Factors affecting performance of air tonometry using the TONOCAP

Sublingual capnometry: a non-invasive measure of microcirculatory dysfunction and tissue hypoxia

With improvement in supportive care patients rarely die from their presenting illness but rather from its sequela, namely sequential multi-organ failure. Tissue hypoxia is believed to be the causation of multi-organ dysfunction syndrome (MODS). The expedient detection and correction of tissue hypoxia may therefore limit the development of MODS. The standard oxygenation and hemodynamic variables (blood pressure, arterial oxygenation, cardiac output) which are monitored in critically ill patients are 'upstream' markers and provide little information as to the adequacy of tissue oxygenation. Global 'downstream' markers such as mixed venous oxygen saturation and blood lactate are insensitive indicators of tissue hypoxia. Sublingual PCO(2) is a regional marker of microvascular perfusion and tissue hypoxia that holds great promise for the risk stratification and end-point of goal directed resuscitation in critically ill patients. This paper reviews the technology and application of sublingual PCO(2) monitoring.

Regional carbon dioxide monitoring to assess the adequacy of tissue perfusion

PURPOSE OF REVIEW

Tissue dysoxia is now widely regarded as the major factor leading to organ dysfunction in critically ill patients. Recent data suggests that early aggressive resuscitation of critically ill patients, which limits and/or reverses tissue dysoxia may prevent progression to organ dysfunction and improve outcome. The traditional clinical and laboratory markers used to assess tissue dysoxia are, however, insensitive and have numerous limitations. Regional carbon dioxide monitoring appears to be ideally suited to monitoring the adequacy of resuscitation. This review provides an update on this evolving technology.

RECENT FINDINGS

Gastric intramucosal carbon dioxide as measured by gastric tonometry has proven to be useful as a prognostic marker, in evaluating the response to specific therapeutic interventions and as an end point of resuscitation. Gastric tonometry is, however, cumbersome and has a number of limitations that may have prevented its widespread adoption. The measurement of carbon dioxide in the sublingual mucosa by sublingual capnometry is technically simple, noninvasive, and provides near instantaneous information. Clinical studies have demonstrated a good correlation between gastric intramucosal carbon dioxide and sublingual mucosa carbon dioxide. Sublingual mucosa carbon dioxide responds more rapidly to therapeutic interventions than does gastric intramucosal carbon dioxide and may be a better prognostic marker.

SUMMARY

Sublingual capnometry may be the ideal technology for guiding early goal directed therapy. This technology may be useful for monitoring tissue oxygenation, titrating therapeutic interventions, and as an end point for resuscitation in critically ill and injured patients.

Performance of automated air tonometry under hypothermia

Intestinal tonometry is used during hypothermic cardio-pulmonary bypass surgery to assess splanchnic perfusion. In an in vitro set-up the performance of automated air tonometry (TONOCAP) was tested for normo- and hypothermia. A 14-FG tonometry catheter was built into a testing chamber (100 cm(3)) perfused with blood from a cardio-pulmonary bypass circuit with P(a)co(2) held at 5.6-5.8 kPa (alpha-stat). P(r)co(2) from the balloon of the tonometry catheter was measured at intervals of 10 min at 37 degrees C and at 25 degrees C by the TONOCAP. Bias (precision) of P(r)co(2) - P(a)co(2 alpha-stat) and P(r)co(2) -P(a)co(2 pH-stat) at 37 degrees C blood temperature were low at 0.23 kPa (0.21) each. Tonometrically measured P(r)co(2) at 25 degrees C significantly differed from P(a)co(2 alpha-stat) bias (precision) of 2.00 kPa (0.11) but was similar to P(a)co(2 pH-stat) (0.30 kPa (0.11)). P(r)co(2) values as measured by the TONOCAP represent pH-stat approach. Identical blood gas management (pH- or alpha-stat) should be used for calculation of mucosal-arterial CO(2) difference (P(r-a)co(2) gap) or calculation of intramucosal pH.

Visceral and Renal Tissue Oxygenation During Supraceliac Aortic Crossclamping and Left Heart Bypass with Selective Organ Perfusion

INTRODUCTION

Left-heart bypass (LHB) and selective organ perfusion (SOP) are used during thoracoabdominal aortic surgery to prevent ischemic damage to the kidneys and visceral organs after supraceliac aortic crossclamping. We studied the hypothesis, in a porcine model, that despite LHB and maximal SOP, visceral and renal ischemia still occurred during surgery.

MATERIAL AND METHODS

Eleven pigs (54-70 kg) were coupled to a non-pulsatile LHB with inflow and outflow at the lower thoracic and distal infrarenal aorta, respectively. After supracoeliac and infrarenal aortic crossclamping, SOP was started using perfusion catheters. The proximal and distal mean aortic blood pressures were kept above 70 and 50 mmHg, respectively, while the mean blood pressure within the SOP system was above 60 mmHg. The visceral and renal tissue oxygenation was measured by intermittent blood gas analysis, from the portal and both renal veins. The jejunal mucosal oxygenation was measured by tonometric measurement of the luminal pCO2.

RESULTS

Measured median blood blood flow through the LHB and the SOP system were 800 and 1140 ml/min, respectively. Median blood flow prior to, and during LHB and SOP through the celiac artery, superior mesenteric artery, and left renal artery were 300 and 240, 762 and 295, and 235 and 235 ml/min, respectively. During 3 h of LHB and SOP no significant changes in the renal tissue oxygenation were noted compared with the physiological situation prior to supracoeliac aortic crossclamping and cannulation. However, in the visceral vascular bed median mixed venous oxygen saturation dropped from 79 to 63% (p<0.001), and median oxygen extraction ratio increased from 26 to 41% (p<0.001). Median tonometric measured intraluminal jejunal pCO2 increased from 9.9 to 12.15 kPa (p>0.05). During 3 h of LHB and SOP no hemolysis was detected, as there was no rise in serum LDH.

CONCLUSION

LHB and SOP preserves renal but not visceral tissue oxygenation during supraceliac aortic crossclamping and does not induce hemolysis.

Nitrous oxide does not affect automated air tonometry in children

PURPOSE

To evaluate the effects of nitrous oxide on automated air tonometry in the clinical setting.

MATERIAL AND METHODS

With approval of the Hospital Ethical Committee and after obtaining informed parental consent, an 8-F tonometry catheter was inserted orogastrically in ten children aged one to three years scheduled for elective surgery with combined regional and general anesthesia. A standardized general anesthesia technique with tracheal intubation was used in all patients and consisted of sevoflurane in oxygen/nitrous oxide (30%/70%; n = 5 patients) or in oxygen/air (FIO(2) 0.3; n = 5 patients). After obtaining steady state gastric CO(2) values (PrCO(2)), fresh gas mixtures were rapidly changed from oxygen/nitrous oxide to oxygen/air (A) or vice versa (B). In addition, balloon pressures were recorded using a pressure transducer. Measurements were performed at intervals of ten minutes with recording of balloon pressures, end-tidal CO(2) (PETCO(2)) and PrCO(2) values. Pr-ETCO(2)-gap were calculated to eliminate influences of changes in PaCO(2).

RESULTS

Changing the fresh gas mixture from N(2)O/O(2) to O(2)/air resulted in a decrease of balloon pressure of -10.4% (113.4 +/- 14.7 mmHg to 101.6 +/- 25.0 mmHg). Changing the fresh gas mixture from O(2)/air to N(2)O/O(2) resulted in an increase of balloon pressures of 6.4% (107.6 +/- 19.3 mmHg to 114.0 +/- 20.3 mmHg). During both fresh gas exchange experiments no significant changes (> 0.2 kPa) in calculated Pr-ETCO(2)-gaps were observed.

CONCLUSIONS

Based on our in vivo data, nitrous oxide during general anesthesia can be used with automated air tonometry and does not affect air tonometric PrCO(2) reading in clinical practice.