Modified head-up tilt test for orthostatic challenge of critically ill patients

The purpose of this study was to assess the cardiovascular response to a modified head-up tilt test for use with the bedridden, critically ill patient. The cardiovascular responses of seven normals and ten critically ill patients to 45 degrees head-up tilt with the legs horizontal (0 degrees) were analyzed. Stroke volume index (SI) and cardiac index (CI) were measured with thoracic electric bioimpedance (TEB). The baseline TEB (Z0) was measured to monitor fluid shift out of the thorax during tilt. BP was measured and systemic vascular resistance index (SVRI) was calculated. Normals responded to the modified head-up tilt with a decreased SI (59 to 44 ml/m2 and CI (3.7 to 2.8 L/min.m2), and an increased SVRI and Z0 (25.5 to 27.9). As a group, the patients showed no significant change with tilt. However, individual analysis revealed a heterogeneous response by the patients. Those patients who demonstrated a caudal shift of blood (increased Z0) had decreased SI. Those with no indication of a caudal shift of blood, presumably due to decreased venous compliance, did not change SI. Thus, this type of modified head-up tilt can be used in the ICU to study more intensely cardiovascular function and control in the bedridden subject.