Fluid shifts. When a person lies flat, blood that has pooled in the legs and abdomen (from gravity) returns to the heart. The extra volume typically causes no problems for healthy persons. In pts with heart failure (chf), however, the failing heart cannot pump the extra fluid so the pressures in the chambers increase. Blood backs up into the lungs causing trouble breathing. This is relieved by sitting up.
Shortness of breath. Shortness of breath in the CHF patient is due to elevated pulmonary artery pressures. Pulmonary artery pressures are more elevated in the supine position as compared to the seated or upright positions. In a patient who is adequately treated, the supine position is not a problem. In a patient who is inadequately treated (decompensated), the supine position will make shortness of breath worse.