Continuous tissue pH and transcutaneous PO2 measurement as an index of tissue perfusion in critically ill patients

Visceral and peripheral tissue perfusion during hypothermic cardiac surgery

Central haemodynamics, blood gases, visceral and peripheral tissue perfusion and oxygenation were studied in six patients undergoing coronary artery bypass grafting (CABG) - including one patient with aortic valve reconstruction and another with CABG and aortic valve reconstruction. The patients were operated upon under moderate haemodilution and systemic hypothermia. Visceral perfusion was indirectly assessed by determining the gastric intramucosal pH (pHi). Peripheral tissue perfusion was assessed by continuous recording of subcutaneous tissue PO 2 (PscO2), laser-Doppler skin red cell flux (RCF), transcutaneous PO2 (PtcO2) and fingertip temperature (Tft) in the upper extremity. During cardiopulmonary bypass (CPB) at the deepest hypothermia, pHi, PaO2 and PtcO2 reached maximum values simultaneously with the lowest calculated oxygen utilization: pHi reached its minimum at the end of the operation. During CPB, PscO2, RCF and Tft decreased markedly, rose during rewarming and fell again at the end of surgery. These data suggest that the patients' visceral perfusion is well maintained during CPB, and the patients develop hypoperfusion and hypoxia of peripheral tissues at the same time. After closing the wounds, pHi, PtcO2 index (=PtcO 2/PaO2) and other peripheral tissue perfusion parameters gained the lowest values indicating impending residual hypothermia and tissue hypoperfusion after rewarming.

Transcutaneous monitor approximates PaCO(2) but not PaO(2) in anesthetized rabbits

OBJECTIVE

To compare the accuracy of transcutaneous (tc) to arterial partial pressure of carbon dioxide (PaCO(2) ) and partial pressure of oxygen (PaO(2) ) in anesthetized rabbits.

STUDY DESIGN

Prospective, randomized, experimental study.

ANIMALS

Eight healthy adult female New Zealand white rabbits weighing 4.05± 0.30 kg.

METHODS

Isoflurane anesthetized rabbits received six treatments in random order; PaCO(2) <35, 35-45, and >45 mmHg and PaO(2)  < 80, 100-200, >200 mmHg. Arterial and transcutaneous measurements were taken after 15 minutes of stabilization at each condition. Linear regression, correlation and Bland-Altman analysis were performed to compare PtcCO(2) to PaCO(2) and PtcO(2) to PaO(2) .

RESULTS

Over a range of measured PaCO(2) values from 21 to 67 mmHg (n=24) mean bias for PtcCO(2) was -1 mmHg and the 95% limits of agreement were -7 to 5 mmHg. The correlation between PtcCO(2) and PaCO(2) was strong with R(2) value of 0.9454. Over the entire range of measured PaO(2) values (46-508 mmHg) mean bias for PtcO(2) was -61 mmHg and the 95% limits of agreement were -226 to 104 mmHg. Correlation was poor with R(2) =0.5969. Comparing PtcO(2) to PaO(2) over a narrower range [PaO(2) < 150 mmHg (n=13)] improved the correlation, with an R(2) value of 0.8518, mean bias of -7 mmHg and 95% limits of agreement from -33 to 19 mmHg.

CONCLUSIONS AND CLINICAL RELEVANCE

In healthy anesthetized rabbits, PtcCO(2) closely approximated PaCO(2) . In contrast PtcO(2) underestimated PaO(2) , particularly at high values. The PtcCO(2) sensor may be a useful noninvasive way to assess adequacy of ventilation in anesthetized rabbits.

Noninvasive tissue oxygen monitoring in surgical and critical care medicine

Noninvasive tissue oxygen monitoring techniques are becoming increasingly important as a means for assessing perfusion and oxygenation in clinical and experimental medicine. Applications of several oxygen monitoring modalities in surgical and critical care medicine are described.