Determination of mean systemic filling pressure: 3 compartments

Determining the volume status is important in critically ill patients. Many
variables to determine volume status, are only validated in sedated, ventilated
patients with regular heart rate and are influenced by the ventilator settings.
The mean systemic filling pressure (the equilibrium pressure in the systemic
circulation when there is no flow) is a measure of volume status. In the
patient in the ICU it is not possible to achieve a situation when there is no
systemic flow.
We are interested in filling pressures of the entire circulation, but also of
the different compartments. At the same time we want to predict fluid
responsiveness with less invasive monitoring.
In the entire circulation the mean systemic filling pressure can only be
estimated. It is calculated by measuring stroke volume variation and central
venous pressure during inspiration and expiration. Another way to calculate
mean systemic filling pressure is by measuring arterial systolic blood pressure
during short periods of inspiratory holds with different airway pressures.
In the arm it is possible to stop blood flow temporarily by inflating a cuff
around the upper arm. A radial artery pressure can be measured in this
situation (arm-occlusion pressure, thus the zero flow pressure in the arm).
Combining arterial blood pressure with transcranial Doppler, it is possible to
extrapolate to a pressure, when there is no blood flow. Zero flow pressure in
the cerebral arteries can thus be determined by synchronized signal analysis of
arterial blood pressure and the spectral augmentation of MCA - blood flow
velocity measured with transcranial Doppler (TCD).

To determine the mean systemic filling pressure, the arm-occlusion pressure,
the cerebral zero flow pressure and their response to changes in volume status
and vasoactive medication.
To evaluate and confirm the value of mean systemic filling pressure compared
with more traditional measures of fluid responsiveness as CVP, PAOP, SVV and
PPV.

This is an observational study during standard treatment.

During 90 minutes hemodynamic and respiratory variables and blood flow
velocities (transcranial doppler) will be recorded during standard care.
During measurements of arm-occlusion prerssure, a cuff around the upper arm
will be inflated during 30 seconds.
There are no additional risks for the patient.