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G. Michagin EJ. Jensen P. Klint-Andersen

The accuracy of gastric tonometry: a matter of mathematical thinking

Received: 12 September 1995 Accepted: 20 May 1996

Sir: Since the introduction of gastric tonometry by Fiddian-Green, the tonometer has been clinically adapted as a useful monitoring tool of systemic ox- ygen needs and gastrointestinal perfusion under different conditions [1]. It has also been shown to be a good prognostic aid in the treatment of critically ill patients [2, 31.

The studies so far have compared measurements from the tonometry meth- od to systemic circulatory parameters; i.e. arterial pH, partial pressure of oxygen in arterial blood (PaO2), bicarbonate (HCOf) , base excess, lactate concentra- tions, and cardiac output [2, 3]. All the studies have used the arterial H C O f and pK a values in the modified Henderson- Hasselbalch equation for the calculation of tonometry pH (pHi). The reason for the substitution of mucosal H C O f with the arterial HCO 3 in the Henderson- Hasselbalch equation is the difficulty in obtaining mucosal HCO 3.

The intracellular space makes up 80% of the body weight and arterial blood measurement will only reflect some of the intracellular events. Accordingly, the substitution of mucosal H C O f with arterial HCO 3 will not always reflect the actual state of HCO;- in the gut, especial- ly not during hypoperfusion or the noflow state to the gut. Furthermore, the use of plasma pK a as the dissociation constant of H C O ; will introduce some analytical bias, as pK a is a conditional constant dependent on the actual pH and temperature. Both parameters are subject to biological variation, which may be ag- gravated under extreme pathophysiological conditions [4]. Ideally, elimination of the arterial HCO 3 and pK a parameters in the equation will improve the mathematical strictness.

The Henderson-Hasselbalch equation is expressed as follows:

. , CaHCOf PHa = Plea +lOg PaCO 2 x up (1)

where up is the solubility coefficient of CO 2 in plasma.

The gastric tonometry equation is a modification of the Henderson- Hasselbalch equation and is expressed as:

pH i = pK a + log CaHCOf (2) PiCOa • UNaCi • F

where the UNaCl is the solubility coeffi- cient of CO 2 in NaC1 and F is a correc- tion factor of equilibration time of the gastric tonometer.

Equations 1 and 2 can be rearranged and combined to derive as:

pH i = pH, +log P , C _._~.~.O~+log up (3) FiC;U 2" P UNaC1

The up value has been determined ex- perimentally as 0.0306 mmol ' l 1 .mmHg-I under normal conditions at 37 ~ [4]. In gastric tonometry the isotonic saline solution is often used and UNaCl can be obtained from the equation:

log u(Aq) = log u(Aqoo)-0.085-I (4)

where a (Aq co) = 0.0329 mmol. 1-1. mmHg-a and the ionic strength, I, for 0.9% NaC1 is 0.154 [4]. The value of UNaCt obtained from Eq. 4 will be 0.0319 mmo1.1 -I -mmHg -1.

Using the ap and UNaCI values in Eq. 3, the following will derive:

pH i = PHa+log PaCO2 -0.0181 (5) PiCO2 �9 F

All parameters are directly measurable and no assumptions are needed, and, in theory, the accuracy of pH i should im- prove considerably. Furthermore, the equation considers the different solubility coefficients of CO 2.

With the rearrangement of the modi- fied Henderson-Hasselbalch equation, the correction factor of equilibration time, F, will remain as a source of incaccuracy. Another contributory source of inac- curacy could be the up, as this coefficient may vary with protein content, ionic strenght, and temperature. When buffer solutions are used to further improve the accuracy of the measurements of gastric PiCO2, one has to consider the change of solubility coefficients in Eq. 5, as different buffer solutions with their respective solubility coefficients will make an impor- tant contribution on the second decimal in the calculation of pH i . However, the disadvantage of using buffer solution has recently been stressed by Takala et al., as the time to obtain the equilibration of gastric tonometry will increase con- siderably due to the increased CO 2 bind- ing capacity [5]. Another advantage of

Eq. 5 is the feasibility of calculating the pH difference, ApH, between mucosal pH i and arterial pH a. The ApH can be expressed as the logarithm of PaCO 2 and PiCO2:

ApH = p H i - p H a = log PaCO2 0.0181 PiCO:- F (6)

Equation 6 possess the ability to reflect the change of splanchnic tissue PCO 2 during hypoperfusion without the consideration of the influence of bicar- bonate, and hence a source of bias is eliminated. With the elimination of pK a and bicarbonate, the pH difference de- pends on the tonometrically measured intramucosal CO 2 and the arterial C Q , which are the sensitive and essential pa- rameters for the early detection of gut ischemia [6].

References 1. Fiddian-Green RG et al (1989) Studies

in splanchnic ischemia and multiple organ failure. In: Marston A, Bulkley GB, Fiddian-Green RG et al (eds) Splanchnic ischemia and multiple organ failure. Mosby, St. Louis, pp 349-363

2. Doglio GR, Pusajo JF, Egurrola MA et al (1991) Gastric mucosal pH as a prognostic index of mortality in criti- cally ill patients. Crit Care Med 19:t037

3. Gutierrez G, Palizas F, Doglio G e t al (1992) Gastric intramucosal pH as a therapeutic index of tissue oxygenation in critically ill patients. Lancet 339:195

4. Siggaard-Andersen O (1974) The acid- base status of the blood, 4th edn, Munksgaard, Copenhagen, pp 28-35

5. Takala J, Parviainen I, Siloaho M e t al (1994) Saline PCO 2 is an important source of error in the assessment of gastric intramucosal pH. Crit Care Med 22:1877

6. Fiddian-Green RG (1995) Gastric intra- mucosal pH, tissue oxygenation and acid-base balance. Br J Anaesth 74:591

G. Michagin (u~) �9 P. Klint-Andersen Department of Anesthesiology and Intensive Care, Odense University Hospital, 5000 Odense C, Denmark FAX: + 45 (66) 113415

P.J. Jensen Department of Anesthesiology, Randers County Hospital, DK-8900 Randers, Denmark