Arterial Effects (Afterload) The intrathoracic blood vessels are exposed to the drop in pressure with inspiration of approximately 7 mmHg. For the left ventricle, the effective pump level drops by that amount, since the bulk of the systemic circulation is outside the chest, at atmospheric pressure. To maintain the same pressure in the peripheral circulation, an increase in pressure generated by contraction of the left ventricle would have to be developed. This slight increase in afterload for the left ventricle has relatively little effect on the stroke volume of the left ventricle under normal conditions of respiration and circulation, but does account for the slight respiratory fall in systolic blood pressure that can be easily measured in the normal subject. By contrast, the right ventricle ejects into the pulmonary circulation within is entirely within the thorax and therefore the right ventricle experiences no change in afterload.
Reflex Effects In healthy subject at rest, inspiration is associated with an acceleration of the heart rate, and a deceleration occurs with expiration. This phenomenon, sinus arrhythmia, occurs under conditions of vagal control over the cardiac pacemaker, via neural radiation from the respiratory centre in the medulla to the cardiovascular center. This has a major effect on stroke volume of the two ventricles, through changes in the filling interval for the ventricles. With inspiratory acceleration, the filling interval is diminished and the stroke volume for both ventricles becomes smaller. With sinus arrhythmia, the varying diastolic interval becomes the single most important determinant of the stroke volume, and effect of enhanced venous return are less obvious. When the subject is stressed, resulting in an increased sympathetic tone and diminished vagal tone, tachycardia results, and the variation of filling interval disappears. At that time, the effect on stroke volume of enhanced venous return to the right heart with inspiration is much more pronounced.
Pleural Pressure During tamponade normal respiration persists, and the wings of intrapleural pressure do not change significantly, an average of 7 mmHg. Thus, in tamponade, the phenomenon of pulsus paradoxus (a drop in systolic blood pressure with inspiration of greater than 10 mmHg) cannot be attributed to a direct transmission of intrathoracic pressure.
Pericardial Pressure The peak-to peak pressure change with respiration in the pericardium is exactly the same as the pleural pressure. However, since pericardium is distended with fluid, its overall pressure is substantially; elevated. This increased pressure reduces the transmural or distending pressure for the heart, and drastically interferes with filling of the two ventricles. The rise in pericardial pressure is not limitless, since all circulation will cease when the pericardial pressure rises above 20 mmHg. Near that point, all of the diastolic pressure throughout the heart will be similar, in both atria and both ventricles.
The pericardial effusion does not shield the heart from the changing intrathoracic pressure of respiration, which is superimposed on the high pressures due to the tamponade. The pericardial pressure will also fluctuate with the cardiac cycle, to a much greater extent than the normal, since the pericardium is quite full, and small changes in volume will produce large changes in pericardial pressure.
The pericardium is capable of a slow stretching, and if the fluid accumulates very slowly, the pressure will not necessarily rise, and sometimes a rather large effusion can be tolerated without tamponade and pulsus.
Venous Return A high rate atrial pressure, as the downstream pressure of the systemic circulation, has a markedly inhibiting effect on systemic venous return. This is the case in tamponade, and shock is usually present with very high venous pressure. The 7 mmHg. Drop with normal inspiration is effective in temporarily increasing venous return to the right heart. Since the stroke volume is markedly limited by the limited diastolic filling, the enhanced venous return will make an obvious improvement in the stroke volume of the right ventricle with inspiration. After 1-2 beats, that stroke volume will appear in the left ventricular output, but that increase will be delayed by 2-3 beats from the onset of inspiration, and therefore systolic blood pressure will be at its lowest during inspiration. The effect of inspiration on the pulmonary veins, as discussed under the normal circulation, is minimized by the fact that the pulmonary vein and the left heart are subjected to the same intrathoracic pressure. An earlier theory of pulsus paradoxus contended that the pulmonary veins actually pooled blood temporarily with inspiration, through an increased transmural pressure, whereas the left ventricle was shielded from the effects of inspiration by the pericardial effusion.
Another theory explaining pulsus paradoxus that remains popular is that the enhanced venous return to the right ventricle with inspiration successfully competes for the fixed volume available within the pericardium. With a fixed total volume, and increase in right ventricular filling would competitively inhibit left ventricular filling, leading to a decrease in the stroke volume which in turn would lead to low systolic pressure during inspiration that is characteristic of pulsus paradoxus. It has been shown in human subjects with tamponade that, indeed, the ventricular septum is displaced into the left ventricular cavity during inspiration. That this could also be attributed to a diminished left ventricular volume of the delayed effect of the expiratory decrease in right ventricular stroke volume. Arterial Effects (Afterload) Since the fall in pleural pressure with tamponade is the same as in the normal, increased afterload cannot reasonably explain the substantially greater all in systolic blood pressure. The slight increase in afterload for the left ventricle could have a negative effect on the left ventricular stroke volume, but both by percentage and by actual volume, the effect would be trivial.
Reflex Effects In tamponade, a shock-like state exists, and invariably there is tachycardia and generalized vasoconstriction, which probably involves the veins as well as the arteries. Without this reflex adjustment, the mean circulatory pressure could not be increased, and circulation would cease at as earlier point in the disorder.