Tonometry

Gastric Tonometry Quick Guide

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What is gastric tonometry?

Gastric tonometry is an innovative monitoring modality which provides vital information on the adequacy of gastric mucosal perfusion. The Datex-Ohmeda Tonocap Monitor and the Tonometry Module for the Datex-Ohmeda S/5 Monitoring system measure gastric mucosal PCO2 (PgCO2).

Why monitor gastric mucosal PCO2?

1. The gastric mucosa is an early victim of blood flow redistribution in shock, trauma, sepsis or major surgery. The stomach is one of the first organs to suffer from hypoperfusion and one of the last to be restored to normality by resuscitation.
   
2. The gastric mucosa is very sensitive to altered perfusion.
   
3. Inadequate splanchnic tissue perfusion plays a major role in the development of sepsis and Multiple Organ Failure (MOF).

Sensitivity of the G - I Tract to Hypoxia

• superficial villus injury in 20 minutes
• destruction of villus in 60 minutes
• transmural gangrene in 8-12 hours
  

Why does PgCO2 increase?

Regional imbalance between CO2 removal and production: a) decreased mucosal perfusion and/or b) increased regional metabolism

The villi covering the gastric and intestinal mucosa are very vulnerable to hypoxia. The special counter- current circulation with decreasing O2 partial pressure towards the tip cannot provide the tip of the villi with sufficient oxygen in the case of decreasing perfusion. The mismatch between mucosal perfusion and regional metabolism leads to regional imbalance between CO2 removal and production. As a result CO2 accumulates in the mucosa. This can be detected early on by measuring the gastric CO2, PgCO2.

In the case of low flow states like hypovolemic and cardiogenic shock, blood flow will be directed to vital organs and vasoconstriction is established in the gastric mucosa. That is why gastric tonometry works as an early warning measurement to detect gastric hypoperfusion prior to systemic variables.

How does gastric tonometry work?

The method is based on a measurement of carbon dioxide partial pressure PCO2 in the stomach or intestine. A special tonometry catheter and Datex-Ohmeda tonometry monitor (Tonocap or Tonometry Module) are used to analyze PCO2 with infrared sensor
technology.

The Tonometrics Catheter is inserted into the stomach. This unique multiple lumen catheter includes a semi-permeable silicone balloon at the distal end of the catheter, which is positioned in the stomach. CO2 freely equilibrates between the gastric mucosa, the gastric lumen and the balloon. A gas sample is drawn from the balloon and analyzed every ten minutes.

Tonometer balloon: permeable to CO2 produced in the gastric mucosa

The value of gastric tonometry

Maintaining adequate tissue perfusion is one of the major goals in patient management throughout the perioperative and critical care process.

• Unique diagnostic information
  Gastric tonometry provides the most reliable information on the adequacy of gastrointestinal perfusion. Information that is unavailable from conventional global monitoring.
• Early identification and intervention
  Changes in PgCO2 often occur hours to days in advance of other parameters, which enables an earlier identification of the problem and earlier consideration of treatment of the patient.
• Improved patient management
  Gastric tonometry is used as an additional valuable end point for resuscitation. This valuable adjunct provides an indication as to whether therapeutic interventions address the demands of the mucosal tissues.

Application areas

Gastric Tonometry is recommended for patients who require gastrointestinal mucosal PCO2 monitoring. This patient population may include:

• trauma patients
• major surgery e.g. cardiac patients
• patients with hemorrhagic shock
• patients with cardiogenic shock
• patients with severe acute respiratory failure
• patients with severe acute pancreatitis
• patients with major burns
• patients weaning from the ventilator

How to optimize monitoring conditions?

It is important that environmental air is removed from the stomach before tonometry measurement, (e.g. after bagging the patient with oxygen), because the air will dilute the CO2 concentration.

Feeding may elevate PgCO2, therefore several experts in the field recommend to discontinue feeding for 1 –2 hours and in septic cases for up to 4 hours. Intermittent enteral feeding often causes spikes on the PgCO2 baseline. Duodenal feeding is recommended.

Low pH ( 1-4 ) of the gastric juice may increase PgCO2 values, e.g. when eventual bicarbonate refluxes from the duodenum react with gastric acid. The possible effect of H2-antagonists or omeprazol on PgCO2 should be individually assessed.

Illustrative examples of Clinical cases

Typical PCO2 values

An example of normal PCO2 values

PCO2 values in Respiratory Acidosis

During respiratory acidosis the high EtCO2 and PaCO2 values will reflect an elevated PgCO2. Normal P(g-a)CO2 gap indicates normal mucosal perfusion in spite of high PgCO2

PCO2 values in Multiple Organ Failure (MOF)

Patient with postoperative pneumonia, ARDS, sepsis, kidney failure -> MOF. High P(g-a)CO2 gap indicates mucosal hypoperfusion

Recommended Interpretation of Values

*) Assuming 4mmHg arterial-alveolar difference in healthy lungs

PgCO2 is influenced by systemic arterial PCO2 values (PaCO2) which in the critically ill population can vary rapidly. Clinical interpretation of changes in PgCO2 can be aided by calculating the gastric-arterial CO2 gap P(g-a)CO2. If end-tidal CO2 (EtCO2) is
measured simultaneously with PgCO2, the minimally invasive marker of gastric perfusion, P(g-Et)CO2 gap, can also be used. Assuming a 4mmHg (0.5 kPa) arterial-alveolar difference in healthy lungs, EtCO2 can represent PaCO2, which removes the
need for blood gas analysis in the calculation of CO2 gap.

In a normally perfused mucosa PgCO2 is close to PaCO2. During inadequate perfusion (hypoperfusion) PgCO2 as well as P(g-a)CO2 gap increases. The gap is reported to be a very sensitive predictor of hypoperfusion. A gap value of 20 mmHg/2.5 kPa should alert the clinician to consider immediate actions for its correction. Increasing P(g-Et)CO2 or P(g-a)CO2 gap values give an early warning of gastrointestinal hypoperfusion problems. It is very useful to follow the trends of the gaps when defining, that therapy should be started to prevent further damage.

Early Intervention

Appropriate fluid administration to ensure adequacy of circulating blood volume and a correct choice of an inotrope could be considered. Successful restoration of splanchnic perfusion within 24 hours may help to reduce further complications and the length of
ICU stay.

Further reading:

J.J. Kolkman, J.A. Otte and A.B.J. Groeneveld. Gastrointestinal luminal PCO2 tonometry: an update on physiology, methodology and clinical applications. British Journal of Anaesthesia 2000; 84 (1): 74-86

J. Takala. Clinical Application Guide of Gastrointestinal Tonometry. Datex-Ohmeda
2000, 894796-3

Thomas Uhlig. Magenmukosa-Tonometrie in der klinischen Praxis. Datex-Ohmeda
2000, 8001240-1

M.A. Hamilton and M.G. Mythen. Intragastric luminal tonometry in intensive care. Datex-Ohmeda Clinical Window Web Journal, Tonometry, September 2000

G. Lebuffe, E. Robin and B. Vallet. Gastric tonometry. Intensive Care Med. 2001;
27:317-319

Last updated: 8 April 2004
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