The Heart Is Not Always in Good Hands: THE POSSIBLE EXPLANATIONS

Posted by James

Although it has been evident that diseases of the lungs affect cardiovascular function, it has not been appreciated how simply and directly the lungs influ­ence the heart because the heart is nestled between them in the chest. It rests on the parietal pleural surfaces of the cardiac indentations of the left and right lung, and on the diaphragm below. In front is the rigid sternum. Normally pleural pressures drop and volumes increase during inhalation while the reverse occurs during exhalation: during mechanical ventilation, pleural pressure and lung volume change in the same direction. At resting end-tidal lung vol­umes the heavy heart can sink deeply into the soft lung tissues in the supine or left and right lateral decubitus positions. However, at high lung volumes (such as at the total lung capacity normally selected for decubitus chest x-ray examination), the heart is held firmly by the relatively tense pleura and dis­tended lung. It is almost incredible that cardiac function is still assessed solely in terms of rate, output, and intracardiac pressures (preload and afterload) without consideration of the pressures around the heart.

Definitions

The pressure in the cardiac fossa around the heart is called the extracardiac pressure. Transmural pres­sure is the difference between the intracardiac pres­sure inside and the extracardiac pressure outside a cardiac chamber. levitra plus

Background from Recent Research

There have been concerns that wedge pressure, measured by balloon occlusion of a pulmonary artery, was not a good reflection of left atrial pressure when pulmonary capillaries were lost or distorted in obstruc­tive pulmonary disease and that, in this situation, apparent elevations of left atrial pressure during ex­ercise, hyperventilation, etc, were spurious. However, careful studies have now shown that the wedge pres­sure is an accurate measure of left atrial pressure under these conditions. Similarly, a correctly ascer­tained jugular venous pressure is undoubtedly a good measure of right atrial pressure. Finally, a traditional misconception is that the volume of the heart changes between systole and diastole. Actually, there is no volume change, although there is a shape change, since the atria fill as the ventricles empty, and vice versa.