Why do you get nocturia in cardiac failure?

Nocturia is the name for needing to urinate excessively at night. It is one of the less obvious clinical features of cardiac failure (it has other causes too, though), and I find that unless you specifically ask your patients about it, they seldom volunteer the information.

The reason that it occurs in the context of cardiac failure is simple. In this condition, by definition, the heart isn't able to adequately perfuse the tissues, and those tissues include the kidneys. At night, however, such patients are obviously (like the rest of us) lying down. With the entire body flat, fluid that has pooled in the extremities during the day finds it easier to return to the heart, and consequently the cardiac output at night in these patients is slightly increased. All this means that the kidneys can filter more blood, but at the expense of making more urine - all at night.