The impact of gravity during head-up tilt

The impact of gravity during head-up tilt, a test often used in the clinic to diagnose patients who suffer from dizziness or frequent episodes of syncope, is not well described. This study uses mathematical modeling to analyze experimental blood pressure data measured at the level of the aorta and the carotid sinuses in a healthy volunteer. During head-up tilt the head is lifted above the heart stimulating gravitational pooling of blood in the lower extremities. This shift in volume is followed by an increase in blood pressure in the lower body, while the pressure in the head decreases, while the pressure at the level of the heart is either constant or increases. At the same time, the normal response to head-up tilt is an increase in heart rate. The change in posture, and subsequent change in heart rate, is believed to be mediated via baroreflex inhibition. Traditional understanding of the baroreceptor system is that inhibition is a result of a blood pressure drop. However, only the carotid sinus blood pressure is decreased during head-up tilt, suggesting that the receptors at this location are more prominent than the receptors in the aortic arch. To explore this hypothesis further, we developed a model predicting hydrostatic height between the two locations. Results from this model were compared with measurements. Furthermore, we show, using a differential equations model predicting blood pressure, that it is possible to predict blood pressure measured at the level of the carotid sinuses using heart rate as an input. Finally, we discuss our results in relation to measurements obtained at the two locations.