduced uptake of calcium ions by the sarcoplasmic reticulum,
resulting in increased contact time between the calcium and actin and myosin filaments.
In rats reduced concentrations of the enzyme calcium adenosinetriphosphatase have been reported, which might explain
such an effect.
There is one report that cardiac output in dogs decreases by 30% from the middle
to last one-third of a dog's lifespan (Mosier 1987). However, there are conflicting
reports from human studies about this effect of ageing and currently it is thought
that cardiac output is probably maintained at rest, but left ventricular ejection
fraction is reduced in elderly human patients during exercise. In other words the
cardiovascular system of geriatric patients has difficulty in adapting to increased
workloads, reinforcing the need to minimise exposure to excessive workload.