Forehead Venous Pressure (FVP) as a Measure of
Respiratory Effort:

Respiratory movements of the thoracic cage with the corresponding change in the volume of the thoracic cavity lead to changes in the intra-thoracic pressure. Since the changes in the intra-thoracic pressure are the driving force for lung expansion/compression, the intra-thoracic pressure is considered a direct measure of respiratory effort, i.e. the effort exerted in order to inhale and/or exhale. However, intra-thoracic pressure affects not only lungs but also the other organs in the thoracic cavity, particularly those with those with wide lumens and thin walls containing little elastic elements such as large veins and esophagus. Thus, the gold standard for quantitatively measuring respiratory effort is using an esophageal catheter with a pressure transducer. Due to invasiveness of the method and its discomfort, esophageal pressure monitoring is rarely used in the routine clinical practice. Instead, indirect measures of respiratory effort are used such as measuring the movements of the thorax and abdomen with piezo-resistive belts, or changes in the volume of the thorax and abdomen with respiratory inductance plethysmography (RIP).

Changes in intra-thoracic pressure affect the large veins on their way through the mediastinum to the right atrium. The relation between the intra-thoracic pressure and venous return is well established in physiology. However, since the jugular veins have no valves, any change in volume of the superior vena cava and brachio-cephalic veins will be directly transmitted to the veins that drain into the jugular veins - among them, to the veins on the forehead. Therefore, the forehead veins can be thought of as a natural 'catheter' through which one can monitor pressure changes in the thoracic cavity.

The animation illustrates the described relationship. With each inhalation intra-thoracic pressure becomes more negative, increasing venous return from the periphery to the superior vena cava and the right atrium - thus, the forehead vein empties itself and its wall nearly collapses. During an exhalation the return of the venous blood is hampered by an increase in the intra-thoracic pressure which makes the pressure and volume in the forehead vein rise. The amplitude of the changes in the forehead venous pressure/volume is proportional to the change in intra-thoracic pressure. Therefore, by measuring the forehead venous pressure/volume, one can effectively measure respiratory effort.