Low gastric intramucosal pH: incidence and significance in intensive care patients

Effects of intra-aortic balloon counterpulsation on parameters of tissue oxygenation

BACKGROUND AND OBJECTIVE

To investigate the effects of intra-aortic balloon pump therapy on parameters of global and regional oxygenation in patients undergoing cardiac surgery.

METHODS

As part of a large surveillance study in cardiac surgery patients (n=266) we retrospectively analyzed the course of 28 patients requiring intra-aortic balloon pump therapy. Patients were grouped according to the time point of pump insertion (during weaning from cardiopulmonary bypass: group early intra-aortic balloon pumping (n=17); after admission to the intensive care unit: group late intra-aortic balloon pumping (n=9). Haemodynamic and tonometric variables, arterial lactate, and use of catecholamines were measured hourly.

MEASUREMENTS AND RESULTS

Cardiac index increased in both groups after intra-aortic balloon pump insertion (2.2+/-0.5 baseline; 3.4+/-0.8 L min-1 m-2 4 h later (group early intra-aortic balloon pumping); 2.8+/-0.5 baseline, 3.6+/-L min-1 m-2 4 h later (group later intra-aortic balloon pumping), each P<0.05), there were no differences between groups. Arterial lactate values increased in group later intra-aortic balloon pumping after pump insertion to a maximum 2 h later (8.4+/-6.1 mmol L-1 baseline; 12.7+/-7.4 mmol L-1, P<0.05), and decreased continuously afterwards. The difference of arterial and gastric CO2 showed a sharp decrease after pump insertion in group later intra-aortic balloon pumping (26.4+/-9.8 baseline; 7.0+/-11.1 mmHg, P<0.05). There were no differences between groups. Epinephrine doses were higher in group later intra-aortic balloon pumping (P<0.05).

CONCLUSIONS

Intra-aortic balloon pump therapy improved global and regional splanchnic oxygenation in cardiac surgery patients with low-cardiac-output syndrome. Gastro-intestinal tonometry could provide additional information concerning tissue oxygenation. Patients with later intra-aortic balloon pump insertion needed more catecholamine therapy to achieve similar haemodynamic values.

Intramucosal PCO2 Measurement as a New Monitoring Method of Free Jejunal Transfer following Pharyngo-laryngo-esophagectomy

The choices for practical monitoring of free jejunal transfer have been quite limited because of its own characteristics, such as buried form, lack of skin surface, and the structure of a hollow viscous tract. Physiologically, it is known that tissue hypoxia caused by compromised perfusion leads to an increase of partial pressure of carbon dioxide (PCO2). Because of its physiological properties, the diffusion of carbon dioxide is always equilibrated between the mucosa of a hollow viscous organ and its lumen. The intramucosal PCO2 (PiCO2) of the gastrointestinal tract can therefore be determined indirectly from the intraluminal PCO2, which is measured with the aid of the tonometer catheter. To develop an optimal monitoring method for free jejunal transfer, the authors proposed the application of PiCO2 measurement by a modified use of a tonometer catheter. Since May of 1999, the authors performed postoperative PiCO2 monitoring on 20 cases of reconstructed pharyngoesophageal tracts in 18 patients who underwent radical tumor resection and one-stage reconstruction at the Shizuoka Red Cross Hospital. All 20 cases were safely monitored by PiCO2 measurement without any complications associated with the use of the tonometer catheter. In the 17 cases that succeeded uneventfully, the mean values of PiCO2 were kept lower than 40 mmHg throughout the monitoring period. On the other hand, the other three cases (15 percent) needed reexploration due to development of vascular complications, which was alerted by an abrupt increase of PiCO2 in each case (229, 130, and 99.6 mmHg). Two of the patients were fortunately successfully treated by immediate reexploration, leading to a 95 percent overall success rate. No false-negative or false-positive cases were observed. The authors' experience suggests that PiCO2 measurement using a tonometer catheter can provide the surgeon with reliable information for evaluating the perfusion and viability of a free jejunal transfer. Simplified manipulation and the objectivity of the numerical data allow stable measurement of PiCO2 and prompt judgment of the adequacy of the perfusion, which could minimize the burden and anxiety of the surgeon, particularly in the early postoperative period.

Accuracy of mucosal pH and mucosal-arterial carbon dioxide tension for detecting mesenteric hypoperfusion in acute canine endotoxemia

OBJECTIVE

To determine the level of mucosal-arterial Pco2 (Pco2 gap) that is both sensitive and specific for the detection of mesenteric hypoperfusion as defined by either a >50% reduction in portal blood flow or release of lactate by the gut.

DESIGN

Animal experiment.

SUBJECTS

Seven anesthetized, intubated, mechanically ventilated, and surgically instrumented mongrel dogs.

INTERVENTION

Escherichia coli endotoxin (1 mg/kg) given intravenously for 5 mins.

MEASUREMENTS AND MAIN RESULTS

Tonometric Pco2, arterial blood gases, arterial and portal venous lactates, and portal and systemic hemodynamic variables were measured. Mucosal pH (pHi) was calculated according to the manufacturers' instructions. From these data, receiver operating characteristics were calculated. Although animals were resuscitated to maintain a constant cardiac output, portal flow decreased from 350+/-101 to 152+/-75 mL/min (p<.01) and the gut released lactate into the portal circulation in all animals. Pco2 gap increased from 13.1+/-3.9 to 40.2+/-39.2 torr (p<.01) and was inversely correlated with portal blood flow (r2 = .20; p<.05). For detection of a >50% reduction in portal blood flow, a Pco2 gap of 20 torr yielded a maximum accuracy of 67% (sensitivity, 55%; specificity, 73%) and was less accurate than a pHi of 7.20, which yielded a maximum accuracy of 76% (sensitivity, 90%; specificity, 70%), although this difference was not significant (p = .24). There was also a correlation between pHi and portal blood flow (r2 = .31; p<.01). For detection of lactate release by the gut, a Pco2 gap of 20 torr was also 67% accurate (sensitivity, 53%; specificity, 78%), whereas a pHi of 7.10 achieved an accuracy of 64% (sensitivity, 40%; specificity, 83%), which was not significantly different.

CONCLUSION

Pco2 gap measurements are neither sensitive nor specific for mesenteric hypoperfusion with regard to total gut blood flow reductions of >50% or the release of lactate into the portal circulation.

Tonometry to estimate intestinal perfusion in newborn piglets

AIM

To determine the correlation between gastric intramucosal pH and superior mesenteric artery (SMA) flow in newborn piglets.

METHODS

Fourteen newborn piglets were randomly assigned to either a control or to an epinephrine group which received 0,1,2,4,0 microg/kg/min of epinephrine for 60 minutes, each dose. Gastric tonometry was performed, SMA flow was measured, and intramucosal pH and the ratio of tonometer pCO(2) over arterial pCO(2) (rCO(2)) were calculated.

RESULTS

Intramucosal pH decreased over time in both groups, but tended to be lower in the epinephrine group. With increasing dose of epinephrine, SMA flow decreased; this in turn increased rCO(2) (p = 0.04) with a tendency to decrease intramucosal pH (p = 0.06).

CONCLUSIONS

Gastric tonometry may be useful in human neonates to evaluate gut ischaemia.

Acidosis and tissue hypoxia in the critically ill: how to measure it and what does it mean

We routinely monitor blood gases to determine the adequacy of ventilation and the presence of acid-base abnormalities. Changes in the blood are easily assessed, but of more importance is the abnormality at tissue level. Defects in acid-base homoeostasis have major effects on protein function, thus affecting tissue and organ performance. We concentrate on the changes seen in critically ill patients with acidosis because they form a large portion of the workload of the average intensive care unit. In addition, such patients have significant morbidity and mortality. The development of acidemia in the critically ill is often attributed to reductions in oxygen utilization, which in the past has generally been regarded as dysregulation of tissue blood supply. Resulting tissue hypoperfusion leads to anaerobic metabolism and lactic acidosis. Carbon dioxide production increases as anaerobically produced hydrogen ions are buffered by extracellular bicarbonate. The effectiveness of tissue perfusion is the target of much research, and in this review we outline factors that affect tissue acid-base status, techniques to measure tissue acid-base status, and explore the relationship between tissue acidosis and hypoxia in the critically ill. However, things are not always as simple as they may first appear.

Gastric tonometry in septic shock

OBJECTIVES

To investigate the prognostic value of intramucosal pH (pHi) and the relation among pHi, arterial pH, base excess, and lactate in children with septic shock.

DESIGN

Children admitted to the paediatric intensive care unit with a diagnosis of septic shock were prospectively enrolled. A gastrointestinal tonometer (Tonometrics Division, Instrumentarium Corporation, Helsinki, Finland) was placed into the stomach and intramucosal pH, arterial pH, base deficit, and lactate were measured on admission and six hours later. Sequential data were analysed on 24 patients (17 survivors, seven non-survivors), median age 46 months (range: 2.8-168 months).

RESULTS

Median pHi on admission was 7.39 (interquartile range 7.36-7.51) in survivors compared with 7.2 (interquartile range 7.18-7.35) in non-survivors (p = 0.01). There was no significant difference in arterial pH, base excess, or lactate among survivors and non-survivors. Admission pHi < 7.32 predicted mortality with sensitivity (57%), specificity (94%), and positive predictive value (80%). Patients with admission pHi < 7.32 who failed to improve > or = 7.32 within six hours (n = 3) had 100% mortality.

CONCLUSION

In children with septic shock the admission pHi is significantly lower in non-survivors. pHi is a better prognostic indicator of mortality than either standard acid-base values or lactate. pHi < 7.32 that does not improve within six hours is associated with a poor prognosis.

The DCO2 measured by gastric tonometry predicts survival in children receiving extracorporeal life support. Comparison with other hemodynamic and biochemical information. Royal Children's Hospital ECMO Nursing Team

STUDY OBJECTIVE

To assess the role of gastric tonometry in monitoring children receiving extracorporeal life support (ECLS) and to determine if DCO2 or pHi in the weaning phase of ECLS predicts survival.

DESIGN

A prospective study of consecutive patients treated with ECLS.

SETTING

A tertiary pediatric ICU that is the ECLS referral center for Australia.

PATIENTS

Twenty consecutive children receiving ECLS for cardiovascular or respiratory failure.

INTERVENTIONS

All children were monitored throughout their ECLS course using a tonometer inserted into the stomach via the orogastric route. The PCO2 in the tonometer balloon was measured every 4 to 6 h and the pHi was calculated using the Henderson-Hasselbalch equation. The DCO2, which is the difference between PCO2 in tonometer saline solution and arterial blood, was calculated. We compared the ability of pHi, DCO2, heart rate, mean arterial pressure, arterial pH, base deficit, and blood lactate to predict death or survival during the weaning phase. Measurements were taken on the lowest level of support, which for veno-arterial extracorporeal membrane oxygenation and ventricular assist device was defined as the lowest ECLS pump flows, and on veno-venous extracorporeal membrane oxygenation was defined as the time of lowest ECLS gas flow. Predictive power was assessed using the receivor operating characteristic (ROC) analysis on the data collected at these times.

RESULTS

In the weaning phase of ECLS, the pHi was significantly lower in children who died (pHi = 7.21; 95% confidence intervals, 7.14 to 7.28) than in those who survived (pHi = 7.38; 95% confidence intervals, 7.28 to 7.47). The DCO2 was significantly higher in children who died (23.6 mm Hg; 95% confidence intervals, 14.3 to 33.1) compared with survivors (4.7 mm Hg; 95% confidence intervals, -0.78 to 10.1). The area under the ROC curve was 0.95 for DCO2 (and 0.88 for pHi). pHi and DCO2 predicted survival better than base deficit (area under ROC curve, 0.82), blood lactate level (0.29), arterial pH (0.65), heart rate (0.62), and mean arterial pressure (0.74).

CONCLUSIONS

DCO2 is a clinically meaningful measurement in children receiving ECLS. A high DCO2 was a good predictor of death in this series. Gastric tonometry may provide a useful measure of the adequacy of regional perfusion and oxygenation in this group of patients.

Tonometry. A review of clinical studies

In the quest for a better monitor of end-organ injury in the critically ill patient, gastrointestinal tonometry to measure intramucosal pH (pHi) and PCO2 has been advocated by several investigators. The authors discuss the principles and practice of gastric tonometry. In addition, the clinical applications of the technique are discussed with close attention to the use of pHi as an indicator of mucosal ischemia, ICU morbidity rates, and ICU mortality rates in surgical and medical patient populations. Although prognostic capability is an important role for tonometry, the principal benefit of the technology is its efficacy as an index of tissue resuscitation and as a guide to medical intervention. Given recent technical advances, gastrointestinal tonometry may gain increasing use in the assessment of tissue perfusion and oxygen metabolism.

The gastric tonometer. A valuable monitor of splanchnic perfusion?

Minimally invasive assessment of the adequacy of perfusion of the gastrointestinal tract has become clinically feasible with the availability of the gastric tonometer. This modified nasogastric tube permits calculation of the pH of the gut mucosal cells; a low tissue pH may indicate tissue hypoxia due to regional hypoperfusion. Such regional hypoperfusion is often undetected by other monitors and, if it occurs intra-operatively, may result in a poor outcome following major surgery. In critical illness, the splanchnic area seems to be particularly vulnerable to hypoperfusion and such a regional oxygen deficit is implicated in the causation of organ dysfunction/failure. Recent studies have begun to define the circumstances in which splanchnic tissue acidosis develops and several therapies have been proposed to reverse the regional oxygen deficit. This review seeks to clarify whether or not the tonometer is a valuable addition to our current monitoring aids.