The increased extracellular fl uid volume caused by sodium and water
retention generates oedema. In left-sided HF, this oedema fi rst occurs
in the lungs. If the right ventricle is still pumping effi ciently, output to
the left side of the heart is too high for the failing left ventricle to cope
with. Stroke volumes and the ejection fraction decrease while the EDV
gradually rises. Blood backs up through the left atrium into the pulmonary
veins and eventually to the pulmonary capillaries. The pulmonary
circuit is a low pressure circuit. If the pressure in the pulmonary capillaries
increases above normal (approximately 20mmHg), fl uid is pushed
from the capillaries into the lung tissues and alveoli, causing impaired
gas exchange and triggering refl exes that give rise to the sensation of
shortness of breath (dyspnoea).