TIME-VARYING LOADING OF THE PULMONARY CIRCULATION - A MODEL TO DESCRIBE HEMODYNAMIC OBSERVATIONS IN THE STIFF LEFT ATRIAL SYNDROME

Objective: To develop a mathematical model that could explain the foll owing observations: that right heart failure can develop in associatio n with no other cardiac abnormality than a severe reduction in the com pliance of the left atrium; and that patients with this syndrome have systolic pulmonary hypertension with left atrial v waves in the absenc e of either mitral regurgitation or left ventricular dysfunction. Desi gn: A model of the pulmonary circulation was designed with a time vary ing terminal hydraulic load, which was varied between a noncomplaint l eft atrium during systole and a complaint left ventricle/left atrium c ombination during diastole. Using representative parameters and a pulm onary arterial flow wave as input, pressures in the pulmonary artery a nd left atrium and right ventricular power output were calculated. Res ults: Pulmonary arterial and left atrial systolic pressures are increa sed as left atrial compliance is reduced. The time varying change in t erminal load results in an increase in systolic pressures, whereas dia stolic pressures remain normal. A decrease in left atrial compliance i ncreases both the nonpulsatile and pulsatile components of pulmonary i nput impedance, whereas only the nonpulsatile component of right ventr icular power output is increased. Conclusions: The time varying pulmon ary load model of the pulmonary circulation, in the presence of a redu ced left atrial compliance results in pulmonary and left atrial pressu res similar to those observed in patients with the stiff left atrial s yndrome. The resulting increase in right ventricular power output coul d be an important factor in the development of right heart failure.