Tissue capnometry as a monitoring strategy for critically ill patients: just about ready for prime time

Real-Time Measurements of Oral Mucosal Carbon Dioxide (POMCO2) Reveals an Inverse Correlation With Blood Pressure in a Porcine Model of Coagulopathic Junctional Hemorrhage

INTRODUCTION

Shock states that occur during, for example, profound hemorrhage can cause global tissue hypoperfusion leading to organ failure. There is an unmet need for a reliable marker of tissue perfusion during hemorrhage that can be followed longitudinally. Herein, we investigated whether longitudinal POMCO2 tracks changes in hemodynamics in a swine model of coagulopathic uncontrolled junctional hemorrhage.

MATERIALS AND METHODS

Female Yorkshire-crossbreed swine (n = 7, 68.1 ± 0.7 kg) were anesthetized and instrumented for continuous measurement of mean arterial pressure (MAP). Coagulopathy was induced by the exchange of 50 to 60% of blood volume with 6% Hetastarch over 30 minutes to target a hematocrit of <15%. A 4.5-mm arteriotomy was made in the right common femoral artery with 30 seconds of free bleeding. POMCO2 was continuously measured from baseline through hemodilution, hemorrhage, and a subsequent 3-h intensive care unit period. Rotational thromboelastometry and blood gases were measured.

RESULTS

POMCO2 and MAP showed no significant changes during the hemodilution phase of the experiment, which produced coagulopathy evidenced by prolonged clot formation times. However, POMCO2 increased because of the uncontrolled hemorrhage by 11.3 ± 3.1 mmHg and was inversely correlated with the drop (17.9 ± 5.9 mmHg) in MAP (Y = -0.4122*X + 2.649, P = .02, r2 = 0.686). In contrast, lactate did not significantly correlate with the changes in MAP (P = .35) or POMCO2 (P = .37).

CONCLUSIONS

Despite the logical appeal of measuring noninvasive tissue CO2 measurement as a surrogate for gastrointestinal perfusion, prior studies have only reported snapshots of this readout. The present investigation shows real-time longitudinal measurement of POMCO2 to confirm that MAP inversely correlates to POMCO2 in the face of coagulopathy. The simplicity of measuring POMCO2 in real time can provide an additional practical option for military or civilian medics to monitor trends in hypoperfusion during hemorrhagic shock.

Monitoring the tissue perfusion during hemorrhagic shock and resuscitation: tissue-to-arterial carbon dioxide partial pressure gradient in a pig model

Background

Despite much evidence supporting the monitoring of the divergence of transcutaneous partial pressure of carbon dioxide (tcPCO₂) from arterial partial pressure carbon dioxide (artPCO₂) as an indicator of the shock status, data are limited on the relationships of the gradient between tcPCO₂ and artPCO₂ (tc-artPCO₂) with the systemic oxygen metabolism and hemodynamic parameters. Our study aimed to test the hypothesis that tc-artPCO₂ can detect inadequate tissue perfusion during hemorrhagic shock and resuscitation.

Methods

This prospective animal study was performed using female pigs at a university-based experimental laboratory. Progressive massive hemorrhagic shock was induced in mechanically ventilated pigs by stepwise blood withdrawal. All animals were then resuscitated by transfusing the stored blood in stages. A transcutaneous monitor was attached to their ears to measure tcPCO₂. A pulmonary artery catheter (PAC) and pulse index continuous cardiac output (PiCCO) were used to monitor cardiac output (CO) and several hemodynamic parameters. The relationships of tc-artPCO₂ with the study parameters and systemic oxygen delivery (DO₂) were analyzed.

Results

Hemorrhage and blood transfusion precisely impacted hemodynamic and laboratory data as expected. The tc-artPCO₂ level markedly increased as CO decreased. There were significant correlations of tc-artPCO₂ with DO₂ and COs (DO₂: r = − 0.83, CO by PAC: r = − 0.79; CO by PiCCO: r = − 0.74; all P < 0.0001). The critical level of oxygen delivery (DO_2crit) was 11.72 mL/kg/min according to transcutaneous partial pressure of oxygen (threshold of 30 mmHg). Receiver operating characteristic curve analyses revealed that the value of tc-artPCO₂ for discrimination of DO_2crit was highest with an area under the curve (AUC) of 0.94, followed by shock index (AUC = 0.78; P < 0.04 vs tc-artPCO₂), and lactate (AUC = 0.65; P < 0.001 vs tc-artPCO₂).

Conclusions

Our observations suggest the less-invasive tc-artPCO₂ monitoring can sensitively detect inadequate systemic oxygen supply during hemorrhagic shock. Further evaluations are required in different forms of shock in other large animal models and in humans to assess its usefulness, safety, and ability to predict outcomes in critical illnesses.

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.

Tissue gas tensions and tissue metabolites for detection of organ hypoperfusion and ischemia

BACKGROUND

The aim of this study was to evaluate how tissue gas tensions and tissue metabolites measured in situ can detect hypoperfusion and differentiate between aerobic and anaerobic conditions during hemorrhagic shock. We hypothesized that tissue PCO(2) (PtCO(2)) would detect hypoperfusion also under aerobic conditions and detect anaerobic metabolism concomitantly with or earlier than other markers.

METHODS

Prospective experimental animal study with eight anesthetized pigs subjected to a continuous blood loss ∼8% of total blood volume per hour until death. We measured cardiac index, organ blood flows, and tissue levels of PO(2), PCO(2), glucose, pyruvate, lactate, and glycerol in intestine, liver, kidney, and skeletal muscle.

RESULTS

With reduction in blood flow to the organs under aerobic conditions, PtCO(2) increased ∼1-4 kPa from baseline. With the onset of tissue hypoxia there was a pronounced increase of PtCO(2), lactate, lactate-pyruvate (LP) ratio, and glycerol. Tissue pH and bicarbonate decreased significantly, indicating that metabolic acid was buffered by bicarbonate to generate CO(2).

CONCLUSION

Moderate tissue hypoperfusion under aerobic conditions is associated with increased PtCO(2), in contrast to metabolic parameters of ischemia (lactate, LP ratio, and glycerol) which remain low. From the onset of ischemia there is a much more rapid and pronounced increase in PtCO(2), lactate, and LP ratio. PtCO(2) can be used as a marker of hypoperfusion under both aerobic and anaerobic conditions; it gives an earlier warning of hypoperfusion than metabolic markers and increases concomitantly with or earlier than other markers at the onset of tissue anaerobiosis.

Sublingual capnometry: a non-invasive measure of microcirculatory dysfunction and tissue hypoxia

With improvement in supportive care patients rarely die from their presenting illness but rather from its sequela, namely sequential multi-organ failure. Tissue hypoxia is believed to be the causation of multi-organ dysfunction syndrome (MODS). The expedient detection and correction of tissue hypoxia may therefore limit the development of MODS. The standard oxygenation and hemodynamic variables (blood pressure, arterial oxygenation, cardiac output) which are monitored in critically ill patients are 'upstream' markers and provide little information as to the adequacy of tissue oxygenation. Global 'downstream' markers such as mixed venous oxygen saturation and blood lactate are insensitive indicators of tissue hypoxia. Sublingual PCO(2) is a regional marker of microvascular perfusion and tissue hypoxia that holds great promise for the risk stratification and end-point of goal directed resuscitation in critically ill patients. This paper reviews the technology and application of sublingual PCO(2) monitoring.

Monitoring therapeutic interventions in critically ill septic patients

Sepsis is the leading cause of admission to intensive care units in the United States. Although the treatment of sepsis is complex and multimodal, nutrition support plays an important role in the management of these patients. The diagnosis of sepsis, disease category, and severity of illness and the change in sepsis severity and organ function over time affect the delivery of nutrition support. This paper reviews the diagnostic criteria of sepsis, the use of "sepsis biomarkers," and regional and global markers of organ function in sepsis and quantitative measures of illness severity and organ dysfunction.

Regional carbon dioxide monitoring to assess the adequacy of tissue perfusion

PURPOSE OF REVIEW

Tissue dysoxia is now widely regarded as the major factor leading to organ dysfunction in critically ill patients. Recent data suggests that early aggressive resuscitation of critically ill patients, which limits and/or reverses tissue dysoxia may prevent progression to organ dysfunction and improve outcome. The traditional clinical and laboratory markers used to assess tissue dysoxia are, however, insensitive and have numerous limitations. Regional carbon dioxide monitoring appears to be ideally suited to monitoring the adequacy of resuscitation. This review provides an update on this evolving technology.

RECENT FINDINGS

Gastric intramucosal carbon dioxide as measured by gastric tonometry has proven to be useful as a prognostic marker, in evaluating the response to specific therapeutic interventions and as an end point of resuscitation. Gastric tonometry is, however, cumbersome and has a number of limitations that may have prevented its widespread adoption. The measurement of carbon dioxide in the sublingual mucosa by sublingual capnometry is technically simple, noninvasive, and provides near instantaneous information. Clinical studies have demonstrated a good correlation between gastric intramucosal carbon dioxide and sublingual mucosa carbon dioxide. Sublingual mucosa carbon dioxide responds more rapidly to therapeutic interventions than does gastric intramucosal carbon dioxide and may be a better prognostic marker.

SUMMARY

Sublingual capnometry may be the ideal technology for guiding early goal directed therapy. This technology may be useful for monitoring tissue oxygenation, titrating therapeutic interventions, and as an end point for resuscitation in critically ill and injured patients.

Assessment of Graded Intestinal Hypoperfusion and Reperfusion Using Continuous Saline Tonometry in a Porcine Model

OBJECTIVE

To evaluate effects of graded intestinal hypoperfusion and reperfusion on intestinal metabolic parameters as assessed by a modified continuous saline tonometry technique.

MATERIALS

Twelve barbiturate-anaesthetized female pigs.

METHODS

Measurements were performed prior to and during three predefined levels of superior mesenteric mean arterial blood pressure (P(SMA) 70, 50 and 30 mmHg, respectively, each 80 min long), obtained by an adjustable clamp around the origin of the superior mesenteric artery, and during reperfusion. We continuously measured jejunal mucosal perfusion (laser Doppler flowmetry), jejunal tissue oxygen tension (PO(2TISSUE); microoximetry) and intramucosal PCO(2) (continuous saline tonometry) and calculated net intestinal lactate production, mesenteric oxygenation, PCO(2) gap (jejunal mucosal PCO(2)-arterial PCO(2)) and pHi.

RESULTS

At P(SMA) 70 and 50 mmHg mesenteric oxygen uptake and net lactate production remained unaltered, in spite of decreased oxygen delivery. At these P(SMA) levels PCO(2) gap increased, while pHi and PO(2TISSUE) decreased. At P(SMA) 30 mmHg pronounced increases in PCO(2) gap and mesenteric net lactate production as well as marked decreases in PO(2TISSUE) and pHi were demonstrated. Data indicate absence of anaerobic conditions at an intestinal perfusion pressure (IPP)> or =41 mmHg, a pHi> or =7.22 or PCO(2) gap< or =15.8 mmHg.

CONCLUSIONS

Continuous saline tonometry detected intestinal ischemia as induced by graded reductions in IPP. A threshold could be defined above which intestinal ischemia does not occur.

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.

Small intestine intramucosal PCO(2) and microvascular blood flow during hypoxic and ischemic hypoxia

OBJECTIVE

To determine whether small intestine intramucosal PCO(2) and mucosal blood flow changes would be different between ischemic and hypoxic hypoxia.

DESIGN

Randomized animal experiment.

SETTING

Research laboratory.

SUBJECTS

Anesthetized, mechanically ventilated, and surgically instrumented pigs.

INTERVENTIONS

Systemic oxygen delivery was lowered in a stepwise manner to decrease it beyond critical oxygen delivery by lowering either FIO(2) or blood volume.

MEASUREMENTS AND MAIN RESULTS

In hypoxic hypoxia pigs (n = 6), arterial oxygen concentration and oxygen delivery decreases were achieved by progressively reducing arterial PO(2) while cardiac index remained unchanged. In ischemic hypoxia pigs (n = 5), oxygen delivery reduction was achieved by progressively reducing cardiac index while arterial PO(2) remained unchanged. In control pigs, oxygen delivery remained unchanged. The lowest oxygen delivery measured in both hypoxia and ischemia experiments was 3.60 +/- 0.26 vs. 2.93 +/- 0.77 mL x kg(-1) x min(-1), respectively (p =.23). At the lowest oxygen delivery level, differences between ischemic hypoxia and hypoxic hypoxia experiments were observed for arterial lactate concentration (468 +/- 308 vs. 1070 +/- 218 mmol/L, respectively; p =.03), mixed venous arterial PCO(2) difference (10 +/- 7 vs. 4 +/- 2 torr, respectively; p =.04), and small intestine mucosal blood flow (6.2 +/- 2.1 vs. 15.7 +/- 7.4 perfusion units, respectively; p =.02). Small intestine intramucosal-arterial difference was higher in ischemic hypoxia than in hypoxic hypoxia (52 +/- 15 vs. 31 +/- 12 torr, respectively; p =.03).

CONCLUSION

Small intestine intramucosal PCO(2) increases may indicate systemic oxygen uptake supply limitation in ischemic and hypoxic hypoxia related to conditions of mucosal flow stagnation and CO(2) generation.

Directly measured tissue pH is an earlier indicator of splanchnic acidosis than tonometric parameters during hemorrhagic shock in swine

OBJECTIVE

To compare tissue pH in the stomach, bowel, and abdominal wall muscle during hemorrhagic shock and recovery using tissue electrodes; also, to compare tissue electrode pH measurements to gastric intramucosal pH (pHi), gastric luminal PCO2, and PCO2 gap (gastric luminal CO2--arterial CO2) measured with an air-equilibrated tonometer.

DESIGN

Prospective animal study.

SETTING

University animal research laboratory.

SUBJECTS

Eight anesthetized, mechanically ventilated Yorkshire swine.

INTERVENTIONS

Hemorrhagic shock was initiated by withdrawing blood over a 15-min period to lower systolic blood pressure to 45 mm Hg. Shock was maintained for 45 mins and was followed by a 5-min resuscitation to normal blood pressure with a blood/lactated Ringer's (1:2) mixture. Recovery was monitored for 60 mins.

MEASUREMENTS AND MAIN RESULTS

pH was measured with electrodes in the submucosa of the stomach, the submucosa of the small bowel, and the abdominal wall muscle. Gastric luminal PCO2 was measured with an air-equilibrated tonometer and pHi and PCO2 gap were calculated. Each organ showed a different sensitivity to shock and resuscitation. The bowel pH responded most rapidly to the onset of hemorrhagic shock and had the largest change in tissue pH. The bowel also showed the most rapid recovery during resuscitation. The submucosal pH of the stomach responded more slowly than the bowel, but faster than the abdominal wall muscle pH, gastric PCO2 gap, or pHi. The smallest changes in organ pH as a result of hemorrhagic shock were seen in the abdominal wall muscle and the stomach as assessed by gastric tonometry.

CONCLUSIONS

Direct measurement of tissue pH indicates that intra-abdominal organ pH varies during hemorrhagic shock. The small bowel pH changes the most in magnitude and rapidity compared with stomach pH or abdominal wall muscle pH. Tonometrically derived parameters were not as sensitive in the detection of tissue acidosis during shock and resuscitation as pH measured directly in the submucosa of the stomach or small bowel.