Carotid Doppler Ultrasound for the Measurement of Intravascular Volume Status

Ultrasound represents an attractive non-invasive method to assess hemodynamic status. Understanding dynamic changes in hemodynamics in situations such as hypovolemia, sepsis, and cardiogenic shock can potentially help improve patient care. However, the inter-rater reliability and accuracy of how various ultrasound measurements reflect dynamic changes in physiology remains incompletely understood. Overall our aims are to investigate the use of ultrasound in a controlled setting, specifically using lower body negative pressure (LBNP), which can simulate hypovolemia at varied levels in human volunteers. Aim 1: To determine the change in carotid blood flow (measured by velocity time integral, VTI) in subjects undergoing simulated hypovolemia at LBNP levels that precede vital sign changes. Hypothesis: Carotid VTI will demonstrate significant changes that precede vital sign changes in simulated hypovolemia. Aim 2: To compare transcranial color Doppler indices of cerebral blood flow with carotid blood flow, as assessed by VTI of the common carotid artery. Hypothesis: Changes in transcranial color Doppler indices of cerebral blood flow will be mirrored by changes in carotid blood flow, indicating carotid VTI is an adequate surrogate for measuring cerebral blood flow in variable states of central hypovolemia. However, if cerebral blood flow remains more constant than carotid blood flow throughout varying levels of hypovolemia, our assumption is that cerebral autoregulation alters the relationship between carotid and cerebral blood flow. The more complex procedure of Transcranial Doppler ultrasound (TCD) must be performed to obtain valid assessments of cerebral blood flow.