Shock/Sepsis/Trauma/Critical care
Investigation of muscle pH as an indicator of liver pH and injury from hemorrhagic shock1

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Abstract

Background

During hemorrhagic shock blood flow to vital organs is maintained by the diversion of blood from both the splanchnic organs and skeletal muscle. In this swine study, we tested the hypotheses that (1) liver and muscle pH are correlated during both shock and resuscitation and (2) muscle pH during shock is an indicator of potential liver injury after resuscitation.

Materials and methods

Hemorrhagic shock was induced over 15 min to lower systolic blood pressure to 40 mm Hg and was maintained for 60 (n = 5) or 90 (n = 5) min. Resuscitation was achieved with shed blood and warm saline to maintain mean pressure >60 mm Hg for 120 min. Liver and muscle pH were measured with microelectrodes throughout the entire shock and resuscitation periods, along with hepatic venous oxygen saturation. Arterial lactate and aspartate aminotransferase were measured at baseline, end of shock, and resuscitation. Correlation between muscle and liver pH was determined. The ability of muscle pH to predict liver injury (40% increase in arterial aspartate aminotransferase) was compared with other predictors: liver pH, arterial lactate, and tonometric-arterial PCO2 gap.

Results

pH values and rates of change were similar in both muscle and liver tissue. Liver pH was well correlated with muscle pH during both shock and resuscitation, R2 = 0.84. Muscle pH predicts potential liver injury with the same sensitivity as blood lactate in this swine shock model.

Conclusions

Minimally invasive measurement of muscle pH warrants further study as a method to assess splanchnic hypoperfusion and resultant injury.

Introduction

Studies of gastric and sublingual capnometry have established that tissue carbon dioxide tension (PCO2) is a sensitive and quantitative indicator of regional hypoperfusion 1, 2, 3, 4, 5, 6, 7. During hemorrhagic shock, increases in PCO2 result from intracellular buffering of excess hydrogen ions and reduced CO2 washout [8]. In this low-oxygen delivery state, hydrogen ions are produced by anaerobic production of lactate and hydrolysis of ATP and other high-energy phosphates [9].

We and others have shown that directly measured tissue pH is also a sensitive indicator of shock and the results of resuscitation 10, 11, 12, 13. Although the monitoring of tissue pH in splanchnic organs is sensitive to the magnitude of hemorrhage, monitoring tissue pH of other organs may also be satisfactory for the early detection of hypoperfusion [6]. Under conditions of reduced blood pressure, studies have shown that blood flow to the skeletal muscle mirrors the reduction in blood flow to the splanchnic organs 14, 15; thus, skeletal muscle is a potential proxy for monitoring perfusion to the gut.

The skeletal muscle is easily and rapidly accessible for application of a noninvasive or minimally invasive probe. We have recently demonstrated the ability to noninvasively measure muscle pH through the skin using near infrared spectroscopy 16, 17, 18. Because of its accessibility, the skeletal muscle is of interest as a monitoring site for the associated low perfusion and hypoxic effects of hemorrhagic shock. Early work established measurable pH changes in the muscle using microelectrodes 19, 20. Recently, we have demonstrated that muscle pH correlates well with bowel and stomach pH during hemorrhagic shock in swine [12]. In this study we tested the hypotheses that (1) liver and muscle pH are correlated during both hemorrhagic shock and fluid resuscitation and (2) muscle pH during shock is an indicator of potential liver injury after resuscitation.

Section snippets

Surgical preparation

Under a protocol approved by the University of Massachusetts Medical School Institutional Animal Care and Use Committee and following guidelines established by the “Guide for the Care and Use of Laboratory Animals” (NIH Publication No. 85–23, revised 1985) 10 Yorkshire swine (24 ± 1 kg) were sedated with a mixture of ketamine/xylazine/telazole and atropine sulfate, intubated, and anesthetized with 1.5% isoflurane in oxygen. The tidal volume was ≈250 mL and respiration rate ≈12 breaths/min.

Results

For the 60-min group, an average of 526 ± 50 mL blood (∼32% blood volume) was removed to create shock, whereas for the 90-min group 498 ± 16 mL blood (∼30% blood volume) was removed. In each animal all the blood was returned during the resuscitation period. The blood was supplemented with an average volume of warm saline equal to 1830 ± 400 mL for the 60-min group and 2030 ± 300 mL for the 90-min group. A one-way analysis of variance showed that there was no difference in the volume of fluid

Discussion

The major finding of this study is that muscle pH correlates well with liver tissue pH during hemorrhagic shock and resuscitation. The absolute values and rates of changes of pH were similar in muscle and liver tissue. In both organs, pH declined more slowly than hepatic venous oxygen saturation in response to hypotension and also increased more slowly after restoration of blood pressure. Muscle pH at the end of shock had reasonable sensitivity and specificity in identifying potential liver

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    1

    This work was supported, in part, by the US Army Medical Research Command through a grant to the Center for Integration of Medicine and Innovative Technology (Boston, MA).

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