Multifaceted Anesthesia

Gastric tonometry today

Prof. B. Vallet, MD, PhD Département d'Anesthésie et de Réanimation II Hôpital Claude Huriez CHU Lille Lille Cedex, France. http://www.chru-lille.fr Click for the Prof .Vallet's Curriculum Vitae and publications.

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Tonometry, an indirect method of monitoring gut perfusion

It is proposed that the detection and reversal of gastrointestinal (GI) ischemia could limit the occurrence of multiple organ dysfunction syndrome (MODS) in critical illness and its associated morbidity and mortality. Circulatory failure is associated with early GI vasoconstriction and redistribution of flow from non-vital to vital organs. Prolonged vasoconstriction may lead to GI mucosal ischemia injury and secondary inflammatory response. This has been hypothesized to provoke distant organ injury and MODS. Gastric tonometry is a unique clinical measurement able to provide information easily and routinely about GI ischemia. By measuring the intramucosal partial pressure of carbon dioxide (PgCO2) tonometry is an intermittent and indirect method of monitoring gut perfusion. See Table 1 for an explanation of some tonometry related terms and abbreviations.

Table 1. A selection of gastric tonometry related terms and abbreviations. Both systemic and pulmonary hemodynamics variables and systemic oxygen derived variables are listed here.

Increase of PgCO2 as an early sign of systemic hypoperfusion

Recently published studies on tonometry provide information on the physiology, technology, and clinical relevance of the method in intensive care unit (ICU) patients. In dogs subjected to stepwise hemorrhage, Guzman et al. [1] clearly demonstrated that the PgCO2 increase before oxygen uptake (VO2) was limited by O2 supply (DO2). The increase in PgCO2 can be related to the reduced washout of CO2, which results from impaired GI mucosal blood flow. This experimental study shows that the increase does not necessarily equal dysoxia, since blood lactate does not increase and arterial pH does not decrease. Dysoxia can be defined as the critical level of DO2 (DO2crit) at which VO2 is no longer sustained.

Nonetheless, an increase in PgCO2 is present and can therefore be considered an early signal of systemic hypoperfusion that can be treatable before the occurrence of cell injury [2, 3].

PgCO2 can be measured using air-automated tonometry

When DO2crit is reached, anaerobic CO2 production contributes to increased PgCO2, while arterial pH decreases and favors a decrease in intramucosal pH (pHi). When pHi is used to guide treatment, therapeutics might prove to be ineffective if pHi is already low [4].

PgCO2 might to some extent provide the clinician with “earlier information” when compared with pHi. The latter gives only “late information” with less ability to help the clinician reverse non-fatal hypoperfusion, although it has a strong predictive value of death with values lower than 7.32. This consideration might explain why outcome among patients entering the ICU with low pHi did not improve in trials where pHi was used as a therapeutic index [4, 5]. In this regard, PgCO2 might prove to be a better therapeutic index.

PgCO2 measurement obtained through a gastric balloon is minimally invasive (Figures 1-2). Determining PgCO2 is made easier today by air-automated tonometry, a technique that allows a short equilibration time (10 minutes). Creteur et al. [6] have demonstrated that this technique can significantly improve the precision of PgCO2 determination when compared with the previously used saline tonometry.

Figure 1. The minimally invasive tonometry catheter in place. Note the inflated gastric balloon (Photo courtesy of Datex-Ohmeda©)	Figure 2. Technical description of the tonometry catheter (Photo courtesy of Datex-Ohmeda©)

Why remove the influence of ventilation from tonometry values?

PgCO2 needs to be compared to PaCO2 (CO2gap) in order to remove the influence of ventilation. A CO2gap of 18 mmHg was found to be prognostic of MODS and death when a cohort study of 114 consecutive trauma patients admitted to the ICU was considered [7]. More recently, a close threshold value of 20 mmHg for CO2gap was similarly shown in ICU-ventilated patients (Figure 3) to be associated with an increased risk of mortality [8]. Any value between 8 mmHg (normal) and 18 mmHg therefore suggests a redistribution of flow, calling for therapeutic intervention.

An alternative to the mucosal-to-arterial CO2 difference P(g-a)CO2 is to relate PgCO2 to end-tidal PCO2 (PetCO2). The System 5 (S/5™) Tonometry Module or Tonocap™ (Datex-Ohmeda, Helsinki, Finland) automatically keep track of PetCO2 and allow semi-continuous monitoring of gut-to-end-tidal PCO2 difference P(g-et)CO2. This difference may be used as a useful index of GI perfusion in critically ill patients [9] or in high-risk surgery [10-12].

A gap of 21 to 25 mmHg is associated with organ failure together with a prolonged need for critical care and hospital stay in that context [10-12]. Whether P(g-a)CO2 and/or P(g-et)CO2 can be used as a therapeutic index still remains entirely unknown.

Tonometry monitoring may be associated with improved outcome in critical care

Although more investigations clearly need to be done, it must be emphasized that tonometry is one of the very few existing techniques in which monitoring was associated with a demonstration of improved outcome. This can be said at least when tonometry-derived pHi was used to direct goal therapy in critically ill patients entering the ICU with a pHi on admission not lower than 7.32.

The CO2 gap [P(g-a)CO2 or P(g-et)CO2] can be proposed to direct therapy as soon as its value goes beyond 12 to 15 mmHg. If an empirical therapeutic decision (fluid challenge, low dose inotrope) is not followed by improvement, the central hemodynamics need to be re-assessed and corrected as necessary.

Figure 3. Kaplan-Meier 28-day survival curves according to the PCO2 gap at hour 24
(after reference 8).

References

1. Guzman JA, Lacoma JF, Kruse JA. Relationship between systemic oxygen supply dependency and gastric intramucosal PCO2 during progressive hemorrhage. J Trauma 1998;44: 696-700
2. Vallet B, Teboul JL, Cain S, Curtis S. Venoarterial CO2 difference during regional ischemic or hypoxic hypoxia. J Appl Physiol 2000;89:1317-1321.
3. Neviere R, Chagnon JL, Teboul JL, Vallet B, Wattel FB. Small intestine intramucosal PCO2 and microvascular blood during hypoxic and ischemic hypoxia. Crit Care Med 2002;30:379-384.
4. Gutierrez G, Palizas F, Doglio G, et al: Gastric intramucosal pH as a therapeutic index of tissue oxygenation in critically ill patients. Lancet 1992;339:195-199.
5. Pargger H, Hampl KF, Christen P, Staender S, Scheidegger D. Gastric intramucosal pH-guided therapy in patients after elective repair of infrarenal abdominal aneurysms: is it beneficial? Intensive Care Med 1998;24:769-76
6. Creteur J, De Backer D, Vincent JL. Monitoring gastric mucosal carbon dioxide pressure using gas tonometry 1997;
7. Miller PR, Kincaid EH, Meredith JW, Chang MC. Threshold values of intramucosal pH and mucosal-arterial CO2 gap during shock resuscitation. J Trauma 1998;45: 868-872
8. Levy B, Gawalkiewicz P, Vallet B, Briancon S, Nace L, Bollaert PE. Gastric capnometry with air-automated tonometry predicts outcome in critically ill patients. Crit Care Med. 2003;31:474-80
9. Lebuffe G, Decoene C, Pol A, Prat A, Vallet B. Regional capnometry with air-automated tonometry detects circulatory failure earlier than conventional hemodynamics after cardiac surgery. Anesth Analg 1999;89:1084-90
10. Lebuffe G, Decoene C, Raingeval X, Lokey JS, Pol A, Warembourg H, Vallet B. Pilot study with air-automated sigmoid capnometry in abdominal aortic aneurysm surgery. Eur J Anaesthesiol. 2001;18:585-92
11. Lebuffe G, Onimus T, Vallet B. Gastric mucosal-to-end-tidal PCO2 difference during major abdominal surgery: influence of the arterial-to-end-tidal PCO2 difference? Eur J Anaesthesiol 2003;20:147-52
12. Lebuffe G, Vallet B, Takala J, Hartstein G, Lamy M, Mythen M, Bakker J, Bennett D, Boyd O, Webb A. European, multi-center, observational study to assess the value of gastric-to-end tidal PCO2 difference in predicting postoperative complications. Anest Analg (being printed)
    

Last updated: 25 March 2004 Created
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