Transcutaneous-Arterial Carbon Dioxide and Microcirculatory Dysfunction

Evaluation of the microcirculation is currently limited. Continual assessment of the tissue carbon dioxide-arterial carbon dioxide (PtC02-aCO2) offers a new, novel and noninvasive method of determining the state of the microcirculation. The investigators will apply two non-invasive devices, the Braedius sidesteam darkfield microscopy (SDF) microscopy device to the sublingual circulation and the transcutaneous PtC02 probe to the forehead in subjects undergoing cardiac surgery with cardiopulmonary bypass. The PtC02-aCO2 gradient will be determined and correlated with the videomicroscopic images of the sublingual microcirculation. Thereafter the incidence of postoperative organ failure and acute kidney injury will be determined and correlated with PtC02-arterial CO2 gradient and videomicroscopic images.

Data will be analyzed by standard descriptive statistical methods.

There is increasing evidence that despite resuscitation and normalization of macrohemodynamic parameters (blood pressure and cardiac output) significant dysfunction of the microcirculation may persist.1, 2 There is also a positive correlation between the severity of microcirculatory dysfunction and outcome in patients with septic shock. In contrast, early improvements in microcirculatory perfusion in response to macrohemodynamic goal directed therapy is associated with an improvement in organ function.3 Monitoring the microcirculation is challenging. The monitoring window (e.g. the sublingual bed) must be reflective of other remote vascular beds. This is more likely to be the case in the setting of systemic diseases such as sepsis and hypovolemic shock. Current research modalities for studying the microcirculation include laser Doppler and videomicroscopy.4 The limitation of these devices is that assessment of the microcirculation has to be performed off-line and targets for microvascular resuscitation have not been established. Therefore, microvascular monitoring is currently restricted to the research arena. Evaluation of the tissue transcutaneous carbon dioxide (PtC02) is a novel, non-invasive, real-time method of assessing the microcirculation.5, 6 The three major determinants of PtC02 are arterial carbon dioxide (PaCO2), oxygen consumption (VCO2) and the tissue blood flow. Under normal conditions, an increased tissue metabolism (thus VCO2) is coupled with an increased tissue perfusion, largely reducing any PtC02 increase ("washout" phenomenon). Therefore, if PaCO2 is constant, an increase in PtC02 reflects an inadequate relationship between metabolism and tissue perfusion. PtC02 thus represents a good estimate of tissue perfusion. To overcome the influence of PaCO2 on PtC02, it is convenient to use the carbon dioxide gap (tissue-arterial carbon dioxide gradient, normal\<7 mmHg). Very high gap values may in addition suggest the presence of tissue hypoxia while moderately elevated gaps may represent either flow stagnation or tissue hypoxia. Cardiac surgery is characterized by significant alteration of the microvascular circulation. These changes are observed in both on-pump and off-pump cases and with pulsatile and non-pulsatile flow. Monitoring the PtC02-arterial CO2 gradient offers a rapid real-time measure of the microvascular abnormalities encountered during the post-cardiopulmonary bypass (CPB) period and their effect on post-CPB organ dysfunction.

Eligibility Criteria:
Inclusion Criteria:

1. Age \> 18 years
2. Elective cardiac surgery
3. Must be able to read and speak English

Exclusion Criteria:

1. Subjects unable/unwilling to give informed consent
2. Emergency surgery
3. Age \< 18 years
4. Pregnant females-self reported
5. Prisoners