Fluid therapy is one of the cornerstones of the treatment of organ failure. The investigators assume that fluid bolus will increase the delivery of oxygen to the cells and resolve the shock.

The purpose of this study is to asses kinetics of the sublingual microcirculation in one place during a fluid bolus. It is expected that fluid therapy after normalization of the red blood cell flow velocity in the microcirculation will result in a decrease in capillary density through the formation of edema in the tissues. This can be considered to be the tipping of potentially beneficial to deleterious effects of fluid therapy.

After cardiac surgery patient will be transferred to the ICU for further stabilisation. Within specific indications the patient will receive a fluid bolus, these indications are hypotension, hyperlactataemia, tachycardia or decreased urine production. The fluid bolus will be 250 ml crystalloids in 15 minutes. The investigators will observe the sublingual microcirculation during this fluid bolus. To asses the red blood cell velocity and capillary vessel density on one spot during this fluid bolus.

Fluid therapy is one of the cornerstones of the treatment of organ failure. The investigators assume that a fluid bolus will increase the delivery of oxygen to the cells and resolve the shock. The evaluation of this treatment is done by clinical signs of organ perfusion en (little used) invasive hemodynamic monitoring. Previous research suggests that in-vivo microscopy of the microcirculation can be of importance to determine the type of shock and the reaction on treatment on tissue level.

Evaluation of the microcirculation was done by measuring red blood cell velocity on three different sites in the sublingual region. This was necessary in order to take heterogeneity into account, it was very hard to fix the camera on one spot during the observation period.

The purpose of this study is to asses kinetics of the sublingual microcirculation in one place during a fluid bolus. The great advantage is that the recordings before and after fluid therapy can be compared with each other as paired data, wherein the subject is its own control. By choosing a time interval that is long enough for the observation of the changes, but short enough for manual fixing of the camera, it seems possible to serve both purposes (15-30 minutes). This seems crucial now because both red blood cell flow as capillary density will be measured. It is expected that fluid therapy after normalization of the red blood cell flow velocity in the microcirculation will result in a decrease in capillary density through the formation of edema in the tissues. This can be considered to be the tipping of potentially beneficial to deleterious effects of fluid therapy.

After cardiac surgery patient will be transferred to the ICU for further stabilisation. Within specific indications the patient will receive a fluid bolus, these indications are hypotension, hyperlactataemia, tachycardia or decreased urine production. The fluid bolus will be 250 ml crystalloids in 15 minutes. The investigators will observe the sublingual microcirculation during this fluid bolus. To asses the red blood cell velocity and capillary vessel density on one spot during this fluid bolus.

Inclusion Criteria:

• > 18 years old
• post-cardiac surgery and need for fluid therapy

Exclusion Criteria:

• recent maxillofacial surgery

Postoperative cardiac surgery patients