Pressure distribution in the thorax during mechanical ventilation and its effects on the circulation

Fluid administration is a daily intervention on the intensive care unit to
improve cardiac output and stabilize circulation in critically ill patients.
Simultaneously, the volume status of the patient is very difficult to assess.
Too little volume leads to inadequate organ perfusion followed by ischemia and
organ failure. Too much volume may worsen heart failure and cause pulmonary and
peripheral edema and contribute to further tissue injury and organ dysfunction.
Although dynamic indices have been shown to be more accurate predictors of
fluid responsiveness, this relevant and complex task is usually guided by
static clinical variables and the specialist*s interpretation due to the fact
that the interpretation of dynamic parameters is not fully developed and that
they are not universally available. This lack of understanding is partially
because of the complex interaction with mechanical ventilation. We hypothesize
that knowing the distribution of ventilatory pressures will make it possible to
index dynamic parameters to tidal volume and improve their predictive value
concerning the volume status of the patient.
In addition, it would be of interest to be able to predict fluid responsiveness
in a non-invasive way, especially in critically ill patients. Up to now,
continuous non-invasive cardiac output monitoring using Nexfin CO in critically
ill patients has not been validated and also not tested for its ability to
predict fluid responsiveness. The present research proposal evaluates the
possibility and accuracy of the model flow analysis obtained by non-invasive
finger arterial pressure measurements to determine fluid responsiveness using
passive leg raising. It will also be compared to a more invasive method (that
is currently used in the clinic) to assess its ability to measure absolute CO
levels accurately. It may make it possible to assess fluid responsiveness in a
non-invasive and patient friendly way.

The primary objective of the study is to evaluate the indexation of the dynamic
parameters for predicting fluid responsiveness at different tidal volumes and
to compare the performance of the individual dynamic indices with an integrated
parameter. The secondary objective of the study is to determine the way
(quantitatively) the ventilatory pressures (as a result of the tidal volume)
are being distributed along the thorax (esophagus, pleura, pericardium) for a
better understanding and interpretation of the measurements of dynamic indices
for fluid responsiveness. The tertiary objective is to determine to what extent
the Nexfin is able to predict volume responsiveness in combination with a
passive leg raising test.

Prospective interventional trail in patients scheduled for open heart surgery.

During surgery, pericardial and pleural drains are routinely placed. For the
patients that participate in the present study, these drains will be
accompanied by small catheters for pressure measurements. On the Intensive Care
Unit the patients are closely monitored and receive regular care from the
attending physician who will not be participating in the study, also they
receive an esophagus catheter to measure the pressure in the lungs. When the
patient is stable (cardiovascular and respiratory), the patients are ventilated
following a specific protocol for a period of 12 minutes. This protocol is
repeated with a different compliance of the patients* thorax. This is
accomplished by a wide rubber band placed around the thorax of the patient. The
band will be adjusted to obtain a decrease of 25-30% in compliance during tidal
volumes of 8 ml/kg. The passive leg raising test is performed in case the
patient is considered to be fluid responsive according to clinical reasons and
fluid resuscitation is started. Clinical reasons and fluid therapy are
interpreted and initiated by the attending physician. The measurements
performed for the present study will not be available for the attending
physician.

During surgery, pericardial and pleural drains are routinely placed. For the
patients that participate in the present study, these drains will be
accompanied by small catheters for pressure measurements. There is no
additional risk or discomfort for the patient.
On the intensive care the patients are normally ventilated with tidal volumes
between 6-10 ml/kg. For a short period (12 minutes in total), a protocol for
mechanical ventilation is used with variable tidal volumes (4-10 ml/kg).
Changes in tidal volume for short periods (3 minutes) will not lead to
discomfort in sedated patients and do not result in additional risk for the
patient. With use of capnography and adjustments of breathing frequency,
respiratory minute volume will be kept constant. The esophagus catheter to
measure the pressure in the lungs is used on a regular base in the clinic to
measure esophageal pressures and does not interfere with regular patient care
and also the elastic bands to influence the compliance of the thorax is
commonly used in clinical practice (mostly in female patients with large
breasts to prevent sternum dehiscence).
The patients do not need to stay in the hospital longer than needed for medical
reasons and no extra hospital visit is necessary.