Tonometry

Tonometry questions and answers

Q: What is the significance of gastric PCO2?
A: Gastric PCO2 (PgCO2) indicates the balance between CO2 production (metabolism) and removal (perfusion). An elevated gastric PCO2, regional hypercapnia, is therefore a marker of inadequate tissue perfusion and/or deranged metabolism.

Q: What is the normal PgCO2value?
A: In a normal situation, PgCO2 approximates arterial PCO2, which means that PgCO2 is about 45 mmHg (6 kPa). But rather than looking at the PgCO2 value, it is recommended to compare it with either arterial PCO2 or with end-tidal CO2.

Q: Why should I compare the PgCO2 value to PaCO2 or EtCO2?
A: A high PgCO2 doesn't necessarily mean gastric hypoperfusion; during e.g. respiratory acidosis or permissive hypercapnia, the global CO2 values are elevated, and PgCO2 will be elevated as well. As long as the gap between the global and regional values remains low there is no regional problem; only an elevated regional-to-global PCO2 gap indicates gastric hypoperfusion.

Q: What is the normal value for gastric-to-arterial PCO2 gap?
A: The P(g-a)CO2 gap should be less than 10 mmHg (1.5 kPa). If it is between 10-20 mmHg (1.5-2.5 kPa) you should start looking for the reason, and if the gap is higher than 20 mmHg (2.5 kPa), cause-related therapy should be provided.

Q: How long can an abnormal PCO2 gap be tolerated?
A: There is no clear answer to this question since tolerance varies from patient to patient. However, it should be noted that the duration of gastric mucosal hypoperfusion is an important factor; a high gap for a brief period (e.g. 30 min) is probably of no major concern (it happens to all of us during exercise), whereas prolonged gastric hypoperfusion may cause irreversible tissue damage. In conclusion, cumulative mucosal damage is caused by both severity and duration of gastric mucosal hypoperfusion.

Q: What is the normal value for gastric to end-tidal PCO2 gap?
A: Since EtCO2 in a stable patient with no lung disease is slightly lower than arterial PCO2, the P(g-Et)CO2 gap is somewhat higher than the P(g-a)CO2 gap. The normal value for the P(g-Et)CO2 gap is thus less than 15 mmHg (2 kPa).

Q: What is the significance of pHi?
A: pHi is an indirect measure of intramucosal pH. It has a regional component, PgCO2, which is a marker of gastric perfusion; the systemic component of pHi is arterial bicarbonate. A low pHi may thus indicate either a regional problem or systemic acidosis, or both.

Q: What is the normal pHi value?
A: pHi is between 7.35 - 7.45 in a normal situation. In many of the studies a value of 7.32 has been used as the lower limit value.

Q: Does an elevated P(g-a)CO2 gap show in the global parameters?
A: Not necessarily. An elevated P(g-a)CO2 gap is related to a gastric mucosal problem, and as the short-term influence of one organ on the global parameters is very small, global parameters may not change until the gastric mucosa has been ischaemic long enough to lead to sepsis and MODS.

Q: How should an elevated PgCO2 be treated?
A: First of all, duodenal bicarbonate reacting with gastric acid should be ruled out as the source of the elevated PgCO2. Make sure that gastric luminal pH>4; if this is not the case, administer an H2 antagonist or omeprazole, and watch for any changes in PgCO2. When gastric luminal pH>4, PgCO2 reflects gastric PCO2, and not luminal chemistry.

The first step in therapy is to assess the cause of mucosal hypoperfusion; if it could be hypovolemia, start volume therapy. If the cause is low cardiac output, try to optimize the pre- and afterload, treat relevant arrhythmias and consider inotropic agents. If the cause is hypotension, increase cardiac output (if low) and use vasopressors if systemic vascular resistance is low.

Q: What is the reason for a negative P(g-a)CO2 gap?
A: There could be two reasons for the negative gaps. The first one is that the PaCO2 was not recorded at actual body temperature. Quite typically blood gas analyzers measure the samples at 37 C but for calculating the tonometry gap, the sample needs to be temperature corrected.

The second reason is that there might be air in the stomach. This can typically be seen in the beginning of an operation after the patient has been bag-mask ventilated before intubation. Some of the air goes into the stomach and the oxygen dilutes the CO2 there. It might even take some hours before the oxygen is absorbed into the circulation. To overcome this, it is recommended that the air is removed by suctioning.

Q: Does suctioning have an effect on the PgCO2?
A: Passive suctioning is recommended since active suctioning may interfere with PgCO2 measurement. It is recommended that active suctioning is discontinued for 30-60 minutes to check the effect on PgCO2.

Q: Does gastric feeding effect PgCO2?
A: Caution should be used when interpreting the PgCO2 during gastric feeding. An increase in PgCO2 in response to gastric feeding may be caused by CO2 generated by a chemical reaction between gastric acid and the feed; to exclude this possibility, a test dose of an acid suppressor (H2 antagonist or omeprazole) should be administered. If PgCO2 remains high following acid suppression, the reason may be gastric mucosal hypoperfusion in response to the increased metabolic demand on the mucosa caused by feeding. In this situation it is recommended to discontinue feeding until PgCO2 has normalized.

Q: Do we need to administer an H2 antagonist or omeprazole while measuring PgCO2?
A: Routine use of acid suppression is not necessary as long as PgCO2 is normal. If PgCO2 is elevated, and the pH of gastric aspirate is less than 4, then give a single iv-dose and observe the effect on PgCO2 for 60-90 minutes.

Q: Can saline be used with the Datex-Ohmeda Tonometry Monitors?
A: No. The saline will damage the filter, whose main purpose is to keep liquid out of the infrared sensor.

Q: Do we need to connect the D-fend Water Trap, although we are not measuring EtCO2?
A: Yes, with Tonocap Monitor you will always need the D-fend Water Trap. There is only one infrared sensor in the Tonocap Monitor for analyzing both PgCO2 and EtCO2. If the D-fend Water Trap is missing it will affect the calibration and therefore the accuracy of the infrared sensor.

Last updated: 1 September 2000
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