Esophageal PCO2 as a monitor of perfusion failure during hemorrhagic shock

Regional capnometry to evaluate the adequacy of tissue perfusion

Tissue hypoperfusion is a major cause of morbidity and mortality in critically ill patients but cannot always be detected by measuring standard whole-body hemodynamic and oxygen-related parameters (e.g., blood pressure, cardiac output, and central venous oxygen saturation). Preclinical and clinical studies have demonstrated that low-flow states are consistently associated with large increases in venous and tissue PCO₂. Monitoring regional PCO₂ with gastric tonometry (PgCO₂) is known to have independent prognostic value for predicting postoperative complications and mortality. The PgCO₂ gap might also be of value as a treatment target (endpoint) in critically ill patients. However, this tool has several limitations and has not yet been developed commercially, thus restricting its use. Regional capnography with sublingual and transcutaneous sensors might be an alternative noninvasive option for evaluating the adequacy of tissue perfusion in critically ill patients. However, further studies are needed to determine whether or not this monitoring technique is of value-particularly as an endpoint for guiding resuscitation. Bladder PCO₂, has only been evaluated in animal studies, and so remains to be validated in patients.

A Comparison of Gastric and Rectal CO₂ in Cardiac Surgery Patients

Critical care nurses assess and treat clinical conditions associated with inadequate oxygenation. Changes in regional organ (gut) blood flow are believed to occur in response to a decrease in oxygenation. Although the stomach is a widely accepted monitoring site, there are multiple methodological and measurement issues associated with the gastric environment that limit the accuracy of P CO2 detection. The rectum may provide nurses with an alternative site for monitoring changes in PCO2 without the limitations associated with gastric monitoring. This pilot study used a repeated measures design to examine changes in gastric and rectal PCO2 during elective coronary artery bypass grafting with cardiopulmonary bypass (CPB) and in the immediate 4-hr postoperative period in 26 subjects. The systemic indicators explained little variation in the regional indicators during protocol. A comparison of rectal and gastric PCO2 revealed no statistically significant differences in the direction or magnitude of change over any phase of cardiac surgery (baseline, CPB, post-CPB). A reduction in both rectal and gastric PCO2 occurred during CPB, and both values trended upward during the post-CPB phase. However, poor correlation and agreement was found between the measures of PCO2 at the two sites. Although clinically important, the cause is unclear. Possible explanations include variation in CO2 production between the gastric and rectal site, differences in sensitivity of the two monitoring instruments, or the absence of hemodynamic complications, which limited the extent of change in PCO2. Further investigation using patients with more profound changes in oxygenation are needed to identify response patterns and possible mechanisms.

Monitoring CO2 in shock states

The primary end point when treating acute shock is to restore blood circulation, mainly by reaching macrocirculatory parameters. However, even if global haemodynamic goals can be achieved, microcirculatory perfusion may remain impaired, leading to cellular hypoxia and organ damage. Interestingly, few methods are currently available to measure the adequacy of organ blood flow and tissue oxygenation. The rise in tissue partial pressure of carbon dioxide (CO2) has been observed when tissue perfusion is decreased. In this regard, tissue partial pressure of CO2 has been proposed as an early and reliable marker of tissue hypoxia even if the mechanisms of tissue partial pressure in CO2 rise during hypoperfusion remain unclear. Several technologies allow the estimation of CO2 content from different body sites: vascular, tissular (in hollow organs, mucosal or cutaneous), and airway. These tools remain poorly evaluated, and some are used but are not widely used in clinical practice. The present review clarifies the physiology of increasing tissue CO2 during hypoperfusion and underlines the specificities of the different technologies that allow bedside estimation of tissue CO2 content.

Buccal capnometry for quantitating the severity of hemorrhagic shock

We have recently demonstrated that measurement of buccal mucosal PCO2 (PBUCO2) is a reliable alternative to sublingual mucosal PCO2 for measuring the severity of hemorrhagic shock. We hypothesized that measurement of PBUCO2 would serve as a continuous and a more sensitive and specific measurement for predicting survival during hemorrhagic shock than conventional measurements and thereby better guide initial management. Four groups of five pentobarbital anesthetized Sprague-Dawley rats were randomly assigned to be bled over 30 min in amounts estimated to be 25%, 30%, 35%, or 40% of total blood volume. With an optical PCO2 sensor applied noninvasively to the mucosa of the left inner cheek, PBUCO2 was continuously measured together with arterial pressure, end-tidal PCO2, and intermittent measurement of cardiac output, arterial blood lactate, and base deficit. Surviving animals had free access to water and food but no other treatment during the 72-h interval after recovery from anesthesia. After an estimated 40% blood loss, all animals died within 1 h. In the remaining animals, arterial pressure, end-tidal carbon dioxide, cardiac index, blood lactate, and base deficit each failed to discriminate among animals with 35%, 30%, and 25% acute blood losses. This contrasted with PBUCO2, which discriminated between the magnitude of massive blood loss and untreated survival. Buccal mucosal PCO2 was predictive of outcome after rapid bleeding when compared with arterial pressure, end-tidal carbon dioxide, cardiac index, arterial blood lactate, and base deficit. This measurement is therefore likely to serve as a useful guide for the immediate management of hemorrhagic shock.

Role of buccal PCO2 in the management of fluid resuscitation during hemorrhagic shock

Arterial pressure is a widely used measurement for estimating the severity of hemorrhagic shock and to guide its management. However, this capability is reduced when very low arterial pressure values cannot be reliably measured by noninvasive methods. Moreover, hypoperfusion may be masked by compensatory hemodynamic changes, and therefore, in the presence of near normal blood pressure, tissue hypoperfusion may progress undetected. Accordingly, hypercarbia is a general phenomenon of perfusion failure, which occurs in coincidence of the onset of hypotension and is promptly reversed with restoration of normal blood flows. Increases in buccal mucosa PCO2 are highly correlated with increases in gastric wall and sublingual mucosa PCO2 and decreases in tissue blood flows during hemorrhagic shock. In both clinical and experimental settings, tissue PCO2 measured in the oral mucosa proved to be a practical and reliable measurement for the diagnosis of circulatory failure states and an indicator of its severity. In contrast to intraarterial pressure, buccal PCO2 discriminated between short- and long-term survival after large-volume blood loss. Buccal PCO2 measurement therefore emerges as a useful predictor for survival and outcome and a useful guide to manage fluid resuscitation during hemorrhagic shock.

Oxygenation monitoring of tissue vasculature by resonance Raman spectroscopy

Resonance Raman spectroscopy offers a mechanism for the noninvasive measurement of in vivo and in situ hemoglobin oxygen saturation (HbO(2)Sat) in living tissue. Clinically informative signals can be provided by resonance enhancement with deep violet excitation. It is notable that fluorescence does not significantly degrade the quality of the signals. During the controlled hemorrhage and resuscitation of rats, signal intensity ratios of oxy- vs. deoxyhemoglobin from sublingual mucosa correlated with co-oximetry values of blood withdrawn from a central venous catheter. The spectroscopic application described here has potential as a noninvasive method for the diagnosis of clinical shock and guidance of its therapy.

A new simple tool for tonometric determination of the PCO2 in the gastrointestinal tract

BACKGROUND AND OBJECTIVE

Following the discovery of the feasibility of assessing the adequacy of the splanchnic perfusion by means of gastrotonometry, this technique became extensively used in clinical practice and in research. At present, high-technology instruments are available for the purpose. However, there is still a need for a simple, cheap tool that is easily applicable for patients of all ages. The present article describes such a tool and presents the results of in vitro and in vivo validation measurements with it.

METHODS

Balloon-free tonometric probes consisting of silicone rubber tubes were developed. In vitro measurements of the uptake of CO2 inside the probes and for comparison in a conventional ballooned intestinal tonometer were made in a closed container maintained at 37 degrees C. In in vivo studies in anaesthetized dogs, the tonometric PCO2 values were determined with a tonometric probe and catheter introduced simultaneously into the ileum of the animals and the results were analysed by the Bland-Altman method.

RESULTS

The in vitro equilibration studies revealed that the filling media inside the probes equilibrated rapidly with the PCO2 content of the chamber. The data obtained from the in vivo investigations with the two different methods demonstrated a statistically significant linear association (correlation coefficient: 0.778, significance: P < 0.001).

CONCLUSION

The new simple tonometric probe described appears to be a reliable tool with certain advantages for gastric tonometry. However, further studies are warranted before its general clinical use.

Gastric and sublingual capnometry

PURPOSE OF REVIEW

Tissue hypoperfusion is a common pathophysiologic process leading to multiple organ dysfunction and death. Increases in tissue PCO2 can reflect an abnormal oxygen supply to the cells, so that monitoring tissue PCO2 by the use of gastric or sublingual capnometry may help identify circulatory abnormalities and guide their correction. This review provides an update on these technologies.

RECENT FINDINGS

Gastric tonometry aims at monitoring PCO2 in the stomach, an organ that becomes ischemic quite early when the circulatory status is jeopardized. Despite substantial initial enthusiasm, this technique has never been widely implemented due to methodological problems. The measurement of sublingual mucosal PCO2 (PslCO2) by sublingual capnometry is technically simple and noninvasive. Experimental studies have suggested that PslCO2 is a reliable marker of tissue perfusion. Clinical studies have demonstrated that high PslCO2 values are associated with impaired microcirculatory blood flow and a worse prognosis in critically ill patients.

SUMMARY

Gastric tonometry was proposed for regional PCO2 monitoring, but it is prone to a number of technical limitations. Sublingual capnometry could offer a valuable alternative for tissue PCO2 monitoring in clinical practice, representing a simple, noninvasive method to monitor tissue perfusion and titrate therapeutic interventions in critically ill patients.

Increases in tissue Pco2 during circulatory shock reflect selective decreases in capillary blood flow*

OBJECTIVES

Tissue Pco2 reflects metabolic alterations due to circulatory failure during circulatory shock. This study addresses simultaneous changes in gastric and buccal tissue Pco2 with changes in microcirculatory blood flow in a rat model of circulatory shock induced by cecal ligation and puncture.

DESIGN

Prospective controlled laboratory study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Male breeder Sprague-Dawley rats.

INTERVENTIONS

Induction of polymicrobial, abdominal sepsis by cecal ligation and puncture.

MEASUREMENTS AND MAIN RESULTS

Tissue Pco2 was continuously measured with the aid of a miniature carbon dioxide electrode. Using orthogonal polarization spectral imaging, recordings of the microcirculation were taken at baseline and hourly intervals until death and compared with sham-operated animals. Gastric and buccal tissue Pco2 values progressively increased in animals after cecal ligation and puncture and terminated in death. Microcirculatory blood flow in vessels >20 microm was well preserved during progression of shock, whereas there was an early and progressive decrease in microcirculatory blood flow in vessels <20 microm, mostly representing capillaries. Tissue Pco2, the tissue Pco2-Paco2 gradient, and blood flow in vessels <20 microm were highly correlated. This contrasted with sham control animals in which no significant hemodynamic, blood gas, lactate, microcirculatory, and tissue Pco2 abnormalities were observed.

CONCLUSIONS

These observations suggest that microcirculatory failure in capillaries appears as an early defect in close association with anaerobic metabolism during progression of circulatory shock in an animal model of septic peritonitis.

EARLY DIFFERENCE IN TISSUE PH AND MICROVASCULAR HEMODYNAMICS IN HEMORRHAGIC SHOCK RESUSCITATION USING POLYETHYLENE GLYCOL-ALBUMIN- AND HYDROXYETHYL STARCH-BASED PLASMA EXPANDERS

The hamster chamber window model was subjected to hemorrhagic shock by the withdrawal of 50% of blood volume (BV). BV was restored 1 h after hemorrhage with a single volume infusion (resuscitation) of 25% BV with polyethylene glycol (PEG)-conjugated bovine serum albumin (Alb) and hydroxyethyl starch (HES). Hemorrhage, shock, and resuscitation were monitored continuously in terms of mean arterial pressure (MAP), microvascular blood flow, capillary perfusion, and tissue pH. Blood samples for laboratory parameters were taken at baseline, shock, and resuscitation. Intravascular and tissue pO2 were assessed after resuscitation, and microvascular oxygen supply and extraction were calculated and corrected for pH effect on hemoglobin saturation. Resuscitation with PEG-Alb restored systemic and microvascular parameters up to the end of the observation period (90 min). HES was identical to PEG-Alb resuscitation during the initial 10 to 15 min, but was not sustained subsequently. The trend of recovery in MAP for HES persisted beyond the time when both function capillary density and tissue pH decreased, thus MAP was not indicative of early microvascular dysfunction. Hemoglobin oxygen saturation estimation showed a significant pH dependence. However, oxygen-dependant parameters corrected for pH varied less than 10% from uncorrected data. Early differences found at the microvascular levels suggest that decisions to amend end-result of resuscitation may be short and on the order of minutes. Furthermore, PEG-Alb appears to provide early and long-term sustained systemic and microvascular recovery when used to restitute perfusion and metabolic conditions after resuscitation from hemorrhagic shock.

Equipment review: the success of early goal-directed therapy for septic shock prompts evaluation of current approaches for monitoring the adequacy of resuscitation

A recent trial utilizing central venous oxygen saturation (SCVO₂) as a resuscitation marker in patients with sepsis has resulted in its inclusion in the Surviving Sepsis Campaign guidelines. We review the evidence behind SCVO₂ and its relationship to previous trials of goal-directed therapy. We compare SCVO₂ to other tools for assessing the adequacy of resuscitation including physical examination, biochemical markers, pulmonary artery catheterization, esophageal Doppler, pulse contour analysis, echocardiography, pulse pressure variation, and tissue capnometry. It is unlikely that any single technology can improve outcome if isolated from an organized pattern of early recognition, algorithmic resuscitation, and frequent reassessment. This article includes a response to the journal's Health Technology Assessment questionnaire by the manufacturer of the SCVO₂ catheter.

Increases in Both Buccal and Sublingual Partial Pressure of Carbon Dioxide Reflect Decreases of Tissue Blood Flows in a Porcine Model During Hemorrhagic Shock

BACKGROUND

Earlier studies had demonstrated that gastric partial pressure of carbon dioxide (PCO2) of the gastric wall is an early indication of perfusion failure. Because hypercarbia is a general phenomenon of perfusion failure, this study investigated whether sublingual and buccal mucosal tissue PCO2 also serve as a sensitive indicator of systemic blood flow during hemorrhagic shock.

METHODS AND RESULTS

Hemorrhagic shock was induced in five male domestic pigs. Buccal and sublingual PCO2 increased from 60 to 129 mm Hg (p < 0.01) in parallel with average decreases in cardiac output to 44% (from 6.1 to 2.8 L/minute; p < 0.01), and mean arterial pressure to 47% (from 115 to 57 mm Hg; p < 0.01), over the 2-hour interval of shock. According to colored microspheres used for measurements, buccal mucosal flow decreased to 35% (from 6.3 to 2.2 mL/minute/100 g; p < 0.01) in close parallel with sublingual blood flow, which decreased to 34% (from 7.2 to 2.5 mL/minute/100 g; p < 0.01). Liver flow decreased to 56% (from 152 to 85 mL/minute/100 g;p < 0.01), and renal flow to 47% (from 272 to 128 mL/minute/100 g;p < 0.01) of baseline values. The procedure yielded to overall comparable volume exchanges in different animals. After reinfusion of shed blood, buccal PCO2, like sublingual PCO2, was restored to approximately baseline values (respectively, from 56 to 71 mm Hg and from 60 to 71 mm Hg; p = not significant), together with arterial pressure, cardiac output, and end-tidal CO2 (EtCO2) (respectively, from 115 to 115 mm Hg, from 6.1 to 6.3 L/minute, and from 35 to 39 mm Hg; p = not significant), but there was delayed reversal of lactic acidosis (from 0.7 to 2.5 mmol/L;p < 0.01). In five unbled control animals, no significant changes were observed over the same interval.

CONCLUSION

The current study extends the rationale for noninvasive measurements to both buccal and sublingual mucosa for diagnosis and quantitation of hemorrhagic shock severity.

Lingual, splanchnic, and systemic hemodynamic and carbon dioxide tension changes during endotoxic shock and resuscitation

Sublingual and intestinal mucosal blood flow and Pco2 were studied in a canine model of endotoxin-induced circulatory shock and resuscitation. Sublingual Pco2 (PsCO2) was measured by using a novel fluorescent optrode-based technique and compared with lingual measurements obtained by using a Stowe-Severinghaus electrode [lingual Pco2 (PlCO2)]. Endotoxin caused parallel changes in cardiac output, and in portal, intestinal mucosal, and sublingual blood flow (Q̇s). Different blood flow patterns were observed during resuscitation: intestinal mucosal blood flow returned to near baseline levels postfluid resuscitation and decreased by 21% after vasopressor resuscitation, whereas Q̇s rose to twice that of the preshock level and was maintained throughout the resuscitation period. Electrochemical and fluorescent Pco2 measurements showed similar changes throughout the experiments. The shock-induced increases in PsCO2 and PlCO2 were nearly reversed after fluid resuscitation, despite persistent systemic arterial hypotension. Vasopressor administration induced a rebound of PsCO2 and PlCO2 to shock levels, despite higher cardiac output and Q̇s, possibly due to blood flow redistribution and shunting. Changes in PlCO2 and PsCO2 paralleled gastric and intestinal Pco2 changes during shock but not during resuscitation. We found that the lingual, splanchnic, and systemic circulations follow a similar pattern of blood flow variations in response to endotoxin shock, although discrepancies were observed during resuscitation. Restoration of systemic, splanchnic, and lingual perfusion can be accompanied by persistent tissue hypercarbia, mainly lingual and intestinal, more so when a vasopressor agent is used to normalize systemic hemodynamic variables.

Methods of monitoring shock

Intensive monitoring is a crucial component of the management of shock. However, there is little consensus about optimal strategies for monitoring. Although the pulmonary artery catheter has been widely used, conflicting data exist about the utility of this device. A variety of other techniques have been developed in hopes of providing clinically useful information about myocardial function, intravascular volume, and indices of organ function. In addition, there is evolving evidence that targeting and monitoring certain physiological goals may be most important early in the course of shock. In this chapter, we examine many of the available monitoring techniques and the evidence supporting their use.

Tissue capnometry: does the answer lie under the tongue?

Increases in tissue partial pressure of carbon dioxide (PCO(2)) can reflect an abnormal oxygen supply to the cells, so that monitoring tissue PCO(2) may help identify circulatory abnormalities and guide their correction. Gastric tonometry aims at monitoring regional PCO(2) in the stomach, an easily accessible organ that becomes ischemic quite early when the circulatory status is jeopardized. Despite substantial initial enthusiasm, this technique has never been widely implemented due to various technical problems and artifacts during measurement. Experimental studies have suggested that sublingual PCO(2 )(P(sl)CO(2)) is a reliable marker of tissue perfusion. Clinical studies have demonstrated that high P(sl)CO(2) values and, especially, high gradients between P(sl)CO(2) and arterial PCO(2) (DeltaP(sl-a)CO(2)) are associated with impaired microcirculatory blood flow and a worse prognosis in critically ill patients. Although some questions remain to be answered about sublingual capnometry and its utility, this technique could offer new hope for tissue PCO(2) monitoring in clinical practice.

Diagnostic Utility of Sublingual Pco2 for Detecting Hemorrhage in Penetrating Trauma Patients

BACKGROUND

Hemorrhage results in early compromise of splanchnic circulation. Studies have shown that sublingual Pco2 (SLCO2) correlates with gut perfusion. We tested SLCO2's ability to detect hemorrhage. We compared SLCO2 with arterial base deficit (BD) and lactate (LAC).

METHODS

This was a prospective study of patients with penetrating torso trauma. SLCO2 was measured at triage. Blood loss was defined as none (group 1), minimal to moderate (<1,500 mL) (group 2), or severe (>/=1,500 mL) (group 3). Data were reported as mean (95% confidence interval) and compared by analysis of variance. Receiver operating characteristic curves compared diagnostic performance between SLCO2, BD, and LAC.

RESULTS

One hundred eight patients were enrolled. There was a significant difference (p < 0.001) in SLCO2 between all blood loss groups: group 1, 46.9 mm Hg (44.9-49.0 mm Hg); group 2, 53.5 mm Hg (50.8-56.2 mm Hg); and group 3, 66.0 mm Hg (53.1-78.9 mm Hg). There were no significant (p > 0.05) differences for receiver operating characteristic curves between SLCO2, BD, or LAC.

CONCLUSION

SLCO2 differentiated blood loss groups. SLCO2 may be useful in triage of penetrating trauma patients.

Detection of ischemia by PCO2 before adenosine triphosphate declines in skeletal muscle

OBJECTIVE

Ischemia is a serious problem in clinical medicine, and effective methods are needed to detect ischemia before the injury becomes irreversible. In experimental studies on several organs, PCO2 was found to increase rapidly after the onset of supply-dependent anaerobic metabolism. A shortcoming of these studies was that PCO2 was not correlated with tissue concentrations of lactate and the energy status in the cell. Thus, in this study we have measured tissue concentrations of lactate, phosphocreatine, and adenosine triphosphate. We hypothesized that during ischemic conditions, PCO2 reflects lactate generation in the cell and not exhausted energy stores per se. If this is the case, PCO2 can be used to detect ischemia before the energy stores are depleted. Consequently, therapy can be instituted at a time when the organ is salvageable.

DESIGN

Prospective laboratory study.

SETTING

University research laboratory.

SUBJECTS

Seven pigs.

INTERVENTIONS

In a porcine model, gluteal skeletal muscles with no-flow ischemia were examined. PCO2 was measured both in situ and in vitro at increasing periods of time. Concomitantly, tissue lactate, adenosine triphosphate, and phosphocreatine were analyzed.

MEASUREMENTS AND MAIN RESULTS

Tissue surface CO2 tension (PtCO2) increased rapidly after onset of ischemia. From a baseline of 63 +/- 3 torr (8.4 +/- 1.2 kPa) under aerobic conditions, it increased to 157 +/- 6 torr (21 +/- 2.2 kPa) after 30 mins of ischemia and 386 +/- 9 torr (51.5 +/- 3 kPa) at 120 mins. The rapid increase of PtCO2 correlated well with increasing values of lactate (r2 >.9) in the tissue. Adenosine triphosphate was essentially unchanged for 45 mins after onset of ischemia, after which it declined. Phosphocreatine decreased earlier than adenosine triphosphate in accordance with the notion that high-energy phosphate groups are transferred from phosphocreatine to adenosine triphosphate.

CONCLUSION

In this porcine model of skeletal muscle ischemia, PtCO2 correlates well with tissue lactate and increases long before the energy stores of phosphocreatine and most notably adenosine triphosphate are severely reduced. Thus, PtCO2 could be monitored to detect and treat earlier stages of ischemia.

Diagnosis and monitoring of hemorrhagic shock during the initial resuscitation of multiple trauma patients: a review

The initial management of the multiple trauma victim requires evaluation for potential hemorrhage and ongoing monitoring to assess the efficacy of resuscitation and avoid complications related to hemorrhagic shock. A variety of strategies exist to assess circulatory status, including hemodynamic monitoring, tissue perfusion measurement, and use of serum markers of metabolism. We review available technologies used to assess fluid status and tissue perfusion in patients with blood loss or hemorrhagic shock, discuss how these methods can be used effectively and efficiently during initial trauma resuscitation to guide therapy and disposition, and suggest directions for future research to improve outcomes by providing more appropriate and timely care and avoiding unnecessary complications.

Sublingual capnometry: an alternative to gastric tonometry for the management of shock resuscitation

Normal vital signs do not reflect the physiologic aberrations after blood loss. Recognition of hypoperfusion during resuscitation can avoid the development of multiple organ failure. Advances in technology enable the clinician to monitor changes, potentially identifying tissue hypoxia much earlier than previously was possible. Gastric tonometry can be quite helpful in the intensive care unit in identifying gastric hypoperfusion, but has considerable drawbacks. The ability to monitor P(SI)CO(2) via sublingual capnometers overcomes some limitations of gastric tonometry and may be a valuable aid in the prehospital phase, the emergency department, and the intensive care unit in identifying end points of resuscitation.

Esophageal capnometry during hemorrhagic shock and after resuscitation in rats

BACKGROUND

Splanchnic perfusion following hypovolemic shock is an important marker of adequate resuscitation. We tested whether the gap between esophageal partial carbon dioxide tension (PeCO2) and arterial partial carbon dioxide tension (PaCO2) is increased during graded hemorrhagic hypotension and reversed after blood reinfusion, using a fiberoptic carbon dioxide sensor.

MATERIALS AND METHOD

Ten Sprague-Dawley rats were anesthetized, tracheotomized, and cannulated in one femoral artery and vein. A calibrated fiberoptic PCO2 probe was inserted into the distal third of the esophagus for determination of luminal PeCO2 during maintained anesthesia (pentobarbital 15 mg/kg per hour), normothermia (38 +/- 0.5 degrees C), and fluid balance (saline 5 ml/kg per hour). Three out of 10 rats were used to determine the limits of hemodynamic stability during gradual hemorrhage. Seven of the 10 rats were then subjected to mild and severe hemorrhage (15 and 20-25 ml/kg, respectively). Thirty minutes after severe hemorrhage, these rats were resuscitated by reinfusion of the shed blood. Arterial gas exchange, hemodynamic variables, and PeCO2 were recorded at each steady-state level of hemorrhage (at 30 and 60 min) and after resuscitation.

RESULTS

The PeCO2-PaCO2 gap was significantly increased after mild and severe hemorrhage and returned to baseline (prehemorrhagic) values following blood reinfusion. Base deficit increased significantly following severe hemorrhage and remained significantly elevated after blood reinfusion. Significant correlations were found between base deficit and PeCO2-PaCO2 (P < 0.002) and PeCO2 (P < 0.022). Blood bicarbonate concentration decreased significantly following mild and severe hemorrhage, but its recovery was not complete at 60 min after blood reinfusion.

CONCLUSION

Esophageal-arterial PCO2 gap increases during graded hemorrhagic hypotension and returns to baseline value after resuscitation without complete reversal of the base deficit. These data suggest that esophageal capnometry could be used as an alternative for gastric tonometry during management of hypovolemic shock.

Gas tonometry for evaluation of gastrointestinal mucosal perfusion: experimental models of trauma, shock and complex surgical maneuvers - Part 1

Substantial clinical and animal evidences indicate that the mesenteric circulatory bed, particularly the gut mucosa, is highly vulnerable to reductions in oxygen supply and prone to early injury in the course of hemodynamic changes induced by trauma, shock, sepsis and several complex surgical maneuvers. Gut hypoxia or ischemia is one possible contributing factor to gastrointestinal tract barrier dysfunction that may be associated with the development of systemic inflammatory response and multiple organ dysfunction syndrome, a common cause of death after trauma, sepsis or major surgeries. Monitoring gut perfusion during experiments may provide valuable insights over new interventions and therapies highly needed to reduce trauma and sepsis-related morbidity and mortality. We present our experience with gas tonometry as a monitor of the adequacy of gastrointestinal mucosal perfusion in clinical and experimental models of trauma, shock and surgical maneuvers associated with abrupt hemodynamic changes, such as aortic occlusion and hepatic vascular exclusion. Next issue we will be presenting our experience with gas tonometry in experimental and clinical sepsis.

Comparison of oesophageal and gastric air tonometry in patients with circulatory failure

BACKGROUND

Gastric PCO2 measured by balloon tonometry can estimate the adequacy of splanchnic perfusion. However, enteral feeding and gastric content can interfere with gastric PCO2 assessment. Tonometry in other sites of the body could avoid these problems. We therefore tested the hypothesis that oesophageal air tonometry would give results similar to gastric tonometry.

METHODS

We studied 20 consecutive patients (mean age 68 (SD 9) [range 49-81] yr, 18 males, SAPS II score 55 (SD 18), ICU mortality 55%) with circulatory disorders during mechanical ventilation in the intensive care unit. Tonometer probes were placed via the nose, one into the stomach and the other in the oesophagus. PCO2 was measured with two automated gas analysers, at admission and 30 min, 1, 2, 3, 32, 40, and 48 h thereafter.

RESULTS

One hundred and forty-eight paired measurements were obtained. Gastric PCO2 was greater than oesophageal PCO2 on admission (7.19 (1.43) vs 5.89 (0.73) kPa, P < 0.01) and subsequently. Differences between the measures correlated (r = 0.67) with the mean absolute value, indicating that overestimation increased as gastric PCO2 increased.

CONCLUSIONS

Oesophageal PCO2 is less than gastric PCO2, and the difference is greater when gastric PCO2 levels are greater. Air tonometry may not measure regional PCO2 levels in the oesophagus satisfactorily. Other methods and sites for carbon dioxide tonometry should be examined.

Skeletal muscle acidosis correlates with the severity of blood volume loss during shock and resuscitation

BACKGROUND

Continuous assessment of tissue perfusion and oxygen utilization may allow for early recognition and correction of hemorrhagic shock. We hypothesized that continuously monitoring skeletal muscle (SM) PO2, PCO2, and pH during shock would provide an easily accessible method for assessing the severity of blood loss and the efficacy of resuscitation.

METHODS

Thirteen anesthetized pigs (25-35 kg) underwent laparotomy and femoral vessel cannulation. Multiparameter fiberoptic sensors were placed in the deltoid (SM) and femoral artery. Ventilation was maintained at a PaCO2 of 40-45 mm Hg. Total blood volume (TBV) was measured using an Evans blue dye technique. Animals were bled for 15 minutes, maintained at a mean arterial pressure (MAP) of 40 mm Hg for 1 hour, resuscitated (shed blood + 2 times shed volume in normal saline) and observed for 1 hour. Four animals served as controls (sham hemorrhage). Blood and tissue samples were taken at each time point.

RESULTS

Blood loss ranged from 28.5-56% of TBV. SM pH and SM PO2 levels fell rapidly with shock. SM PO2 returned to normal with resuscitation; however, SM pH did not return to baseline. SM PCO2 significantly rose with shock, but returned to baseline promptly with resuscitation. There was a significant correlation between SM pH and blood volume loss at end shock (r2 = 0.73, p < 0.001) and recovery (r2 = 0.84, p < 0.001). Animals (n = 2) whose SM pH did not recover to 7.2 were found to have ongoing blood loss from biopsy sites and persistent tissue hypercarbia despite normal MAP.

CONCLUSION

Continuous multiparameter monitoring of SM provides a minimally invasive method for assessing severity of shock and efficacy of resuscitation. Both PCO2 and PO2 levels change rapidly with shock and resuscitation. SM pH is directly proportional to lost blood volume. Persistent SM acidosis (pH < 7.2) and elevated PCO2 levels suggest incomplete resuscitation despite normalized hemodynamics.

Dopamine-1 receptor stimulation attenuates the vasoconstrictive response to gut ischemia

The effects of fenoldopam, a dopamine-1 (DA-1) receptor agonist, were studied in two groups of anesthetized dogs before and after induction of splanchnic ischemia by way of hemorrhage. During the first portion of the experiment, both groups received fenoldopam (1.5 microg x kg(-1) x min(-1)) for 45 min followed by a 45-min washout. During the second portion, hemorrhage (10 ml/kg) was induced, followed by no intervention in group I (controls) and restarting of the fenoldopam infusion in group II. Prehemorrhage, fenoldopam increased composite portal blood flow by 33% (P < 0.01). After hemorrhage-induced splanchnic ischemia, fenoldopam restored portal vein blood flow to near baseline, maintained the splanchnic fraction of cardiac output, and attenuated the rise in gut mucosal PCO(2). DA-1 receptor stimulation increased portal blood flow and redistributed blood flow away from the serosal layer in favor of the mucosa during basal conditions and after hemorrhage, suggesting a more concentrated distribution of splanchnic DA-1 receptors within the mucosal layer vasculature. Fenoldopam maintained splanchnic blood flow during hypoperfusion and attenuated the splanchnic vasoconstrictive response to hemorrhage.

Comparison of the response of saline tonometry and an automated gas tonometry device to a change in CO2

OBJECTIVE

To examine the speed of response of saline tonometry and an automated gas tonometry system by using standard tonometry catheters.

DESIGN

In vitro validation study.

SETTING

Experimental research laboratory.

INTERVENTIONS

Tonometry catheters were placed in a test chamber designed to simulate the lumen of a hollow viscus and were exposed to a rapid change in CO2 from 0% to 5% or 10%. Measured CO2 over time was fit to a mathematical model to determine the response time constant (the time to reach 63% of the final value) for each system.

MEASUREMENTS AND MAIN RESULTS

Response time to a change in CO2 was significantly faster with the automated gas system than with traditional saline tonometry. The mathematical time constant for a 5% change in CO2 in a gas environment was 2.8 mins (95% confidence interval, 2.6-3.0 mins) for the gas and 6.3 mins (95% confidence interval, 5.8-7.3 mins) for the saline technique. These times were longer for the CO2 change in a liquid environment: The time constant was 4.6 mins (95% confidence interval, 4.5-4.7 mins) for the gas system and 7.8 mins (95% confidence interval, 7.15-8.6 mins) for the saline tonometry. There was a significantly lower final equilibration value for the CO2 measurement with saline tonometry. There was essentially no difference in time constants for each system for a 5% change compared with a 10% CO2 change, except for a slightly faster time constant for the gas tonometry system with a 5% change in the gas environment (5%: 2.8 mins vs. 10%: 3.3 mins).

CONCLUSIONS

The automated gas tonometry system has a significantly faster response to a change in CO2 than conventional saline tonometry.

Comparisons between sublingual and gastric tonometry during hemorrhagic shock

STUDY OBJECTIVES

To compare sublingual tissue PCO(2), a disarmingly simple and noninvasive measurement of the severity of perfusion failure, with gastric tonometric PCO(2) during hemorrhagic shock in five male domestic pigs weighing between 35 and 40 kg.

DESIGN

Prospective animal study.

SETTING

Animal laboratory in a research institution.

PARTICIPANTS

Domestic pigs.

INTERVENTIONS

Hemorrhagic shock was induced by a modification of the Wigger's method. BP was maintained at 50 mm Hg for 120 min followed by reinfusion of shed blood at a rate of 100 mL/min with the aid of an infusion pump.

MEASUREMENTS AND RESULTS

During bleeding, the mean arterial pressure decreased from an average of 127 to 42 mm Hg, and cardiac output decreased from 7.7 to 2.4 L/min. Arterial blood lactate concentration concurrently increased from 1.2 to 13.9 mmol/L. Sublingual PCO(2) (PslCO(2)) increased from 59 to 105 mm Hg, and gastric PCO(2) increased from 61 to 111 mm Hg. The correlation between time-coincident sublingual and gastric measurements of PCO(2) was r = 0.91 (p<0.0001). Bland-Altman analyses demonstrated a close correspondence between the two measurements. The reinfusion of shed blood promptly reversed the hemodynamic abnormalities and reestablished gastric and PslCO(2) to near baseline values. This contrasted with a delayed reversal of lactic acidosis.

CONCLUSIONS

Under experimental conditions of hemorrhagic shock, sublingual capnometry yielded measurements that were interchangeable with those of gastric tonometry.

End-points of resuscitation how much is enough?

Fluid resuscitation after traumatic hemorrhage has historically been instituted as soon after injury as possible. Patients suffering from hemorrhagic shock may receive several liters of crystalloid, in addition to colloid solutions, in an attempt to normalize blood pressure, heart rate, urine output, and mental status, which are the traditional end-points of resuscitation. Current theory and recent investigations have questioned this dogma. Resuscitation goals may be different between when the patient is actively hemorrhaging, and once bleeding has been controlled. Newer markers of tissue and organ system perfusion may allow a more precise determination of adequate resuscitation.

Tonometry of partial carbon dioxide tension in gastric mucosa: use of saline, buffer solutions, gastric juice or air

Tonometry of gastric mucosal partial carbon dioxide tension (PCO2) has been forwarded as a clinically applicable tool to monitor regional perfusion adequacy during circulatory failure. The technique is still not used widely, partly because of methodological problems. Current measurement principles are reviewed, with help of the report on PCO2 measurements in gastric juice and tonometer in this issue.

Effects of hyper- and hypoventilation on gastric and sublingual PCO(2)

We investigated the effects of hyper- and hypoventilation on gastric (Pg(CO(2))) and sublingual (Psl(CO(2))) tissue PCO(2) before, during, and after reversal of hemorrhagic shock. Pg(CO(2)) was measured with ion-sensitive field-effect transistor sensor and Psl(CO(2)) with a CO(2) microelectrode. Under physiological conditions and during hemorrhagic shock, decreases in arterial (Pa(CO(2))) and end-tidal (PET(CO(2))) PCO(2) induced by hyperventilation produced corresponding decreases in Pg(CO(2)) and Psl(CO(2)). Hypoventilation produced corresponding increases in Pa(CO(2)), PET(CO(2)), Pg(CO(2)), and Psl(CO(2)). Accordingly, acute decreases and increases in Pa(CO(2)) and PET(CO(2)) produced statistically similar decreases and increases in Pg(CO(2)) and Psl(CO(2)). No significant changes in the tissue PCO(2)-Pa(CO(2)) gradients were observed during hemorrhagic shock in the absence or in the presence of hyper- or hypoventilation. Acute changes in Pg(CO(2)) and Psl(CO(2)) should, therefore, be interpreted in relationship with concurrent changes in Pa(CO(2)) and/or PET(CO(2)).

Comparison of skeletal muscle PO2, PCO2, and pH with gastric tonometric P(CO2) and pH in hemorrhagic shock

OBJECTIVES

To monitor PO2, PCO2, and pH in the interstitium of skeletal muscle (PmO2, PmCO2, and pHm) during hemorrhage, shock, and resuscitation using fiber-optic sensors and to compare Pco2 and pH in the interstitium of gastric mucosa (PrCO2 and pHi) obtained using gastric CO2 tonometry.

DESIGN

Prospective, controlled observational study in an acute experimental preparation.

SETTING

Physiology laboratory in a university medical school.

SUBJECTS

Nine mongrel dogs (20 to 35 kg).

INTERVENTIONS

Anesthesia was induced with pentobarbital (25 mg/kg iv) and maintained (10 mg/hr) after hemorrhagic shock. Mechanical ventilation was established to maintain baseline PaCO2 approximately 35 torr. Arterial, venous, and pulmonary artery catheters were placed. Blood flow probes were placed around the right femoral artery and vein. A probe (0.5 mm in diameter) with fiber-optic PO2, PCO2, and pH sensors was placed percutaneously in the adductor muscle of the right thigh. A gastric tonometer catheter was placed in the stomach lumen. After baseline data collection, controlled hemorrhage to mean arterial pressure (MAP) of 45 to 50 mm Hg was maintained for 1 hr. Shed blood was then reinfused. Blood gas, hemodynamic, and gastric tonometric data were collected during shock and reinfusion at 30-min intervals and hourly after reinfusion for 4 hrs. Normothermia was maintained.

MEASUREMENTS AND MAIN RESULTS

PmO2 decreased rapidly from 42 +/- 13 torr (mean +/- sD) to 13 +/- 9 torr within 15 mins and to 6 +/-4 torr within 30 mins of MAP reaching 45 mm Hg, and it recovered to baseline with reinfusion. pHm decreased gradually from 7.23 +/-0.09 to 6.89 +/- 0.25 during the 1-hr shock period and increased slowly toward baseline after reinfusion. pHi decreased from 7.43 +/- 0.14 to 6.91 +/- 0.23, and on average it returned to baseline 2 hrs after reinfusion. PmCO2 increased from 50 +/- 12 to 113 +/- 49 torr, increased further to 124 +/- 73 torr during reinfusion, and returned slowly toward baseline after reinfusion. PrCO2 increased from 35 +/- 8 to 60 +/- 19 torr and returned to baseline within 15 mins after reinfusion. During shock and reinfusion, oxygen delivery, mixed venous PO2, mixed venous oxygen saturation, and PmO2 responded with similar time courses. After reinfusion, on average, PmO2 exceeded baseline PmO2 and mixed venous PO2, and oxygen availability exceeded demand, suggesting an oxygen consumption defect. On average, PmCO2 and pHm did not return to baseline values 4 hrs after reinfusion, suggesting the persistence of anaerobic metabolic effects in skeletal muscle beyond the relatively short time that is required to reestablish baseline MAP, blood flow rates, oxygen delivery, PrCO2, and pHi.

CONCLUSIONS

PmO2, PmCO2, and pHm, monitored simultaneously using fiber-optic sensors in a single, small probe placed percutaneously, appear to indicate greater severity of shock and more prolonged resuscitation than conventional systemic or gastric tonometric variables.

Sublingual capnometry: a new noninvasive measurement for diagnosis and quantitation of severity of circulatory shock

OBJECTIVE

To investigate the feasibility and predictive value of sublingual Pco2 (P(SL)CO2) measurements as a noninvasive and early indicator of systemic perfusion failure.

DESIGN

A prospective, criterion study.

SETTING

Emergency department and medical and surgical intensive care units of an urban community medical center.

PARTICIPANTS AND PATIENTS

Five normal human volunteers and 46 patients with acutely life-threatening illness or injuries.

INTERVENTIONS

Intra-arterial or automated cuff blood pressure and arterial blood lactate (LAC) were measured concurrently with P(SL)CO2.

RESULTS

P(SL)CO2 in five healthy volunteers was 45.2 +/- 0.7 mm Hg (mean +/- sD). Twenty-six patients with physical signs of circulatory shock and LAC >2.5 mmol/L had a P(SL)CO2 of 81 +/- 24 mm Hg. This contrasted with patients admitted without clinical signs of shock and LAC of <2.5 mmol/L who had a P(SL)CO2 of 53 +/- 8 mm Hg (p < .001). The initial P(SL)CO2 of 12 patients who died before recovery from shock was 93 +/- 27 mm Hg, and this contrasted with 58 +/- 11 mm Hg (p < .001) in hospital survivors. Increases in P(SL)CO2 were correlated with increases in LAC (r2 = .84; p < .001). When P(SL)CO2 exceeded a threshold of 70 mm Hg, its positive predictive value for the presence of physical signs of circulatory shock was 1.00. When it was <70 mm Hg, it predicted survival with a predictive value of 0.93.

CONCLUSION

P(SL)CO2 may serve as a technically simple and noninvasive clinical measurement for the diagnosis and estimation of the severity of circulatory shock states.

Decreases in organ blood flows associated with increases in sublingual PCO2 during hemorrhagic shock

Earlier studies demonstrated that not only the stomach but also the esophageal wall served as an appropriate site for estimating the severity of circulatory shock by using tonometric methods. We then conceived of the option of sublingual tonometry. In the present study, we tested the hypothesis that the changes in sublingual PCO2 serve as indicators of decreases in blood flow to sublingual and visceral tissue. In Sprague-Dawley rats, sublingual PCO2 increased from 50 to 127 Torr and arterial blood lactate increased from 0.9 to 11.2 mmol/l during bleeding. Sublingual blood flow simultaneously decreased to approximately 32% of preshock values. After reinfusion of shed blood, organ blood flows and sublingual PCO2 were promptly restored to near-baseline values. There were corresponding decreases in blood flows in the tongue, stomach, jejunum, colon, and kidneys during hemorrhagic shock. Increases in sublingual PCO2 were highly correlated with decreases in sublingual blood flow (r = 0.80), tongue blood flow (r = 0.81), gastric blood flow (r = 0.74), jejunal blood flow (r = 0.65), colon blood flow (r = 0.80), and renal blood flow (r = 0.75). Unbled control animals demonstrated no significant changes. Therefore, we anticipate that sublingual tonometry will provide a useful, noninvasive alternative for monitoring visceral PCO2.

Sublingual capnometry for diagnosis and quantitation of circulatory shock

We investigated sublingual tissue PCO2 during hemorrhagic and septic shock. Hemorrhagic shock was induced in 10 rats. Sublingual PCO2 increased from 45 to 125 mm Hg and arterial pressure declined from 138 to 49 mm Hg, end-tidal PCO2 decreased from 35 to 13 mm Hg, and cardiac index fell from 290 to 77 ml/min/kg. Arterial blood lactate increased from 0.9 to 15.8 mmol/L. Gastric PCO2 was measured in five animals and it increased from 46 to 87 mm Hg. No significant changes were observed in eight "sham" bled animals including the five animals in which gastric PCO2 was measured. Highly significant linear correlations (p < 0.001) between sublingual PCO2 and gastric PCO2 (r = 0.71), cardiac index (r = -0.74), and arterial lactate (r = 0.59) were documented. We subsequently investigated sublingual PCO2 in five animals in which sepsis was induced by intravenous infusion of live Staphylococcus aureus. Like hemorrhagic shock, highly significant linear correlations were observed between end-tidal PCO2 and cardiac index and between sublingual PCO2 and arterial blood lactate. Sublingual PCO2 promises to serve as a technically simple, noninvasive, and rapid response quantitator of severity of circulatory shock states.