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

Cerebral perfusion and metabolism with mild hypercapnia vs. normocapnia in a porcine post cardiac arrest model with and without targeted temperature management

Aim

To determine whether targeting mild hypercapnia (PaCO2 7 kPa) would yield improved cerebral blood flow and metabolism compared to normocapnia (PaCO2 5 kPa) with and without targeted temperature management to 33 °C (TTM33) in a porcine post-cardiac arrest model.

Methods

39 pigs were resuscitated after 10 minutes of cardiac arrest using cardiopulmonary bypass and randomised to TTM33 or no-TTM, and hypercapnia or normocapnia. TTM33 was managed with intravasal cooling. Animals were stabilized for 30 minutes followed by a two-hour intervention period. Hemodynamic parameters were measured continuously, and neuromonitoring included intracranial pressure (ICP), pressure reactivity index, cerebral blood flow, brain-tissue pCO2 and microdialysis. Measurements are reported as proportion of baseline, and areas under the curve during the 120 min intervention period were compared.

Results

Hypercapnia increased cerebral flow in both TTM33 and no-TTM groups, but also increased ICP (199% vs. 183% of baseline, p = 0.018) and reduced cerebral perfusion pressure (70% vs. 84% of baseline, p < 0.001) in no-TTM animals. Cerebral lactate (196% vs. 297% of baseline, p < 0.001), pyruvate (118% vs. 152% of baseline, p < 0.001), glycerol and lactate/pyruvate ratios were lower with hypercapnia in the TTM33 group, but only pyruvate (133% vs. 150% of baseline, p = 0.002) was lower with hypercapnia among no-TTM animals.

Conclusion

In this porcine post-arrest model, hypercapnia led to increased cerebral flow both with and without hypothermia, but also increased ICP and reduced cerebral perfusion pressure in no-TTM animals. The effects of hypercapnia were different with and without TTM.(Institutional protocol number: FOTS, id 14931).

Considering whole-body metabolism in hyperpolarized MRI through 13 C breath analysis-An alternative way to quantification and normalization?

PURPOSE

Hyperpolarized [1-¹³ C]pyruvate MRI is an emerging clinical tool for metabolic imaging. It has the potential for absolute quantitative metabolic imaging. However, the method itself is not quantitative, limiting comparison of images across both time and between individuals. Here, we propose a simple signal normalization to the whole-body oxidative metabolism to overcome this limitation.

THEORY AND METHODS

A simple extension of the model-free ratiometric analysis of hyperpolarized [1-¹³ C]pyruvate MRI is presented, using the expired ¹³ CO₂ in breath for normalization. The proposed framework was investigated in two porcine cohorts (N = 11) subjected to local renal hypoperfusion defects and subsequent [1-¹³ C]pyruvate MRI. A breath sample was taken before the [1-¹³ C]pyruvate injection and 5 min after. The raw MR signal from both the healthy and intervened kidney in the two cohorts was normalized using the ¹³ CO₂ in the expired air.

RESULTS

¹³ CO₂ content in the expired air was significantly different between the two cohorts. Normalization to this reduced the coefficients of variance in the aerobic metabolic sensitive pathways by 25% for the alanine/pyruvate ratio, and numerical changes were observed in the bicarbonate/pyruvate ratio. The lactate/pyruvate ratio was largely unaltered (<2%).

CONCLUSION

Our results indicate that normalizing the hyperpolarized ¹³ C-signal ratios by the ¹³ CO₂ content in expired air can reduce variation as well as improve specificity of the method by normalizing the metabolic readout to the overall metabolic status of the individual. The method is a simple and cheap extension to the hyperpolarized ¹³ C exam.

Cerebral perfusion and metabolism with mean arterial pressure 90 vs. 60 mmHg in a porcine post cardiac arrest model with and without targeted temperature management

AIM

To determine whether targeting a mean arterial pressure of 90 mmHg (MAP90) would yield improved cerebral blood flow and less ischaemia compared to MAP 60 mmHg (MAP60) with and without targeted temperature management at 33 °C (TTM33) in a porcine post-cardiac arrest model.

METHODS

After 10 min of cardiac arrest, 41 swine of either sex were resuscitated until return of spontaneous circulation (ROSC). They were randomised to TTM33 or no-TTM, and MAP60 or MAP90; yielding four groups. Temperatures were managed with intravasal cooling and blood pressure targets with noradrenaline, vasopressin and nitroprusside, as appropriate. After 30 min of stabilisation, animals were observed for two hours. Cerebral perfusion pressure (CPP), cerebral blood flow (CBF), pressure reactivity index (PRx), brain tissue pCO2 (PbtCO2) and tissue intermediary metabolites were measured continuously and compared using mixed models.

RESULTS

Animals randomised to MAP90 had higher CPP (p < 0.001 for both no-TTM and TTM33) and CBF (no-TTM, p < 0.03; TH, p < 0.001) compared to MAP60 during the 150 min observational period post-ROSC. We also observed higher lactate and pyruvate in MAP60 irrespective of temperature, but no significant differences in PbtCO2 and lactate/pyruvate-ratio. We found lower PRx (indicating more intact autoregulation) in MAP90 vs. MAP60 (no-TTM, p = 0.04; TTM33, p = 0.03).

CONCLUSION

In this porcine cardiac arrest model, targeting MAP90 led to better cerebral perfusion and more intact autoregulation, but without clear differences in ischaemic markers, compared to MAP60.

INSTITUTIONAL PROTOCOL NUMBER

FOTS, id 8442.

Near real-time bedside detection of spinal cord ischaemia during aortic repair by microdialysis of the cerebrospinal fluid

OBJECTIVES

Spinal cord ischaemia (SCI) remains the most devastating complication after thoraco-abdominal aortic aneurysm (TAAA) repair. Its early detection is crucial if therapeutic interventions are to be successful. Cerebrospinal fluid (CSF) is readily available and accessible to microdialysis (MD) capable of detecting metabolites involved in SCI [i.e. lactate, pyruvate, the lactate/pyruvate ratio (LPR), glucose and glycerol] in real time. Our aim was to evaluate the feasibility of CSF MD for the real-time detection of SCI metabolites.

METHODS

In a combined experimental and translational approach, CSF MD was evaluated (i) in an established experimental large animal model of SCI with 2 arms: (a) after aortic cross-clamping (AXC, N = 4), simulating open TAAA repair and (b) after total segmental artery sacrifice (Th4-L5, N = 8) simulating thoracic endovascular aortic repair. The CSF was analysed utilizing MD every 15 min. Additionally, CSF was collected hourly from 6 patients undergoing open TAAA repair in a high-volume aortic reference centre and analysed using CSF MD.

RESULTS

In the experimental AXC group, CSF lactate increased 3-fold after 10 min and 10-fold after 60 min of SCI. Analogously, the LPR increased 5-fold by the end of the main AXC period. Average glucose levels demonstrated a 1.5-fold increase at the end of the first (preconditioning) AXC period (0.60±0.14 vs 0.97±0.32 mmol/l); however, they decreased below (to 1/3 of) baseline levels (0.60±0.14 vs 0.19±0.13 mmol/l) by the end of the experiment (after simulated distal arrest). In the experimental segmental artery sacrifice group, lactate levels doubled and the LPR increased 3.3-fold within 30 min and continued to increase steadily almost 5-fold 180 min after total segmental artery sacrifice (P < 0.05). In patients undergoing TAAA repair, lactate similarly increased 5-fold during ischaemia, reaching a maximum at 6 h postoperatively. In 2 patients with intraoperative SCI, indicated by a decrease in the motor evoked potential of >50%, the LPR increased by 200%.

CONCLUSIONS

CSF is widely available during and after TAAA repair, and CSF MD is feasible for detection of early anaerobic metabolites of SCI. CSF MD is a promising new tool combining bedside availability and real-time capacity to potentially enable rapid detection of imminent SCI, thereby maximizing chances to prevent permanent paraplegia in patients with TAAA.

Transcutaneous PCO2 monitoring in critically ill patients: update and perspectives

The physiology of venous and tissue CO₂ monitoring has a long and well-established physiological background, leading to the technological development of different tissue capnometric devices, such as transcutaneous capnometry monitoring (TCM). To outline briefly, measuring transcutaneous PCO₂ (tcPCO₂) depends on at least three main phenomena: (I) the production of CO₂ by tissues (VCO₂), (II) the removal of CO₂ from the tissues by perfusion (wash-out phenomenon), and (III) the reference value of CO₂ at tissue inlet represented by arterial CO₂ content (approximated by arterial PCO₂, or artPCO₂). For this reason, there are, at present, roughly two clinical uses for tcPCO₂ measurement: a respiratory approach where tcPCO₂ is likely to estimate and non-invasively track artPCO₂; and a hemodynamic under-estimate use where tcPCO₂ can reflect tissue perfusion, summarized by a so-called "tc-art PCO₂ gap". Recent research shows that these two uses are not incompatible and could be combined. The spectrum of indications and validation studies in ICUs is summarized in this review to give a survey of the potential applications of TCM in critically ill patients, focusing mainly on its potential (micro)circulatory monitoring contribution. We strongly believe that the greatest benefit of measuring tcPCO₂ is not to only to estimate artPCO₂, but also to quantify the gap between these two values, which can then help clinicians continuously and noninvasively assess both respiratory and hemodynamic failures in critically ill patients.

Ischemia/reperfusion injury in porcine intestine - Viability assessment

AIM

To investigate viability assessment of segmental small bowel ischemia/reperfusion in a porcine model.

METHODS

In 15 pigs, five or six 30-cm segments of jejunum were simultaneously made ischemic by clamping the mesenteric arteries and veins for 1 to 16 h. Reperfusion was initiated after different intervals of ischemia (1-8 h) and subsequently monitored for 5-15 h. The intestinal segments were regularly photographed and assessed visually and by palpation. Intraluminal lactate and glycerol concentrations were measured by microdialysis, and samples were collected for light microscopy and transmission electron microscopy. The histological changes were described and graded.

RESULTS

Using light microscopy, the jejunum was considered as viable until 6 h of ischemia, while with transmission electron microscopy the ischemic muscularis propria was considered viable until 5 h of ischemia. However, following ≥ 1 h of reperfusion, only segments that had been ischemic for ≤ 3 h appeared viable, suggesting a possible upper limit for viability in the porcine mesenteric occlusion model. Although intraluminal microdialysis allowed us to closely monitor the onset and duration of ischemia and the onset of reperfusion, we were unable to find sufficient level of association between tissue viability and metabolic markers to conclude that microdialysis is clinically relevant for viability assessment. Evaluation of color and motility appears to be poor indicators of intestinal viability.

CONCLUSION

Three hours of total ischemia of the small bowel followed by reperfusion appears to be the upper limit for viability in this porcine mesenteric ischemia model.

Effects of Levosimendan on Cellular Metabolic Alterations in Patients With Septic Shock: A Randomized Controlled Pilot Study

Introduction:

Mitochondrial dysfunction and consequent cellular energetic failure play a key role in the development of sepsis-related organs failure. Evidence suggests that the pleiotropic effects of levosimendan may positively affect cellular metabolism during septic shock.

Objectives:

To investigate changes in the concentration of glucose, lactate, pyruvate, and glycerol in the extracellular fluid of the skeletal muscle following levosimendan administration in patients with septic shock.

Methods:

The study was designed as a prospective, double-blind, controlled, clinical pilot trial and performed in a multidisciplinary intensive care unit. After achieving normovolemia and a mean arterial pressure of at least 65 mm Hg, 20 septic shock patients were randomized to receive either levosimendan 0.2 μg/kg/min (n = 10), or dobutamine 5 μg/kg/min as active comparator (n = 10). Interstitial tissue concentrations of lactate, pyruvate, glucose, and glycerol were obtained by using muscle microdialysis. All measurements, including data from right heart catheterization, were obtained at baseline and every 6 h for the following 72 h after randomization.

The trial is registered with Clinicaltrials.gov, number NCT02963454.

Results:

Compared with dobutamine, levosimendan increased interstitial tissue pyruvate concentration (153.3 ± 73 and 187. 2 ± 13.5 vs. 210.7 ± 76.2 and 161 ± 64.6; P < 0.05), and lactate clearance (55 vs. 10). Lactate/pyruvate ratio was lower in the levosimendan group at the end of study period (37. 7 ± 18.9 and 29.3 ± 12.7 vs. 10.9 ± 4.5 and 31.4 ± 13. 2; P < 0.05).

Conclusion:

Although we investigated a small number of patients, our preliminary results suggest that levosimendan may improve cellular metabolic alterations in patients with septic shock.

Cerebral Effects of Targeted Temperature Management Methods Assessed by Diffusion-Weighted Magnetic Resonance Imaging

The aim of this randomized porcine study was to compare surface targeted temperature management (TTM) to endovascular TTM evaluated by cerebral diffusion-weighted magnetic resonance imaging (MRI): apparent diffusion coefficient (ADC), and by intracerebral/intramuscular microdialysis. It is well known that alteration in the temperature affects ADC, but the relationship between cerebral ADC values and the cooling method per se has not been established. Eighteen anesthetized 60-kg female swine were hemodynamically and intracerebrally monitored and subsequently subjected to a baseline MRI. The animals were then randomized into three groups: (1) surface cooling (n = 6) at 33.5°C using EMCOOLSpad^®, (2) endovascular cooling (n = 6) at 33.5°C using an Icy^® cooling catheter with the CoolGard 3000^®, or (3) control (n = 6) at 38.5°C using a Bair Hugger™. The swine were treated with TTM for 6 hours followed by a second MRI examination, including ADC. Blood and microdialysate were sampled regularly throughout the experiment, and glucose, lactate, pyruvate, glycerol, and the lactate/pyruvate ratio did not differ among groups, neither intracerebrally nor intramuscularly. Surface cooling yielded a significantly lower median ADC than endovascular cooling: 714 (634; 804) × 10^-6 mm²/s versus 866 (828; 927) × 10^-6 mm²/s, (p < 0.05). The surface cooling ADC was lowered to a range usually attributed to cytotoxic edema and these low values could not be explained solely by the temperature effect per se. To what extent the ADC is fully reversible at rewarming is unknown and the clinical implications should be further investigated in clinical studies.

Ethyl pyruvate protects against sepsis by regulating energy metabolism

Background

Ethyl pyruvate (EP) is a derivative of pyruvic acid that has been demonstrated to be a potential scavenger of reactive oxygen species as well as an anti-inflammatory agent. In this study, we investigated the protective effects of EP and its role in regulating the energy metabolism in the livers of cecal-ligation-and-puncture-induced septic mice.

Methods

The animals were treated intraperitoneally with 0.2 mL of Ringer’s lactate solution or an equivalent volume of Ringer’s lactate solution containing EP immediately after cecal ligation and puncture. Each mouse in the Sham group was only subjected to a laparotomy. At 30-, 60-, 180-, and 360-minute time points, we measured the histopathological alterations of the intestines, and the plasma levels of interleukin (IL)-1β, IL-6, IL-10, and tumor necrosis factor-α, and the total antioxidative capacity, malondialdehyde content, and lactate and lactate/pyruvate levels in livers. Furthermore, we detected the levels of adenosine triphosphate, total adenylate, and energy charge in the livers.

Results

Our results demonstrated that the administration of EP significantly improved the survival rate and reduced intestinal histological alterations. EP inhibited the plasma levels of IL-1β, IL-6, and tumor necrosis factor-α and increased the IL-10 level. EP significantly inhibited the elevation of the malondialdehyde, lactate, and lactate/pyruvate levels and enhanced the total antioxidative capacity levels in the liver tissues. The downregulation of the adenosine triphosphate, total adenylate, and energy charge levels in the liver tissues was reversed in the septic mice treated with EP.

Conclusion

The results suggest that EP administration effectively modulates the energy metabolism, which may be an important component in treatment of sepsis.

Tissue ischemia microdialysis assessments following severe traumatic haemorrhagic shock: lactate/pyruvate ratio as a new resuscitation end point?

BACKGROUND

Intensive care of severe trauma patients focuses on the treatment of haemorrhagic shock. Tissues should be perfused sufficiently with blood and with sufficient oxygen content to ensure adequate tissue oxygen delivery. Tissue metabolism can be monitored by microdialysis, and the lactate/pyruvate ratio (LPR) may be used as a tissue ischemia marker. The aim of this study was to determine the adequate cardiac output and haemoglobin levels that avoid tissue ischemia.

METHODS

Adult patients with serious traumatic haemorrhagic shock were enrolled in this prospective observational study. The primary observed parameters included haemoglobin, cardiac output, central venous saturation, arterial lactate and the tissue lactate/pyruvate ratio.

RESULTS

Forty-eight patients were analysed. The average age of the patients was 39.8 ± 16.7, and the average ISS was 43.4 ± 12.2. Hb < 70 g/l was associated with pathologic arterial lactate, ScvO2 and LPR. Tissue ischemia (i.e., LPR over 25) developed when CI ≤ 3.2 l/min/m(2) and Hb between 70 and 90 g/l were observed. Severe tissue ischemia events were recorded when the Hb dropped below 70 g/l and CI was 3.2-4.8 l/min/m(2). CI ≥ 4.8 l/min/m(2) was not found to be connected with tissue ischemia, even when Hb ≤ 70 g/l.

CONCLUSION

LPR could be a useful marker to manage traumatic haemorrhagic shock therapies. In initial traumatic haemorrhagic shock treatments, it may be better to maintain CI ≥ 3.2 l/min/m(2) and Hb ≥ 70 g/l to avoid tissue ischemia. LPR could also be a useful transfusion trigger when it may demonstrate ischemia onset due to low local DO2 and early reveal low/no tissue perfusion.

Myocardial tissue CO2 tension detects coronary blood flow reduction after coronary artery bypass in real-time†

BACKGROUND

Coronary stenosis after coronary artery bypass grafting (CABG) may lead to myocardial ischaemia and is clinically difficult to diagnose. In a CABG model, we aimed at defining variables that detect hypoperfusion in real-time and correlate with impaired regional ventricular function by monitoring myocardial tissue metabolism.

METHODS

Off-pump CABG was performed in 10 pigs. Graft blood flow was reduced in 18 min intervals to 75, 50, and 25% of baseline flow with reperfusion between each flow reduction. Myocardial tissue Pco2 (Pt(CO2)), Po2, pH, glucose, lactate, and glycerol from the graft supplied region and a control region were obtained. Regional cardiac function was assessed as radial strain.

RESULTS

In comparison with baseline, myocardial pH decreased during 75, 50, and 25% flow reduction (-0.15; -0.22; -0.37, respectively, all P<0.05) whereas Pt(CO2) increased (+4.6 kPa; +7.8 kPa; +12.9 kPa, respectively, all P<0.05). pH and Pt(CO2) returned to baseline upon reperfusion. Lactate and glycerol increased flow-dependently, while glucose decreased. Regional ventricular contractile function declined significantly. All measured variables remained normal in the control region. Pt(CO2) correlated strongly with tissue lactate, pH, and contractile function (R=0.86, R=-0.91, R=-0.70, respectively, all P<0.001). New conductometric Pt(CO2) sensors were in agreement with established fibre-optic probes. Cardiac output was not altered.

CONCLUSIONS

Myocardial pH and Pt(CO2) monitoring can quantify the degree of regional tissue hypoperfusion in real-time and correlated well with cellular metabolism and contractile function, whereas cardiac output did not. New robust conductometric Pt(CO2) sensors have the potential to serve as a clinical cardiac monitoring tool during surgery and postoperatively.

Anaerobic metabolism associated with traumatic hemorrhagic shock monitored by microdialysis of muscle tissue is dependent on the levels of hemoglobin and central venous oxygen saturation: a prospective, observational study

Background

Traumatic hemorrhagic shock resulting in tissue hypoxia is a significant cause of morbidity and mortality in polytraumatized patients. Early identification of tissue hypoxia is possible with microdialysis. The aim of this study was to determine the correlation between a marker of tissue hypoxia (L/P; lactate to pyruvate ratio) and selected parameters of systemic oxygen delivery (Hb; hemoglobin) and oxygen extraction (ScvO₂; central venous oxygen saturation). We also investigated the severity of tissue hypoxia over the course of care.

Methods

Adult patients with traumatic hemorrhagic shock were enrolled in this prospective, observational study. Microdialysis of the peripheral muscle tissue was performed. Demographic data and timeline of care were collected. Tissue lactate, pyruvate, glycerol, glucose levels, hemoglobin, serum lactate and oxygen saturation of the central venous blood (ScvO₂) levels were also measured.

Results

The L/P ratio trend may react to changes in systemic hemoglobin levels with a delay of 7 to 10 hours, particularly when systemic hemoglobin levels are increased by transfusion. Decrease in tissue L/P ratio may react to increase in ScvO₂ with a delay of up to 10 hours, and such a decrease may signify elimination of tissue hypoxia after transfusion. We also observed changes in the L/P trend in the 13 hours preceding a change in the hemoglobin level. Fluid administration, which is routinely used as a first-line treatment of hypovolemic shock, can cause hemodilution and decreased hemoglobin. When ScvO₂ decreases, increase in L/P ratio may precede the ScvO₂ trend by 10 or 11 hours. An increase in the L/P ratio is an early warning sign of insufficient tissue oxygenation and should lead to intensive observation of hemoglobin levels, ScvO₂ and other hemodynamic parameters. Patients who were treated more rapidly had lower maximal L/P values and a lower degree of tissue ischemia.

Conclusion

The L/P ratio is useful to identify tissue ischemia and can estimate the effectiveness of fluid resuscitation. An increase in the L/P ratio is an early warning sign of inadequate tissue oxygenation and should lead to more detailed hemodynamic and laboratory monitoring. This information cannot usually be obtained from global markers.

Monitoring tissue perfusion, oxygenation, and metabolism in critically ill patients

Alterations in oxygen transport and use are integral to the development of multiple organ failure; therefore, the ultimate goal of resuscitation is to restore effective tissue oxygenation and cellular metabolism. Hemodynamic monitoring is the cornerstone of management to promptly identify and appropriately manage (impending) organ dysfunction. Prospective randomized trials have confirmed outcome benefit when preemptive or early treatment is directed toward maintaining or restoring adequate tissue perfusion. However, treatment end points remain controversial, in large part because of current difficulties in determining what constitutes "optimal." Information gained from global whole-body monitoring may not detect regional organ perfusion abnormalities until they are well advanced. Conversely, the ideal "canary" organ that is readily accessible for monitoring, yet offers an early and sensitive indicator of tissue "unwellness," remains to be firmly identified. This review describes techniques available for real-time monitoring of tissue perfusion and metabolism and highlights novel developments that may complement or even supersede current tools.

Hepatic and abdominal carbon dioxide measurements detect and distinguish hepatic artery occlusion and portal vein occlusion in pigs

Hepatic artery (HA) occlusion and portal vein (PV) occlusion are the most common vascular complications after liver transplantation with an impact on mortality and retransplantation rates. The detection of severe hypoperfusion may be delayed with currently available diagnostic tools. Hypoperfusion and anaerobically produced lactic acid lead to increases in tissue carbon dioxide. We investigated whether the continuous assessment of the intrahepatic and intra-abdominal partial pressure of carbon dioxide (PCO(2) ) could be used to detect and distinguish HA and PV occlusions in real time. In 13 pigs, the HA and the PV were fully occluded (n = 7) or gradually occluded (n = 6). PCO(2) was monitored intrahepatically and between loops of small intestine. The hepatic and intestinal metabolism was assessed with microdialysis and PV as well as hepatic vein blood samples, and the results were compared to clinical parameters for the systemic circulation and blood gas analysis. Total HA occlusion led to significant increases in hepatic PCO(2) and lactate, and this was accompanied by significant decreases in the partial pressure of oxygen and glucose. PV occlusion induced a significant increase in intestinal PCO(2) (but not hepatic PCO(2) ) along with significant increases in intestinal lactate and glycerol. Gradual HA occlusion and PV occlusion caused steady hepatic and intestinal PCO(2) increases, respectively. Systemic clinical parameters such as the blood pressure, heart rate, and cardiac output were affected only by PV occlusion. In conclusion, even gradual HA occlusion affects liver metabolism and can be reliably identified with hepatic PCO(2) measurements. Intestinal PCO(2) increases only during PV occlusion. A combination of hepatic and intestinal PCO(2) measurements can reliably diagnose the affected vessel and depict the severity of the occlusion, and this may emerge as a potential real-time clinical monitoring tool for the postoperative course of liver transplantation and enable early interventions.