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
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.